omega-Conotoxin GVIA-resistant neurotransmitter release in postganglionic sympathetic nerve terminals

Intracellular recording techniques were used to study neurotransmitter release in the guinea-pig isolated vas deferens. Low concentrations of the irreversible and selective N-type calcium channel blocker omega-conotoxin GVIA have previously been shown to block excitatory junction potentials evoked by low frequencies (less than or equal to 1 Hz) of nerve stimulation. Here we report a component of action potential-evoked release which is insensitive to high concentrations of omega-conotoxin GVIA. We have termed this component ''residual release'' and show (i) it is positively frequency-dependent, (ii) its magnitude is dependent on both the train length and interval between trains, (iii) ''residual release'' can be modulated through prejunctional alpha(2)-adrenoceptors and (iv) ''residual release'' is insensitive to many calcium entry blockers but abolished by omega-grammotoxin SIA and cadmium ions. Although noradrenaline is released by nerve action potentials, residual excitatory junction potentials were abolished by alpha,beta-methylene-ATP and therefore resulted entirely from the actions of neuronally released ATP acting through postjunctional P-2x-purinoceptors. The results suggest that calcium entry through a novel, pharmacologically uncharacterized voltage-dependent calcium channel is responsible for ''residual release'' in sympathetic nerve terminals. It seems that in response to single or short trains of nerve action potentials, N-type calcium channels dominate the release process. However, at higher frequencies other voltage-dependent calcium channels are recruited and these may have an important role to play in triggering the mechanisms underlying frequency-dependent facilitation.