DIAPHRAGMATIC ENERGETICS DURING PROLONGED EXHAUSTIVE EXERCISE

The present study was carried out to examine diaphragmatic O2 extraction and lactate and ammonia production during prolonged exhaustive exercise. Experiments were performed on nine healthy exercise-conditioned ponies in which catheters had been implanted in the phrenic vein previously. Blood-gas variables and lactate and ammonia concentrations were determined on simultaneously obtained arterial and phrenic-venous blood samples at rest and during 30 min of exertion at 15 mph + 7% grade (heart rate, 200 beats/min; approximately 90% of maximum). Arterial O2 tension and saturation were maintained near resting value but CO2 tension decreased markedly with exercise, and because of increased hemoglobin concentration, arterial O2 content rose. Concomitantly, phrenic-venous O2 tension, saturation and content decreased markedly (23.6 +/- 1 mm Hg, 24.5 +/- 2%, 5.2 +/- 0.3 ml/dl at 3 min of exertion) and significant fluctuations did not occur as exercise duration progressed to 30 min. Diaphragmatic arteriovenous O2 content difference and O2 extraction rose from 4 +/- 0.3 to 16 +/- 0.5 ml/dl and from 30 +/- 3 to 75 +/- 1% at 3 min of exercise, and significant deviations did not occur as exercise duration progressed. Arterial lactate and ammonia levels increased during exercise, indicating their release from working limb muscles. Phrenic-venous values of lactate and ammonia did not exceed arterial values. Ponies sweated profusely and were unable to keep up with the belt speed in the last 4 to 5 min of exercise. Constancy of phrenic arteriovenous O2 content difference in exercise indicated ability to adjust perfusion in diaphragm so as to adequately meet its O2 needs. This and the observation that diaphragm did not have net release of lactate or ammonia suggest its metabolism in prolonged exhaustive exercise remained aerobic.