The effects of temperature on relaxation in frog skeletal muscle: the role of parvalbumin

Isolated muscle fibers from Rana temporaria tibialis anterior muscles were microinjected with aequorin. The force responses and the Ca²⁺ transients associated with twitch and tetanic contractions were studied at several temperatures. The declines of the Ca²⁺ transients were well described by single exponential equations and the effects of temperature were complex (multi-exponential). To determine if these temperature effects on the Ca²⁺ transients were influenced by the Ca²⁺ indicator itself, samples of the injected aequorin were studied in vitro using a Gibson stopped-flow apparatus. The quenching of aequorin luminescence with either EGTA or de-calcified Rana temporaria parvalbumin were mono-exponential. These overall quenching reactions had single exponential temperature dependencies. The effects of temperature on the declines of the single fiber Ca²⁺ transients did not appear to be influenced by the kinetics of the aequorin reaction. The disparity in the effects of temperature on the single fiber Ca²⁺ transients versus the in vitro quenching of aequorin luminescence with parvalbumin, were interpreted to indicate that in twitch and tetanic contractions of these fibers, it was unlikely that soluble Ca²⁺ binding proteins played a major role in the regulation of myoplasmic Ca²⁺.