The influence of basal heart rate (HR) on the effects of inotropic (dobutamine infusion) and chronotropic (atrial pacing) stimuli during acute myocardial ischemia was assessed by measurement of intramural carbon dioxide partial pressure (PCO2) in open-chest dogs undergoing transient 10-minute left anterior descending coronary artery occlusions. In protocol I, in 5 dogs anesthetized with pentobarbital alone, HR increased from 153 +/- 10 to 182 +/- 7 beats/min between experimental coronary occlusions, but the increase in ischemic zone intramural carbon dioxide tension (delta PmCO2) was not altered by this significant increase in HR. In protocols II to V, dogs were anesthetized with combinations of morphine, thiamylal and pentobarbital and had a basal average HR of 81 beats/min. Atrial pacing in protocol II (13 dogs) increased HR from 76 +/- 21 to 134 +/- 19 beats/min (p less than 0.001); left ventricular (LV) myocardial oxygen consumption (MVO2) rose from 3.9 +/- 1.6 to 4.9 +/- 1.4 ml/min/100 g (p less than 0.05), and delta PmCO2 rose from 42 +/- 14 to 50 +/- 15 mm Hg (p less than 0.01), indicating more severe ischemic injury during the second experimental coronary occlusion. In protocol III, 11 dogs received 20 micrograms/kg/min of dobutamine before the second experimental occlusion, which significantly (p less than 0.02) increased HR, LV dP/dt and MVO2; delta PmCO2 increased from 46 +/- 13 to 63 +/- 18 mm Hg (p less than 0.01). The 7 dogs in protocol IV received 3.9 +/- 1.9 micrograms/kg/min of dobutamine, titrated such that HR was unchanged (84 +/- 10 vs 81 +/- 15 beats/min), but LV dP/dt increased by 92% (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)