Decreased neutrophil beta adrenergic receptors in the neonate

Using cord blood neutrophils (PMN) the maturity of the beta adrenergic receptor has been assessed. [3H]-Dihydroalprenolol isoproterenol-induced cyclic AMP generation to characterize receptor function. Data from six neonates and six adult controls revealed that neonatal PMN had significantly (P less than 0.001) fewer receptor sites (728 +/- 43 receptors/cell) compared to the adult (1302 +/- 68 receptors/cel); however, the affinity constants (Kd) were very similar (0.78 +/- 0.14 nM compared to 0.61 +!- 0.05 nM). Neonatal PMN also generated less isoproterenol-induced cyclic AMP than adult cells at all concentrations, reaching significance (P less than 0.005) at 10(-4) - 10(-6) M isoproterenol concentrations. No difference in the isoproterenol concentrations stimulating 50% of the maximal cyclic AMP generation (EC50) was found between the two populations. These data indicate that the neonatal PMN has decreased beta adrenergic receptor sites. No evidence of a receptor--adenylate cyclase coupling defect was detected. These findings could reflect ontogenic differences between the neonate and adult PMN. The effect of increased catecholamine secretion during the stress of delivery resulting in 'down regulation' of the beta adrenergic receptor cannot presently be ruled out.