The study of the contribution of the nicotinic receptor to the stimulus-secretion coupling process at high spatio-temporal resolution has not been performed yet. The whole-cell secretory response evoked by short pulses of acetylcholine (ACh) in human chromaffin cells was examined for the first time in the present study, using a new protocol based on quickly switching from the voltage-clamp to the current-clamp configuration of the patch-clamp technique.

It was found that Ca²⁺ entry through the nicotinic receptor at hyperpolarized membrane potentials contributed as much to the exocytosis (100.4±27.3 fF) evoked by 200 ms pulses of the agonist, as Ca²⁺ flux through voltage-dependent Ca²⁺ channels (VDCC) at depolarized membrane potentials. The nicotinic current triggered a depolarization event, which was blocked by 10 microM mecamylamine. When a long ACh stimulus (15 s) was applied, the nicotinic current at the end of the pulse reached a value of 15.45 ± 3.6 pA, but the membrane potential depolarization still remained at the "plateau" stage until removal of the agonist. Perfusion with 200 microM Cd²⁺ during the 15 s ACh pulse completely abolished the plasma membrane depolarization at the end of the pulse. These data reflect that VDCC were recruited by the small current flowing through the desensitized nicotinic receptor to maintain the depolarization, attributing again a relevant role to this receptor in the exocytosis. Acknowledgements: Grants BFU2005-00743/BFI and 11/BCB/003 (A.A). FPI-CAM fellowship (APA).