LACTIC-ACIDOSIS AND DIAPHRAGMATIC FUNCTION IN-VITRO

Diaphragm fatigue occurs during heavy exercise. Acidosis leads to skel etal muscle fatigue, yet the diaphragm is not a net producer of either lactic acid or hydrogen ions. We tested the hypothesis that hydrogen ion and lactic acid concentrations similar to those seen in arterial b lood at maximal exercise decrease contractility of the in vitro isolat ed rat diaphragm. Diaphragm strips were exposed to a control solution for 15 min and then to one of the following treatment solutions: contr ol (C, pH = 7.4) or 10 mM lactic acid buffered to pH 7.4 (L74), pH 7.2 (L72), pH 7.1 (L71), or pH 6.8 (L68). After 15 min, the force-frequen cy relationship of the strip was measured. The strips were then stress ed with 75 contractions at 25 Hz (250-ms train duration) at the rate o f one per second and the force-frequency curve was measured after 15 m in of recovery. The L74, L72, and L71 strips responded similarly to th e C strips at all times and frequencies. Decrements in force associate d with acidosis were only seen in L68. Within L68, we found decreases in force at stimulation frequencies < 100 Hz. These data suggest that physiologic levels of exogenous hydrogen ions are not a primary cause of in vitro diaphragm fatigue.