Recovery of fatigued Xenopus muscle fibres is markedly affected by the extracellular tonicity.

In previous studies it has shown that isolated Xenopus muscle fibres may enter a long-lasting, reversible state of severely depressed tetanic force when recovering from fatigue produced by repeated tetani. The mechanism behind this postcontractile depression (PCD) has been studied further by exposing rested and fatigued fibres to a hypertonic (1.2 x normal tonicity) or a hypotonic (0.8 x) solution. In the rested state the average tetanic tension increased by 9% in the hypotonic solution and was reduced by 8% in the hypertonic solution. After fatiguing stimulation similar alterations of tonicity resulted in changes of tetanic tension of about 40% in easily fatigable fibres (type 1; n = 21); an increased tonicity always resulted in reduced tension, whereas decreased tonicity gave an increased tension output. Similar results were obtained in fatigue-resistant fibres (type 2; n = 4), but here the force depression caused by hypertonicity appeared to be irreversible. Thus, fibres were markedly more sensitive to changes of the extracellular tonicity during the recovery period. It is suggested that this increased sensitivity reflects alterations in the signal transmission between t-tubules and sarcoplasmic reticulum.