Facilitation of noradrenaline release by isoprenaline is not mediated by angiotensin II in mouse atria and rat tail artery.

The hypothesis that facilitation of noradrenaline release by prejunctional beta-adrenoceptors is linked to the local generation of angiotensin II was investigated in rat tail artery and mouse atria which were incubated with [3H]-noradrenaline. The outflow of radioactivity induced by electrical field stimulation of the tissues was taken as an index of noradrenaline release. In mouse atria, angiotensin II (0.01 microM) enhanced the stimulation-induced (S-I) noradrenaline release and this was blocked by saralasin (0.1 microM). Isoprenaline (0.01 microM) also enhanced the S-I release of noradrenaline, but this was not blocked by either saralasin (0.1 microM and 0.3 microM) or captopril (5 microM). Furthermore, the facilitatory effects of angiotensin II (0.1 microM) and isoprenaline (0.1 microM) were additive. In rat tail artery, the facilitatory effect of angiotensin II (0.01 microM) on noradrenaline release was blocked by saralasin (0.1 microM). The facilitatory effect of isoprenaline (0.1 microM) was blocked by propranolol (1 microM) but not by saralasin (0.1 microM) nor captopril (5 microM). These results indicate that beta-adrenoceptor-mediated facilitation of noradrenaline release occurs independently of angiotensin II in mouse atria and rat tail artery.