A method for the determination of immunoreactive somatostatin in rat plasma is described. Blood specimens were collected into aprotinin and EDTA. Plasma was separated, immediately diluted with acidified acetone and ultrasonicated. The resultant supernatant was lyophilised. The dilution curve of the material thus extracted was parallel to that of synthetic somatostatin. The material was eluted mainly in a similar position to that of synthetic somatostatin on Sephadex G-25(0 column chromatography. The somatostatin immunoreactivity was degraded significantly from the pre-incubated value of 846 + 86 pg/ml (n = 4, mean _+ SEM) to 102 __ 16 pg/ml in the same manner as that of synthetic somatostatin when incubated with one ml of fresh rat plasma at 37 ~ for 30 min. The mean recovery in quadruplicate of immtmoreacfive somatostatin at concentrations of 100, 200 and 400 pg/ml was 83 ___ 7, 95 ___ 4 and 76 ___ 4%, respectively. Using this method, plasma immunoreactive somatostatin responses to arginine, glucose and glucagon infusion were measured in pentobarbital anaesthetized rats. The mean basal plasma immtmoreactive somatostatin concentration in the jugular vein was 35 + 3 pg/ml (n = 7), while that in the hepatic portal vein was 120 + 17 pg/ml (n = 7). Infusion of arginine, glucose and glucagon all resulted in 2-3 fold increases in portal plasma immunoreactive somatostatin concentration.

In order to clarify the physiologic role of somatostatin in insulin release, rat pancreatic islets treated by somatostatin antiserum were incubated in media containing various concentrations of glucose. Insulin release from antiserum-treated islets was significantly elevated above that from nontreated ones at 3.3 and 8.3 mM glucose, while the former was not different from the latter at 16.7 mM glucose. It is suggested that somatostatin plays an important role in the regulation of insulin release in the physiologic range of glucose concentration.

The concentration of thyrotrophin-releasing hormone (TRH) immunoreactivity was determined in pancreatic islets and acini in the rat. In addition, timecourse changes in TRH in response to an iv injection of streptozotocin (65 mg/kg body weight) with or without nicotinamide (500 mg/kg body weight) were examined in the whole pancreas. Furthermore, pancreatic TRH was measured in diabetic rats treated with insulin for 3 weeks.The TRH concentration in rat islets was 42-fold higher than in exocrine glands, indicating that the majority of pancreatic TRH is of islet origin. The mean concentration of pancreatic TRH decreased to 60 and 65% of the respective control values at 4 and 7 h after administration of streptozotocin, respectively. At 24 h, it fell to 10% of control values without significant changes in TRH levels in the hypothalamus and gastrointestinal tract. In contrast, no significant change in pancreatic TRH was noted in rats given combined treatment with streptozotocin and nicotinamide. The injection of streptozotocin alone resulted in severe hypoglycaemia at 7 h and hyperglycaemia at 24 h, whereas neither resulted from the combined treatment. Insulin therapy had no influence on the decreased TRH concentrations in the diabetic pancreas.These results suggest that TRH may be localized to the B cells of pancreatic islets, and that the marked reduction in TRH in diabetic pancreases is not a metabolic consequence of insulin deficiency. Thyrotrophin-releasing hormone (TRH) immuno¬ reactivity and biological activity has been shown to be present throughout the gastrointestinal tract and pancreas in rats (Morley et al. 1977), with the largest amount occurring in the islets of Langer¬ hans (Martino et al. 1978). On the other hand, streptozotocin exerts a diabetogenic action through the selective destruction of B cells of pancreatic islets (Junod et al. 1967;Kazumi et al. 1979Kazumi et al. , 1980b, which is prevented by combined treatment with nicotinamide (Junod et al. 1969;Kazumi et al. 1979;Stauffacher et al. 1970). Recently, Martino et al. (1978) reported that marked decreases in TRH in whole pancreas and in isolated islets were found in rats 7-10 days after induction of streptozotocin diabetes. Therefore, the present study was de¬ signed to investigate acute effects of a diabetogenic dose of streptozotocin alone or streptozotocin with nicotinamide on the concentration of TRH immu¬ noreactivity in rat pancreas. In addition, effects of insulin replacement therapy on pancreatic TRH were examined in diabetic rats. Materials and MethodsMale Wistar rats, weighing 210-290 g and fed rat chow ad libitum, were used throughout the study. They were fasted for 16 h prior to the injection of streptozotocin (Upjohn, Kalamazoo, Michigan). After the injection, ani¬ mals which were to be killed after 4 and 7 h remained fasting until that time, while the others were allowed 1 To whom requests for reprints should be addressed.

Abstract.To elucidate abnormalities in the feedback relationships between plasma glucose and plasma insulin levels in diabetic patients, we have introduced the vector autoregressive modeling method as a new for tool feedback analysis. This technique was applied to plasma glucose and insulin level data from a series of 977 frequently-sampled oral glucose tolerance tests (FS-OGTT). Neither special instruments nor medications were used in FS-OGTT. We were able to predict the degree of the plasma glucose response occurring after an impulse-like increase in plasma insulin at 1mU/mL, as well as the plasma insulin response triggered by an impulse-like increase in plasma glucose at 1mg/dL, in the form of "impulse response curves". The predicted impulse response curve of glucose to insulin gradually changed from negative to positive with incremental changes in the fasting plasma glucose level, reflecting increased insulin resistance. Furthermore, the response of insulin to glucose decreased in a stepwise fashion with the incremental changes in the fasting plasma glucose level. Our findings confirm the usefulness of impulse response curves as clinical indicators. In addition, analytical data point to a possible contribution of excessive hepatic glucose production to the pathogenesis of the insulin resistance in non-insulin-dependent diabetes mellitus. (Keio J Med 47 (1): 28-36, March 1998)

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