Inhibitory effects of alpha, beta-methylene ATP on nerve-mediated contractions of the nictitating membrane in reserpinised cats.

Residual responses of the cat nictitating membrane to nerve stimulation were obtained after reserpine pretreatment (40% of controls), in spite of a pronounced reduction in noradrenaline content. The putative ATP-receptor desensitising agent alpha, beta-methylene ATP (alpha, beta-MATP), administered intraarterially through the lingual artery produced a contraction of the nictitating membrane and subsequently inhibited the residual responses evoked by sympathetic nerve stimulation in reserpinised cats. These doses of alpha, beta-MATP did not modify the contractions evoked by exogenous noradrenaline (i.a.) but antagonized the contractions of the nictitating membrane elicited by beta, gamma-methylene ATP, which is an agonist at P2 receptors. These results are compatible with a co-transmitter role for ATP in the neurally mediated contractile responses of the nictitating membrane following depletion of endogenous noradrenaline stores by pretreatment with reserpine.