Catecholamine exocytosis during low frequency stimulation in mouse adrenal chromaffin cells is primarily asynchronous and controlled by the novel mechanism of Ca2+ syntilla suppression

Although the importance of asynchronous exocytosis is becoming clearer, not enough is known about its roles and mechanisms. Here we describe the nature of exocytosis in mouse adrenal chromaffin cells during low frequency physiological stimulation, i.e. 0.5Hz, providing new views. We report that less than 10% of all catecholaminergic exocytosis during low frequency stimulation is synchronized to a simulated action potential (sAP), i.e. the dominant phase is asynchronous. This asynchronous phase of exocytosis does not require Ca2+ influx, requires the ryanodine receptor, RyR2, and comprises exocytic events with characteristics similar to those of spontaneous events. We propose a novel mechanism of disinhibition wherein APs inhibit Ca2+ syntillas, relieving their inhibition of spontaneous exocytosis, which leads to an increase in the asynchronous phase of elicited exocytosis. The work has the specific physiological implication that basal sympathetic tone associated with the rest and digest' state is set in part by Ca2+ syntillas. Furthermore, there is evidence that this regulation of exocytosis by Ca2+ syntillas may be a general mechanism that extends to neurons. AbstractAdrenal chromaffin cells (ACCs), stimulated by the splanchnic nerve, generate action potentials (APs) at a frequency near 0.5Hz in the resting physiological state, at times described as rest and digest'. How such low frequency stimulation in turn elicits sufficient catecholamine exocytosis to set basal sympathetic tone is not readily explained by the classical mechanism of stimulus-secretion coupling, where exocytosis is synchronized to AP-induced Ca2+ influx. By using simulated action potentials (sAPs) at 0.5Hz in isolated patch-clamped mouse ACCs, we show here that less than 10% of all catecholaminergic exocytosis, measured by carbon fibre amperometry, is synchronized to an AP. The asynchronous phase, the dominant phase, of exocytosis does not require Ca2+ influx. Furthermore, increased asynchronous exocytosis is accompanied by an AP-dependent decrease in frequency of Ca2+ syntillas (i.e. transient, focal Ca2+ release from internal stores) and is ryanodine sensitive. We propose a mechanism of disinhibition, wherein APs suppress Ca2+ syntillas, which themselves inhibit exocytosis as they do in the case of spontaneous catecholaminergic exocytosis.