The goals of the present study were to characterize pharmacologically and biophysically the whole set of voltage dependent Ca²⁺ channels (VDCC) in human chromaffin cells, and to investigate the efficacy of each Ca²⁺ channel to control the exocytosis of neurotransmitters. The perforated-patch configuration of the patch-clamp technique was employed. Nifedipine, w-conotoxin GVIA and w-conotoxin MVIIC, added sequentially and accumulatively, were used to block L, N, and P/Q-type channels, respectively.

Two distinct populations of cells sorted out by their predominant Ca²⁺ channel charge density, N or P/Q, were discovered. In the N group, N and P/Q channels accounted respectively for 51.4±2.9% and 18±2.7%. In the P/Q group this proportion was 20.4±1.4% and 45.6±1.5%, respectively. Also, after blocking L, N and P/Q channels, a resistant current with a fast inactivation time constant (21.8±1.4 ms, n=20) was found to be present. Inmunocytochemistry performed with a Cav 2.3 antibody confirmed the existence of an R-type channel. L- and R-type contributions were not significatively different between groups, accounting for 14.5±1% and 17.7±1%, respectively.

In relation to the exocytosis of neurotransmitters, the percentage of secretion attributed to each Ca²⁺ channel was similar to the percentage of Ca²⁺ current charge carried by it, with the exception of the L-type Ca²⁺ channel, which was more efficiently coupled to the exocytosis (34±4% secretion controlled by this channel).

Acknowledgements: Work supported by grants Nordm; BFU2005-00743/BFI and 11/BCB/003 awarded to A.A. APA holds a FPI-CAM fellowship.