Hypersensitivity of cardiac beta-adrenergic receptors after neonatal treatment of rats with 6-hydroxydopa.

The cardiac beta-adrenergic response to noradrenaline (NA) was investigated in the rat neonatally s.c. treated with 6-hydroxydopa (6-OHDOPA, 75 mg/kg, 0, 2 and 4 days after birth). 6-OHDOPA treatment significantly (P < 0.05) reduced the NA content by 32% of the control in whole heart and markedly potentiated the isoproterenol (0.17 mg/kg, i.p.)-induced stimulation of ornithine decarboxylase activity which had been blocked by a preceding i.p. injection of propranol (10 mg/kg) at the adult stage. Scatchard analysis of the specific [3H]-dihydroalprenolol ([3H]DHA0 binding indicated that 6-OHDOPA induced an increase in the maximum number of specific binding sites of [3H]DHA but did not alter the dissociation constant. In addition, 6-OHDOPA treatment resulted in a potentiation of the positive inotropic response to NA but not to isoproterenol in isolated atria. It is suggested that 6-OHDOPA treatment produces an increased density of beta-adrenergic receptor sites relevant to the hypersensitivity of beta-receptors to NA.