[Effects of CO2 on the rat diaphragm in vitro]

Acidosis produced by an increase in CO2 concentration decreases contractility and increases the tendency to fatigue in striated muscle. It is not clear a what levels of CO2 this effect takes place, or in what experimental models it can be observed. In this study we examine the effect of increasing concentrations of CO2 on contractility and fatigue in strips of rat diaphragm in vitro. Six strips of diaphragm from a rat of low-weight were kept in organ bath and supramaximal stimuli of 1, 10, 50 and 100 Hz were applied while CO2 concentration was changed from 5% (29 mmHg), to 10% (48.3 mmHg) to 20% (102 mmHg). To study fatigue we applied 45 series of 50 Hz stimuli per minute for 15 minutes. We obtained these results: a) Contractility. Force of contraction was less with 20% CO2 than with 5% under a stimulus of 1 Hz. No significant differences were found for other concentrations or frequencies. b) Fatigue. Force of contraction decreases as CO2 concentration increased and significant differences were found for all 3 concentrations. The conclusions were: a) an increase in CO2 from 5% to 20% produces a significant decrease in force of contraction under conditions of low-frequency stimuli and b) increases in CO2 concentration from 5% to 10 to 20% increase the muscle's susceptibility to fatigue.