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J. Anim. Sci. 2002. 80:2168-2175
© 2002 American Society of Animal Science

Metabolism of propionate and 1,2-propanediol absorbed from the washed reticulorumen of lactating cows1

N. B. Kristensen2, A. Danfær, B. A. Røjen, B.-M. L. Raun, M. R. Weisbjerg and T. Hvelplund

Danish Institute of Agricultural Sciences, Department of Animal Nutritionand Physiology, Box 50, DK-8830 Tjele, Denmark

2 Correspondence:
E-mail:
nielsb.kristensen{at}agrsci.dk.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
To investigate the metabolism of 1,2-propanediol (PPD) in lactating cows independently of normal rumen microbial metabolism, three ruminally cannulated lactating Holstein cows were subjected to three experimental infusion protocols under washed reticulo-ruminal conditions in a Latin square design. Reticulo-ruminal absorption rates were maintained for 420 min by continuous intraruminal infusion of VFA and PPD. With the control treatment, 1,246 ± 39 mmol/h of acetate and 213 ± 5 mmol/h of butyrate were absorbed from the reticulorumen. With the propionate treatment, 1,148 ± 39 mmol/h of acetate, 730 ± 23 mmol/h of propionate and 196 ± 5 mmol/h of butyrate were absorbed from the reticulorumen. With PPD treatment, 1,264 ± 39 mmol/h of acetate, 220 ± 5 mmol/h of butyrate and 721 ± 17 mmol/h of PPD were absorbed from the reticulorumen. Glucose irreversible loss rate (ILR), as well as the relative enrichment of plasma lactate and alanine, were determined by primed continuous infusion of [U-13C]glucose in a jugular vein. Treatments did not affect (P > 0.10) the plasma concentrations of glucose (4.2 ± 0.1 mmol/L), alanine (0.14 ± 0.01 mmol/L), or insulin (80 ± 25 pmol/L). The plasma concentration of lactate was higher (P < 0.05) with both propionate (0.84 ± 5 mmol/L) and PPD treatment (0.81 ± 5 mmol/L) compared with the control treatment (0.29 ± 0.5 mmol/L). The plasma concentration of pyruvate was higher (P < 0.05) with the propionate treatment (0.09 ± 0.01 mmol/L) compared with the control treatment (0.03 ± 0.01 mmol/L). The plasma concentration of 3-hydroxybutyrate was lower (P < 0.05) with the propionate treatment (0.15 ± 0.03 mmol/L) compared with the control treatment (0.40 ± 0.03). With the PPD treatment, the plasma concentrations of pyruvate and 3-hydroxybutyrate were in between the other treatments and tended (P < 0.10) to be different from both. The plasma concentration of PPD increased throughout the infusion period with the PPD treatment and reached a concentration of 4.9 ± 0.6 mmol/L at 420 min. The ILR of glucose was not affected (P > 0.10) by treatments (441 ± 35 mmol/h). The relative 13C enrichment of plasma lactate compared with that of glucose decreased (P < 0.05) with the PPD treatment compared with the control treatment (44 to 21 ± 3%). It was concluded that PPD has a low rate of metabolism in cows without a normal functioning rumen, although about 10% of the absorbed PPD was metabolized into lactate.

Key Words: Dairy Cattle • Gluconeogenesis • Metabolism • Propionic Acid • Propylene Glycol • Ruminants


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
It has been established for decades that 1,2-propanediol (PPD; propylene glycol) affects the metabolism of dairy cows (Johnson, 1954). However, the metabolic pathways of PPD in the cow are still not known and especially the relative importance of intraruminal and extraruminal pathways needs to be established. Emery et al. (1967) reported from a study based on a single cow that PPD was glucogenic in the classical sense (Krebs, 1963), that PPD administered orally primarily was absorbed intact from the rumen and that PPD was subsequently metabolized in the liver. It has been repeatedly observed that oral administration of PPD increases the ruminal propionate concentration (Waldo and Schultz, 1960; Clapperton and Czerkawski, 1972; Cozzi et al., 1996). Nevertheless, it is generally believed that the main site of metabolism is in the cow and not in the rumen as suggested by Emery et al. (1967) and Clapperton and Czerkawski (1972). There are, however, some contradictions to the suggestion that PPD is glucogenic and readily taken up by the liver: a) the enrichment of plasma glucose does not peak until 3 h after administration of radio labeled PPD (Emery et al., 1967), b) the irreversible loss rate of glucose did not increase following an oral dose of 520 g PPD (Palmquist and Brunengraber, 1997), and c) PPD continues to affect rumen metabolism for 12 h following an oral dose (Clapperton and Czerkawski, 1972). Our hypothesis was that a comparison between the glucogenic properties of propionate and PPD absorbed from the washed reticulorumen would enable an evaluation of the problem to distinguish between intraruminal and extraruminal pathways of PPD metabolism in lactating cows. The purpose of the present work was to study the effects of intraruminally administered propionate and PPD on some aspects of the intermediary metabolism in lactating cows under washed reticulorumen conditions.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Implications
 Literature Cited
 
The present experiment complied with the Danish Ministry of Justice, Law no. 382 (June 10, 1987), Act no. 726 (September 9, 1993) concerning experiments with animals and care of experimental animals.

Animals and Feeding

The three Holstein cows (BW 854, 697 and 741 kg; milk yield 14, 20 and 25 kg/d; days in milk 514, 356 and 309) used in the present experiment were selected from the lowest yielding part of our herd of rumen cannulated cows. These cows were selected because of uncertainty about how the milk yield and cow performance might be affected by the removal of the rumen contents for more than 8 h. Between sampling days, cows were given ad libitum access to a total mixed ration (in % of DM: corn silage, 32.6; rape seed cake, 25.0; ground barley, 20; grass silage, 10.0; barley straw, 6.6; barley silage, 5.0; CaCO3, 0.4; mineral mix, 0.4). The mineral mix contained 22% Ca, 5.5% P, 4% Mg, 5% Na, 600 IU/kg vitamin A, 160 IU/kg vitamin D3, 4500 PPM Zn, 4000 PPM Mn, 900 PPM Cu, 225 PPM I, 25 PPM Co, and 20 PPM Se.

Experimental Procedure and Sampling

Cows were subjected to three experimental treatments (control, propionate and PPD; Table 1Go) in a Latin square design. Only one cow was sampled per day. With the control treatment, only acetate and butyrate were absorbed from the reticulorumen (i.e., nonglucogenic substrates). The control treatment was compared to a positive control (propionate) and PPD. At least 1 wk passed between two samplings with the same cow to reduce potential carryover effects. On sampling days cows were fitted with temporary catheters in both jugular veins (polyethylene tubing i.d. 0.85 mm, Meadox Surgimed A/S, Stenløse, Denmark). The reticulorumen was emptied and washed with 3 x 10 L of tap water and 2 x 10 L of isotonic saline. Water, saline, and buffers were heated to a temperature of between 39 and 40°C before intraruminal administration. Thirty liters of rumen buffer was added to the rumen (Table 1Go) and continuous intraruminal infusion of rumen infusate (1.54 ± 0.02 L/h; Table 1Go) as well as gassing (30% CO2/70% N2) of the buffer in the rumen was initiated. The gas was finely distributed in the rumen buffer by forcing (1.5 bar) it through a helium filter from a HPLC solvent conditioner (LKB, Bromma, Sweden). To ensure that the rumen infusate was immediately mixed with the rumen buffer and did not cause any local damage to the reticulo-ruminal epithelium, the infusion tubing was mounted with the exit on the surface of the helium filter. A third tube with a 15-cm free floating end was mounted for obtaining rumen samples without the need to open the rumen fistula. The infusion/gassing device was kept 5 cm from the bottom of the ventral rumen sack by a lead (150 g). After initiation of intraruminal infusion and gassing, the position of the device was checked by hand and the rumen fistula was closed. The stopper used, in addition to the three attached tubes, had a 2-mm hole for equilibration of the pressure in the rumen. Because of the leaky stopper in the rumen cannula, the cows had to be prevented from lying on their left side during rumen infusions.


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  		Table 1. Composition of rumen buffers and rumen infusate (mmol/L)
 
The blood samples for determination of the background abundance of 13C in plasma metabolites were obtained after initiation of intraruminal infusion. Five to ten minutes after initiation of the intraruminal infusion a primed i.v. infusion of [U-13C]glucose was initiated. The start of the infusion of the priming dose was defined as time zero. The [U-13C]glucose infusate was prepared from 0.95 g of D-[U-13C]glucose (min. 99% 13C, Campro Scientific, Veenendaal, The Netherlands) dissolved in 500 mL of sterile saline. The solution was sterile filtered (Steritop, Millipore, Bedford, MA) into an autoclaved bottle and used within 24 h. The priming dose was 0.88 ± 0.06 mmol ([U-13C]glucose excess) and the infusion rate was 0.52 ± 0.01 mmol/h ([U-13C]glucose excess). The intraruminal infusion of rumen infusate and the i.v. infusion of [U-13C]glucose was maintained for > 420 min except on one sampling day (1 cow, control treatment) in which the infusion was terminated after 300 min. Every 60 min, 20 mL of rumen buffer was collected, rumen pH was measured immediately, and the sample was stored below -20°C until analysis.

Blood was sampled from the catheter opposite of the infusion catheter at times 10, 20, 30, 45, and then every 30 min until 420 min after the initiation of the primed continuous infusion of [U-13C]glucose. One aliquot of blood was stabilized with Na-heparin (Becton Dickinson, Brøndby, Denmark) and another aliquot with Na-heparin/NaF (Becton Dickinson). Blood samples were stored on ice until the end of the experiment when blood plasma was separated by centrifugation (3,000 x g, 20 min, 4°C) and stored below -20°C until analysis. After the collection of blood and rumen samples at time 420 min, the rumen was emptied and washed with 10 L of saline. The rumen buffer and saline collected was weighed and sampled for determination of VFA and PPD absorbed by the cows. The original rumen content was reintroduced to the rumen. During the experiment the rumen content was stored in 50-L plastic bowls and isolated with straw to keep it warm.

Following PPD treatments, urine was collected for 24 h and a blood sample was collected after termination of urine collection.

Analytical Procedures

The 13C abundance in plasma glucose was determined by GLC/isotope ratio mass spectrometry (Delta S, Finnigan MAT, Bremen, Germany) on the pentaacetate derivative of glucose using a CP SIL 19CB GLC column (30 m, 0.44 µm, 0.32 mm; Chrompack, analytical instruments as, Værløse, Denmark). The 13C abundance in the pentaacetate derivative was corrected for fractionation and carbon added during derivatization, as previously described (Tetens et al., 1995). The glucose derivative was prepared from 200 µL of Na-heparin/NaF stabilized plasma deproteinized by addition of 400 µL 0.15 M Ba(OH)2 and 400 µL of 0.20 M ZnSO4. After centrifugation (17,000 x g, 20 min, 4°C) the supernate was passed through a cation exchange resin (0.25 g, Dowex 50W X8, H+form, Sigma St. Louis, MO) in series with an anion exchange resin (0.25 g, AG1 X8, formate form, Bio-Rad Lab., Richmond, CA). The glucose was eluted with 2 x 2.5 mL of water and dried overnight in a vacuum centrifuge. One hundred microliters acetic anhydride plus 50 µL pyridine were added to the samples and they were incubated at 90°C for 60 min. The excess of reagent was stripped off in a stream of N2 at room temperature and the glucose derivative suspended in 100 µL of chloroform. Plasma glucose concentrations were determined on plasma stabilized with Na-heparin by the glucose oxidase procedure (Sera-Pak glucose, Bayer, Tarrytown, NY). Plasma concentrations of 3-hydroxybutyrate, pyruvate, lactate, alanine and insulin as well as the 13C abundance in plasma lactate and alanine were determined, as previously described (Kristensen et al., 2000). The concentration of VFA in rumen buffers was determined, as described by Kristensen (2000). The concentration of PPD in rumen buffers, blood plasma, and urine was determined by the procedure described by Needham et al. (1982).

Calculations and Statistical Methods

Atomic fraction excess (AFE) was calculated as the atomic fraction (AF, [13C / (12C + 13C)]) in samples - AF (background). The AF (background) was measured in samples collected before initiation of [U-13C]glucose infusion. The irreversible loss rate (ILR) of glucose was calculated as: infusion rate of [U-13C]glucosei x AFE of infused glucose/AFE of plasma glucoseij (Index i = sampling day 1 to 9 and index j = sample 1 to 17). The relative enrichment (RE) of plasma lactate and alanine compared with plasma glucose was calculated as: 100 x AFEij (lactate or alanine)/AFEij (glucose). The absorption rate of VFA and PPD from the reticulorumen was calculated as: (amount added to the rumen with rumen buffers and rumen infusate - amount removed with rumen buffer and wash water)/time of infusion. One cow did not get a primer infusion of [U-13C]glucose in the original design, and this experimental sampling day was replaced by a new sampling day at the end of the trial. Because no period effect was observed on any parameter tested, this modification of the design was paid no further attention. Treatment comparisons were analyzed, on means generated for the interval 120 to 420 min by ANOVA, in a 3 x 3 Latin square design (treatment, period, and cow) using the GLM procedure, of SAS (SAS Inst. Inc., Cary, NC). Treatment means were compared, using the PDIFF option of the LS MEANS statement of the GLM procedure, when an overall treatment effect was detected (P < 0.05). Effects of time of infusion were analyzed by comparing mean values of the interval 120 to 180 min, with mean values of the interval 360 to 420 min, by a two-tailed paired t-test using the means procedure of SAS. A significant difference was declared for P < 0.05.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Implications
 Literature Cited
 
Despite our initial concerns, the experimental protocol did not affect the milk production, feed intake or the apparent well-being of the cows during the days following sampling (data not shown).

Rumen Variables

The rumen pH was not affected by time of infusion (P > 0.10) but was higher (P < 0.05) with the propionate treatment (7.37 ± 0.02), compared with the control (7.23 ± 0.02) and the PPD (7.21 ± 0.02) treatments. The rumen concentrations, absorption rates, and the fractional absorption of infused VFA and PPD are shown in Table 2Go. The absorption rate of acetate and butyrate was not affected (P > 0.10) by treatment and the cows absorbed similar amounts of propionate and PPD on their respective treatments, enabling a direct comparison of the metabolism of these two nutrients. The ruminal concentrations of VFA and PPD stabilized (data not shown) after 2 to 3 h of infusions, at a level slightly below the initial concentrations in the rumen buffer (Table 1Go and Table 2Go). However, this deviation from steady state was considered to be minimal and to not affect treatment comparisons. All VFA and PPD disappearing from the rumen are assumed to have been absorbed in agreement with low recovery of VFA at the duodenum (Kristensen, 2001). By removing the normal rumen content, we assume that we can evaluate animal metabolism of PPD independently of the microbial metabolism, although there are still microbial activity in the gastrointestinal tract distal to the rumen and on the rumen epithelial surface.


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  		Table 2. Reticuloruminal concentrations, absorption rates, and percentage absorbed of volatile fatty acids and 1,2-propanediol (PPD) infused into the washed reticulorumen of lactating cows
 
Isotope dilution

In the present experiment, the mean 13C enrichment of plasma glucose was about 0.13% (AFE 0.00128 ± 0.00005). Even though the analyzed derivative of glucose was diluted with 10 carbons this low level of enrichment is possible when using a GLC–isotope ratio mass spectrometer (Tissot et al., 1990), since it also enables relatively inexpensive measurements of glucose ILR in large animals. Figure 1Go shows the time course of AFE/infusion rate of [U-13C]glucose. Based on a preliminary experiment, a slightly decreasing ILR of glucose was expected during the infusion period. It was, therefore, decided that the primer should enrich the cows above the expected quasi steady state level in order to be able to use the inflection point of the enrichment curve to identify: when the observed changes in AFE reflected initial distribution of the infused tracer and when the changes in AFE reflected changes in the ILR of glucose. Figure 1Go shows that the inflection point of the enrichment curve is reached between 60 and 120 min after initiation of [U-13C]glucose infusion. The data analysis was therefore based on means of the period 120 to 420 min of [U-13C]glucose infusion. Changes in the AFE of glucose after 120 min were relatively small compared to the time scale, and were assumed not to affect the validity of glucose ILR estimates. The ILR of glucose was not affected (P > 0.10) by treatment and the numerical trend shown in Figure 1Go and Table 3Go reflects that one cow (the cow with the highest milk yield) responded with an increased ILR to the propionate treatment (711 mmol/h), compared with the control treatment (391 mmol/h). Except for this one cow on the propionate treatment, all observations on glucose ILR were within the interval of 373 to 452 mmol/h (n = 8). As also shown in Table 3Go, there was not even a numerical tendency towards an increased ILR in the PPD treatment compared with the control treatment. In fed cows, hepatic gluconeogenesis from propionate is generally assumed to account for 50 to 60% of the glucose ILR (Danfær et al., 1995). Due to the expected quantitative importance of propionate absorbed from the rumen, and the rapid change of the dietary conditions of the cows induced by removing the normal ruminal content, we had expected a measurable difference between control and propionate treatments. In fact, the propionate absorbed with the propionate treatment could account for about 68% of the ILR of glucose. However, even in high yielding cows the liver may contain 3% glycogen (Bergman, 1971). Assuming the liver weight is 2% of BW (Johnson et al., 1990), the cows in the present experiment had on average at least 2.5 moles of glucose available for maintaining the plasma glucose concentration and glucose ILR. The glycogen stores of the cows were therefore at least at a level comparable to the potential glucose production from the infused propionate. This may explain the lack of an observable difference in glucose ILR between the propionate and the control treatment. Another explanation would be an increased gluconeogenesis from AA possibly originating from protein breakdown in the liver. Therefore, larger treatment effects could be expected by increasing the duration of the infusion periods, although the decrease (P < 0.001) in the ILR of glucose showed no consistent pattern with the treatments (mean decrease in ILR of glucose 68 ± 12 mmol/h). We were not able to demonstrate a direct glucogenic effect of PPD in the present study, but the lack of a propionate effect on the glucose ILR precludes the conclusion that PPD can be considered nonglucogenic in cows. Brunengraber and Palmquist (1997) also found no effect on glucose ILR from an oral dose of PPD. However, the lack of a propionate effect indicates that the glucose ILR approach may be of limited value, at least in low or moderate yielding cows, because of the ability of the cows to maintain a constant ILR of glucose despite a sudden removal of glucogenic substrates from the rumen.



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  		Figure 1. Carbon-13 enrichment of plasma glucose following primed continuous infusion of [U-13C]glucose in v. jugularis. The enrichment is given as atomic fraction excess (AFE) divided by infusion rate of [U-13C]glucose to enable direct treatment comparisons. Treatments were: 1) control (squares; washed reticulorumen added 30 L of bicarbonate buffer containing acetate and butyrate + continuous intraruminal infusion of acetate and butyrate), 2) Propionate (circles; control + propionate), and 3) PPD (triangles; control + 1,2-propanediol). Each value is the mean of three cows with bars denoting ± SE except the last four data points with the control treatment, which is the mean of only two cows.

 

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  		Table 3. Irreversible loss rate (ILR) of glucose, relative 13C enrichment (RE) of plasma lactate and alanine compared with glucose, and plasma concentrations
 
The RE of alanine was not affected (P > 0.10) by treatment (Table 3Go). Treatments tended (P = 0.07) to affect the RE of lactate, and data in Figure 2Go shows a stringent decrease in the RE of lactate with the PPD treatment compared with the control treatment. Again, the overall F-test is weak because the cows reacted differently to the propionate treatment. However, because the difference between the control and the PPD treatment is so consistent, it seems reasonable to refuse the hypothesis that RE of lactate is not different with the control and the PPD treatments (P < 0.05; pairwise comparison of the control and PPD treatments). This means that a smaller fraction of the lactate carbon originates from glucose with the PPD treatment compared with the control treatment; this could be caused by metabolism of absorbed PPD into lactate. This is in agreement with the earlier demonstrated lactate-to-PPD interconversions in nonruminants (Miller and Bazzano, 1965), and the increased plasma lactate concentrations in cows dosed orally or i.v. with PPD (Palmquist and Brunengraber, 1997). The metabolism of PPD into lactate via alcohol and aldehyde dehydrogenases was also in agreement with the recovery of labeled carbon from PPD in glucose in the study of Emery et al. (1967). However, this could also have been caused by intraruminal metabolism of PPD into propionate. The ILR of lactate was not measured in the present experiment. In other studies, this parameter varied from 30–100% of the glucose ILR (Van der Walt et al., 1983; Perry et al., 1994). If it is assumed that: a) ILR of lactate is 30% of the glucose ILR (carbon basis), and b) the increase in non-glucose carbon flux to lactate is due to metabolism of PPD into lactate, then metabolism of 7.5% of the PPD dose or 54 mmol/h of PPD could account for the observed decrease in the RE of lactate. This rate of metabolism is lower than previously found: Emery et al. (1967), 20%/h in the rumen alone; Giesecke (1974), 14 to 27%/h in the rumen alone; Palmquist and Brunengraber (1997), 28 to 45%/h after oral dose of PPD]. This suggests that the rumen does have a predominant role in PPD metabolism.



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  		Figure 2. Relative enrichment (RE) of plasma lactate compared with plasma glucose following primed continuous infusion of [U-13C]glucose in v. jugularis. Treatments were: control (squares; washed reticulorumen added 30 L of bicarbonate buffer containing acetate and butyrate + continuous intraruminal infusion of acetate and butyrate), and PPD (triangles; control + 1,2-propanediol). Each value is the mean of three cows with bars denoting ± SE except the last four data points with the control treatment, which is the mean of only two cows.

 
Plasma and Urine Concentrations

The plasma concentration of PPD increased (Figure 3Go) during the infusion period with the PPD treatment and reached a level of 4.9 ± 0.6 mmol/L. In samples obtained with control or propionate treatments PPD was not detectable in plasma or urine. If it is assumed that the distribution space of PPD in the cows is 90% (the substance is soluble in both fat and water), the plasma concentration at the end of the study indicates that as much as 67% of the absorbed PPD could have been stored in the body. Even though it appears that PPD can be metabolized via lactate, as indicated by the decreased RE of plasma lactate, either the affinity for PPD or the capacity for its metabolism by that pathway is limited. The PPD concentration in urine collected during the 24 h following the end of the PPD treatments was 16 ± 3 mmol/L. The urine collection was not attended during the nights, and quantitative urine collection was probably only obtained for one of the cows. From this cow 25 L of urine was collected in 24 h. If it is assumed that this value, as well as the urine concentration, is representative for all three cows, then about 8% of the absorbed PPD was excreted into the urine. No PPD could be detected in blood plasma obtained 24 h after the end of the PPD infusions. It can be assumed that at least 90%, or about 4.5 moles of PPD, was metabolized by the cows during the infusion period and the following 24 h. This assumption is based on: a) less than 10% of the absorbed PPD could be accounted for in the urine, b) no PPD was left in the blood plasma after 24 h, and c) there is no reason to believe that PPD would be present in the feces in any higher concentrations than in the blood plasma. This leads to a mean metabolic rate of PPD of 145 mmol/h, or about 3 times the estimated metabolism of PPD into lactate. It has generally been assumed that the quantitative most important metabolic pathways for PPD had to be within the cow because of the rapid absorption rate from the rumen (Emery et al., 1967; Clapperton and Czerkawski, 1972). However, in these previous studies absorption of PPD and its dilution in the body did not prevent PPD from diffusing back into the rumen as soon as the intraruminal metabolism had reduced the ruminal concentration. Because of the delayed PPD metabolism under washed reticulorumen conditions, it is tempting to speculate that a major part of the PPD metabolism in the 24-h period following the PPD treatment (where the cows had a normal functioning rumen) could take place in the rumen. Adaptation of the rumen microbiota to PPD has previously been shown not to be a prerequisite for ruminal metabolism of PPD (Clapperton and Czerkawski, 1972; Giesecke, 1974). This would also be in agreement with the higher metabolism rate of PPD found in cows with normal rumen (see above) compared with the present study.



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  		Figure 3. Plasma concentration of 1,2-propanediol in cows with 30 L of bicarbonate buffer containing acetate, butyrate, and 1,2-propanediol added to the washed reticulorumen; intraruminal infusion of these nutrients maintains a constant absorption rate throughout the study. Each value is the mean of three cows with bars denoting ± SE.

 
The plasma concentrations of glucose and alanine were not affected (P > 0.10) by treatment (Table 3Go). However, the glucose concentration tended (P = 0.07) to decrease during the time of infusion (4.3 to 4.1 ± 0.1 mmol/L). The small numerical magnitude of the decrease in glucose concentration does not violate the assumption of steady state conditions underlying the isotope dilution method. The insulin concentration in plasma was not affected (P > 0.10) by treatment, but decreased (P < 0.05) during the infusion period (88 to 70 ± 21 pmol/L). The numerical treatment effect on the plasma insulin concentration shown in Table 3Go is apparently large. However, only the highest yielding cow (also the only cow responding with increased glucose ILR with the propionate treatment) responded with a large increase in the insulin concentration when comparing the control (27 pmol/L) and the propionate (220 pmol/L) treatments. The plasma concentration of pyruvate and lactate increased (P < 0.05) with the propionate treatment, compared with control. The plasma lactate concentration increased (P < 0.05) with the PPD, but was not different (P > 0.10) when comparing the PPD and the propionate treatments. However, the plasma concentration of pyruvate with the PPD treatment was apparently between the levels of the other two treatments (Table 3Go). The effects of PPD on plasma concentrations of lactate and pyruvate as well as on the RE of lactate all support that PPD is partly glucogenic under washed reticuloruminal conditions. The plasma concentration of 3-hydroxybutyrate decreased (P < 0.05) with the propionate treatment and tended (P < 0.10) to differ from both the control and the propionate treatments with the PPD treatment. Increased plasma insulin concentrations have been reported in a number of studies where PPD has been dosed orally in ruminants (Studer et al., 1993; Christensen et al., 1997; Palmquist and Brunengraber, 1997; Bremmer et al., 2000). This effect was completely absent in the present study, suggesting that PPD is not a direct insulin secretagogue as discussed by Studer et al. (1993). It is, however, noteworthy that we observed a tendency of a depression in the plasma 3-hydroxybutyrate concentration with the PPD treatment. This depression was not due to differences in butyrate absorption and hence may be explained by either an increased tissue affinity for 3-hydroxybutyrate or a decreased ketogenesis from endogenous fatty acids. Contrary to other studies showing a decrease in the plasma NEFA (Studer et al., 1993; Palmquist and Brunengraber, 1997) or plasma 3-hydroxybutyrate concentration (Hamada et al., 1982) following PPD treatment, it can be deduced from the present study that the effect on 3-hydroxybutyrate is not exclusively related to rumen metabolism of PPD into propionate. This would suggest that PPD itself, or an endogenous metabolite of PPD, affects lipid metabolism.

In conclusion, our data suggests: 1) PPD is partly metabolized into lactate and 2) PPD decreases the fat mobilization and/or increases the tissue affinity for 3-hydroxybutyrate directly or via an endogenous metabolite. We were not able to demonstrate that PPD was glucogenic in excess of the fraction metabolized into lactate. However, the experiment did not show that PPD could not be glucogenic in excess of the lactate response because also the propionate treatment failed to give a consistent response in the glucose ILR compared with the control. The accumulation of PPD in the blood plasma suggests that PPD is not efficiently metabolized in cows under washed reticulorumen conditions. From this observation, it can be inferred that under normal conditions the rumen microbiota are responsible for the majority of PPD metabolism in cattle.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
Literature Cited
 
This study questions well-established concepts of the metabolism of 1,2-propanediol (propylene glycol) in ruminants. Even when readily absorbed and partly metabolized into lactate, 1,2-propanediol was not efficiently metabolized in cows under washed reticulorumen conditions. Under normal feeding conditions, 1,2-propanediol may affect the ruminant via intraruminal metabolism, through not yet identified endocrine responses, or a combination thereof.


   Footnotes
 
1 The skillful technical assistance of E. Serup and H. Pedersen is gratefully acknowledged. We wish to thank J. Adamsen and P. Løvendahl for the analyses of plasma insulin as well as J. B. Clausen and T. Larsen for the analyses of plasma glucose concentrations. The Danish Agricultural and Veterinary Research Council supported this study (grant 9801555). Back

Received for publication November 15, 2001. Accepted for publication February 25, 2002.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 


Bergman, E. N. 1971. Hyperketonemia-ketogenesis and ketone body metabolism. J. Dairy Sci. 54:936–948.[Abstract/Free Full Text]

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