Inhibition of nicotine-induced relaxation of the bovine retractor penis muscle by β-adrenoceptor antagonists

The relative potency in inhibiting nicotine-induced relaxation of the bovine retractor penis muscle (BRP) was estimated for the racemates of seven β-adrenoceptor antagonists, both of the optical isomers of propranolol, and lidocaine. The order of potency of the drugs studied was (+)-propranolol>(−)- propranolol>propranolol>alprenolol>metoprolol>lidocaine>acebutolol>pindolol >sotalol>atenolol. It is concluded that the inhibition of the relaxation was not due to blockade of β-adrenoceptors but to the nonspecific effects of the β-adrenoceptor antagonists. It is also concluded that the neurotransmitter(s) which was (were) released from the non-adrenergic non-cholinergic inhibitory nerves in the BRP did not relax the muscle by activating the β-adrenoceptors. It is suggested that the β-adrenoceptor antagonists inhibited the release of the inhibitory neurotransmitter(s) by a mechanism which is significantly correlated to their lipophilicity.