Dependence of fatigue properties on the pattern of stimulation in the rat diaphragm muscle.

Diaphragm fatigue was studied in isolated phrenic nerve diaphragm strips from 28 Swiss Albino rats. Three procedures were used to estimate the isometric twitch characters and force frequency responses to fatigue of the rat diaphragm at different rates of phrenic nerve stimulation. Diaphragm fatigue was induced by using low frequency stimulation (0.2 ms pulse duration, at 5 Hz frequency, 3 min), high frequency stimulation (0.2 ms pulse duration, at 50 Hz frequency, 3 min), and by production brief submaximal contraction (25 Hz, for 160 ms at the rate of 1/s for 45 contractions). Tension was reduced to 26.67 +/- 5.10% and 6.59 +/- 2.64% and 68.69 +/- 2.45% of the initial value at the end of the low and high frequency and brief submaximal stimulation, respectively. In all the fatigue experiments, twitch tension and tetanic tension decreased, contraction and 1/2 relaxation time prolonged and force-frequency curves shifted to the right. The most significant changes were observed in low frequency fatigue whereas the most moderate ones were recorded in brief submaximal fatigue. It was concluded that fatigue in the rat diaphragm depended on the frequency and duration of stimulation as well as on the number of stimuli delivered to the muscle. Various mechanisms of muscle fatigue are described in the discussion to explain the observations made in the present investigation.