Study of two alkylating derivatives: the p-isothiocyanato- and the p-methylisothiocyanato-clonidine

Pharmacological experiments with isolated rat aorta and radioligand binding studies in rat cerebral membranes were performed with the p-isothiocyanato (p-NCS) and p-methylisothiocyanato (p-CH2-NCS) derivatives of clonidine in order to assess their selectivity for alpha 1- and alpha 2-adrenoceptors, and to characterize their ability to alkylate alpha-adrenoceptors. Preincubation of rat aortic strips with both derivatives produced non-parallel rightward shifts in the dose-response curves of noradrenaline and significantly depressed the maximum response in a manner characteristic of irreversible receptor antagonists. The p-CH2-NCS derivative was slightly more potent than the p-NCS derivative. Further analysis of the data indicated that treatment of rat aorta with a 30 microM concentration of the p-CH2-NCS derivative alkylated all but 2.4 percent of the alpha-adrenoceptors, whereas a 100 microM concentration of the p-NCS derivative was required to produce a similar degree of alpha-adrenoceptor alkylation. Radioligand binding studies indicate an apparent 2 fold alpha2-adrenoceptor selectivity for the p-NCS derivative. In contrast, the p-CH2-NCS derivative displayed 7 fold selectivity for alpha 1-adrenoceptors. Interestingly, both alkylating derivatives of clonidine produced dose-dependent contractile responses in rat aorta with pD2 values of 6.30 and 5.56 for the p-NCS and p-CH2-NCS derivatives, respectively, relative to a pD2 of 7.67 for clonidine. The order of potency of the two alkylating derivatives of clonidine for producing contraction of rat aorta is the opposite of that for antagonizing the contractile effects of noradrenaline. The results suggest that the p-NCS and p-CH2-NCS derivatives of clonidine non-competitively antagonize noradrenaline by irreversibly alkylating alpha-adrenoceptors.