Identification of the neuroeffector transmitter in jejunal branches of the rabbit mesenteric artery

Summary

Vasoconstriction or excitatory junction potentials (e.j.ps) evoked by nerve stimulation (15 field pulses at 2 Hz every 3 min) were recorded in rabbit isolated jejunal arteries. The resting diameter of the arteries and its decrease in response to stimulation was measured by a photoelectric method. Vasoconstriction was insensitive to prazosin 0.1 or 1 μmol/l. Yohimbine 1 μmol/l considerably enhanced, whereas α,β-methylene ATP (α,β-meATP) 1 μmol/l abolished the contractile response. In order to test the effect of exogenously applied transmitter candidates, noradrenaline (0.1–1 μmol/l) and ATP (10–30 μmol/l) were added in concentrations which evoked a vasoconstriction comparable to that induced by electrical stimulation. The action of noradrenaline was prevented by prazosin 0.1 μmol/l, but was unaffected by both yohimbine 1 μmol/l and α,β-meATP 1 μmol/l. α,β-meATP 1 μmol/l depressed the effect of ATP. The e.j.ps evoked by a train of 15 pulses showed facilitation up to the third response and thereafter depression; a partial summation was also observed. Prazosin 0.1 μmol/l did not change the e j.p. amplitudes. By contrast, when yohimbine 0.1 or 1 μmol/l was added to the prazosin-containing medium, both the late e j.ps in the train and the summation were enhanced in a concentration-dependent manner. α,β-meATP 1 μmol/l almost abolished the e.j.ps. In conclusion, in rabbit jejunal arteries, stimulation of postganglionic sympathetic nerves may release noradrenaline together with ATP which is probably the sole neuroeffector transmitter under our conditions. Transmitter release seems to be modulated by the activation of presynaptic α₂-adrenoceptors. Under the stimulation conditions of the present experiments the released transmitter does not activate postsynaptic α₁-adrenoceptors.