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Department of Physiology, University of Sydney, NSW, Australia.

1. Parvalbumin (PA) has been proposed to take up Ca2+ and enhance skeletal muscle relaxation in brief contractions; as the duration of the contraction is increased, PA will become saturated with Ca2+ and no longer contribute to relaxation which therefore will be slowed. The rate of Ca2+ loading of PA is determined by the Mg2+ off rate (about 4 s-1 at 22 degrees C). In the present study we produced prolonged tetani in intact, single fibres of Xenopus frogs while measuring force and the free myoplasmic [Ca2+] ([Ca2+]i) with indo-1. 2. Mean rate constants of slowing of force relaxation with increasing tetanus duration ranged between 3.2 and 4.8 s-1, thus, similar to the Mg2+ off rate of PA. 3. The amplitude of the tail of [Ca2+]i after tetani increased with tetanus duration. This increase developed with a rate constant similar to the Mg2+ off rate of PA 4. Steady-state force-[Ca2+]i curves were produced from tetani of various frequencies and tetani produced when force was depressed after fatiguing stimulation. These curves were used to convert [Ca2+]i records into Ca(2+)-derived force. Relaxation of Ca(2+)-derived force was slowed following a time course similar to that of real force. The lag between Ca(2+)-derived and real force during relaxation was not affected by tetanus duration. 5. Tails of elevated [Ca2+]i after tetani were used to analyse the function of the SR Ca2+ pumps. This analysis showed a marked decline in the rate of Ca2+ uptake with prolonged tetani. 6. In conclusion, in Xenopus fibres the slowing of relaxation with increasing tetanus duration can be explained by altered Ca2+ handling due to PA Ca2+ loading and impaired SR Ca2+ uptake. This contrasts to our previous results in mouse fibres and the difference can be explained by a markedly lower rate of SR Ca2+ uptake resulting in higher tetanic [Ca2+]i in Xenopus fibres.

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