We studied the influence of inotropic agents on prompt and transient left ventricular (LV) diastolic relaxation dysfunction produced by superimposition of pacing tachycardia on low-flow ischemia, using an isolated, blood-perfused and isovolumic (balloon-in-LV) rabbit heart preparation. The LV balloon volume was adjusted to produce an LV end-diastolic pressure (EDP) of 15 mmHg and was held constant thereafter. Coronary perfusion pressure was adjusted to 100 mmHg during baseline and to 20 mmHg during low-flow ischemia of 6 min. At baseline, isoproterenol and ouabain were administered to cause moderate and similar rises (14 +/- 3 and 16 +/- 4% above baseline values, respectively) in maximum + dp/dt of LVP with no change in LVEDP. In control hearts which received no drug, superimposition of 5-min pacing tachycardia on low-flow ischemia produced a significant and transient increase in LVEDP under constant LV volume (from 13.4 +/- 0.4 to 24.7 +/- 3.3 mmHg, p less than 0.01). In the hearts which received isoproterenol it did not change LVEDP (from 14.0 +/- 0.4 to 16.2 +/- 1.0 mmHg, NS). In contrast, the ouabain hearts showed a further increase in LVEDP (from 13.7 +/- 0.8 to 29.9 +/- 4.6 mmHg, p less than 0.01). LV developed pressure, myocardial oxygen consumption or myocardial lactate production during pacing tachycardia superimposed on the low-flow ischemia did not differ significantly among the 3 groups. Thus, isoproterenol markedly improved transient LV relaxation dysfunction produced by superimposition of pacing tachycardia on low-flow ischemia, in which an equipotent inotropic dose of ouabain exaggerated the relaxation dysfunction. These results suggest that calcium overload rather than ATP depletion per se contributes to transiently impaired diastolic relaxation by pacing tachycardia and low-flow ischemia.