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 micron. 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 micron, where thick and thin filaments fully overlap, the [Ca2+]i corresponding to 50 % tension generation ([Ca2+]50) was 1.09 +/- 0.02 microM (mean +/- S.E.M., n = 61 contractions). At a SL of 2.9 micron, where overlap is approximately 50 %, the [Ca2+]50 was significantly lower, 0.69 +/- 0. 02 microM (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 micron 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 micron and 0.17 +/- 0.01 pCa units (n = 22) for a SL of 2.9 micron, 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.
Claflin
D R
DR
Department of Anesthesia Research Laboratories, Harvard Medical School, Brigham & Women's Hospital, 75 Francis Street, Boston, MA 02115-6195, USA. claflin@zeus.bwh.harvard.edu
Morgan
D L
DL
Julian
F J
FJ
D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D009119:Muscle Contraction; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011896:Rana temporaria; D012518:Sarcomeres / Q000187:drug effects / Q000502:physiology*; D013314:Stress, Mechanical
D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007211:Indoles; C049925:fura-2-am; D000252:Calcium-Transporting ATPases; D002118:Calcium; D016257:Fura-2; C000543:cyclopiazonic acid