Relaxation of Non-Contracted Smooth Muscle by Atrial Natriuretic Peptide

Atrial natriuretic peptide (alpha-hANP-99-126), 1 x 10(-8) to 5 x 10(-10) M was able to further relax the non-contracted aortic smooth muscle of rabbit after complete recovery from a previous challenge with human angiotensin II (AII), 1 x 10(-6) M. The relaxation was directly proportional to the response of the ring or strip to the previous challenge with AII. ANP does not have effect on basal tension in isolated strips or rings of the rabbit aorta not previously exposed to AII. Norepinephrine (NE), 10(-7) M was less potent in inducing reactivity of the vessel to ANP, ie, only a small relaxant effect on basal tension could be observed. A similar vasorelaxant effect of ANP on basal tension could be obtained in the absence of extracellular calcium: Ca(2+)-free Ringers' solution containing 2 mM ethylene-glycol-bis (beta-aminoethyl ether)N',N' tetraacetic acid (EGTA-Ringer). In contrast, sodium nitroprusside 10(-8) M does not affect basal tension. Present results demonstrate the role of the physiological state of the vessel in the reactivity to ANP.