Endothelin may play an important role in modulating myocardial contractility under certain pathophysiological conditions. To determine whether endothelin beneficially modulates myocardial contractility in the common clinical condition of acidosis, we compared the effects of endothelin-1 on intracellular Ca2+transients and isometric contractions under normal (extracellular pH [pHo] 7.4) and acidotic (pHo6.4) conditions in ferret papillary muscles (n=33) loaded with the Ca2+-regulated bioluminescent indicator aequorin. A pHoof 6.4 was induced by replacing 92% of HCO3−with Cl−in the bathing medium. The effects of endothelin at pHo6.4 differed from the effects at pHo7.4 in that 1) the minimally effective concentration of endothelin was 30-fold lower (1 × 10−10M at pHo6.4; 3 × 10−9M at pHo7.4) and the concentration-response curve of endothelin was significantly shifted to the left with a decrease in log EC50from −7.83±0.13 to −8.92±0.10 (p<0.001), indicating an increased sensitivity of myocardium to endothelin; 2) endothelin produced an increase of ≈375% in tension development at pHo6.4 (≈62% at pHo7.4) (p<0.001) without increasing peak [Ca2+]i(≈13% increase at pHo7.4,p<0.001), indicating an increase in myofilament Ca2+responsiveness; and 3) endothelin significantly abbreviated (≈ −19%,p<0.001) the prolonged intracellular Ca2+transient induced by acidosis (pHo6.4). In addition, pretreatment with 10 μM of the Na+-H+exchange inhibitor 5-(N-methyl-N-isobutyl)-amiloride significantly attenuated endothelin-induced effects on the intracellular Ca2+transient and contraction during acidosis. Results indicate that 1) acidosis decreased myofilament Ca2+responsiveness and prolonged the intracellular Ca2+transient, whereas endothelin enhanced myofilament Ca2+responsiveness and abbreviated the intracellular Ca2+transient by decreasing intracellular H+via Na+-H+exchange during acidosis; and 2) endothelin exerts its cardiotonic effects at much lower concentrations during acidosis, presumably due to altered behavior of the receptor. Taken together, these findings suggest that endothelin could beneficially reverse acidosis-induced negative inotropic and lusitropic effects on the intracellular Ca2+transient and myocardial contractility.