RAT DIAPHRAGM DURING POSTNATAL-DEVELOPMENT .2. RESISTANCE TO LOW-FREQUENCY AND HIGH-FREQUENCY FATIGUE

The effects on the rat diaphragm of fatigue induced by low- and high-f requency stimulation (at 5 Hz for 1.5 min and 75 Hz for 1 min) were ex amined during postnatal development. Experiments were performed on iso lated costal diaphragm strips. Before stimulation, twitch contraction time and half relaxation time were longest in the neonate and decrease d significantly between weeks 1 and 6. Correspondingly, the specific t witch tension (corrected for cross-sectional area) increased progressi vely with age. After either low- or high-frequency fatigue, the force recovery was complete in 1- and 2-week-old rats, whereas the force pro duction progressively decreased in older rats. In addition, the neonat e diaphragm further enhanced its force selectively after high-frequenc y fatigue. It is concluded that the rat diaphragm is comparably resist ant to fatigue during the early postnatal period, whether fatigue is i nduced by low- or by high-frequency stimulation. This suggests that po stnatal changes in diaphragm contractile and fatigue properties may be related to changes in the process of force production. The possibilit y is discussed that a higher total muscle oxidative potential and the mechanisms leading to force potentiation in the neonate might explain the fatigue resistance.