pA2 values of selective beta-adrenoceptor antagonists on isolated atria demonstrate a species difference in the beta-adrenoceptor populations mediating chronotropic responses in cat and guinea-pig.

pA2 values for atenolol (beta 1-selective) and alpha-methylpropranolol (beta 2-selective) have been determined on isolated atria of cat and guinea-pig using noradrenaline (beta 1-selective) and fenoterol (beta 2-selective) as agonists. On guinea-pig atria, the pA2 values did not vary with the agonist used. On cat atria the pA2 for atenolol was greater with noradrenaline than with fenoterol and the pA2 for alpha-methylpropranolol was greater with fenoterol than with noradrenaline. Fenoterol was 20 times more potent on cat than on guinea-pig atria whereas noradrenaline was approximately equipotent in the two species. The results have been intrepreted as suggsesting that both cat and guinea-pig atria contain one receptor type in common (beta 1) but that only cat atria contain beta 2-adrenoceptors as well.