Amrinone Relaxes Potassium-Induced Contracture of Failing Right Ventricular Muscle of Cats

The effects of amrinone (5.3 X 10(-4) M) on isometric contraction and K+-induced contracture force of right ventricular muscle isolated from normal cats (n = 6) and cats in right ventricular failure (RVF, n = 6), 3-14 days after partial pulmonary artery ligation, were studied. Peak isometric contractile force (Po) and maximal rate of force development (dP/dt) of RVF muscles (1.38 +/- 0.21 g/mm2; means +/- SEM, and 11 +/- 1 g/s/mm2, respectively) were significantly lower than normal muscles (2.46 +/- 0.41 g/mm2, p less than 0.025, and 24 +/- 3 g/s/mm2, p less than 0.005, respectively). Duration of contraction (DC) was significantly longer in RVF muscles (442 +/- 32 ms) than in normal muscles (361 +/- 11 ms, p less than 0.025). Times to peak twitch force (TTP) and peak K+-induced contracture force (Pc) of RVF and normal muscles were not different. Amrinone increased Po and dP/dt significantly in normal muscles (+78 +/- 24%; means +/- SEM; p less than 0.01, and +53 +/- 17%, p less than 0.025, respectively), but not RVF muscles (+11 +/- 16% and +8 +/- 19%, respectively). TTP and DC of normal muscle were unchanged by amrinone, whereas TTP was unchanged while DC was shortened (-12 +/- 3%, p less than 0.025) in RVF muscle. Amrinone relaxed Pc similarly and significantly in normal (-28 +/- 7%, p less than 0.005) and RVF (-26 +/- 4%, p less than 0.025) muscles. These results suggest that part of amrinone's salutary action in the failing heart occurs through modification of myocardial relaxation.