Beta-adrenoreceptor-mediated modulation of vasoconstriction in rat isolated perfused mesenteric arteries.

Pressor responses to bolus injections of noradrenaline (NA) were studied, in the isolated perfused rat mesenteric arterial bed, in the presence of beta-adrenoreceptor agonists and antagonists, in an attempt to identify a possible beta-adrenoreceptor mediated modulation of catecholamine-induced vasoconstrictor effects. NA-induced responses were potentiated in the presence of timolol and (-)propranolol and suppressed in the presence of (-)isoprenaline; (+)isoprenaline was less effective against the NA-induced responses. Timolol attenuated the effects of (-)isoprenaline on NA-induced responses but not those of the stereoisomer (+)isoprenaline. It is concluded that the NA-induced pressor effect in the rat mesenteric vasculature is the net result of vasoconstrictor alpha- and vasodilator beta-adrenoreceptor activation and that the interaction of the two opposing adrenoreceptor-mediated effects represents a 'physiological antagonism'.