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

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 mu m. 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 degrees C. 2. At a SL of 2.2 mu m, where thick and thin filaments fully overlap, the [Ca2+](i) corresponding to 50% tension generation ([Ca2+](50)) was 1.09 +/- 0.02 mu M (mean +/- S.E.M., n = 61 contractions). At a SL of 2.9 mu m, where overlap is approximately 50 %, the [Ca2+](50) was significantly lower, 0.69 +/- 0.02 mu M (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 XL of 2.2 mu m 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 mu m and 0.17 +/- 0.01 pCa units (n = 22) for a XL of 2.9 mu m, 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.