Cardiac β-adrenoceptors (βAR) were studied using membranes prepared at birth (day 0) and at days 7, 10, 15, 21, 30, 45 and 60. Saturation experiments using the antagonist ligand (¹²⁵I)-iodocyanopindolol (ICYP) allowed the determination of βAR number (Bmax) and ICYP dissociation constant (K_d), while (-) isoproterenol competition curves of ICYP binding, performed in the absence or presence of Gpp(NH)p (10⁻⁴M), were used to measure the relative proportions of high and low affinity states of the βAR for the agonist and to assess the ability of βAR to couple with the GTP-binding protein.

Rat cardiac βAR evolved at 3 distinct periods: during the first period (days 0–10), the receptor density and ICYP K_d were half that of adults, and βAR were present only in an homogenous high affinity state. The second period (days 15–21) was characterized by a progressive increase in βAR number and ICYP K_d, while analysis of (-)isoproterenol competition curves indicated that βAR were poorly coupled to the GTP-binding protein. In the third period (days 30–60), ICYP Bmax and K_d were respectively 53.9±1.2 fmoles/mg proteinand 106.4±2.9 pM, while analysis of (-)isoproterenol competition curves showed the existence of high and low affinity binding states in equal proportions in the absence Gpp(NH)p, and of one homologous low affinity state of the receptor in its presence.

These data indicate that βAR follow a post-natal evolution marked by an increase in βAR density concomittant with a decrease in affinity toward the antagonist ligand ICYP, accompanied by the progressive appearance of a poorly-coupled βAR. However, the number of efficiently coupled receptors was found to be similar in adult and newborn rats.