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

Abstract

Abstract pA2 values for atenolol (β1-selective) and α-methylpropranolol (β2-selective) have been determined on isolated atria of cat and guinea-pig using noradrenaline (β1-selective) and fenoterol (β2-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 α-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 interpreted as suggesting that both cat and guinea-pig atria contain one receptor type in common (β1) but that only cat atria contain β2-adrenoceptors as well.