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Activation of insulin signaling and cell cycle intermediates is required for adult [beta]-cell proliferation. Here, we report a model to study [beta]-cell proliferation in living rats by administering three different doses of dexamethasone (0.1, 0.5, and 1.0 mg/kg ip, DEX 0.1, DEX 0.5, and DEX 1.0, respectively) for 5 days. Insulin sensitivity, insulin secretion, and histomorphometric data were investigated. Western blotting was used to analyze the levels of proteins related to the control of [beta]-cell growth. DEX 1.0 rats, which present moderate hyperglycemia and marked hyperinsulinemia, exhibited a 5.1-fold increase in [beta]-cell proliferation and an increase (17%) in [beta]-cell size, with significant increase in [beta]-cell mass, compared with control rats. The hyperinsulinemic but euglycemic DEX 0.5 rats also showed a significant 3.6-fold increase in [beta]-cell proliferation. However, DEX 0.1 rats, which exhibited the lowest degree of insulin resistance, compensate for insulin demand by improving only islet function. Activation of the insulin receptor substrate 2/phosphatidylinositol 3-kinase/serine-threonine kinase/ribosomal protein S6 kinase pathway, as well as protein retinoblastoma in islets from DEX 1.0 and DEX 0.5, but not in DEX 0.1, rats was also observed. Therefore, increasing doses of dexamethasone induce three different degrees of insulin requirement in living rats, serving as a model to investigate compensatory [beta]-cell alterations. Augmented [beta]-cell mass involves [beta]-cell hyperplasia and, to a lower extent, [beta]-cell hypertrophy. We suggest that alterations in circulating insulin and, to a lesser extent, glucose levels could be the major stimuli for [beta]-cell proliferation in the dexamethasone-induced insulin resistance. [beta]-cell growth; glucocorticoid; hyperglycemia; hyperinsulinemia; insulin resistance

To understand the secretory mechanisms and physiological role of insulin in the tear film, the present study examined 1) the time course of insulin secretion in the tear film under glucose intravenous stimulation, 2) the glucose- and carbachol-induced insulin secretion from isolated lachrymal gland (LG), 3) the effect of insulin on glucose consumption by the cornea, and 4) the expression of insulin, pancreatic duodenal homeobox-1 (PDX-1), and glucose transport proteins (GLUTs) in LG tissue. The insulin level in the tear film of 8-wk-old male Wistar rats increased from 0.6 [+ or -] 0.45 to 3.7 [+ or -] 1.3 ng/ml in the initial minutes after glucose stimulation. In vitro assays demonstrated that higher glucose concentrations from 2.8 to 16.7 mM, 200 [micro]M carbachol, or 40 mM KCl significantly increased insulin secretion from lacrimal glands compared with controls, but did not detect C-peptide as measured by RIA. Glucose consumption by corneal tissue, evaluated by radiolabeled D-[U-[sup.14]C]glucose uptake, was 24.07 [+ or -] 0.61 and was enhanced to 31.63 [+ or -] 3.15 nmol x [cornea.sup.-1] x 2 [h.sup.-1] in the presence of 6 nM insulin (P = 0.033) and to 37.5 [+ or -] 3.7 nmol x [cornea.sup.-1] x 2 [h.sup.-1] in the presence of 11.2 mM glucose (P = 0.015). Insulin and PDX-1 mRNA was detected in LG. Insulin was located in the apical areas of acinar cells by immunoperoxidase and the expression of GLUT-1, but not PDX-1, was confirmed by Western blot. These findings suggest that insulin secretion in the tear film is influenced by local stimuli such as nutrient and neural inputs and that this hormone plays a metabolic role in ocular surface tissues. These data also indicate that under normal conditions the insulin secreted by LG is stored, but it is not clear that is locally produced in the LG. glucose transportertear filmocular surface

A low protein diet has been shown to affect the amount and activity of several enzymes and to decrease insulin secretion by islets isolated from rats fed such a diet. To understand the mechanisms involved in this phenomenon, we investigated the effects of forskolin, a stimulator of adenylyl cyclase, on insulin secretion by pancreatic islets from rats fed a normal (17%; NP) or low (6%; LP) protein diet for 8 wk. Isolated islets were incubated for 1 h in Krebs-bicarbonate solution containing 8.3 mmol glucose/L, with or without 10 [micro]mol forskolin/L The forskolin-induced insulin secretion was higher in islets from NP rats than in those from LP rats (P < 0.05). Western blotting revealed that the amount of the [alpha] catalytic subunit of protein kinase A (PKA[alpha]) was 35% lower in islets from LP rats than in islets from NP rats (P < 0.05). Moreover, PKA[alpha] mRNA expression was reduced by 30% in islets from LP rats (P < 0.05). Our results indicated a possible relationship between a low protein diet and a reduction in PKA[alpha] expression. These alterations in PKA[alpha] may be responsible in part for the decreased insulin secretion by islets from rats fed a low protein diet. KEY WORDS: * low protein diet * forskolin * insulin secretion * protein kinase [alpha] * gene expression

Undernutrition has been shown to affect the autonomic nervous system, leading to permanent alterations in insulin secretion. To understand these interactions better, we investigated the effects of carbamylcholine (CCh) and phorbol 12-myristate 13-acetate (PMA) on insulin secretion in pancreatic islets from rats fed a normal (17%; NP) or low (6%; LP) protein diet for 8 wk. Isolated islets were incubated for 1 h in Krebs-bicarbonate solution containing 8.3 mmol glucose/L, with or without PMA (400 nmol/L) and CCh. Increasing concentrations of CCh (0.1-1000 [micro]mol/L) dose dependently increased insulin secretion by islets from both groups of rats. However, insulin secretion by islets from rats fed the NP diet was significantly higher than that of rats fed the LP diet, and the dose-response curve to CCh was shifted to the right in islets from rats fed LP with a 50% effective concentration (E[C.sub.50]) of 2.15 [+ or -] 0.7 and 4.64 [+ or -] 0.1 [micro]mol CCh/L in islets of rats fed NP and LP diets, respectively (P < 0.05). PMA-induced insulin secretion was higher in islets of rats fed NP compared with those fed LP. Western blotting revealed that the protein kinase (PK)C[alpha] and phospholipase (PL)C[[beta].sub.1] contents of islets of rats fed LP were 30% lower than those of islets of rats fed NP (P < 0.05). In addition, PKC[alpha] mRNA expression was reduced by 50% in islets from rats fed LP. In conclusion, a reduced expression of PKC[alpha] and PLC[[beta].sub.1] may be involved in the decreased insulin secretion by islets from LP rats after stimulation with CCh and PMA. KEY WORDS: * low protein diet * carbamylcholine * insulin secretion * protein kinase C[alpha] * phospholipase C[beta]1.