Abstract

We set out to characterize the types of Ca²⁺ channels that mediate release of the predominant excitatory (acetylcholine) and inhibitory (norepinephrine) neurotransmitters in canine bronchi, using electrically evoked contractions and relaxations, respectively, as indicators of this release. We found that the selective N-type Ca²⁺ channel blocker (ω-conotoxin GVIA) eliminated electrically evoked contractions in a dose-dependent fashion (half-maximal inhibition in the presence of 1–5 nM) but had no significant effect on those evoked by exogenously added acetylcholine. Selective blockers of P-type Ca²⁺ channels (ω-agatoxin TK; 10⁻⁸ to 10⁻⁷ M) or ofl-type Ca²⁺ channels (nifedipine; 10⁻⁸ to 10⁻⁶ M) had no significant effect on the responses to neurally released or exogenously added acetylcholine. Likewise, electrically evoked relaxations were blocked by ω-conotoxin GVIA (10⁻⁷ M) but not by ω-agatoxin TK (10⁻⁷ M) or nifedipine (10⁻⁷ M); none of these Ca²⁺ channel blockers had a significant inhibitory effect on isoproterenol-triggered relaxations. We conclude that excitatory and inhibitory neurotransmission in canine bronchi is mediated predominantly by N-type Ca²⁺ channels, with little or no contribution from L-, P-, Q-, or T-type channels.