Capacitance measurements of exocytosis in mouse pancreatic α‐, β‐ and δ‐cells within intact islets of Langerhans

Capacitance measurements of exocytosis were applied to functionally identified α‐, β‐ and δ‐cells in intact mouse pancreatic islets. The maximum rate of capacitance increase in β‐cells during a depolarization to 0 mV was equivalent to 14 granules s −1 , <5% of that observed in isolated β‐cells. β‐Cell secretion exhibited bell‐shaped voltage dependence and peaked at +20 mV. At physiological membrane potentials (up to ∼−20 mV) the maximum rate of release was ∼4 granules s −1 . Both exocytosis (measured by capacitance measurements) and insulin release (detected by radioimmunoassay) were strongly inhibited by the L‐type Ca 2+ channel blocker nifedipine (25 μ m ) but only marginally (<20%) affected by the R‐type Ca 2+ channel blocker SNX482 (100 n m ). Exocytosis in the glucagon‐producing α‐cells peaked at +20 mV. The capacitance increases elicited by pulses to 0 mV exhibited biphasic kinetics and consisted of an initial transient (150 granules s −1 ) and a sustained late component (30 granules s −1 ). Whereas addition of the N‐type Ca 2+ channel blocker ω‐conotoxin GVIA (0.1 μ m ) inhibited glucagon secretion measured in the presence of 1 m m glucose to the same extent as an elevation of glucose to 20 m m , the L‐type Ca 2+ channel blocker nifedipine (25 μ m ) had no effect. Thus, glucagon release during hyperglycaemic conditions depends principally on Ca 2+ ‐influx through N‐type rather than L‐type Ca 2+ channels. Exocytosis in the somatostatin‐secreting δ‐cells likewise exhibited two kinetically separable phases of capacitance increase and consisted of an early rapid (600 granules s −1 ) component followed by a sustained slower (60 granules s −1 ) component. We conclude that (1) capacitance measurements in intact pancreatic islets are feasible; (2) exocytosis measured in β‐cells in situ is significantly slower than that of isolated cells; and (3) the different types of islet cells exhibit distinct exocytotic features.