CALCIUM AND BARIUM-DEPENDENT EXOCYTOSIS FROM THE RAT INSULINOMA CELL-LINE RINM5F ASSAYED USING MEMBRANE CAPACITANCE MEASUREMENTS AND SEROTONIN RELEASE

Electrophysiological measurements of cell capacitance (C-m) and bioche mical assays of [H-3] serotonin ([H-3]5-hydroxytryptamine or [H-3]5-HT ) release were combined to study the control of secretion in rat insul inoma RINm5F cells. Depolarizing pulses produced C-m changes (Delta C- m), indicative of exocytosis, with the same voltage and Ca2+ dependenc y as the inward Ca2+ currents (I-Ca). Ba2+ was able to substitute for Ca2+ in stimulating exocytosis, but not endocytosis. However, both the relative potency and kinetics of Ca2+-versus Ba2+-triggered exocytosi s differed significantly. 5-HT synthesis and uptake were demonstrated in RINm5F cells. This allowed the use of [H-3]5-HT to study hormone re lease from eel populations. [H-3]5-HT was released in a depolarization -, Ca2+- and time-dependent manner. Ba2+ also substituted for Ca2+ in depolarization-induced [H-3]5-HT release. Thapsigargin, used to deplet e Ca2+ stores, had no effects on Ca2+-triggered C-m increases, but Ca2 +-triggered [H-3]5-HT release was abolished. Ba2+-triggered [H-3]5-HT release, however, was only slightly affected by Ca2+ store depletion. Ba2+ was found to act directly as a secretagogue of [H-3]5-HT in intac t cells, but not in C-m measurements of voltage-clamped cells, suggest ing that cell depolarization is a prerequisite for this action.