ラット膵島B細胞の刺激-放出連関の温度依存性に関する研究 Temperature effects on stimulus-secretion coupling in rat pancreatic B cells

博士論文

The present study was designed to examine 1) the correlation between the secretory response of insulin and the dynamics of cytoplasmic calcium concentration([Ca2+]i) induced by glucose and 2) the effects of lowering temperature on the processes proximal and distal to Ca2+ signaling in pancreatic B cells. Glucose (15mM) caused a biphasic increase in insulin secretion in perifused pancreatic islets o f rat at 35℃: that is, the secretory response consisted of a transient increase followed by a nadir (first phase) and gradual increase (second phase). Glucose caused a biphasic increase in [Ca2+]i in fura-2-loaded islets at 35℃, a transient increase(first phase) followed by a gradual increase (second phase) on which recurrent Ca2+ spikes were frequently superimposed. Time courses of the first and the second phases of the [Ca2+]i increase corresponded to those of the insulin secretion, suggesting that the biphasic insulin secretion is underlain by the biphasic [Ca2+]i increase. The insulin secretion and the [Ca2+]i inerease were decreased by lowering temperature to 30 or 25℃ with a temperature coefficient (Q10) of 11.6 and 1.8, respectively. The ratio of glucose-induoed insulin secretion to the [Ca2-]i increase (IS/Ca) was calculated to represent the efficiency o f Ca2+ to cause exocytosis. The Q10 value of the ratio of IS/Ca was calculated to be 3.7 timcs grcater(6.6) than that of the [Ca2+]i increase, suggcsting that the exocytotic process is more sensitive to temperature than the [Ca2+]i increase. The effects of lowering temperature on the insulin secretion and the [Ca2+]i increase induced by high K+(30mM), carbamylcholine (CCh; 100mM), and glibenclamide (2mM) were also examined. The Q10 values of the ratios of IS/Ca in the responses to high K+, CCh, and glibenclamide were 5.6, 3.8, and 13.0, respectively. These values were greater than the Q10 values of corresponding [Ca2+]i responses (1.2, 1.4, and 1.8, respectively), suggesting that the inhibition of insulin secretion could be attributed to temperature sensitivity of the exocytotic process regardless of the cause of [Ca2+]i increase. It is concluded that: 1) the glucose-induced biphasic insulin secretion is underlain by the biphasic [Ca2+]i increase;2) lowering tempcrature inhibits both processes proximal and distal to Ca2+ signaling in response to glucose; and 3) the latter process is more sensitive to temperature than the former in rat pancreatic B cells.