The beta-adrenoceptors responsible for a direct relaxation of the uncontracted nictitating membrane of the anaesthetized cat.

Direct relaxation of the nictitating membrane of the anaesthetized cat was produced by close intra-arterial injections of single doses of isoprenaline and other beta-adrenoceptor agonists. This response was relatively resistant to blockade by doses of propranolol that almost abolished the corresponding positive chronotropic and vasodepressor responses. Construction of dose-response curves, however, revealed antagonism of all three parameters by propranolol and different sensitivities of the parameters to isoprenaline. The absence of blockade by practolol, although negative evidence, permitted the nictitating membrane response to be classified as a beta2-adrenoceptor effect, which was supported by the full agonist activity of salbutamol. The blockade of the nictitating membrane response by propranolol was however weaker than of the beta2-adrenoceptor mediated vasodepressor response. Contractile responses were observed with higher doses of isoprenaline and after intravenous injection. These were probably indirect effects induced by local changes in blood pressure at the nictitating membrane due to the concommitant vasodepressor response.