[Pharmacological characterization of chicken cardiac beta-adrenoceptors]

Pharmacological properties of cardiac beta-adrenoceptors of embryonic chick (16-18 day in ovo), hatched chick (3-5 days after hatching) and adult chicken (20 weeks after hatching) were investigated. Pharmacological analysis using subtype specific agonists and antagonists showed that cardiac beta-adrenoceptors of embryonic chick and hatched chick could not be classified as beta 1- or beta 2-adrenoceptors. Scatchard analysis of specific [125I]-iodocyanopindolol (ICYP) binding revealed ICYP bound to two different population of binding sites (high and low affinity) in all stages; Kd and Bmax of these sites remained unchanged before and after hatching, although Bmax but not Kd was found to increase in adult chicken. Displacement of ICYP with propranolol (a nonspecific beta-antagonist), ICI 118,551 (a specific beta 2-antagonist) and atenolol (a specific beta 1-antagonist) indicated the presence of two affinity binding sites for all of the antagonists. Ki values for these antagonists at the high affinity binding sites were almost identical to pA2 values obtained from their antagonistic effects on isoproterenol inotropic response, suggesting that the high affinity binding sites for ICYP bear the pharmacological significance. Significant increase in sensitivity to the inotropic effects of beta-adrenoceptor agonists was observed in the hatched chick heart, whereas Ki values calculated from displacement of ICYP with these agonists in the presence of GppNHp were not changed. Dobutamine which was a Partial agonist in the embryonic chick heart, was found to behave as a full agonist in the hatched chick heart. These result suggested that chicken cardiac beta-adrenoceptors show different properties from either beta 1- or beta 2-adrenoceptors reported in mammalian hearts, and coupling between binding and adenylate cyclase might be more efficient after hatching.