Oxidative and glycolytic ATP formation of rabbit papillary muscle in oxygen and nitrogen

Contraction-related O2 consumption of rabbit papillary muscles was determined at 20 degrees C by measuring change in saline PO2 during and after trains of 120 twitches at 0.125-1 Hz in a microrespirometer. Although anoxic cores occurred at twitch frequencies greater than 0.2 Hz, no lactate was found in saline after twitch train. To measure lactate accumulation in muscle, fully oxygenated muscles were frozen at rest and during steady-state twitches at 0.2 Hz. We also measured nucleotides and creatine (Cr) compounds. There were no differences in lactate, ATP, and phosphocreatine (PCr) content between the resting and active muscles. When a P-to-O2 ratio of 6.3 is assumed, aerobic ATP formation was compared with glycolytic ATP formation during anoxia at a stimulus frequency of 0.2 Hz. The latter value was obtained by freezing muscles between 6 and 25 min after changing from O2- to N2-saturated saline. Withdrawal of O2 caused the ratio of PCr to total Cr to fall in less than 6 min from 0.77 to 0.23, while ATP remained at approximately 15 mumol/g dry wt. Force fell initially within 4 min to approximately 70% of control value, decreasing thereafter more slowly to approximately 40% at 20 min. From the relationship between amount of lactate formed and duration of anoxia, rate of anaerobic ATP formation was calculated assuming a P-to-lactate ratio of 1. We found that despite continuing contractile activity, anaerobic ATP formation was less than that required by a fully oxygenated resting muscle and was about the same magnitude as the estimated ATP hydrolysis for the contractions in N2. We conclude that in fully oxygenated rabbit papillary muscles no net lactate is produced during stimulation and that in anoxia anaerobic glycolytic capacity may not provide sufficient ATP for processes other than the uptake of Ca by the sarcoplasmic reticulum and cross-bridge cycling.