Inhibition of oxotremorine-induced desensitization of guinea-pig ileal longitudinal muscle in Ca2+-free conditions.

The aim of this study was to investigate the differences between oxotremorine-induced and acetylcholine (ACh)-induced desensitization, particularly under Ca2+-free conditions, in guinea-pig ileal longitudinal muscle, and to elucidate the different mechanisms of desensitization that might exist between these two muscarinic agonists. Pretreatment of the tissue with 10(-7)-10(-5) M oxotremorine (desensitizing treatment) in normal Tyrode solution caused desensitization of the responses to ACh, as did the desensitizing treatment with ACh. However, Ca2+-free conditions significantly reduced oxotremorine-induced desensitization, contrary to the previous findings that Ca2+-free conditions enhanced ACh-induced desensitization. The desensitizing treatment with oxotremorine caused suppression of the responses to high K+ (tonic phase), as did the ACh treatment. Ca2+-free conditions removed this suppression, whereasthis condition enhanced ACh-induced suppression of the K+ response. A protein kinase C inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (10(-4) M) had no effect on oxotremorine-induced desensitization of the ACh response. The results suggest that a voltage-gated Ca2+ channel was involved in oxotremorine-induced desensitization, as in ACh-induced desensitization, but that the process of inactivation of Ca2+ channels was different between oxotremorine and ACh, and that oxotremorine-induced desensitization was due not only to Ca2+ channel, but also to other unknown factors. Protein kinase C did not participate in oxotremorine-induced desensitization.