Calcium- and barium-dependent exocytosis from the rat insulinoma cell line RINm5F using membrane capacitance measurements and serotonin release

Richmond, Codignola, Cooke and Sher
Pflügers' Arch 432: 258-269 (1996)

Summary

Electrophysiological measurements of cell capacitance (C_m) and biochemical assays of [³H] serotonin ([³H]5-hydroxytryptamine or [³H]5-HT) release were combined to study the control of secretion in rat insulinoma RINm5F cells.  Depolarizing pulses produced C_m changes (δC_m), indicative of exocytosis, with the same voltage and Ca²⁺ dependency 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 exocytosis differed significantly.  5-HT synthesis and uptake were demonstrated in RINm5F cells.  This allowed the use of [³H]5-HT to study hormone release from cell populations.  [³H]5-HT was released in a depolarization-, Ca²⁺- and time-dependent manner.  Ba²⁺ also substituted for Ca²⁺ in depolarization-induced [³H]5-HT release.  Thapsigargin, used to deplete Ca²⁺ stores, had no effects on Ca²⁺-triggered C_m increases, but Ca²⁺-triggered [³H]5-HT release was abolished.  Ba²⁺-triggered [³H]5-HT release, however, was only slightly affected by Ca²⁺ store depletion.  Ba²⁺ was found to act directly as a secretagogue of [³H]5-HT in intact cells, but not in C_m measurements of voltage-clamped cells, suggesting that cell depolarization is a prerequisite for this action.

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