This study examined whether an immunohistochemical method examining the subcellular localization of STAT5 could be used to characterize the activation of the JAK2/STAT5 pathway by prolactin (PRL) in intact cells or tissues. In the Ins-1 beta-cell line, STAT5A and STAT5B were distributed almost equally in the cytoplasm and the nucleus in unstimulated cells. STAT5A was also detected along the border of cells and in the perinuclear region. After exposure to PRL, the redistribution from the cytoplasm to the nucleus was much higher for STAT5B compared to STAT5A. This translocation represented 12% of the STAT5A and 22% of the STAT5B originally located in the cytoplasm before stimulation. In isolated rat islets of Langerhans, PRL stimulated the nuclear translocation of both STAT5A and STAT5B only in beta-cells. The expression of the PRL receptor only by beta-cells was confirmed with a rabbit polyclonal antiserum raised against the rat PRL receptor. It was estimated that 4% of STAT5A and 9% of STAT5B originally located in the cytoplasm was translocated to the nucleus after stimulation. The presence of a functional JAK2/STAT5 signaling pathway in all islet cells was demonstrated by the nuclear translocation of STAT5B in all islet cells (i.e., alpha-, beta-, and delta-cells) after stimulation with fetal calf serum. The nuclear translocation and tyrosine phosphorylation of STAT5B was biphasic, with an initial peak within 30 min, a nadir between 1 and 3 hr, and prolonged activation after 4 hr. In contrast, the tyrosine phosphorylation of STAT5A was also biphasic but its nuclear translocation peaked within 30 min and was then reduced to a level slightly above that observed before PRL stimulation. This method is able to detect changes in STAT5 activation as small as 2% of the total cell content. These observations demonstrate the utility of this approach for studying the activation of STAT5 in a mixed population of cells within tissues or organs. In addition, the dose response for the nuclear translocation of STAT5B in normal beta-cells was similar to those for changes in proliferation and insulin secretion in isolated rat islets. Therefore, the subcellular localization can be used to monitor the activation of STAT5 and it may be a key event in the upregulation of the pancreatic islets of Langerhans during pregnancy.

To elucidate the effect of nitric oxide (NO) on the blood flow of the pancreatic islets, the NO synthase inhibitor NG-nitro-L-arginine (N-arg; 25 mg/kg BW) was administered iv to rats 10 min before pancreatic blood flow was measured with a nonradioactive microsphere technique. In male Sprague-Dawley rats, N-arg induced a marked decrease in islet blood flow (16 +/- 4 vs. 44 +/- 8 microliters/min.g pancreas; P < 0.001) and a less pronounced decrease in whole pancreatic blood flow (0.27 +/- 0.04 vs. 0.43 +/- 0.06 ml/min.g; P < 0.05), leading to a markedly decreased fractional islet blood flow (5.5 +/- 0.9% vs. 10.3 +/- 1.3%; P < 0.02). In a second experiment, injection of D-glucose (300 mg/kg BW, iv) in male Sprague-Dawley rats induced a selective increase in islet blood flow (P < 0.05). Such an increase has previously been shown to be mediated by a vagal cholinergic mechanism. Administration of N-arg to these rats resulted in decreased pancreatic (P < 0.05), islet (P < 0.001), and fractional (P < 0.001) islet blood flow, which did not differ from those observed in normoglycemic rats after treatment with N-arg. Furthermore, we studied the mechanism behind the previously described increase in islet blood perfusion, mediated by the vagus nerve, in F1-hybrids of the GK (Goto-Kakizaki) rat, a spontaneous animal model of noninsulin-dependent diabetes mellitus. Administration of N-arg to female GK rats resulted in decreases in islet (P < 0.001), pancreatic (P < 0.01), and fractional islet blood flow (P < 0.001) to the levels observed in female Wistar rats treated in parallel. These data are consistent with the possibility that NO is an important physiological regulator of islet blood flow. Furthermore, the vagally dependent high levels of islet blood flow demonstrated in the GK rat appear to be mediated by a mechanism involving NO.

Previous studies have indicated that various conditions under which an increased functional load is posed on the pancreatic islets, e.g. partial pancreatectomy and continuous glucose infusions, may influence the microcirculation of the pancreas. To investigate further the effects of elevated functional demand on the islets, the blood perfusion of the whole pancreas and the pancreatic islets was measured with a microsphere technique in an animal model presenting impaired glucose tolerance and mild hyperglycemia, namely F1-hybrids of the spontaneously non-insulin-dependent diabetic GK-Wistar rat. Normal Wistar rats served as controls. All hybrids had a pathological intraperitoneal glucose tolerance test 1 week before the blood flow measurements, which were performed in 10-12-week-old rats. Both the whole pancreatic and the islet blood flows were increased in the hybrids compared to controls. The fractional islet blood flow, i.e. the fraction of whole pancreatic blood flow diverted through the islets, also was increased in the hybrid rats (12.6 +/- 0.6% vs 9.8 +/- 0.5% in controls, p < 0.01). A bilateral abdominal vagotomy performed 30 min before the blood flow measurement markedly decreased the blood flow values of the islets and the whole pancreas in both groups of rats. After vagotomy, the islet blood flow in the hybrid rats was similar to that of the vagotomized control animals (8.2 +/- 0.8 and 7.5 +/- 1.4%, respectively). It is concluded that the increased pancreatic and islet blood perfusion observed in F1-hybrids of the GK-Wistar rat depends on a mechanism mediated by the vagus nerve.

The present study aimed to compare longitudinal variations in islet blood perfusion in rats with different degrees of impairment of glucose metabolism. For this purpose, mildly diabetic Goto-Kakizaki (GK) rats, glucose intolerant F(1) hybrids of GK and Wistar (W) rats (H), and control W rats were examined at 5 wk, 12 wk, or 1 yr of age, using the microsphere technique for blood flow measurements. W rats showed progressively increasing islet blood flow (IBF) throughout the experiment. Both GK and H rats demonstrated increasing IBF between 5 and 12 wk. However, H rats showed no further increment in IBF at 1 yr, whereas GK rats displayed a pronounced decrease in IBF between 12 wk and 1 yr of age. The augmented IBF seen in older W rats may constitute an adaptation to the increasing demand for insulin secretion in aging rats. The inability to adapt to the increased demand for insulin secretion by upregulation of islet blood flow could contribute to the progressive deterioration of glucose metabolism seen in the aging GK rat.

The aim of the present study was to evaluate the effects of diet-induced obesity on pancreatic islet blood perfusion in normal Wistar rats. Furthermore, we investigated to what extent any obesity-associated changes in islet blood flow could be reversed after reversion to a normal diet with normalization of body weight. Young adult female Wistar rats were offered a palatable mixed high-caloric diet (cafeteria diet) in addition to standard pelleted chow. Age-matched control rats received standard pelleted chow only. After 4 weeks the diet-treated rats had a body weight of approximately 15% more than that of the controls. All diet-treated rats had decreased glucose tolerance and increased serum insulin concentrations, but basal blood glucose concentrations were similar in anesthetized diet-treated and control rats. Whole pancreatic and islet blood flow rates were measured with a microsphere technique. The islet blood flow as well as fractional islet blood flow were increased (P < 0.01) in rats fed the cafeteria diet, while blood perfusion of the whole pancreas was similar to that of the control rats. In a second experiment, rats received the cafeteria diet for 4 weeks and were then fed standard pelleted food alone for another 3 weeks, while controls received standard diet for 7 weeks. After this period total body weight, retroperitoneal fat pad weight and glucose tolerance were similar to those of the controls. Whole pancreatic blood flow was unchanged as compared with that of control rats. However, both islet blood flow (P < 0.01) and fractional blood flow (P < 0.01) were increased. We conclude that diet-induced obesity in rats is associated with decreased glucose tolerance, hyperinsulinemia and a specific increase in absolute and fractional islet blood perfusion. This increase persists for at least 3 weeks after the diet is withdrawn despite normalization of body weight and glucose tolerance.