CARDIAC β‐ADRENOCEPTORS DURING NORMAL GROWTH OF MALE AND FEMALE RATS

1 A binding assay involving (-)-[3H]dihydroalprenolol (DHA) and KCl-washed cardiac membranes were used to assess the numbers and affinities of beta-adrenoceptors in hearts from male and female rats varying in age from about 2 weeks to 18 months. 2 Although female rats grow more slowly and attain lower adult weights than male rats, heart weights increased in approximate proportion to body weight with little sex difference. 3 As heart weight increased about three fold, beta-receptors increased three fold. Since the number of myocardial cells is believed to be nearly constant during postnatal growth, the numbers of receptors/cell presumably increases similarly. 4 As heart weight increased, the number of beta-receptors per g of tissue decreased according to the equation: total pmol/g = 4.33 - 1.43 x heart weight, equally in males and females. 5 Dissociation constants for DHA (2 to 4 nM) remained the same, and equal, in male and female rats during their growth. 6 An excellent correlation was found between the decline in beta-receptors/g tissue during growth and the decline in the area of the external sarcolemma/g tissue. The data suggest that the number of receptors per unit area remains constant during growth, and thus that cell surface area is a major factor determining normal numbers of receptors per cardiocyte.