Identification of beta-adrenergic receptors using [3H]dihydroalprenolol in fetal sheep heart: direct evidence of qualitative similarity to the receptors in adult sheep heart

The fetal sheep heart responds to beta-adrenergic stimuli; however, in vivo studies show the response of the fetal heart is less than that of the adult heart. We used [3H]dihydroalprenolol (DHA) to study directly beta-adrenergic receptors in heart particulates of fetal sheep at term and adult sheep. [3H]DHA binding to fetal heart particulates was rapid, reversible (t 1/2 = 2.9 +/- 0.3 min), stereoselective, saturable (101.2 +/- 7.4 fmoles/mg protein), and of high affinity (4.8 +/- 0.4 nM). The rank order of agonists competing for [3H]DHA binding was isoproterenol (0.32 +/- 0.10 microM) greater than epinephrine (1.19 +/- 0.23 muM) approximately equal to norepinephrine (2.67 +/- 0.69 muM), which is compatible with beta 1-adrenergic potencies. [3H]DHA also bound to the adult sheep heart in a manner expected for beta 1-receptors. No difference in the binding affinity of [3H]DHA or agonists' competition was demonstrated between the fetal and adult sheep heart. Comparison of the concentration of beta-adrenergic receptors in fetal and adult hearts was confounded by the choice of the denominator for unit expression. The concentration was higher in the adult when expressed as a function of protein content or 5'-nucleotidase activity (0.52 +/- 0.07 versus 1.12 +/- 0.06). However, there was no difference when tissue weight, Na+ - K+-ATPase, or NaF-stimulated adenylate cyclase was used. Furthermore, isoproterenol-stimulated adenylate cyclase and cardiac contractile response to a threshold dose of isoproterenol were identical in the fetal and adult sheep heart. We conclude that beta-receptors can be studied with [3H]DHA in the fetal sheep heart, this receptor is qualitatively similar to the beta-receptor in the adult sheep heart, and it is unlikely that there is a difference in the concentration of beta-adrenergic receptors in fetal and adult sheep heart.