J. Anim Sci. 2007. 85:101-110. doi:10.2527/jas.2006-130
© 2007 American Society of Animal Science
ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Insulin sensitivity during pregnancy, lactation, and postweaning in primiparous gilts1
M.-C. Père2 and
M. Etienne
INRA, UMR Livestock Production Systems, Animal and Human Nutrition, 35590 Saint-Gilles, France
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Abstract
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The objectives were to examine changes in the insulin response during pregnancy, lactation, and postweaning in an experiment involving 10 primiparous Landrace x Large White gilts. Gilts were catheterized at 50 d of pregnancy, and tests were conducted at approximately 59 d of pregnancy (midpregnancy; MP), 106 d of pregnancy (end of pregnancy; EP), 17 d of lactation (L), and 9 d after weaning (PW), respectively. Changes in plasma glucose, insulin, and NEFA concentrations were studied after 3 different tests: ingestion of 1.3 kg of feed (meal test); a glucose tolerance test; and 2 euglycemic, hyperinsulinemic clamp tests, in which 20 and 55 ng of insulin·kg of BW–1·min–1 were infused during 150 min. Fasting concentrations of plasma glucose were less during L than during the other stages (P < 0.001). Concentrations of glucose and insulin increased after ingestion of the meal and decreased thereafter. Plasma insulin returned to basal concentrations at all stages, whereas glucose reached basal concentrations before the end of the meal at the PW test only. Postprandial concentrations of plasma glucose and area under the curve for insulin were greater during L than at the other stages (P < 0.05); both tended to be greater during EP than during MP or after weaning. Concentrations of NEFA were greater during L than at other stages before as well as after a meal (P < 0.001). Glucose half-life was greatest during L, least during MP and PW, and intermediate during EP. Compared with other stages, insulin secretion during the tolerance tests seemed to be delayed during L and, to a lesser extent, at EP. Irrespective of insulin dose, glucose infusion rates during the clamps did not differ between MP and PW, and were greater than during EP and L (P < 0.001). Plasma concentrations of NEFA decreased less rapidly during L than during the other stages. Gilts from EP developed a state of insulin resistance that was further accentuated during L. Changes in insulin responsiveness at MP, EP, and L may be an adaptation that allows gilts to acclimate to the increasing demand of glucose by the growing conceptus and the even greater demands of lactation.
Key Words: gilt • glucose tolerance • insulin resistance • lactation • nonesterified fatty acid • pregnancy
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INTRODUCTION
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Increased nutritional requirements of reproductive females that result from the development of fetuses or from milk production require important adaptations of the dams. Requirements during pregnancy have been studied more than those during lactation. Uterine blood flow has been reported to increase throughout gestation in ewes (Caton et al., 1983
), cows (Reynolds and Ferrell, 1987), and sows (Père and Etienne, 2000). In addition, physiological and metabolic changes occur throughout pregnancy.
During the last third of gestation, a decrease in insulin sensitivity, which is progressive and reversible, has been shown in women, rats, guinea pigs, rabbits, and ewes (Ryan et al. 1985; Leturque et al., 1987; Catalano et al., 1991). This adaptation is thought to develop to spare glucose for the pregnant uterus. Its occurrence has also been shown in sows (Père et al., 2000), but its amplitude seems to be less than in other species. This may help to explain the limited body energy reserves present in the newborn pig.
Lactation is a period of intense nutrient mobilization because of the considerable needs for milk synthesis. It is hypothesized that during lactation, females are also resistant to insulin to support the transfer of glucose to the udder. There are few data available to test this hypothesis, but insulin resistance has been reported at the beginning of lactation in goats (Grizard et al., 1988; Debras et al., 1989).
The purpose of the present experiment was to study changes in insulin sensitivity in primiparous gilts during the middle and end of pregnancy, lactation, and postweaning.
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MATERIALS AND METHODS
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Animals and Diets
All procedures were performed according to current French legislation on experimental animal care (authorization to experiment on living animals No. 04740 delivered by the French Ministry of Agriculture to M.-C. Père). The experiment was performed on 10 primiparous crossbred Landrace x Large White gilts. Gilts were inseminated with semen of a Large White boar at 230 ± 5 d of age after estrus synchronization (Regumate, Roussel-Uclaf, Paris, France). During the whole experiment, gilts were tethered and kept in farrowing crates (2.4 x 2.0 m) with slatted floors.
From insemination to parturition and after weaning, gilts were offered 2.6 kg/d of a standard pregnancy diet containing 12.5 MJ of DE/kg, 13.0% CP, and 0.6% lysine (as-fed basis). During lactation, females received a standard lactation diet (13.0 MJ of DE/kg, 17.4% CP and 0.9% lysine; as-fed basis). The diet composition is given in Table 1. Diets were pelleted and offered as 2 equal meals twice daily at 0900 and 1600. Feed allowance progressively increased between parturition and d 4 of lactation from 2.5 kg/d to a maximum of 5.0 kg/d, which was considered close to the ad libitum intake. This management allowed limiting feed refusals and equalized feed intake between primiparous gilts. Water was available ad libitum.
Experimental Procedure and Measurements
Surgical Procedure.
Surgery was performed at 50 d of pregnancy under general anesthesia induced with sodium thiopenthal (10 mg/kg of BW, given i.v.) and maintained with 2 to 5% halothane (Fluothane, Pitman-Moore, 77100 Meaux, France) in oxygen (2 to 3 L/min). An indwelling catheter (Silastic, Dow Corning Corporation, Midland, MI; 2.16 mm o.d., 1.02 mm i.d.) was implanted in the right external jugular vein, and an indwelling Tygon catheter (Tygon Tubing, Cole-Parmer Instrument Co., Vernon Hills, IL; 2.29 mm o.d., 1.27 mm i.d.) was inserted in the carotid artery for a distance of 35 cm. Catheters were gathered together, tunneled under the skin, externalized on the dorsal surface of the neck, and stored in a small bag sutured to the skin. The duration of surgery never exceeded 1 h. Postsurgical recovery, as assessed by feed intake of the gilt, required less that 1 d. Catheters were flushed 3 times weekly with a 10-mL normal saline solution (154 mM NaCl) containing 200 IU of heparin/mL.
Sampling.
The same sampling protocol was applied at each sampling time. Before a blood sample was taken via the arterial catheter, 5 mL of blood were withdrawn and discarded to eliminate dilution from the heparin block. Two to three milliliters of blood were then collected on ice with heparinized syringes and immediately centrifuged for 2 min at 8,500 x g at 4°C. Supernatant was divided into 2 subsamples and stored at –20°C until further analysis. After sampling, a 5-mL saline solution with heparin (20 IU/mL) was injected into the catheters to prevent blood clot development.
Experimental Design.
Three tests (a meal test, a glucose tolerance test, and an euglycemic, hyperinsulinemic clamp test) were used at 4 physiological stages: at approximately 59 d (midpregnancy, MP) and 106 d of pregnancy (end of pregnancy, EP), at 17 d of lactation (L), and at 9 d postweaning (PW), respectively. The BW of gilts before each physiological stage were 181.8 ± 3.7, 214.7 ± 2.3, 195.7 ± 3.5, and 179.8 ± 4.4 kg (mean ± SEM) at stages MP, EP, L, and PW, respectively. The 3 tests were applied during successive days at each physiological stage. They began in the morning after an overnight fasting period of 16 to 18 h.
Meal tests consisted of measuring plasma glucose, insulin, and NEFA concentrations after ingestion of a meal of 1.3 kg. Concentrations were measured in the carotid artery at 15 and 5 min before the meal, at 15-min intervals from 15 to 180 min, and at 240 min after the initiation of the meal (time 0). Concentration of plasma NEFA was measured at 15 and 5 min before and 15, 30, 45, 60, 75, 105, 135, 180, and 240 min after the beginning of the meal.
Glucose tolerance tests consisted of infusion of 0.5 g of glucose/kg of BW (1.665 M sterile glucose, Braun, Boulogne, France) through the jugular catheter. Infusion lasted about 5 min, after which 20 mL of a saline solution (154 mM NaCl) was injected to rinse the catheter. The first blood sample (considered as time zero) was taken immediately after infusion. Blood samples were also collected 30 and 15 min before the test and at 3-min intervals from 3 to 21 min, at 5-min intervals from 25 to 50 min, and at 10-min intervals until 90 min after time zero. All samples were analyzed for plasma glucose and insulin concentration.
Two euglycemic, hyperinsulinemic clamp tests were carried out at each physiological stage according to procedures described by De Fronzo et al. (1979) and Burnol et al. (1983b), with 2 rates of insulin infusion (human insulin, 40 U/mL, Actrapid, France; 20 and 55 ng·kg of BW–1·min–1). In a preliminary experiment, it was shown that glucose utilization in gilts was maximal at the greater insulin infusion rate. Insulin was diluted in about 35 mL of blood plasma obtained from the same gilt before the clamp test. The amount of insulin added was determined so that the infusion rate of the mixture was similar (150 to 170 µL/min) for all the clamps, regardless of the insulin dose and BW of the gilt. Euglycemia was defined for each female as the average of blood glucose concentrations measured every 15 min during the 60 min preceding the clamp study. During the clamp, a loading dose of insulin (80 or 220 ng of insulin/kg of BW for the 20 or 55 ng·kg of BW–1·min–1 infusion rates, respectively) was injected through the jugular catheter and followed immediately by the continuous infusion (syringe pump; kdS model 260, KD Scientific, Boston, MA) at a constant rate corresponding to the insulin dosage tested.
The duration of the infusion period was 150 min. The arterial concentration of glucose was maintained at the basal preinfusion concentration by infusing a glucose solution (1.665 M sterile glucose) at a variable rate through the jugular catheter with an IPC-04 peristaltic pump (Ismatec SA, Zürich, Switzerland). Glucose infusion began about 5 min after the beginning of the insulin infusion. The flask containing the glucose solution was kept on a scale. At 5-min intervals during the entire glucose clamp procedure, its weight was registered to measure the glucose infusion rate (GIR), and a 1-mL arterial blood sample was taken. Blood glucose was immediately measured on that sample by the glucose oxidase method using a glucose analyzer (YSI, Yellow Springs Instrument Co., Yellow Springs, OH) to adjust GIR. Every 15 min, 2 to 3 mL of blood was taken to immediately measure the blood packed-cell volume and for subsequent determination of plasma insulin and glucose concentrations, whereas NEFA were determined every 30 min.
Chemical Analyses
Plasma glucose and NEFA were determined by enzymatic methods adapted for a multianalyzer Cobas Mira apparatus (Roche, Basel, Switzerland). Glucose concentration was determined according to the glucose deshydrogenase method (Bergmeyer, 1974) using a commercial kit (reference 61273, BioMérieux, Marcy-l’Etoile, France). Plasma NEFA concentration was determined using an enzymatic kit (reference 46551, C-test Wako, Unipath, Dardilly, France). Assay sensitivities for glucose and NEFA were 19 and 3 µM, respectively. Plasma concentration of insulin was measured with a commercial RIA kit (CIS Bio International, 92192 Gif sur Yvette Cedex, France). The sensitivity was 3 µU/mL, and intra- and interassay CV were 6 and 19% at 64 µU/mL, respectively.
Calculations and Statistical Analysis
Arterial glucose, NEFA, and insulin concentrations measured in the 3 tests were analyzed by ANOVA with physiological stage (MP, EP, L, or PW) and time from meal or from glucose injection or from the beginning of insulin infusion as the main effects. Experimental unit was individual gilt. The model to test for differences in plasma glucose, insulin, and NEFA was physiological stage, gilt nested within physiological stage, and time. Differences between physiological stages were tested with the error term for gilt nested within physiological stage, and time was tested with the residual error. For the effects of physiological stage and of sampling time, means were separated by F-protected LSD. Statistical analyses were conducted using the GLM procedures of SAS (SAS Inst. Inc., Cary, NC).
For each tolerance test, the fasting concentrations of glucose and insulin corresponding to means of the –30 and –15-min values were calculated. Glucose half-life was estimated from individual regression equations relating the logarithm of glucose concentrations to the time between time 0 and the time at which the concentration passes through the fasting concentration on the declining portion of the curve.
For the meal test profiles as for the tolerance tests, the area under the curve (AUC) for insulin was calculated by linear interpolation of insulin concentrations between the measurements, using the fasting insulin concentration as the base line. This estimation was carried out between time 0 and the time at which the insulin concentration returned to the fasting level for the tolerance tests, and between 0 and 135 min or 0 and 240 min after the meal test. In the case of tolerance tests, the time required to reach 25, 50, or 75% of this area was also estimated by interpolation. Glucose infusion rate during the clamps (mg of glucose·kg of BW–1·min–1) was calculated during periods 60 to 90, 90 to 120, 120 to 150, and 60 to 150 min. The effect of physiological stage on all of these values was tested by using the GLM procedure of SAS (SAS Inst. Inc.), and means were separated by F-protected LSD.
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RESULTS
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Meal Tests
Plasma Concentrations of Glucose and Insulin During Feed Withdrawal.
Basal concentrations of glucose were less during lactation compared with all other physiological stages (3.95 vs. 4.90, 5.00, and 4.62 mM at the MP, EP, and PW stages, respectively; P < 0.001; Figure 1, panel A). Basal concentrations of insulin were not affected by physiological stage and averaged 8.7 µU/mL (Figure 1, panel B).
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Figure 1. Effects of physiological stage (MP = mid-pregnancy; EP = end of pregnancy; L = lactation; PW = postweaning) on plasma concentrations (means ± SEM) of glucose, insulin, and NEFA before (time < 0) and after meal tests. The meal was given at time 0. (A) Mean concentrations of glucose were lower during fasting and greater between 30 and 120 min and at 180 and 240 min after meals during L than after those during PW and EP (P < 0.05), during L than EP from 45 to 90 min and at 240 min (P < 0.01), and greater during EP than PW between 45 and 75 min and 150 and 180 min (P < 0.05). (B) Mean concentrations of insulin were greater during L than at all other stages between 60 and 105 min, and greater during EP than PW between 45 and 75 min (P < 0.05). (C) Mean concentrations of NEFA were greater during L than at all other stages at all sampling times (P < 0.001).
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Plasma Concentrations of Glucose After Meal Tests.
Plasma glucose concentration increased from 30 min after the initiation of the meal irrespective of the physiological stage (P < 0.001). It was maximal at 30 min at the PW stage and at 45 min at the other stages (Figure 1
, panel A). During lactation, glucose concentration remained elevated and did not differ from the maximum between 30 and 90 min. At all stages, the mean glucose concentration decreased after the mean peak concentration was attained but remained greater than the basal concentration, except at the PW stage where mean basal concentrations were reached at 240 min after the initiation of the meal. Between 30 and 120 min and at 180 and 240 min, glucose concentrations were greater during L than at MP and PW (P < 0.05 to P < 0.001), and during EP from 45 to 105 min and at 240 min (P < 0.01). From 30 to 75 min and 150 to 180 min, glucose concentration was greater at the EP than after weaning (P < 0.05) and intermediate at MP.
Plasma Insulin After Meal Tests.
Mean concentrations of insulin increased beginning at 15 min after the meals (P < 0.01) and were maximal at 30 min for the PW stage, and at 45 min for the other stages (Figure 1, panel B). Insulin returned to basal concentrations at 90 min for the EP and PW stages, at 105 min for the MP stage, and at 120 min for the L stage. Insulin concentrations were greater during L than for the other stages between 60 and 105 min after the meal (P < 0.05 to P < 0.01). Mean concentrations were greater at EP than after weaning between 45 and 75 min (P < 0.05), and intermediate at MP. Areas under the curves for insulin between 0 and 135 and 0 and 240 min depended on the stage of pregnancy (P < 0.05). The AUC was greater during L than in MP or PW, and intermediate at EP.
NEFA After Meal Tests.
Plasma NEFA varied with sampling time (P < 0.001; Figure 1, panel C) and started to decrease 15 min after the meal in stages MP and PW, 30 min after the meal in L, and 45 min at EP. Irrespective of the time after the meal, NEFA concentrations remained greater during L than during the other stages (P < 0.001).
Glucose Tolerance Tests
As observed for the meal tests, fasting glucose concentrations were less during L than during the other physiological stages (Figure 2, panel A). Glucose injection induced hyperglycemia similarly for all physiological stages (27.4 ± 0.4 mM). Glucose concentration decreased rapidly thereafter and returned to the fasting concentration after 25 min for stages MP and PW, and after 30 min at the end of pregnancy. It then continued to decrease for these 3 stages, became less than the basal level between 30 and 70 min, 35 and 50 min, and 45 and 50 min for the MP, PW, and EP stages, respectively (P < 0.05). Glucose concentration returned to the fasting concentration after 60 min (EP and PW) or 80 min (MP) after the end of the glucose injection. During L, the basal concentration was reached after 60 min, and primiparous gilts were never hypoglycemic thereafter. Plasma glucose was greater during L than during the other physiological stages between 9 and 50 min (P < 0.001). From 15 to 35 min, it was greater at EP than at MP (P < 0.05), and it was less after weaning than for any other stage from 6 to 35 min (P < 0.05).
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Figure 2. Effects of physiological stage (MP = mid-pregnancy; EP = end of pregnancy; L = lactation; PW = postweaning) on plasma concentrations (means ± SEM) of glucose and insulin after intravenous injection of 0.5 g of glucose/kg of BW. The first blood sample was drawn at time 0, immediately after glucose injection. (A) Glucose concentrations were greater between 9 and 50 min after glucose injection during L than during all other stages (P < 0.001), greater during EP than PW between 15 and 35 min (P < 0.01), and less during PW than all other stages from 6 to 35 min (P < 0.05). (B) Time needed to reach 25, 50, or 75% of the area under the insulin curve was greatest during L, and greater during EP than MP and PW (P < 0.001).
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Glucose injection caused hyperinsulinemia that was maximal at 12 min for stage EP, between 12 and 15 min for MP, between 3 and 15 min for PW, and between 0 and 30 min for L (Figure 2, panel B). It then decreased and returned to basal concentrations within 25 min after the end of the glucose injection for PW, within 30 min for EP and MP, and within 35 min for L. Insulin concentration was greater during lactation between times 25 and 60 min than for the other stages (P < 0.05 to P < 0.0001). The AUC for insulin was calculated between the end of the injection and the time when insulin concentration reached the preinfusion level. It did not differ between physiological stages. However, the time needed to reach 25, 50, or 75% of this area depended on the stage (P < 0.001; Table 2). It was greatest for L and greater for EP than for MP and PW (P < 0.001). Glucose half-life differed between the 4 stages (P < 0.0001; Table 2). It was greatest for L, least for MP and PW, and intermediate for EP.
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Table 2. Effects of physiological stage on the time (min) required to reach 25, 50, and 75% of the area under the insulin curve after an injection of 0.5 g of glucose/kg of BW and on glucose half-life (min)
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Euglycemic Hyperinsulinemic Clamps
During the clamp procedure, insulin infusion increased plasma insulin concentrations to steady state concentrations from 30 to 150 min after the start of infusion (Figure 3). These concentrations differed according to the rate of insulin infusion (P < 0.001) and depended on physiological stage (P < 0.05). Irrespective of the infusion rate of insulin, insulin concentration was greater for EP than for L and intermediate for MP and PW. During pregnancy, clamps were conducted so as to maintain plasma glucose concentrations at the average of that observed during fasting. However, as the basal concentration of glucose was less during L than during other physiological stages, the average of the values measured during pregnancy in each gilt was taken as baseline for that stage. Target concentrations of glucose were reached and maintained stable during the clamps (Figure 4). Steady state concentrations of arterial plasma glucose and insulin were reached 30 to 60 min after the initiation of the glucose clamp; therefore, the quantity of glucose administered was considered only after the first hour of infusion. In the present experiment, GIR needed to be doubled to maintain euglycemia when females were standing. To study gilt metabolism under standardized conditions and to avoid interactions with activity, values recorded when these changes occurred, representing 8% of whole measurements, were not included in the calculations. Their inclusion would have increased the total amount of glucose infused by about 3%. Irrespective of the physiological stage and rate of insulin infusion, GIR increased gradually during the first 120 min, reaching a plateau between 120 and 150 min for the insulin infusion rate of 55 ng·kg of BW–1·min–1. Mean quantities of glucose infused between 120 and 150 min were greater than between 60 and 90 min (P < 0.05). Irrespective of the insulin infusion rate and the period of the clamp considered, GIR was affected in a similar way by physiological stage (P < 0.001). It did not differ between stages EP and L, or between MP and PW, and was less for the first 2 stages than for the latter 2 (Figure 5).
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Figure 3. Effects of physiological stage (MP = mid-pregnancy; EP = end of pregnancy; L = lactation; PW = postweaning) on plasma concentrations (means ± SEM) of insulin during euglycemic, hyperinsulinemic clamps for insulin infusion rates of 20 and 55 ng·kg of BW–1·min–1, respectively. Insulin infusion began at time 0. Irrespective of the rate of insulin infusion, plasma insulin concentrations were greater during EP than during L, and intermediate during MP and PW (P < 0.05).
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Figure 4. Effects of physiological stage (MP = mid-pregnancy; EP = end of pregnancy; L = lactation; PW = postweaning) on plasma concentrations of glucose and on glucose infusion rate (GIR) during euglycemic, hyper-insulinemic clamps for insulin infusion rates of 20 (A) and 55 (B) ng·kg of BW–1·min–1. Values are means ± SEM. Insulin infusion began at time 0. Irrespective of the rate of insulin infusion, GIR from 60 to 150 min was less during EP and L than during MP and PW (P < 0.001). The upper curves in panels A and B refer to the blood glucose data, and the lower curves refer to the GIR data.
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Figure 5. Effects of physiological stage (MP = mid-pregnancy; EP = end of pregnancy; L = lactation; PW = postweaning) on glucose infusion rate (GIR) between 60 and 150 min during euglycemic, hyperinsulinemic clamps for insulin infusion rates of 20 and 55 ng·kg of BW–1·min–1, respectively. Values are means ± SEM. Irrespective of insulin infusion rate, GIR during MP and PW was greater than during EP and L (P < 0.001).
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Irrespective of the rate of insulin infusion, hyperinsulinemia under euglycemic conditions decreased plasma NEFA concentrations similarly (Figure 6). Plasma concentrations of NEFA, expressed as a percentage of their basal concentration, were affected by physiological stage during the first hour of the clamp (P < 0.001 to P < 0.05). For example, at 30 min for the 55 ng·kg of BW–1·min–1 infusion rate, NEFA concentrations declined less during L (40%) than during the other stages and less during EP (52%) than during the PW and MP stages (82 and 88%, respectively; P < 0.001).
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Figure 6. Effects of physiological stage (MP = mid-pregnancy; EP = end of pregnancy; L = lactation; PW = postweaning) on plasma concentrations of NEFA (expressed as a percentage of the basal concentration) during euglycemic, hyperinsulinemic clamps for insulin infusion rates of 20 (A) and 55 (B) ng·kg of BW–1·min–1, respectively. Values are means ± SEM. Insulin infusion began at time 0. Plasma NEFA concentrations decreased less rapidly during L than during all other stages at 30 and 60 min (P < 0.001 to P < 0.05), and during EP than MP or PW at 30 min (P < 0.01 to P < 0.05).
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DISCUSSION
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Postprandial concentrations of glucose for MP and EP were similar to those observed in pregnant women (Kühl, 1991; Piva et al., 1991) and pregnant nulliparous sows (Le Cozler et al., 1998; Père, 2001). However, they differed from those reported for multiparous pregnant sows fed a similar quantity (1.25 kg/meal) of the same diet used in the present experiment (Père et al., 2000). In that study, postprandial hyperglycemia was much less marked than in the present work and was followed by a transitory hypoglycemia and a return to basal concentrations thereafter. In the current study, postprandial glucose remained greater than basal concentrations throughout. Conversely, insulin concentrations changed in a similar manner in the current and previous experiments. Therefore, insulin may be less effective for controlling circulating glucose in primiparous than in multiparous sows.
Similar to multiparous sows (Père et al., 2000), fasting glucose was not affected by the stage of pregnancy in primiparous gilts, whereas it has been shown to decrease near parturition in other species (humans: Spellacy and Goetz, 1963; rats: Leturque et al., 1981; rabbits: Gilbert et al., 1984). On the other hand, plasma glucose was less in primiparous gilts in the current studies during L than during pregnancy, which is in agreement with Le Cozler et al. (1998) in sows and Burnol et al. (1983a) in rats. This is probably related to the considerable uptake of glucose by the udder for lactose synthesis (Dourmad et al., 2000). Moreover, plasma NEFA concentrations were considerably greater during L than during other physiological stages in the fasting and the fed state. This demonstrates that body lipids are intensively mobilized to contribute to the high energy requirements of lactating female swine (Etienne et al., 1985).
Although the same quantity of feed was used during meal tests conducted across all physiological stages, postprandial concentrations of glucose and insulin differed among physiological stages. At stages with the greatest postprandial increases in glucose, insulin concentration and insulin AUC were greater, indicating a lower effectiveness of insulin to stimulate glucose uptake. This was especially the case during L where the differences with the other stages were most marked. An intermediate pattern was observed at the end of pregnancy, whereas reduced concentrations of glucose in association with reduced insulin secretion was observed at MP and PW. In primiparous gilts, the glucose-lowering effects of insulin were less marked at EP than at MP. Similar results have been reported in multiparous sows (Père et al., 2000). A decreased efficiency of insulin is most evident during lactation, whereas after weaning, the situation returns to that observed at MP. Postprandial increases in glucose tended to be lower and peak insulin concentrations were reached earlier during the PW compared with MP, and PW was the only stage where concentrations of glucose returned to preprandial concentrations before 240 min after the meal. Therefore, insulin seemed to be even more effective at PW than during MP.
The clearance rate of plasma glucose after injection of similar quantities of glucose/kg of BW differed between physiological stages. Glucose half-life was longer during lactation than during MP and PW, and intermediate at the EP. Values measured during pregnancy were similar to those obtained in multiparous sows (Père et al., 2000). The increase in glucose half-life at the end of pregnancy agrees with the results of George et al. (1978), Bouillon-Hausman et al. (1986), and Schaeffer et al. (1991) in sows. Le Cozler et al. (1998) also found that glucose half-life at 13 d of lactation was greater than at 106 d of pregnancy (18.9 vs. 13.0 min) in primiparous gilts having large body reserves. During glucose tolerance tests, insulin secretion estimated by the AUC did not vary during the reproductive cycle. However, compared with other stages, insulin secretion was delayed during lactation, and to a lesser extent at the end of the pregnancy, as shown by the longer time needed for insulin to return to basal concentrations. Similar results have been reported in late pregnant sows (George et al., 1978; Schaeffer et al., 1991; Père et al., 2000) and in rats (Leturque et al., 1980). Therefore, the present results confirm the progressive development of insulin resistance at the end of pregnancy. This phenomenon is further accentuated during lactation and is accompanied by increased energy requirements. This could be related to the greater concentrations of NEFA observed during lactation. Indeed, the increase of plasma NEFA concentrations during pregnancy has been associated with the development of insulin resistance in rabbits (Gilbert et al., 1991, 1993). Furthermore, our results show that insulin resistance disappears relatively quickly after weaning. In ruminants, insulin resistance that has developed at the end of pregnancy continues at the beginning of lactation and decreases thereafter (Debras et al., 1989; Bell, 1995; Bell and Bauman, 1997). In sheep, Vernon et al. (1990) suggested that lactation results in insulin resistance in skeletal muscle, affecting glucose uptake by the muscle, whereas glucose uptake by the mammary gland remains unchanged. This could facilitate the preferential utilization of glucose by the mammary gland. Burnol et al. (1986, 1987) also showed that in rats the mammary gland is more sensitive to insulin than other tissues such as white adipose tissue or muscle.
In the current study, GIR needed to be doubled to maintain euglycemia during clamps when primiparous gilts were standing. This agrees with results showing that the energy cost of activity is 4 to 5 times greater in sows than in ruminant species and that standing position increases their maintenance requirement by almost 100% (Noblet et al., 1993). Similar to the other tests, the euglycemic hyperinsulinemic clamps demonstrated the presence of insulin resistance at the end of pregnancy and during lactation. However, contrary to results of other tests, the intensity of this resistance did not differ between these stages. The same rates of insulin infusion induced a reduced hyperinsulinemia in lactation than at the end of the pregnancy. This may have been due to a greater metabolic clearance of insulin during lactation, as reported in goats (Grizard et al., 1988). Moreover, the reference euglycemic concentration used for the clamp was the same during lactation and pregnancy, whereas basal concentrations of glucose were less in lactation. The quantity of glucose infused would thus have been less if concentrations of glucose had been maintained at the baseline observed during lactation. Furthermore, hyperinsulinemia resulted in a marked decline of NEFA that was less rapid during L than during the EP stage. This confirms that sows remain resistant to insulin during lactation and return to a normal degree of sensitivity after weaning. This change is of great magnitude and occurs relatively rapidly. However, would insulin resistance persist after lactation in sows that have mobilized their body reserves to a great extent? If so, it could explain the longer weaning-to-estrus interval in these females. Insulin has been reported to positively influence growth and development of granulosa cells (Booth, 1990) and follicular growth as indicated by increased steroid production in vitro (Purvis et al., 1997) and in vivo (Whitley et al., 1998), and reduces follicular atresia (Matamoros et al., 1990) in cyclic gilts. Persistence of insulin resistance in sows after weaning, in association with chronically elevated NEFA, could decrease insulin action and may explain the absence of a positive effect of insulin in feed-restricted sows during lactation in earlier experiments (Quesnel and Prunier, 1998). Collectively, the results of these studies suggest that primiparous females are less sensitive to insulin than multiparous sows, and this may contribute to an explanation for the lengthened postpartum anestrous interval commonly observed in primiparous females.
In conclusion, similar to mature sows, gilts become resistant to insulin at the end of pregnancy. This phenomenon may even be more pronounced than in multiparous sows. The condition is characterized by greater plasma glucose and insulin concentrations after a meal, increased glucose half-life, a delayed return of insulin to basal concentrations after an i.v. glucose load, and greater amounts of glucose infused during euglycemic hyperinsulinemic clamps. Primiparous females became even more resistant to insulin during lactation, and this was accompanied by greater plasma concentrations of NEFA. These adaptations may be related to the increased glucose requirements of the mammary gland during lactation. Primiparous gilts attain a normal sensitivity to insulin after weaning. However, it can be hypothesized that insulin resistance would persist in females that have extensively mobilized body energy reserves during lactation, which as a consequence can affect the interval to return to estrus.
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Footnotes
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1 The authors gratefully acknowledge C. David, V. Beaumal, J.-C. Hulin, and Y. Lebreton for their efficient technical assistance, J. Grizard (Unité de Recherche Nutrition et Métabolisme Protéique, INRA, 63122 Saint-Genès-Champanelle, France) and J. Simon (Unité de Recherches Avicoles, INRA, 37380 Nouzilly, France) for their advice concerning the experiment, and J. van Milgen (Unité Mixte de Recherches Systèmes d’Elevage, Nutrition Animale et Humaine, INRA, 35590 Saint-Gilles, France) for reading the manuscript. 
2 Corresponding author: Marie-Christine.Pere{at}rennes.inra.fr
Received for publication March 6, 2006.
Accepted for publication June 1, 2006.
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Abstract
INTRODUCTION
