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ANIMAL NUTRITION |
Unidad de Nutrición Animal, Estación Experimental del Zaidín (CSIC), Camino del Jueves s/n, 18100 Armilla, Granada, Spain
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: breed difference • Iberian pig • lysine deficiency • whole-body protein turnover
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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We have reported that the Iberian pig, a primitive breed, has a reduced capacity for protein deposition at maximal growth compared with conventional pig breeds (Nieto et al., 2002a
). However, the reasons for this lower capacity for lean tissue gain have not been elucidated. Different studies have compared the responses of pig breeds to nutrient intake in terms of animal performance, maximum protein deposition, and body composition, with the most extreme breed comparison reported for Large White x Landrace vs. Chinese Meishan (Kyriazakis et al., 1993, 1994). To our knowledge, no attempt has been made to determine the responses in whole-body protein turnover to nutrient intake for different breeds.
Our hypothesis was that under similar nutritional and physiological conditions, whole-body protein turnover and the protein synthesis:protein deposition ratio (S/PD) differ between Iberian and Landrace breeds. These mechanisms then would drive the dissimilar protein deposition rates. In addition, the effect of dietary Lys deficiency on whole-body protein turnover was examined because this is the prevalent condition found during the fattening period of the Iberian pig in the Mediterranean forest, where the main feed source is oak acorn, which provides only 37% of the available Lys present in an ideal protein (Nieto et al., 2002b).
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; NRC, 1998). The 2 concentrations of CP in the experimental diets were chosen to account for breed differences in the capacity for protein accretion (Nieto et al., 2002a). The Lys-deficient diet was supplied at 12% CP for Ib and 16% CP for Ld pigs. Therefore, Lys deficiency in each breed was tested under its optimal dietary protein concentration.
The optimal diets were produced by supplementation of a basal diet (Table 1) with L-Lys (as free base, minimum assay 98.5%) to that required for the ideal protein; i.e., 70 g/kg of protein (ARC, 1981). Other essential AA were also added with the same purpose (Table 1). Based on the analysis performed on wheat and corn gluten meal, the relative proportions of essential AA in the diets were found to be close to those in the ideal protein, except for Lys in the Lys-deficient diet. Specifically, the Lys:CP ratios were 71.2 and 74.1 for diets A12 and A16, respectively, and 26.8 and 24.5 for the Lys-deficient diet at 12 and 16% CP, respectively. In this way, the effect of a deficit of Lys in an otherwise optimum AA-balanced diet was studied.
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). Because only 10 metabolic cages were available, the experiment was performed in 4 replicates, each with 4 to 6 animals per breed, including all treatment combinations. Initial BW were 27.6 ± 0.6 kg for Ib and 22.7 ± 0.4 kg for Ld pigs. The diets were formulated with wheat and corn gluten meal and supplied 13.3 to 13.9 kJ of ME/g of DM (Table 1
). The pigs were housed throughout the study in individual metabolic cages in an environmentally controlled room (21 ± 1°C).
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, in an attempt to achieve similar intakes per kilogram of BW in both breeds. This reduction in feed allowance was based on the comparatively greater intake capacity of the Ib breed (Nieto et al., 2001; Morales et al., 2002). Daily food allowance was divided into 6 hourly portions (0400, 1000, 1600, and 2200). On the last day of adaptation, a bladder catheter (Foley-type No. 12, Rüsch médica España, S.A., Madrid, Spain) was implanted after the administration of a sedative [0.1 mL/kg of BW; Stresnil (Azaperone, 40 mg/mL), Esteve Laboratories, Barcelona, Spain]. Once the pigs recovered, urine was collected over several hours to determine the natural abundance of 15N in urinary ammonia and urea. On the next day, a single dose of [15N]-Gly (7.5 mg/kg of BW) was administered using an esophageal catheter, and immediately afterward, total urinary output was collected for the determination of 15N excretion. The collection periods were every 12 h for the first 72 h and then every 24 h for the next 48 h. Each urine sample was collected in 20 mL of 4.5 M H2SO4. The weight corresponding to each urinary sample was recorded, and a portion (10% of volume) was frozen at –20°C until analyzed. During the 5-d period, feces were collected daily for the determination of N balance and digestibility of energy and nutrients.
Statistical Analysis
All statistical analyses were performed with the GLM procedure (SAS Inst. Inc., Cary, NC) for a randomized complete block design. The experimental data were subjected to two 2 x 2 ANOVA. In the first analysis, the terms of the model contained the effects of breed, adequate diets differing in protein concentration (DPC; A12 vs. A16), and their interaction. In the second analysis, the terms of the models were breed, dietary AA pattern, (AAP; adequate vs. inadequate Lys content), and their interaction. When significant interactions were found, significant differences between means for each of the 4 treatment combinations were declared at P < 0.05 using the Student-Newman-Keuls’ test (Newman, 1939; Keuls, 1952).
Analytical Procedures
All analyses were performed in duplicate. The DM content of feeds and feces was determined by using standard procedures (AOAC, 1990), and total N in feed, urine, and freeze-dried samples of feces was determined with a Kjeldahl procedure using mineralization (Block digestor S-509, Selecta, Barcelona, Spain), distillation units (Büchi Laboratoriums Technik AG, Flawil, Switzerland), and titration units (Metrom AG, Herisau, Switzerland).
Amino acids in feeds were determined after protein hydrolysis in 6 M HCl plus 1% phenol in sealed, evacuated tubes at 110°C for 24 h, by HPLC according to the Waters PicoTag method (Cohen et al., 1989) with precolumn derivatization with phenylisothiocyanate using a Waters 2695 separation module (Waters Cromatografía, S. A., Madrid, Spain). The Cys and Met were determined as cysteic acid and methionine sulphone, respectively, obtained after oxidation with performic acid before 6 M HCl hydrolysis (Moore, 1963). A Millenium 32 chromatography manager system (Waters Cromatografía) was used for gradient control and data processing.
Analyses of GE were performed on freeze-dry samples of feces, diets, and urine using an adiabatic bomb calorimeter (Gallenkamp Autobomb CBA 305, Loughborough, Leicestershire, UK). In the case of the urine, samples were freeze-dried in a polyethylene sheet of known energy value and their GE values obtained by difference. Whenever an analysis was made on freeze-dried material, a DM determination was performed on an aliquot sample, in a ventilated oven, by standard procedures (AOAC, 1990) to determine residual water content after freeze-drying, and the corresponding analytical result was expressed on a DM basis. Concentration of urinary urea and ammonia were determined by Sigma methods 171 and 535-A, respectively (Sigma-Aldrich Co., Madrid, Spain).
15 N Analyses
An adaptation of the method described by Read et al. (1982) was followed. For urea and ammonia separation, a cation exchange resin (AG59 WX8 200 H+ form, Sigma-Aldrich) was converted to the Na/K form by stirring 100 g of resin 3 times for 15 min each in 600 mL of 0.1 M NaOH and, finally, once in 300 mL of 0.5 M NaOH. The resin was then washed to neutrality with Milli-Q water (18.2 M
, Millipore Ibérica, Madrid, Spain) and stirred 3 times for 15 min each in 600 mL of 0.2 M Na/K phosphate buffer adjusted to pH 7.4. The resin was again washed to neutrality, made up to a total weight of 400 g with Milli-Q water and stored at 4°C.
To extract the urinary NH3-N, an aliquot of urine containing approximately 3,000 µg of NH3-N was passed through a chromatography column (Poly-Prep 731–1550, BioRad, Madrid, Spain) containing 0.5 mL of the Na/K resin and then washed with 1 mL of Milli-Q water. The eluate, including the wash, contained the urea fraction. The resin was then washed with 2 x 1 mL of Milli-Q water and the eluant discarded. The ammonia bound to the column was eluted with 1 mL of 1 M KOH and recovered in a 1.5-mL Eppendorff tube (Eppendorf Ibérica, Madrid, Spain) containing 25 µL of 9 M H2SO4 and 20 µL of bromphenol blue (1% in Milli-Q water) and stored at –20°C until analyzed for 15N enrichment.
The eluate containing the urea fraction was subjected to enzymatic hydrolysis (Urease Type VI Jack Beans, 105,000 units/g, Sigma-Aldrich Co.) to obtain ammonia. Urease solution (400 units/mL) was prepared in sodium phosphate buffer, pH 7. An eluate volume containing approximately 3,000 µg of urea N was added to a 10-mL, open Vacutainer tube (Becton Dickinson, Madrid, Spain) containing 2 mL of sodium phosphate buffer, pH 7. The tubes were closed with a rubber stopper, and 100 µL of urease solution was injected through the stopper. After 1 h of incubation at 30°C with the tubes upside down to ensure no ammonia losses, 0.5 to 1 mL of 1 M HCl (depending on the amount of eluate incubated) were injected to stop the reaction. The solution was then passed through the chromatography columns as described previously for the ammonia-N extraction.
Both ammonia fractions, derived from urea or present in the urine, were prepared for combustion in an elemental analyzer (NA1500 NC, Fisons, ThermoFinnigan, S.A., Madrid, Spain) connected to a continuous-flow mass spectrometer (MAT DeltaplusXL, ThermoFinningan, S.A.). Twenty microliters of the samples were adsorbed in Cromosorb W (Fisons, ThermoFinningan, S.A.) onto a tin capsule for combustion. The 15N enrichment in each ammonia fraction was obtained after deducting the natural 15N abundance.
Calculations
Whole-body protein turnover was measured following the end product method (Waterlow et al., 1978) using a single oral dose of [15N]-glycine (Fern et al., 1981). This simplified model assumes the tracer is distributed between protein synthesis and AA oxidation in the same proportion as total body AA (Russell et al., 2003). It also assumes that 15N released from protein degradation is not reincorporated into protein during the collection period. From these assumptions, it follows that the proportion of label excreted in the end product (urinary ammonia or urea) to the total dose given is the same as the contribution of unlabelled N excreted as that end product to the total flux:
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where Ex is the amount of N excreted in urinary ammonia or urea, in grams per day; d is the dose of 15N given orally, in grams; ex is the total amount of 15N excreted in urine as ammonia or urea, in grams, during the collection period; and Q is total whole-body N flux, in grams per day.
The rate of whole-body N flux was calculated separately for urinary ammonia and urea as end products because these often have different 15N enrichment, or estimated by the arithmetic mean (Fern et al., 1981), using the standard equation of Waterlow et al. (1978):
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Absolute rates of whole-body protein synthesis (g of protein synthesized/d) and breakdown (g of protein degraded/d) were derived from the following expression:
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where I is the rate of N entering the pool from the diet (absorbed N), B is the rate of whole-body protein breakdown, S is the rate of whole-body protein synthesis, and U is total urinary N excretion, all expressed as g/(kg0.75·d).
To determine the time at which the urinary 15N excretion declined to negligible values under our experimental conditions, a group of 6 animals was randomly selected to obtain the excretion curve of 15N from 12 to 120 h after administration of the isotope. The results indicated that more than 95% of 15N was excreted by 60 h after isotope administration for labeled urinary ammonia and urea (data not shown). Based on this observation, a pooled sample, representative of the combined sampling periods from 12 to 60 h after isotope administration, was used for the rest of the experimental animals to determine 15N enrichment in urinary ammonia and urea.
To express the results in fractional terms, the fractional rate of protein growth (FPR), fractional rate of protein synthesis (FSR), and fractional rate of protein degradation (FBR) indicate daily protein deposition, protein synthesis, and degradation, respectively, relative to whole-body protein. For this purpose, the following equations were used (Waterlow et al., 1978):
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and
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Whole-body protein content was estimated by multiplying BW by a factor of 0.17 for Ld (ARC, 1981) and 0.146 for Ib gilts (Nieto et al., 2002a).
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). In spite of the reduction practiced in feed allowance to achieve 90% of the theoretical ad libitum intake of the Ib pig, some Ld gilts failed to consume their whole ration. For this reason, DM intakes [g/(kg0.75·d)] were 8 to 9% greater in Ib pigs (P < 0.001, Table 3). However, feed intake was not affected by dietary protein concentration (P = 0.70, Table 3). Intakes of N and Lys followed differences in total intake between breeds and, obviously, in dietary protein concentration (P < 0.001). The ADG was expressed on a metabolic body size basis to correct for differences in BW. The Ld gilts gained faster (P < 0.001), but gain was not affected by dietary protein concentration for either breed.
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Nitrogen retention (NR), as g of N/(kg0.75·d) and as the efficiency of retention of ingested N (NR:NI), was greater in Ld gilts (P = 0.029 and P < 0.001, respectively). Nitrogen retention [g of N/(kg0.75·d)] increased with DPC (P = 0.002) by 27% in Ld pigs and by 12% in the Ib breed. The efficiency of retention of ingested N was not significantly affected by DPC. Nitrogen retention per gram of Lys intake was 22% greater in Ld gilts (P < 0.001) but not affected by dietary protein concentration.
No breed x DPC interactions were detected for protein metabolism variables except for urinary N (Table 4). Protein synthesis and protein degradation expressed on a metabolic body size basis (Table 4) were greater in Ld (P = 0.011 and P = 0.018, respectively) than Ib gilts. Protein synthesis increased with greater dietary protein concentration (P = 0.036), whereas protein degradation only showed a tendency to increase (P = 0.089). Urinary N [expressed as g of N/(kg0.75·d)] (Table 4) resulted in a breed x dietary protein concentration interaction (P = 0.045). Both breeds excreted more urinary N when 16% protein was fed, but Ld gilts increased to a lesser degree than Ib gilts (P < 0.05). The S/PD was maintained fairly constant with no effect of breed or dietary protein concentration. On average 5.4 g of protein were synthesized per gram of protein deposited regardless of breed or dietary treatment.
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). Across breeds, fractional rate of protein growth and fractional rate of protein synthesis increased with 16% protein (P = 0.004 and P = 0.044, respectively), whereas fractional rate of protein degradation tended to increase (P = 0.099). Despite no significant breed x DPC interactions, fractional rate of protein growth and fractional rate of protein synthesis increased numerically in Ld pigs.
Lysine Deficiency Responses
Significant breed x AAP interactions were obtained for most variables due to the greater effect of Lys deficiency on the Ld vs. the Ib gilts (Tables 3 and 4). The intake of DM (and N) was not affected by Lys deficiency in Ib pigs but was reduced by 12 to 13% in Ld pigs (P < 0.001; Table 3).
The Lys-deficient diet reduced ADG by 51 and 20% in Ld and Ib pigs, respectively (P < 0.001; Table 3). With the Lys-deficient diet, the apparent digestibility of N was diminished (P = 0.035) as was DE:GE (P = 0.064). The ratio ME:DE was not affected in Ib pigs but was slightly reduced in the Ld breed leading to a breed x AAP interaction (P = 0.006).
There was a greater decrease in N retention on a metabolic body size basis for Ld than Ib gilts when the gilts were fed Lys-deficient diets resulting in a breed x AAP interaction (0.62 vs. 1.50 and 0.67 vs. 1.13; P = 0.002; Table 3). The reduction in N retained relative to intake was approximately 50 to 40% for Ld and Ib gilts, respectively, resulting in an interaction between breed and AAP (P = 0.049). The N retained per g of Lys intake was 50% greater when feeding Lys-deficient diet compared with adequate Lys. There was no effect of breed (P < 0.001).
Interactions between breed and AAP were observed for protein synthesis and protein degradation, expressed in a metabolic body size basis, when gilts were fed Lys-deficient diets due to the greater reduction obtained in Ld gilts than Ib gilts (Table 4, P = 0.012 and P = 0.039, respectively).
Urinary N excretion on a metabolic body size basis increased by 35% in both breeds when fed a Lys-deficient diet (P < 0.001). Despite the marked decrease of protein synthesis on metabolic body size basis when a Lys-deficient diet was fed, the resultant S/PD was increased by 31 to 54% in both pig breeds (P = 0.012, Table 4).
Fractional rates of protein growth, of synthesis and degradation were also reduced when the gilts were fed a Lys-deficient diet (Table 4). Because Ld gilts were affected to a greater extent than Ib gilts, a breed x AAP interaction resulted for fractional rate of protein growth (P = 0.011) and fractional rate of protein synthesis (P = 0.037). Fractional rate of protein degradation was not affected by breed but decreased by 19 and 36% with the Lys-deficient diet (P < 0.001).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). We previously reported the lower capacity for lean tissue growth of the growing Ib pig compared with modern pig breeds (Nieto et al., 2002a), but the mechanism is unknown.
The comparison of protein metabolism between animal breeds (or species) with different growth potential is not a simple issue. Because of the substantial changes in protein turnover occurring during development (Lobley, 1993), it is important that animals are comparable for age or physiological state. In the case of the present work, animals only differed by about 18% in weight and 12% in age. The Ib gilts have a greater voluntary feed intake than Ld gilts, which might confound the intake of nutrients (Nieto et al., 2001; Morales et al., 2002). To overcome this difference in appetite, a 10% reduction of theoretical ad libitum feed intake of the Ib gilts was used as a controlled feed intake for gilts of both breeds. Effects of such a change on efficiencies of protein utilization would be expected to be small. In this respect, in the growing Ib pig (15 to 50 kg of BW) we found that a reduction in the level of intake from 0.95 to 0.80 of ad libitum caused no effect on N retained relative to intake and a decrease of only 3.5 percentage units in N retained relative to N absorbed (Nieto et al., 2002a).
The increase in N digestibility when dietary protein concentration increased is probably due to the smaller proportional contribution of endogenous N excretion when increasing the dietary N intake (Lenis et al., 1999), but also to the greater proportion of corn gluten meal, with greater N digestibility than wheat (FEDNA, 2003), in the 16% CP diet. The results for N retention observed in Ib gilts in terms of protein deposition and NR:NI ratio are similar to observations from our laboratory in growing castrated male Ib pigs (Nieto et al., 2002a), despite differences in methodology and physiological state of the experimental animals. In these earlier studies, we demonstrated that during a similar stage of growth (15 to 50 kg of BW), the maximum deposition of protein was achieved with a diet supplying 129 g of ideal protein/kg of DM, fed at levels close to ad libitum. Increasing the supply of dietary protein to 175 g of ideal CP/kg of DM reduced NR:NI ratio from 0.37 to 0.22. In the present experiment, when dietary protein concentration was increased from 12 to 16%, N retention improved slightly in the Ib breed (11.5%) whereas this increase was proportionally greater in Ld gilts (27.1%). However, no change in NR:NI ratio was observed. These observations suggest that the Ib gilts, which were in an earlier stage of growth than the castrated males used in our previous study, approached maximum capacity for protein deposition with the 12% CP diet and therefore that they require less protein per kilogram of feed than fast growing leaner breeds (Campbell et al., 1985; Bikker et al., 1994; Kemm et al., 1995). Due to differences in growth potential, muscle accretion, and mature BW, Landrace gilts could meet their Lys requirements only with the A16 treatment, which provided 11.8 g of Lys per kilogram of diet, a concentration slightly greater than the recommended allowance of NRC (1998).
In the Ld gilts, the improved N retention with greater dietary protein concentration was due to the increased protein synthesis and protein degradation, but greater protein synthesis. This is comparable with earlier data reported for Large White x (Large White x Ld) pigs (Reeds et al., 1981) where increases in dietary protein resulted in increased protein synthesis, protein degradation, and protein deposition. Responses in protein synthesis for the Ib pigs to increased dietary protein content were smaller than for Ld, but this may relate to the closeness to the maximum protein deposition at 12% CP (i.e., protein metabolism within the Ib pig was close to maximal with little room for further improvement). This suggests a constraint within the protein synthetic machinery and that no mechanism operates in these young animals to raise protein deposition by reducing protein degradation. The constraint in protein synthesis might be imposed by the lower protein mass of Ib pigs compared with Ld pigs. A relevant indication of this lower body protein mass is the 20 to 30% smaller muscle size found in Ib gilts compared with Ld gilts of similar BW and fed the same balanced AA pattern diets to those used in the present experiment (Rivera-Ferre et al., 2005). It is possible that the provision of additional protein to Ib pigs requires additional energy to oxidize the excess AA with no benefit for protein deposition. This would be in agreement with Bikker et al. (1994) who observed in gilts with a high genetic potential for lean gain that at levels of Lys and protein intake beyond those required to maximize protein deposition, protein and lipid deposition remained constant, and that greater protein intakes caused a continued decrease in lipid gain suggesting that the dietary protein excess was deaminated at the expense of lipid deposition.
Although examination of S/PD revealed no differences caused by breed or dietary protein concentration, this may be confounded by the large contribution of protein synthesis to body maintenance. Change in synthesis:deposition (Lobley, 1998) ratio on increasing dietary protein concentration for Ib is 5.23 and for Ld is 3.88, indicating a greater marginal efficiency of protein deposition for Ld gilts. This is similar to marginal efficiencies of 4.6, 1.6, and 0.6 for Large White x (Large White x Ld) pigs when a basal diet was supplemented with protein, carbohydrates, or fat, respectively (Reeds et al., 1981).
Protein dynamics when expressed as fractional rates for growth, synthesis, and degradation did not differ for breeds, which implies that both breeds are synthesizing and degrading similar amounts of protein when expressed relative to total body protein. Breed differences may have occurred at an earlier stage of development, and by the time of measurements, protein mass of both breeds may have been genetically determined by an earlier event. In other species, similar observations were obtained in lean and obese Zucker rats (Lobley et al., 1978), but lower fractional protein degradation rates were reported in fast-growing rats (Bates and Millward, 1981) and chickens (Klasing and Calvert, 1987) compared with corresponding slower-growing strains. More recently, in cattle, no differences in muscle fractional rate of protein degradation between Aberdeen Angus and Charolais finishing steers were observed when corrections for differences in body fatness were applied (Lobley, 1998; Lobley et al., 2000). In tissue studies performed in Ib and Ld pigs (Rivera-Ferre et al., 2005), we observed greater fractional protein synthesis rate in muscle, but not in liver or duodenum, in Ib compared with Ld pigs. Although these results appear to be contradictory to those found in the current study at the whole-body level, it should be considered that whole-body fractional rate of protein synthesis depends on both the specific fractional rate of protein synthesis and the protein mass of the contributing tissues. The Ib pigs had smaller muscles and greater viscera than the Ld pigs (Rivera-Ferre et al., 2005). So, the sum of the tissue event described may well lead to the whole-body observations of the current study.
In both breeds, Lys deficiency decreased protein synthesis as well as protein degradation, whether expressed in absolute terms [g of N/(kg0.75·d)] or as fractional rates (per 100 g of body protein). This finding confirms previous observations (Salter et al., 1990; Roy et al., 2000) that the addition of Lys to Lys-deficient diets leads to a greater protein degradation resulting from greater increases in protein synthesis than in protein degradation. However, Fuller et al. (1987) reported that the improvement in protein deposition observed when Lys-deficient diets were supplemented with this AA was related to a substantial reduction in protein degradation. In the current experiment, the relative proportions of protein synthesized:protein degraded changed with the Lys deficiency. When adequate AA pattern diets were used, protein degradation was 81% of protein synthesis, but with Lys-deficient diets this ratio increased to approximately 85%. The ratio of S/PD was elevated when feeding Lys-deficient diets, indicating that when the imbalanced AA pattern was fed, the decrease in protein synthesis was accompanied by an ever larger decline in protein deposition.
In conclusion, at similar live BW and dietary balanced protein intakes, greater rates of protein deposition, synthesis, and degradation in Ld gilts than in Ib gilts were attributed to presumable differences in body protein mass. The ratio of S/PD provides an average value of 5.4 across treatments and breeds although the incremental data show a greater marginal efficiency for Ld pigs with dietary protein concentration. Lysine-deficient diets dramatically affected all variables of protein metabolism studied, with greater effects on the Ld breed.
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Footnotes |
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2 Presented in part in: Rivera-Ferre, M. G., R. Nieto, and J. F. Aguilera. 2003. Whole body protein turnover of Iberian and Landrace pigs fed adequate or amino acid deficient diets. Pages 805–808 in Progress in Research on Energy and Protein Metabolism. Symposium on Energy and Protein Metabolism and Nutrition. EAAP Publication no. 109, Rostock-Warnemünde, Germany.
3 Corresponding author: rosa.nieto{at}eez.csic.es
Received for publication July 26, 2005. Accepted for publication July 31, 2006.
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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