The effect of acidosis on insulin-induced relaxation was studied in thoracic aortic rings (from Wistar-Kyoto (WKY) rats) with (+ED) or without (-ED) endothelium. The rings were mounted in normal (pH 7.4) or acidotic (pH 7.2) Krebs solution for isometric tension recording. Phenylephrine (PE, 3.0 microM)-contracted tissues were exposed to insulin in the presence or absence of various inhibitors. Insulin exerted similar concentration-dependent relaxation of +ED tissues in normal and acidotic pH. Endothelium denudation, significantly (p<0.05) reduced insulin effect in normal, but not acidotic pH. Under normal pH, treatment with L-NAME or methylene blue significantly (p<0.05) reduced insulin responses in the +ED (but not the -ED) tissues. The insulin effect was also significantly (p<0.05) inhibited by tetraethylammonium (TEA; BK(Ca) blocker), 4-Aminopyridine (4-AP; K(V) channel blocker), combined treatments (L-NAME+4-AP+TEA, in +ED tissues) or (4-AP+TEA, in -ED tissues). In either +ED or -ED tissues, indomethacin (cyclo-oxygenase inhibitor), glibenclamide (K(ATP) channel blocker), barium chloride (K(ir) channel blocker) or Ouabain (a Na(+)/K(+)-ATPase inhibitor) had no effect. Except for methylene blue (effect on +ED tissues), none of the drug treatments inhibited insulin vasodilator effect in acidosis (+ED or -ED tissues). These data indicate that insulin exerts an endothelium-dependent and -independent vasodilatation in rat aorta which in normal pH is mediated via BK(Ca) and K(v) channels, including the EDNO-cGMP cascade. Acidosis abolishes the endothelium-dependent relaxation mechanism unraveling a novel mechanism that is as efficacious and is cGMP-, but not EDNO-, BK(Ca)- or K(v)-mediated.