Ca influx and clearance at hyperpolarized membrane potentials modulate spontaneous and stimulated exocytosis in neuroendocrine cells.

Neuroendocrine adrenal chromaffin cells release neurohormones catecholamines in response to Ca entry via voltage-gated Ca channels (VGCCs). Adrenal chromaffin cells also express non-voltage-gated channels, which may conduct Ca at negative membrane potentials, whose role in regulation of exocytosis is poorly understood. We explored how modulation of Ca influx at negative membrane potentials affects basal cytosolic Ca concentration ([Ca]) and exocytosis in metabolically intact voltage-clamped bovine adrenal chromaffin cells. We found that in these cells, Ca entry at negative membrane potentials is balanced by Ca extrusion by the Na/Ca exchanger and that this balance can be altered by membrane hyperpolarization or stimulation with an inflammatory hormone bradykinin. Membrane hyperpolarization or application of bradykinin augmented Ca-carrying current at negative membrane potentials, elevated basal [Ca], and facilitated synchronous exocytosis evoked by the small amounts of Ca injected into the cell via VGCCs (up to 20 pC). Exocytotic responses evoked by the injections of the larger amounts of Ca via VGCCs (> 20 pC) were suppressed by preceding hyperpolarization. In the absence of Ca entry via VGCCs and Ca extrusion via the Na/Ca exchanger, membrane hyperpolarization induced a significant elevation in [Ca] and asynchronous exocytosis. Our results indicate that physiological interferences, such as membrane hyperpolarization and/or activation of non-voltage-gated Ca channels, modulate basal [Ca] and, consequently, segregation of exocytotic vesicles and their readiness to be released spontaneously and in response to Ca entry via VGCCs. These mechanisms may play role in homeostatic plasticity of neuronal and endocrine cells.