Energetics and mechanics of frog skeletal muscle in hypotonic solution

Hypotonic solutions are known to potentiate muscle force production and increase actomyosin ATPase activity in solution. As such, both the rate and amount of ATP hydrolysis should increase during contraction. This was tested indirectly by measuring force and energy liberation in Rana pipiens semitendinosus muscles at 0 degrees C in hypotonic solution. Force and the amount and rate of energy liberation increased. This increase is consistent with the interpretation that the rate of ATP hydrolysis is increased in hypotonic solution. Muscles, stretched to beyond myofilament overlap, liberate a substantial fraction of the energy liberated at maximal myofilament overlap. This energy liberation, the activation heat, is thought to reflect the energy utilized to cycle Ca2+. Hypotonic solution decreased the amplitude and the rate of the activation heat, suggesting that the amount and rate of Ca2+ cycled by the sarcoplasmic reticulum is reduced. Thus in hypotonic solution, force production and the rate of ATP hydrolysis by the cross bridges are potentiated despite an apparent decrease in Ca2+ cycling.