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Pancreatic mass, cellularity, and -amylase and trypsin activity in feedlot steers fed diets differing in crude protein concentration^1,2

Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1

	Abstract

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Twenty-four yearling beef steers (initial BW = 510 ± 4.9 kg) predominantly of Angus breeding were used in a randomized complete block design to determine the effect of dietary CP concentration on pancreatic cellularity, mass, and -amylase and trypsin activities. Treatment diets were formulated to contain 8.8, 11.0, 13.2, and 15.4% CP. Soybean meal and Top Soy (ruminal bypass soybean meal) were used as supplemental protein sources to ensure that MP intake was increased with increasing dietary CP concentrations. Steers were penned in groups of 4 (1 steer per treatment) and individually fed at 2.5x the NE_m requirement by using Calan gates for 28 d before tissue collection. Four steers (1 pen) were slaughtered per week. Pancreases were weighed, subsampled, frozen in liquid N₂, and stored at –80°C until analyses for DNA, RNA, and protein concentrations, and -amylase and trypsin activities. Pancreatic weight (g and g/kg of BW) did not differ among treatment groups. Pancreatic DNA concentration (mg/g) decreased linearly (P = 0.06) with increasing CP concentration. Pancreatic protein (g/pancreas) increased linearly (P = 0.08) with increasing dietary CP concentration. Pancreatic -amylase activity (U/g, U/mg of DNA, U/g of protein, U/pancreas, and U/kg of BW) increased linearly (P 0.04) with increasing dietary CP concentration. Pancreatic trypsin activity (U/g, U/g of DNA, U/g of protein, U/pancreas, and U/kg of BW) increased linearly (P 0.09) with increasing dietary CP concentration. Pancreatic -amylase and trypsin activities (U/mg of RNA) responded quadratically (P 0.09), with the greatest -amylase activity observed in the 13.2% CP treatment. These data indicate that increasing dietary CP concentration decreases pancreatic cell numbers and also increases the concentration and content of pancreatic -amylase and trypsin activities. Changes in cell number and size may be important factors regulating digestive enzyme production in the pancreas of cattle.

Key Words: beef cattle • pancreas • -amylase • trypsin • dietary protein

	INTRODUCTION

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Improving the digestibility of feeds can dramatically influence the efficiency of nutrient utilization. A better understanding of the mechanisms regulating small intestinal digestion could lead to the development of feeding protocols that improve small intestinal and total tract digestive efficiency.

The pancreas produces a complement of enzymes, including -amylase and trypsin, which are responsible for the partial hydrolysis of nutrients (starch and protein, respectively) in the small intestine. In high-concentrate feedlot diets, significant amounts of dietary starch reach the small intestine (Theurer, 1986), where there may be limitations in starch digestion potentially because of inadequate -amylase secretion (Swanson and Harmon, 2002; Harmon et al., 2004). Pancreatic -amylase content or secretion may decrease with increasing postruminal starch infusion (Chittenden et al., 1984; Walker and Harmon, 1995; Swanson et al., 2002b) and may increase with increasing postruminal casein infusion (Richards et al., 2003), but the response may differ depending on whether starch is present or absent (Swanson et al., 2002a, 2004). However, postruminal casein infusion (Richards et al., 2003; Swanson et al., 2004) or different dietary protein sources (Khorasani et al., 1990) may not influence pancreatic trypsin secretion. The mechanisms by which pancreatic digestive enzymes are regulated by diet in ruminants is not well understood, but it likely occurs through altered expression of digestive enzymes (Swanson et al., 2000, 2002a) or changes in tissue mass (Wang et al., 1998; Swanson et al., 2002a). Little is known about how diet influences pancreatic growth and cellularity. This experiment was conducted to examine how feeding increasing concentrations of dietary CP in finishing diets influences pancreatic mass, cellularity, and -amylase and trypsin activities in yearling beef cattle.

	MATERIALS AND METHODS

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Animals and Dietary Treatments

The experiment was approved by the University of Guelph Animal Care Committee. Twenty-four yearling steers predominantly of Angus breeding were blocked by BW and assigned to 6 pens of 4 (1 steer per treatment). Steers were adapted to a 12.1% CP (DM basis) diet based on high-moisture corn before the beginning of the trial. Treatment diets were formulated to contain 8.8, 11.0, 13.2, and 15.4% CP (Table 1) and were offered for 28 d before tissue collection. Soybean meal and Top Soy (ruminal bypass soybean meal, Shur-Gain, Guelph, Ontario, Canada) were used as supplemental protein sources to ensure that MP intake was increased with increasing dietary CP concentrations. Increases in CP concentration in the diets were accomplished by replacing high-moisture corn with soybean meal and Top Soy (approximate undegradable intake protein = 65% of CP), resulting in a decrease in starch concentration as CP concentration increased. Steers were individually fed at 2.5x the NE_m requirements (NRC, 1996) by using Calan gates.

Feed samples were collected weekly. At the conclusion of the feeding period, steers were weighed on 2 consecutive days. Steers were not fasted before slaughter. Four steers (1 pen) were slaughtered per week (2 on Monday and 2 on Tuesday) at the University of Guelph meat laboratory. After evisceration, the pancreas was removed, weighed, and a sample from the midpancreas was subsampled for the analyses of DNA, RNA, and protein concentrations, and -amylase and trypsin activities.

Dietary Analyses

Diet samples were dried in a 55°C oven, ground to pass a 1-mm screen, and analyzed for DM by standard procedures (AOAC, 1990). Diet N concentrations were determined by using a Leco N analyzer (Leco Corporation, St. Joseph, MI) and the percentage of CP was calculated by multiplying the percentage of N x 6.25. Concentrations of ADF were determined by the method of Robertson and Van Soest (1981) with an Ankom fiber analyzer (Ankom Technology Corp., Fairport, NY). Starch was analyzed by using the starch gelatinization and hydrolysis method (Hall, 2000).

Pancreatic RNA, DNA, and Protein Analysis

Diphenylamine and orcinol procedures were used, respectively, to analyze for DNA and RNA concentrations (Burton, 1955; Schrader and O’Malley, 1982; Swanson et al., 1999). Type I DNA from calf thymus and type IV RNA from calf liver were used as standards (Sigma-Aldrich, Oakville, Ontario, Canada). True protein concentration was determined by using the procedure of Lowry et al. (1951), with BSA used as the standard. Total DNA, RNA, and protein contents were calculated by multiplying tissue concentrations by fresh tissue weight. Concentration and content of DNA were used as an index of tissue hyperplasia (change in cell number), and RNA:DNA and protein:DNA ratios were used as indexes of tissue hypertrophy (change in cell size; Baserga, 1985; Swanson et al., 1999). This approach has been widely used as an indicator of cellularity in visceral tissues (e.g., Burrin et al., 1992; Sainz and Bentley, 1997; Baldwin et al., 2004; Hersom et al., 2004; Scheaffer et al., 2004).

Pancreatic Enzyme Analysis

Pancreatic tissue (1 g) was homogenized in 0.9% NaCl (9 mL) with a polytron (Brinkmann Instruments Inc., Westbury, NY). Activity of -amylase was determined with the procedure of Wallenfels et al. (1978) by using a kit from Teco Diagnostics (Anaheim, CA). Trypsin activity was assayed by using the method described by Geiger and Fritz (1986) after activation with 100 U/L of enterokinase (Sigma-Aldrich, Oakville, Ontario, Canada) (Glazer and Steer, 1977; Swanson et al., 2002a). Analyses were adapted for use on a PowerWave XS microplate spectrophotometer (BioTek Instruments Inc., Winooski, VT). One unit (U) of enzyme activity equals 1 µmol of product produced/min. Enzyme activity data are expressed as units per gram of wet tissue, units per milligram of DNA, units per milligram of RNA, units per gram of protein, kilounits per pancreas, and units per kilogram of BW.

Statistical Analysis

Data were analyzed as a randomized complete block design by using the GLM procedure (SAS Inst. Inc., Cary, NC). The model included block (week of slaughter; pen of 4 steers) and the effects of dietary treatment. Contrast statements were used to test for linear and quadratic effects of dietary CP concentration. Contrast coefficients were determined with the IML procedure of SAS by using the measured dietary CP concentrations (Table 1) as the tested variables. Differences were considered significant when P < 0.10. Differences were considered a trend toward significance when 0.13 P > 0.10.

	RESULTS

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Final BW, total BW gain, and ADG increased linearly (P 0.05) with increasing dietary CP concentration (Table 2). Dry matter intake (kg/d) increased linearly (P = 0.05) and tended to increase linearly (P = 0.10) with increasing dietary CP concentration. Pancreas weight (g and g/kg of BW) did not differ (P 0.21) among treatment groups (Table 3). Pancreatic DNA concentration (mg/g) decreased linearly (P = 0.06) with increasing CP concentration. Pancreatic DNA content (g/pancreas and mg/kg of BW) did not differ (P 0.13) among treatments. Pancreatic RNA concentration (mg/g) responded quadratically (P = 0.03), with concentration decreasing up to 10.7% CP and increasing thereafter. Pancreatic RNA content (g/pancreas and mg/kg of BW) did not differ (P 0.31) among treatments. Pancreatic protein concentration (mg/g) did not differ (P 0.11) among treatments. Pancreatic protein content (g/pancreas) increased linearly (P = 0.08) with increasing dietary CP concentration, although when expressed per kilogram of BW (mg/kg of BW), pancreatic protein content did not differ (P 0.15) among treatments. Neither pancreatic RNA:DNA or protein:DNA differed (P 0.13) among treatments.

View this table: [in this window] [in a new window]   Table 4. Influence of dietary CP concentration on pancreatic -amylase and trypsin activity in yearling steers1

	DISCUSSION

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The experiment was designed so that 2 treatments were below and 2 treatments were above the expected requirements for MP (NRC, 1996). The linear increase in final BW, BW gain, and ADG suggests that this was not the case, although the number of animals used per treatment may not have been sufficient to examine animal growth responses accurately. The small linear increase in DMI as dietary CP concentration increased was due to the experimental design, because steers were fed at similar energy levels relative to NE_m requirements. Therefore, as soybean meal and Top Soy replaced corn in the diet, intake needed to be increased slightly so that steers were receiving similar energy levels. The small difference in DMI among treatments likely had minimal effects on growth or pancreatic function.

Changes in pancreatic mass or cell size and number may have dramatic effects on the overall production of pancreatic digestive enzymes. For example, in a previous experiment, casein infused postruminally for 10 d elicited an increase in pancreatic mass of nearly 2-fold in steer calves (Swanson et al., 2002a), whereas pancreatic -amylase and trypsin concentrations (U/g pancreas) did not differ. The observed increase in total enzyme content was the result of increased tissue size rather than changes in tissue enzyme concentration. In the current experiment, pancreatic tissue mass was not influenced by dietary CP. The lack of response on pancreatic mass in the current experiment could be because starch flowing to the small intestine inhibited the induction by dietary protein (Swanson et al., 2002a, 2004) or because of differences in AA composition between dietary or microbial sources (Clark et al., 1992) in the current experiment vs. that of casein (Ensminger et al., 1990) infused in the previous experiment (Swanson et al., 2002a). Some research suggests differing effects on pancreatic -amylase concentration depending on AA composition of the test diet in nonruminants. Hashimoto and Hara (2004) reported that feeding a diet high in non-branched-chain AA increased -amylase concentration, whereas feeding a diet high in branched-chain AA decreased -amylase concentration as compared with a low-protein-concentration control diet in rats. Pancreatic mass was increased in rats fed the high-protein diets containing non-branched-chain or branched-chain AA as compared with the low-protein control diet.

Although there were no differences in pancreatic mass among dietary treatments, pancreatic DNA concentration decreased as dietary CP increased. This suggests that pancreatic cell numbers per milligram decreased. There also was a tendency (P = 0.13) for protein:DNA to increase with increasing dietary CP concentration, suggesting that cell size was increased. Little is known about how diet influences pancreatic cellularity in cattle, although others have reported changes in DNA concentration or content and protein:DNA caused by the physiological state or diet in pigs and rats (Pelletier et al., 1986; Mubiru and Xu, 1998). The pancreas is a mixed exocrine-endocrine gland, with the exocrine portion (acinar cells) of the gland constituting approximately 84% of the volume of the tissue (Gorelick and Jamieson, 1994). Although changes in cell number and size could be specific for different cell types, it is likely that the response was at least partially due to changes occurring within the larger acinar cell population. How cell number and size are related to pancreatic exocrine function has not been extensively studied, although there are some indications that changes in cell shape induced by the actin-myocin cytoskeleton may function to regulate secretion (Torgerson and McNiven, 2000). Decreases in cell number could result in reduced rates of cell proliferation and ion transport activity to maintain tissue mass and cellular function (Cavalier-Smith, 2005; Savage et al., 2007). This likely would result in a lower metabolic rate and proportion of energy used for maintenance functions. This potentially could result in more energy being directed toward digestive enzyme production. -Amylase and trypsin activities per milligram of DNA increased linearly with increasing dietary CP concentration, suggesting that individual cells were producing greater amounts of digestive enzymes.

The data from this experiment suggest that increasing dietary CP concentration increases the concentration (U/g) and content (kU/pancreas and U/kg of BW) of -amylase. However, with increasing CP concentration, there also was a concomitant decrease in dietary starch concentration. Therefore, it is also possible that the linear increase in -amylase concentration and content was due to decreased starch flow to the small intestine, resulting in less inhibition of pancreatic -amylase production (Chittenden et al., 1984; Kreikemeier et al., 1990; Walker and Harmon, 1995; Swanson et al., 2002b). However, increasing protein flow to the small intestine through casein infusion also has increased -amylase content or secretion in previous studies (Richards et al., 2003; Swanson et al., 2002a, 2004), suggesting that a response may be caused by increased protein supply. It is likely that the increase in -amylase content and concentration in this experiment was due to both an increase in dietary CP concentration and a decrease in dietary starch concentration. The linear increase in pancreatic trypsin activity with increasing dietary CP concentration is consistent with results observed in nonruminants (Brannon, 1990; Scheele, 1994), although results have not been consistent in ruminant trials (Richards et al., 2003; Swanson et al., 2004).

A high-moisture corn-based diet was used for this experiment, which potentially could have lessened the impact of postruminal starch flow on pancreatic -amylase content compared with feeding a diet based on dry-rolled or whole corn. Owens et al. (1986) reported that an average of 86% of dietary starch from high-moisture corn disappears from the rumen, resulting in 14% of dietary starch reaching the small intestine. A greater proportion of dietary starch from dry-rolled and whole corn passes to the small intestine (approximately 28 and 41%, respectively). Assuming that 14% of dietary starch reached the small intestine in our study, we estimate that 773 g (1.49 g/kg of BW), 762 g (1.40 g/kg of BW), 704 g (1.34 g/kg of BW), and 690 g (1.25 g/kg of BW) of starch reached the small intestine daily for the 8.5, 10.7, 12.3, and 14.5% CP diets, respectively. Although the flow of starch was less than in some of our previous infusion studies (Swanson et al., 2002a, 2004) and the relative difference in starch flow among treatments was relatively small, the impact of the level of small intestinal starch flow cannot be overlooked. In addition, some previous results from infusion studies have suggested a negative impact on pancreatic -amylase secretion when partially hydrolyzed starch was infused at a daily level of 1.38 g/kg of BW (Swanson et al., 2002b), which is similar to the estimated starch flow in the current experiment. It is possible that results caused by increasing dietary CP concentration may differ when feeding dry-rolled or whole corn diets because increased starch flow potentially could result in a more negative impact of postruminal starch on pancreatic -amylase production. In addition, although results have been conflicting (Richards et al., 2003), increasing protein flow may have less of an effect on pancreatic -amylase content or secretion in the presence of high levels of postruminal starch flow (Swanson et al., 2002a, 2004).

The tendency for -amylase content (kU/pancreas and U/kg of BW) to respond quadratically (P 0.12), with the greatest concentration observed in steers receiving 12.3% CP, suggests that increasing dietary CP may increase pancreatic -amylase concentration linearly up to a point at which additional CP will result in no additional -amylase production. The response in -amylase caused by increasing dietary CP concentration could be related to dietary protein requirements because pancreatic -amylase concentration and content did not increase further after MP requirements were met (NRC, 1996). However, our data suggest that pancreatic trypsin concentration increases linearly as dietary CP increases to at least 14.3% CP. Although -amylase:trypsin did not differ among treatments, our data do suggest a potential differential response between -amylase and trypsin to higher dietary CP concentrations.

The quadratic effect observed for pancreatic -amylase and trypsin activity when expressed per milligram of RNA (U/mg of RNA) suggests that, at high dietary CP levels, the cells require less total RNA to maintain digestive enzyme production. It also could suggest that a greater proportion of protein synthesis is being directed toward digestive enzyme production.

In conclusion, these data indicate that increasing dietary CP concentrations from 8.5 to 14.5% CP linearly increase pancreatic -amylase and trypsin concentration and content. This may result in increased capacity of the small intestine to digest starch and protein. However, this response may plateau when expressed per milligram of RNA because high dietary CP concentration (14.5% CP) did not elicit an additional increase in -amylase or trypsin. This could indicate that at higher CP intakes, a larger proportion of protein synthesis is being directed toward digestive exocrine enzymes. Additionally, cell number may be decreased with increasing dietary CP concentrations and -amylase and trypsin activities per milligram of DNA increased, suggesting that individual cells were producing more digestive enzymes. Changes in cell number and size may be important factors regulating digestive enzyme production in the pancreas of cattle.

	Footnotes

 
¹ The authors thank Charlie Watson and the staff at the Elora Beef Research Centre (University of Guelph) and Brian McDougall and the staff at the University of Guelph Meat Laboratory for their assistance.

² Financial support was provided by the Ontario Cattlemen’s Association (Guelph, Ontario, Canada), the Natural Sciences and Engineering Council of Canada (Ottawa, Ontario, Canada), and the Ontario Ministry of Agriculture, Food and Rural Affairs (Guelph, Ontario, Canada).

³ Corresponding author: kswanson{at}uoguelph.ca

Received for publication August 13, 2007. Accepted for publication December 28, 2007.

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