In in vivo experiments on postsynaptic ganglia, agents were administered directly to the cardiac sympathetic ganglia via the right subclavian artery in spinal dogs and changes in heart rate were utilized as indicators of ganglionic function.
In in vitro experiments on presynaptic ganglia, acetylcholine liberated from the preganglionic sites of isolated dog stellate ganglion was measured by HPLC with ECD.To determine endogenous thromboxane A_2 (TXA_2) formation, TXB_2 accumulation was quantitated in the aliquots of incubation buffer by enzyme immunoassay using a kit. Positive chronotropic responses to dimethylphenylpiperazinium and McN-A-343 i.a. administered to the ganglia were inhibited by endothelin-1 (0.05-0.2 mug) and endothelin-3 (0.5-2 mug). The inhibition induced by endothelins was antagonized by endothelin ET_A receptor antagonist, BQ-123. The inhibition was not affected by pretreatment with a cyclooxygenase inhibitor, indomethacin, voltage sensitive Ca^<2+> and K^+ channel anta
gonists, nifedipine and 4-aminopyridine and ATP dependent K^+ channel antagonist, glybenclamide. However, the inhibion induced by endothelins was antagonized by pretreatment with a low conductance Ca^<2+>-activated K^+ channel antagonists, d-tubocurarine and apamin and scyllatoxin, but not by a high conductance Ca^<2+>-activated K^<2+> channel antagonists, charybdotoxin and iberiotoxin. The amount of acetylcholine released by preganglionic stimulation was reduced dose-dependently after exposure to endothelin-3 (10^<-9>-10^<-6>M). The similar reductions were also mimicked by exposure to exogenous STA_2 (stable TXA_2), U-46619 (TXA_2/PGH_2 receptors agonist) and prostaglandin E_2, but not by prostaglandin F_2alpha and I_2.
However, the reduction elicited by endothelin-3 was antagonized by pretreatment with a phospholipase A_2 inhibitor, dexamethasone and methylprednisolone, a cyclooxygenase inhibitors, aspirin and indomethasin, a TXA_2 synthetase inhibitor, OKY-046 and a specific TXA_2 receptor antagonist, S-145, but not by a specific prostaglandin E_2 receptor antagonist, SC-19220. Endothelin-3 (10^<-7> M) stimulated the OKY-046- and indomethacin-sensitive formation of TXA_2 in the ganglia. These results suggest that endothelins inhibit ganglionic transmission ; postsnaptically desensitising a nicotinic and muscarinic pathways via acting on the ET_A receptor operated low conductance Ca^<2+>-activated K^+ channel at postganglionic membrane and presynaptically reducing acetylcholine release from presynaptic nerve terminals by the activation of endogenous TXA_2 production. Less