The Effects of Adrenoceptor Agonists and Antagonists on Plasma Potassium Concentration in Anaesthetized Guinea-pigs, Rabbits and Rats

An intravenous K+-sensitive electrode has been used to monitor plasma [K+] changes induced by alpha- and beta-adrenoceptor agonists in anaesthetized guinea-pigs, rabbits and rats. The effects of phentolamine and propranolol on these responses were studied. In the guinea-pig both alpha- and beta-adrenoceptor agonists produced a biphasic response consisting of an initial rapid increase in [K+] which was followed, within 1 min, by a fall below baseline. The antagonist studies indicated that in this species both phases of the response could be elicited by either alpha- or beta-adrenoceptor activation. In the rabbit the responses were both slower and smaller than those seen in the guinea-pig and required larger agonist doses. In addition it was found that the increase in plasma [K+] was alpha-adrenoceptor-mediated while the subsequent fall was seen only with beta-adrenoceptor activation. In the rat triphasic changes in plasma [K+] were seen consisting of an initial decrease which was alpha-adrenoceptor-mediated, followed by an increase and then a second fall which was elicited by beta-adrenoceptor stimulation. The increase in plasma [K+] was only slightly reduced by either alpha- or beta-adrenoceptor antagonists. Apamin, a toxin from bee venom which blocks Ca2+-activated K+-channels, was found to block the hyperkalaemic phase of the response in the guinea-pig and rabbit but had no effect in the rat. It is concluded that there are marked species differences in the effects of adrenoceptor agonists on plasma [K+] in vivo.