EFFECTS OF COCAINE ON INCREMENTAL TREADMILL EXERCISE IN HORSES

Four mature horses were used to test the effects of two doses (50 and 200 mg) of intravenously administered cocaine on hemodynamics and selected indexes of performance [maximal heart rate (HR(max)), treadmill velocity at HR(max), treadmill velocity needed to produce a blood lactate concentration of 4 mmol/l, maximal mixed venous blood lactate concentration, maximal treadmill work intensity, and test duration] measured during an incremental treadmill test. Both doses of cocaine increased HR(max) similar to 7% (P < 0.05). Mean arterial pressure was 30 mmHg greater (P < 0.05) during the 4- to 7-m/s steps of the exercise test in the 200-mg trial. Neither dose of cocaine had an effect on the responses to exertion of right atrial pressure, right ventricular pressure, or maximal change in right ventricular pressure over time. Maximal mixed venous blood lactate concentration increased 41% (P < 0.05) with the 50-mg dose and 75% (P < 0.05) with the 200-mg dose during exercise. Administration of cocaine resulted in decreases (P < 0.05) in the treadmill velocity needed to produce a blood lactate concentration of 4 mmol/l from 6.9 +/- 0.5 and 6.8 +/- 0.9 m/s during the control trials to 4.4 +/- 0.1 m/s during the 200-mg cocaine trial. Cocaine did not alter maximal treadmill work intensity (P > 0.05); however, time to exhaustion increased by similar to 92 s (15%; P < 0.05) during the 200-mg trial. The earlier onset of lactate accumulation and the greater maximal lactate concentration observed during the cocaine trials may have been due to the greater run times during the test or to peripheral vasoconstriction, which may also explain the increase in mean arterial pressure observed during the 200-mg cocaine trial.