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ATP causes postjunctional potentiation of noradrenergic contractions in the portal vein of guinea‐pig and rat *

Superfusion of the portal vein of rat and guinea‐pig with Krebs' solution maintained at 25°C greatly inhibits spontaneous contractions of the preparations and allows contractile responses to ATP and noradrenaline to be measured accurately. Under these conditions, continuous superfusion with ATP (10−5 M), a concentration which had no effect on either basal tension or spontaneous activity, caused a significant shift to the left of the concentration‐response curve to exogeneous noradrenaline in both tissues. The mechanism of this potentiation induced by ATP may differ in the two tissues since in the rat portal vein potentiation appeared to be rapidly reversed by superfusing with ATP‐free solution, whereas in the guinea‐pig portal vein a further concentration‐response curve to noradrenaline, in the absence of ATP, was still significantly shifted to the left compared with the control curve. However, potentiation in the rat portal vein may have had a longer duration than is suggested by the results since control concentration‐response curves to noradrenaline in this tissue showed a progressive shift to the right which, although not significant, is likely to have affected the apparent time course of potentiation. It is concluded that ATP can potentiate contractions to exogeneous noradrenaline in the portal vein of rat and guinea‐pig via an, as yet, unidentified postjunctional mechanism.