Noradrenaline, somatostatin and opioids inhibit activity of single HVA/N-type calcium channels in excised neuronal membranes

Unitary barium currents were measured in outside-out patches of membrane from acutely dissociated guinea-pig submucosal neurones. A single set of HVA/N-like calcium channels with unitary conductance in 110 mM barium of approximately 10 pS was activated by depolarization; channel activity did not show inactivation during 100 ms duration depolarizing pulses. Somatostatin, [Met⁵]enkephalin and α₂-adrenoceptor agonists, which are known to partially inhibit macroscopic calcium currents in these neurones, decreased frequency of single channel opening per depolarizing pulse without altering unitary conductance. Agonists were effective only when GTP was included in the patch pipette. These experiments directly demonstrate that an excised membrane patch can sustain local convergence of multiple receptors to a single calcium channel.