Inhibition of Ca-spikes in rat preganglionic cervical sympathetic nerves by sympathomimetic amines

1. Propagated Ca-spikes were recorded from isolated cervical sympathetic nerve trunks of the rat when bathed in a solution containing 5 mM Ca2+, 0.5 or 1 microM tetrodotoxin (to block Na currents) and 1 mM 4-aminopyridine (to reduce K currents). 2. Spikes persisted when external Ca2+ was replaced with Sr2+ or Ba2+, but were blocked by the addition of the following inorganic Ca-channel blockers (in descending order of potency): Cd2+ greater than La3+ greater than Ni2+ greater than Co2+ greater than Mn2+ greater than Mg2+. 3. Ca-spike amplitude was reduced by up to 90% by (-)-noradrenaline (IC50 1.5 microM). The following sympathomimetic amines imitated this effect (in descending order of potency): clonidine greater than or equal to (-)-adrenaline greater than or equal to [(-)-noradrenaline] greater than or equal to dopamine greater than (-)-phenylephrine greater than or equal to (+/-)-amidephrine. 4. Ca-spike inhibition by (-)-noradrenaline was antagonized by phentolamine (pA2 6.5). Yohimbine was about 10 times weaker than phentolamine; (+/-)-propranolol (1 microM) and prazosin (10 microM) had no clear effect. 5. (-)-Noradrenaline reduced the amplitude of the compound action potential recorded from the superior cervical sympathetic ganglion following supramaximal preganglionic trunk stimulation when recorded in normal Krebs solution and hyperpolarized the ganglion with respect to the post-ganglionic trunk. Depression of the transmitted ganglionic action potential was antagonized by phentolamine (5 microM) but not by yohimbine (1 microM); in contrast 1 microM yohimbine completely prevented the ganglionic hyperpolarization. (-)-Noradrenaline did not hyperpolarize the preganglionic cervical sympathetic nerve trunk under these recording conditions. 6. It is suggested that inhibition of transmitter release from sympathetic preganglionic fibres produced by noradrenaline results from a depression of the voltage-gated Ca current in the fibres and/or their terminals, and that this action is mediated by an alpha-adrenoceptor which does not fully conform to either alpha 1 or alpha 2 subtypes.