Facilitation of noradrenaline release from sympathetic nerves in rat anococcygeus muscle by activation of prejunctional β-adrenoceptors and angiotensin receptors

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Abstract
1 Isolated preparations of rat anococcygeus muscle were incubated with [3H]-noradrenaline and the efflux of radioactivity induced by stimulation of intramural sympathetic nerves was used as a measure of release of transmitter noradrenaline. Isometric contractile responses were also measured. 2 Angiotensin I (0.03 μm) and angiotensin II (0.03 μm) produced non-sustained contractile responses and enhanced the stimulation-induced (S-I) effluxes of radioactivity as well as the contractile responses to electrical stimulation. These effects were blocked by the angiotensin II receptor antagonist saralasin (0.03 μm), and the effect of angiotensin I, but not angiotensin II, was blocked by the angiotensin converting enzyme inhibitor captopril (0.1 μm). 3 The findings indicate that there are both pre- and postjunctional receptors for angiotensin II and that angiotensin I is converted to angiotensin II in the anococcygeus muscle preparation. 4 Isoprenaline (0.1 μm) slightly enhanced the S-I efflux of radioactivity, and produced a greater enhancement after neuronal uptake blockade with desipramine (0.03 μm) and α-adrenoceptor blockade with phentolamine (1 μm). 5 The facilitatory effect of isoprenaline on S-I efflux of radioactivity was abolished by propranolol (0.3 μm), but was not affected by low concentrations of saralasin (0.03 μm) or captopril (0.1 μm) which abolished the effect of angiotensin I. The findings suggest that isoprenaline acts directly on prejunctional β-adrenoceptors to enhance S-I noradrenaline release, rather than indirectly by releasing angiotensin II from within the tissue. Higher concentrations of saralasin (0.1 μm) or captopril (5 μm) did block the facilitatory effect of isoprenaline. The significance of this finding is not clear.