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The effect of length on the relationship between tension and intracellular [Ca²⁺] in intact frog skeletal muscle fibres

Dennis R. Claflin, David L. Morgan * and Fred J. Julian

1. The relationship between tension and intracellular calcium concentration ([Ca²⁺]_i) in intact frog skeletal muscle fibres was determined at two fibre lengths, corresponding to mean sarcomere lengths (SL) of 2·2 and 2·9 µm. Tension and [Ca²⁺]_i were recorded during the slow decline of tension following stimulation in the presence of cyclopiazonic acid (CPA), a sarcoplasmic reticulum Ca²⁺-uptake pump inhibitor. [Ca²⁺]_i was estimated by injecting the K⁺ salt form of the fluorescent dye fura-2 into the fibres. Experimental temperature was 3·0 °C.

2. At a SL of 2·2 µm, where thick and thin filaments fully overlap, the [Ca²⁺]_i corresponding to 50 % tension generation ([Ca²⁺]₅₀) was 1·09 ± 0·02 µM (mean ± s.e.m., n = 61 contractions). At a SL of 2·9 µm, where overlap is approximately 50 %, the [Ca²⁺]₅₀ was significantly lower, 0·69 ± 0·02 µM (n = 22 contractions). This is in agreement with previous results from skinned fibres.

3. The relationship between tension and [Ca²⁺]_i was very steep, as reported previously from experiments at a SL of 2·2 µm in which the membrane permeant acetoxymethyl ester form of fura-2 was used. The fall in tension from 90 to 10 % occurred in 0·12 ± 0·01 pCa units (mean ± s.e.m., n = 61) for a SL of 2·2 µm and 0·17 ± 0·01 pCa units (n = 22) for a SL of 2·9 µm, corresponding to Hill coefficients of 15·4 and 10·9, respectively.

4. We conclude that the increase in sensitivity of tension to [Ca²⁺] that occurs in skinned skeletal muscle fibres upon stretch also occurs in intact fibres, that the steepness of the relation between tension and [Ca²⁺]_i in intact fibres reported previously cannot be attributed to the use of the acetoxymethyl ester form of fura-2 to report [Ca²⁺]_i, and that the steepness decreases as myofilament overlap decreases.

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