The Readily Releasable Pool of Vesicles in Chromaffin Cells Is Replenished in a Temperature-Dependent Manner and Transiently Overfills at 37°C

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Abstract
Maturation of exocytic vesicles to the release-ready state is regulated by several factors, including intracellular calcium concentration ([Ca2+]int) and the state of protein phosphorylation. Here we investigated the effects of temperature on the recovery from depletion of the readily releasable pool (RRP) of vesicles in adrenal chromaffin cells. Exocytosis and [Ca2+]int were monitored by combined membrane capacitance and fura-2 measurements. At higher temperatures, a faster pool refilling and a larger RRP size were observed. The time constants of the recovery from depletion ranged from 3.6 to 1.1 sec (22 and 37°C, respectively) yielding a Q10 of 2.3. The changes of the Ca2+ signal between the different temperatures could not account for the differences in recovery kinetics. At 32 and 37°C, we observed a transient overfilling of the RRP after pool depletion, which stands in clear contrast to the sustained secretory depression seen at lower temperatures. The overshoot in RRP size was very prominent in cells with lower basal [Ca2+]int, hence with a large difference between prestimulus and poststimulus [Ca2+]int. In cells with higher basal [Ca2+]int, the pool was larger under steady-state conditions but showed less overfilling on stimulation. We conclude that vesicle maturation is markedly accelerated at physiological temperature, thus allowing for a rapid adaptation of the pool size to the relatively short-lived Ca2+ transient.