BETA(2)-ADRENERGIC RECEPTOR AND ANGIOTENSIN-II RECEPTOR MODULATION OF SYMPATHETIC NEUROTRANSMISSION IN HUMAN ATRIA

The aim of the present study was to investigate beta-adrenergic receptor and angiotensin II (Ang II) receptor modulation of norepinephrine release in human atria. Slices of human atrial appendages were incubated with [H-3]norepinephrine, superfused with Krebs-Henseleit solution, and electrically stimulated in superfusion chambers. Pretreatment of the tissue with 6-hydroxydopamine (1.2 mmol/L) before the [H-3]norepinephrine incubation to destroy sympathetic nerves reduced the uptake of radioactivity and abolished the stimulation-induced (S-I) outflow of radioactivity. Furthermore, S-I outflow of radioactivity was prevented by the addition of tetrodotoxin (1 mu mol/L) to and omission of extracellular Ca2+ from the superfusion solution. Separation of [H-3]norepinephrine from its metabolites revealed that the S-I outflow of radioactivity was mainly composed of intact [H-3]norepinephrine. Thus, the S-I outflow of radioactivity was taken as an index of norepinephrine release. Isoproterenol (0.001 to 0.1 mu mol/L) dose-dependently enhanced the S-I outflow of radioactivity. The concentration-response curve of isoproterenol was shifted to the right by the selective beta(2)-adrenergic receptor antagonist ICI 118551 (0.01 and 0.1 mu mol/L) but not by the beta(1)-adrenergic receptor-selective antagonist atenolol (0.3 and 30 mu mol/L). Ang II (0.001 to 1.0 mu mol/L) also dose-dependently enhanced S-I outflow of radioactivity. The facilitatory effect of Ang II was blocked by either the peptide Ang II receptor antagonist saralasin (1.0 mu mol/L) or EXP 3174 (0.1 mu mol/L), the in vitro active form of the nonpeptide Ang II receptor antagonist losartan. The cell-permeable cAMP analogue 8-bromo-cAMP (30 to 300 mu mol/L) dose-dependently enhanced S-I outflow of radioactivity. A combination of a saturating concentration of 8-bromo-cAMP (300 mu mol/L), the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX, 90 mu mol/L), and isoproterenol (0.1 mu mol/L) did not enhance S-I outflow of radioactivity to a greater extent than did isoproterenol (0.1 mu mol/L) alone. In contrast, a combination of 8-bromo-cAMP (300 mu mol/L), IBMX (90 mu mol/L), and Ang II (0.1 mu mol/L) enhanced S-I outflow of radioactivity to a greater extent than did Ang II (0.1 mu mol/L) alone. The data suggest that norepinephrine release from sympathetic nerves in human atria is facilitated through prejunctional beta(2)-adrenergic receptors and Ang II receptors. Prejunctional beta(2)-adrenergic receptors but not Ang II receptors seem to be linked to activation of an adenylate cyclase pathway. (Circ Res. 1994;74:434-440.)