Ion interaction at the pore of Lc‐type Ca 2+ channel is sufficient to mediate depolarization‐induced exocytosis

Abstract The coupling of voltage‐gated Ca 2+ channel (VGCC) to exocytotic proteins suggests a regulatory function for the channel in depolarization‐evoked exocytosis. To explore this possibility we have examined catecholamine secretion in PC12 and chromaffin cells. We found that replacing Ca 2+ with La 3+ or other lanthanide ions supported exocytosis in divalent ion‐free solution. Cd 2+ , nifedipine, or verapamil inhibited depolarization‐evoked secretion in La 3+ , indicating specific binding of La 3+ at the pore of L‐type VGCC, probably at the poly‐glutamate (EEEE) locus. Lanthanide efficacy was stringently dependent on ionic radius with La 3+  > Ce 3+  > Pr 3+ , consistent with a size‐selective binding interface of trivalent cations at the channel pore. La 3+ inward currents were not detected and the highly sensitive La 3+ /fura‐2 imaging assay (∼1 p m ) detected no La 3+ entry, cytosolic La 3+ build‐up, or alterations in cytosolic Ca 2 . These results provide strong evidence that occupancy of the pore of the channel by an impermeable cation leads to a conformational change that is transmitted to the exocytotic machinery upstream of intracellular cation build‐up (intracellular Ca 2+ concentration). Our model allows for a tight temporal and spatial coupling between the excitatory stimulation event and vesicle fusion. It challenges the conventional view that intracellular Ca 2+ ion build‐up via VGCC permeation is required to trigger secretion and establishes the VGCC as a plausible Ca 2+ sensor protein in the process of neuroendocrine secretion.