Hypothalamic atrial natriuretic peptide secretion plasticity: differential modulation of alpha and beta adrenoceptors

Increasing evidence suggests that atrial natriuretic peptide (ANP), a 28 amino acid peptide with biologically active 4-28 and 5-28 congeners, modulates salt-water homeostasis at both peripheral and central levels. In rats, immunoreactive (ir) ANP is found in hypothalamic (HT) neurons of preoptic and paraventricular regions rich in aminergic innervation. Employing a well-characterized perifusion model of rat HT explants, the acute effects of norepinephrine (NE) on HT release of irANP were examined. Pulsatile administration (20 min) of NE (10(-7) to 10(-5) M) induced a dose-related release of irANP. The stimulatory effect of 10(-5) M NE (2.66 +/- 0.54 pg/ml/HT, means +/- SE, n = 12) was abolished in the presence of 10(-7) M propranolol, a beta-antagonist, but was 50% higher when administrated with 10(-5) M phentolamine, an alpha-antagonist. Administration of equivalent doses of propranolol or phentolamine alone, consistently suppressed (40% below basal secretion rate, BSR) or stimulated (50% above BSR) irANP release, respectively. In addition, infusion of isoprenaline (10(-5) M), a beta-agonist, enhanced BSR by 45%, whilst phenylephrine (10(-5) M), an alpha-agonist, suppressed it by 25%. We conclude that in rat hypothalami (1) occupancy of the beta-adrenoceptor by its agonist stimulates irANP release, (2) alpha- and beta-adrenoceptors modulate irANP secretion in an opposing manner, and (3) the basal release of irANP is a product of the activation of alpha- and beta-adrenoceptors by their endogenous ligands.