A stereological investigation of rat endocrine pancreas after a long-term low-protein diet

Leucine supplementation augments insulin secretion in pancreatic islets of malnourished mice

OBJECTIVES

We investigated the influence of leucine supplementation on insulin secretion and on some proteins related to insulin secretion in malnourished mice.

METHODS

Swiss mice (aged 21 days) received isocaloric normo-17% (NP) or 6% low-protein (LP) diet for 120 days. Half of the NP and LP mice received 1.5% leucine in the drinking water during the last 30 days (NPL and LPL, respectively).

RESULTS

The LP mice were hypoinsulinemic compared with the NP group, whereas LPL mice exhibited increased insulinemia in the fed state versus LP mice. The LP mouse islets were less responsive to 22.2 mM glucose, 100 microM carbachol (Cch), and 10 mM leucine than the NP group. However, LPL islets were more responsive to all these conditions compared with the LP group. The muscarinic type 3 receptor, (M3R) Cabeta2, and PKC-alpha protein contents were reduced in LP compared with NP islets but significantly higher in LPL than LP islets. The p-AKT/AKT ratio was higher in LPL compared with LP islets.

CONCLUSIONS

Leucine supplementation increases insulin secretion in response to glucose and leucine and to agents that potentiate secretion, such as Cch, in malnourished mice. The enhanced levels of M3R, Cabeta2, and PKC-alpha proteins, as well as of the p-AKT/AKT ratio, may play a role in this process.

Dexamethasone treatment in vivo counteracts the functional pancreatic islet alterations caused by malnourishment in rats

The effects of dexamethasone (Dex) on the metabolic parameters, peripheral insulin, and glucose sensitivity in vivo as well as on islet function ex vivo of rats submitted to low-protein diet were analyzed. Dexamethasone (1.0 mg/kg body weight) was administered intraperitoneally daily to adult Wistar rats fed on a normal-protein diet or low-protein diet (LPD) for 5 days, whereas control rats fed on a normal-protein diet or low-protein diet (LP) received saline alone. At the end of the experimental period, LP rats showed a significant reduction in serum insulin, total serum protein, and serum albumin levels compared with rats fed on a normal-protein diet (P<.05). All these parameters tended to be normalized in LPD rats (P<.05); furthermore, these rats exhibited increased serum glucose and nonesterified fatty acid levels compared with LP rats (P<.05). Rats submitted to the low-protein diet demonstrated normal peripheral glucose sensitivity and improved peripheral insulin sensitivity, which was reversed by Dex treatment. A reduced area of islets from LP rats was partially recovered in LPD rats (P<.05). At 16.7 mmol/L glucose, insulin secretion from LPD islets was also partially recovered and was significantly higher than that from LP islets (P<.05). In conclusion, induction of insulin resistance by Dex treatment reverses most of the metabolic alterations in rats submitted to a low-protein diet. In addition, several islet functions were also improved by Dex, confirming the plasticity of pancreatic islets in adverse conditions.

Changes of islet size and islet size distribution resulting from protein-malnutrition in lean (Fa/Fa) and obese (fa/fa) Zucker rats

Potential alterations in islet size and islet size distribution resulting from protein malnutrition were studied in lean (Fa/Fa) and obese (fa/fa) Zucker rats. The purpose was to investigate whether the distribution of enlarged islets in obese rats was altered by low-protein feeding. Four-week-old, male, lean and obese Zucker rats were fed either a diet containing 20% (w/w) protein (control diet) or a diet containing 5% (w/w) protein (low-protein diet) for 3 weeks. Pancreata were dissected at autopsy and immunostained for insulin. Islet size and distribution were determined by morphometric analysis. Bodyweight gain, food intake, and serum insulin and glucose were also measured. After 3 weeks on the diets, serum insulin was significantly lower in both lean (-75%) and obese (-54%) rats fed low protein compared with that in controls. However, obese rats were still hyperinsulinemic compared with lean rats. Protein malnutrition resulted in a shift in distribution of islets to smaller size both in lean and in obese rats, with an increase in the population of small islets (< or = 100 microns2) and a decrease in the population of large islets (> 20,000 microns2). In lean and obese rats fed low protein, beta-cell weight was significantly lower, beta cell volume fraction tended to decrease, and islet number per section area was significantly elevated when compared with controls. Taken together, these results show that protein deficiency alters the endocrine pancrease in both lean and obese Zucker rats. Although the decrease in islet size and the shift in distribution to smaller islets most likely contribute to the decrease in serum insulin concentration, these changes appear insufficient to normalize hyperinsulinemia in the obese Zucker rat.