Short-Term Changes in the Ca2+-Exocytosis Relationship during Repetitive Pulse Protocols in Bovine Adrenal Chromaffin Cells

Abstract

Stimulus-secretion coupling was monitored with capacitance detection in bovine chromaffin cells recorded in perforated patch mode and stimulated with trains of depolarizing pulses. A subset of stimulus trains evoked a response with a Ca²⁺-exocytosis relationship identical to that obtained for single depolarizing pulses (Engisch and Nowycky, 1996). Other trains evoked responses with enhanced or diminished Ca²⁺ efficacy relative to this input–output function. The probability of obtaining a particular Ca²⁺-exocytosis relationship was correlated with the amount of Ca²⁺ entry per pulse, such that shorter pulses or smaller currents were associated with the greatest efficacy, and longer pulses and larger currents with the lowest efficacy. Apparent enhancements in Ca²⁺ efficacy were not caused by residual Ca²⁺ summing between pulses, because decreasing the interval between pulses usually reduced efficacy in the same cell; conversely, increasing the interval between pulses did not prevent an enhanced Ca²⁺-exocytosis relationship. Apparent decreases in Ca²⁺ efficacy were not caused by depletion of an available pool of release-ready vesicles, because an equivalent amount of total Ca²⁺entry during a single long depolarizing pulse usually evoked a much larger secretory response in the same cell. Finally, there were no striking differences in global Ca²⁺ levels monitored with the fluorescent indicator Fura Red that could account for apparent changes in Ca²⁺ efficacy during repetitive stimulus protocols. It appears that in chromaffin cells, the Ca²⁺-exocytosis relationship is subject to activity-dependent changes during a stimulus train and can be modulated up or down from a basal state accessed by single pulse stimulations.