Hypertrophied hearts have enhanced susceptibility to ischemia-induced contractile dysfunction. To explore the mechanisms of this phenomenon, we studied the effect of acidosis on the Ca2+ sensitivity of the contractile proteins and on Ca2+ accumulation by the sarcoplasmic reticulum (SR) in chemically (saponin) skinned cardiac fibers obtained from normal and pressure-overloaded hypertrophied rat left ventricles. Left ventricular pressure overload was induced by partial ligation of the abdominal aorta 6-8 wk before study. Age- and weight-matched normal rats served as controls. Pressure overload increased the left ventricular weight-to-body weight ratio by 48%. Reduction in pH shifted the pCa-tension curve to the right similarly in normal and hypertrophied preparations, and there was no difference in pCa-tension relationship at pH 7.0 or 6.5 between the two groups. However, reducing the pH of 1 microM Ca2(+)-loading solution from 7.0 to 6.5 decreased the amount of Ca2+ accumulated in the SR to 66.2 +/- 3.0% in normal fibers and 43.2 +/- 4.0% in hypertrophied fibers (P less than 0.01). We conclude that the enhanced susceptibility of hypertrophied hearts to ischemia-induced diastolic dysfunction may be partly explained by the greater depressing effect of acidosis on Ca2+ accumulation by the SR.