KINETICS OF STIMULUS-COUPLED SECRETION IN DIALYZED BOVINE CHROMAFFIN CELLS IN RESPONSE TO TRAINS OF DEPOLARIZING PULSES

Stimulus-secretion coupling in bovine chromaffin cells was investigate d with whole-cell patch-clamp recordings and capacitance detection tec hniques to monitor exocytosis in response to trains of depolarizing pu lses. Two kinetically discrete modes of exocytotic responses were obse rved. In one mode, the first depolarization of a train elicited a larg e increase in membrane capacitance (C-m; mean similar to 70 fF). This secretory mode was characterized by small Ca2+ requirements, relative insensitivity to the pipette Ca2+ chelator concentration, and rapid de pletion of the secretory response. This mode of stimulus-secretion cou pling was labile and was seen only in response to the first and, occas ionally, the second stimulus train of whole-cell recordings. The secon d type of exocytotic response persisted for the remainder of the whole -cell recordings and consisted of two distinct phases. During the earl iest pulses of a stimulus train, Ca2+ entry did not evoke C-m increase s. Instead, C-m responses were elicited by later pulses, despite dimin ished Ca2+ entry per pulse caused by Ca2+ channel inactivation. The se cretory phase was initiated after a specific ''threshold'' amount of C a2+ had entered the cell, which was determined by the concentration, b ut not the binding kinetics, of the Ca2+ chelator in the pipette. In b oth the early and the secretory phases, the response of the cell was p roportional to cumulative Ca2+ entry, regardless of current amplitude, pulse duration, or number of pulses. Threshold-type secretory kinetic s has been described previously in peptide-secreting neurohypophysial (NHP) nerve terminals (Seward et al., 1995). Secretory kinetics with m inimal Ca2+ requirements has not been observed in that preparation. Ch romaffin cells appear to possess a broader repertoire of stimulus-secr etion coupling modes than NHP terminals.