Differences in the accessibility of the β-adrenergic receptor in isolated hepatocytes from foetal and adult rats

Abstract

β-receptor number (measured by [ 3H]-CGP 12 177 binding) and β-adrenergic response (measured by isoproterenol stimulated glucose liberation and isoproterenol stimulated adenylate cyclase activity) were compared in hepatocytes isolated from foetal (on day 22 of gestation), adult female and adult male rats. β-receptor numbers in crude membrane preparations of hepatocytes from adult female and adult male rats were found to be nearly equal (15.5 and 15.1 fmol/mg), but in crude membrane preparations of foetal rats β-adrenergic receptor number was significantly higher (34.3 fmol/mg). Determination of number of β-adrenergic surface receptors of intact hepatocytes showed relative high values in foetal rats (about 22,000/cell) and adult female rats (about 20,000/cell), but in male rats the number was less (about 6500/cell). Glucose liberation was stimulated by isoproterenol to the same extent in hepatocytes isolated from adult female and foetal rats (about 150% over basal), whereas no effect was found in hepatocytes isolated from adult male rats. Dose-response curves showed that in foetal rat hepatocytes glucose release was already increased by 10 −8 M isoproterenol, whereas in female rat hepatocytes at least 10 −6 M isoproterenol was required. Adenylate cyclase was stimulated by isoproterenol in lysates of hepatocytes from adult female rats by about 180% and from foetal rats by about 250%. No effects were observed using lysates of hepatocytes from adult male rats. We interpret the observed differences of β-adrenergic responses between adult female and male rats as being primarily caused by different accessibility of the β-receptor to the β-agonist isoproterenol in intact hepatocytes. There are no differences in the number of accessible β-adrenergic cell surface receptors in foetal and female rats, but the surface receptor density is significantly higher in foetal rats.