SUMMARY1. Experiments were done to test the hypothesis that parvalbumin (PA) promotes relaxation in frog skeletal muscle. Single fibres and purified PA from Rana temporaria skeletal muscle were used to determine the relationship between PA concentration ([PA]), Ca2" and Mg2" dissociation rates from PA and changes in rate of relaxation as a function of tetanus duration at 0°C. 3. Relaxation rate from an isometric tetanus slows progressively as a function of tetanus duration with an exponential time course and a rate constant of 1P18 + 0 35 s-' (n = 17). Time course of recovery of relaxation rate after a prolonged tetanus is exponential with a rate constant of 0O12 + 0-02 s-1 (n = 14).4. The extent of recovery of relaxation rate after a prolonged tetanus was correlated with total [PA] in fibres (correlation coefficient (r) = 0-80, n = 7; P < 005).5. Dissociation rate constants for Mg2+ and Ca2+ from purified PA are 0-93+0-02 s-' (n = 5) and 0 19+0-01 s-' (n = 5), respectively. Dissociation rate constants were not significantly different for PA isoforms IVa and IVb. These rate constants are similar to the rate constants determined for the time courses of slowing and recovery of relaxation rate, respectively. 6. Results suggest that the time courses of slowing and recovery of relaxation rate may be controlled, to a large extent, by Mg2+ and Ca2+ dissociation from PA, respectively. This evidence supports a role for PA in facilitating relaxation during a tetanus in frog skeletal muscle at 0 'C.

SUMMARY1. Influence of temperature on relaxation rate as a function of isometric tetanus duration and on Ca2" and Mg2" dissociation rates from purified parvalbumin (PA) was examined to test the hypothesis that PA promotes relaxation in frog skeletal muscle. Single fibres and PA IVB from Rana temporaria skeletal muscle were utilized.2. The magnitude of slowing of relaxation rate with increasing tetanus duration, relative to the final, steady value of relaxation rate, is 3-fold greater at 0 than at 10 OC.3. In the 0-10°C range, the Qlo for relaxation rate increases from 2-3 to 3-7 with increasing tetanus duration.

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