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Department of Anesthesia Research Laboratories, Harvard Medical School, Brigham and Women's Hospital, Boston, MA 02115-6195, USA.

1. The relationship between intracellular calcium concentration, [Ca2+]i, and fixed-end tension was investigated in intact single muscle fibres from frogs. A slow decline of tension was produced by cyclopiazonic acid (CPA), a sarcoplasmic reticulum Ca2+ uptake pump inhibitor. The fluorescent dyes fura-2 and furaptra (mag-fura-2) were used to estimate [Ca2+]i. 2. Neither the steepness nor the position of the curve changed consistently over a wide range of tension decay times from a few seconds to over 100 s. For these near-steady-state curves, the 10-90% tension change occurred, on average, in 0.07 pCa units, corresponding to a Hill coefficient > 25, much steeper than previously reported. Possible artifacts could reduce that to 15. 3. Methoxyverapamil (D600) reduces the calcium released in response to an action potential. Contractions with D600 and CPA had a slow rise composed of many small steps, and a slow fall. Comparing rise and fall showed little or no hysteresis in the tension-[Ca2+]i relationship. 4. A model involving co-operativity between the binding of Ca2+ and myosin to thin filaments is shown to produce a tension-pCa relationship that is substantially altered by the mean rate constant for detachment of myosin cross-bridges, which in turn is likely to be affected by sarcomere movements. 5. Such a model is shown to be capable of reproducing the small rise in [Ca2+]i previously reported during the late phase of fixed-end relaxation of intact fibres.

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