Nitric Oxide as a Positive Inotropic Agent in Isolated Rat Hearts

The purpose of this study was to examine the inotropic effects of nitroprusside (NP), a direct nitric oxide (NO) donor, in isolated rat hearts. Langendorff-perfused hearts ( n = 5), paced at 6 Hz, were subjected to 15 min of equilibration (EQ) followed by infusion of NP, producing a coronary artery concentration of 1 × 10 -5 M. Coronary flow, left ventricular developed pressure (DP), end-diastolic pressure, and contractility and compliance (±dP/dt) were monitored throughout the experiment by a computerized data acquisition system. Myocardial oxygen consumption (MVO 2) was measured at the end of EQ and after 2 1 2 min of NP infusion. Myocardial efficiency was calculated as the quotient of DP divided by MVO 2. Values are expressed as the mean ± SEM. Paired t tests were used to calculate statistical significance. Values for parameters monitored at end EQ and at 2 1 2 min NP infusion showed that there was a 93% increase in coronary flow, 18, 17, and 16% increases in developed pressure, contractility, and compliance, respectively, no significant change in end-diastolic pressure, a 49% increase in myocardial oxygen consumption, and a 21% decline in myocardial efficiency ( P < 0.05 for all differences). We conclude that in the isolated rat heart, NO behaves as a positive inotrope.