Plasma membrane depolarization as a determinant of the first phase of insulin secretion

The role of plasma membrane depolarization as a determinant of the initial phase of insulin secretion was investigated. NMRI mouse islets and beta-cells were used to measure the kinetics of insulin secretion, ATP and ADP content, membrane potential, and cytosolic free Ca(2+) concentration ([Ca(2+)](i)). The depolarization of metabolically intact beta-cells by KCl corresponded closely to the theoretical values. In contrast to physiological (glucose) or pharmacological (tolbutamide) ATP-sensitive K(+) (K(ATP)) channel block, KCl depolarization did not induce action potential spiking. The depolarization by 15 mM K(+) (21 mV) corresponded to the plateau depolarization by 50 or 500 microM tolbutamide; that by 40 mM K(+) (41 mV) corresponded to the action potential peaks. Nifedipine and diazoxide abolished action potentials but not KCl depolarization, suggesting that the depolarizing strength of 15, but not 40 mM K(+) corresponds to that of K(ATP) channel closure. K(+) (40 mM) induced a massive secretory response in the presence of 5 mM glucose, whereas 15 mM K(+), like 50 microM tolbutamide, was only slightly effective, even though a marked increase in [Ca(2+)](i) was produced. Raising glucose from 5 to 10 mM in the continued presence of 15 mM K(+) resulted in a strongly enhanced biphasic response. The depolarization pattern of this combination could be mimicked by combining basal glucose with 15 mM K(+) and 50 microM tolbutamide; however, the secretory response to these nonnutrients was much weaker. In conclusion, the initial secretory response to nutrient secretagogues is largely influenced by signaling mechanisms that do not involve depolarization.