Ph.D. Candidate:  Rafaela Stimmelmayr *

Supervisor:  Robert. G. White.

Insulin, secreted by the pancreas is a key hormone in the regulation of food intake, nutrient storage and nutrient partitioning, and is linked to animal growth (Houpt, T.R. 1974; Roberts et al. 1994; Grovum, 1995). Little is known about seasonal insulin cycles in species that undergo prominent annual changes in voluntary food intake, varying from hyperphagia in summer to highly regulated hypophagia in the winter (White and Trudell, 1980). Such changes are associated with many hormonal changes and these are well studied in reindeer (Rangifer tarandus) (Bubenik et al. 1998) and hyperinsulimia is recorded for summer (Larsen et al. 1985). We are testing the hypothesis that seasonal hyperinsulimia in reindeer is endogenous and not a consequence of voluntary food intake, and that observed seasonal fattening is related to increasing plasma insulin levels. This study specifically addresses the period of regulated hypophagia of winter, when doses of insulin are hypothesized to increase food intake and/or fattening unless the animal becomes seasonally resistant to insulin.

Adult female reindeer (n=8; >2yr; non-pregnant) were kept in 4 out door pens (2 single pens; 2 group pens) at the Large Animal Research Station. Animals were provided with ad libitum pelleted food (UAF Reindeer Ration; Alaska Mill and Feed Company, Anchorage, Alaska; dry matter values range from 15-15.6% digestible protein, 9.6-10.8% acid detergent fiber, 77.5-78.5% TDN) and snow. Animals were allocated to either of 2 groups (treatment n=4 and control n=4) evenly matched for body mass (BM), a body condition index (Gerhart et al. 1996) and maximum subcutaneous backfat (MAXFAT) for a pre-conditioning period of 5 weeks. The Institutional Animal Care and Use Committee at UAF approved the study. All animals were weighed daily (nearest 0.5 kg). Chest and abdominal girth measurements (index of alimentary fill) were collected on alternating days. A real-time portable scanner (Technicare Model 210DX) with a linear 5MHz transducer was used to measure maximum ultrasonic fat thickness along a longitudinal line between the tuber coxae and the tuber ischii (Stimmelmayr, 1998). Sampling interval was every other day. Body condition scores were recorded as an alternative measure of body fatness. Daily feed intakes in 2 individuals were measured (nearest 1 g). Individuals were rotated on a daily basis through a single feeding pen equipped with an electronic scale and 24-hour feeding activities were recorded. Sampling interval was 5 minutes. Food residues were measured daily (nearest 1 g). To assess the influence of socialization on feeding behavior animals were observed randomly 4 times a day. Three criteria were used to define meals: minimum amount eaten (50g), maximum time during which the minimum amount must be eaten (5 min), and the minimum interval during which no feed was eaten (>15 min < 30 min). Eating that occurs between meals is designated as nibbling. These criteria determine the initiation and termination of meals and intermeal intervals (Baile, 1975). Relations between variables were determined with polynomial regressions. Saline and insulin were injected subcutaneously (pre-scapular) one time daily at 9-11am for 21 days. The dosages and groups were as follows: treatment 1, saline (volume corresponding to insulin injection); treatment 2, insulin 1 IU/kg body weight. Data were analyzed using a two-factor analysis of variance with repeated measurement design, followed by post hoc tests if appropriate (SAS 6.12 for Windows). Significance level was set at P<0.05.

Chronic insulin treatment had a significant effect on backfat depth (P=0.0321) but did not affect, food intake, alimentary fill, and body mass (P>0.05). Average meal frequency (SEM) in control group per day was 7.4 +/- 0.33 with a range of 3-11 meals/day. There is a strong correlation between meal size and % of total number of meals (r=0.96; P< 0.001). Reindeer preferably ate small meals (50-250 grams). Meal size increased with duration of the pre-meal interval (r= 0.99; P< 0.001). We found no correlation between meal size and post-meal interval (r=0.70; P>0.05). However, the after-meal interval does not exceed on average 200 minutes.

The response of insulin on backfat depth in reindeer was two phasic with an initial decrease followed by a steady increase-suggesting reindeer in winter develop tolerance (habituation) to acute effects of insulin. Insulin also had no effect on daily food intake, again suggesting appetite in reindeer during winter is resistant to insulin. However, these data are consistent with the possible role of insulin in nutrient partitioning as seen by its transient effect on backfat depth.

Baile, C. A.1975. Control f eed intake in ruminants. In: Digestion and Metabolism in the ruminant. (eds). I.W.McDonald and A.C. I. Warner. The University of New England Publishing House, Armidale, Australia. pp 333-350.

Bubenik, G.A., Schams, D., White, R.G., Rowel, J., Blake, J., and Bartos. L. 1998. Seasonal levels of metabolic hormones in male and female reindeer. Comp. Biochem. Physiol. (in press).

Gerhart, K.L., White, R.G., Russell, D. E. and Cameron, R. D. 1996. Estimating fat content of caribou from body conditions cores. J. Wildl. Manage. 60:713-718.

Grovum, R. 1995. Mechanisms explaining the effects if short chain fatty acids on feed intake in ruminants-osmotic pressure, insulin and glucagon. In: Ruminant physiology: Digestion, Metabolism, Growth and Reproduction. (eds) Engelhardt, W. v.; Leonhard-Marek, S.; Breves, G., and Giesecke, D. Ferdinand Enke Verlag Stuttgart, West Germany. pp.173-205

Houpt , T.R. 1974. Stimulation of food intake in ruminants by 2-deoxy-D-glucose and insulin. Am. J. Physiol. 227 (1):161-167.

Larsen , T.S., Lagercrantz, H., Riemersma, R.A., and Blix, A.S. 1985. Seasonal Changes in blood lipids, adrenaline; noradrenaline, glucose and insulin in Norwegian reindeer. Acta Physiologica. Scandinavia. 124 (1):53-59 .

Roberts, T.J, Azain, M.J. Hausman, G.J., and Martin, R.J. 1994. Interaction of insulin and somatotropin on body weight gain, feed intake, and body composition in rats. Am. J. Phys. Vol. 267: E293-E299.

R. Stimmelmayr, 1998. In-vivo Determination of Backfat Depth in Reindeer (R.tarandus) using Real-Time Ultrasonography; Alaska Chapter The Wildlife Society Annual Meeting. Girdwood, Alaska January 16, 1998 (abstract).

White, R.G. and Trudell, J. 1980. Habitat preference and forage consumption by reindeer and caribou near Atkasook, Alaska. Arctic and Alpine Research. 12(4):551-529.

 
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