Calcium-independent and voltage-dependent exocytosis in mouse chromaffin cells

MOYA-DÍAZ, JOSE; GONZÁLEZ INCHAUSPE, CARLOTA; MARENGO, FERNANDO D

Cairns

Congreso; 25th meeting of the International society of Neurochemistry; 2015

A brief depolatization resembling an action potential applied on mouse chromaffin cells in conditions of complete inhibition of calcium currents (ICa2+) induced a moderate increase in capacitance. To study this phenomenon more systematically we applied square depolarizations (from -80 to +10 mV) of variable durations in presence of (i) 0 mM external nominal Ca+2 or (ii) 5 mM calcium + 100 μM cadmio. In both conditions we measured an increase in capacitance that saturated at 17±2 and 14±1 fF respectively at 100 ms pulse duration, in the absence of measurable ICa2+ and cytosolic calcium transients. To buffer any contaminant calcium that might enter to the cell or be released from internal stores we made experiments in 0 extracellular calcium + 4 mM intracellular BAPTA, obtaining again a significant exocytosis (14±2 fF) in response to 100 ms depolarizations. The calcium release blocker 2-APB or a pretreatment with the SERCA inhibitor thapsigargin were both unable to block this exocytosis process (15±2fF and 14±2 fF, respectively). Moreover, this calcium-independent exocytosis process followed a sigmoid dependence with membrane potential, reaching the 50% of the saturating value at approximately -30 mV. When this vesicle pool was completely depleted by application of a 100 ms depolarization, it recovered with a time constant of 1.04±0.18 s. In agreement with this result, synchronous exocytosis at 0 extracellular calcium did not decrease noticeably at low frequency stimulation (0,2-0,5 Hz), but the application of trains at higher frequencies (2-5 Hz) induced a pronounced decrease in this parameter. These results suggest the existence of a calcium independent, but membrane potential dependent, mechanism of secretion in chromaffin cells that would be relevant at low frequencies.