Figure 2. Concentration-response relations for positive inotropic responses to milrinone in isolated (A) right ventricular preparation (RV), and (B) left atrial preparation (LA) of the guinea pig at pH values of 7.4, 7, or 6.6. Muscles were electrically driven at 2 Hz. Data are shown as means +/- SD of seven to nine experiments for RV and seven experiments for LA. Baseline isometric contractions of RV at pH 7.4, 7, and 6.6 were 0.037 +/- 0.018 g, 0.032 +/- 0.027 g, and 0.024 +/- 0.009 g, respectively. Baseline isometric contractions of LA at pH 7.4, 7, and 6.6 were 0.280 +/- 0.178 g, 0.152 +/- 0.105 g, and 0.081 +/- 0.039 g, respectively. *P < 0.05 versus pH 7.4. (dagger)P < 0.05 versus pH 7.