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Effects of calcium and substrate on force‐velocity relation and energy turnover in skinned smooth muscle of the guinea‐pig.
Author(s) -
Arner A,
Hellstrand P
Publication year - 1985
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1985.sp015621
Subject(s) - isometric exercise , chemistry , calcium , biophysics , taenia coli , crossbridge , muscle contraction , calmodulin , isotonic , anatomy , myosin , medicine , biochemistry , biology , organic chemistry
Mechanical properties and rate of ATP breakdown (JATP) have been determined in the chemically skinned guinea‐pig taenia coli at 22 degrees C. The influence of varied [Ca2+], [Mg ATP] and muscle length were investigated. The shortening response after a step decrease in force (isotonic quick release) was highly curvilinear in the first 100‐200 ms. This effect was shown to be a time‐dependent response to the force step and not primarily caused by the shift along the length‐force relation associated with shortening. Maximal shortening velocity (Vmax) decreased gradually following the release. At pCa (= ‐log [Ca2+]) 4.5, Vmax at 20 and 1000 ms after release was 0.49 +/‐ 0.07 and 0.041 +/‐ 0.004 (mean +/‐ S.E. of mean, n = 5) lengths s‐1 respectively. Unloaded shortening velocity obtained from length steps of different magnitude (slack test) also showed a gradual decrease after the release, consistent with the isotonic release results. Increasing [Ca2+] from the relaxed state at pCa 9 (1 microM‐calmodulin present) gave increased isometric force to a maximum at pCa 4.5. Half‐maximal response was obtained at pCa 6.1. JATP at maximal force at pCa 4.5 was about 3 times the basal rate at pCa 9. The relation between JATP and force was highly non‐linear, with a marked increase in JATP with little alteration in force at the highest [Ca2+]. When force was reduced to zero at pCa 4.5 by shortening the muscle to 0.3 L0 (L0 being the length giving maximal active force), JATP decreased by about 30%. At two levels of [Ca2+] giving similar force (pCa 5.75 and 4.5) the energetic tension cost obtained by length variations was lower at the low [Ca2+]. At pCa 6.0, Vmax and force were decreased to the same extent relative to their values at pCa 4.5. At pCa 5.75, where there was no reduction in force but a 25% decrease in isometric JATP, Vmax was unchanged relative to pCa 4.5. Force, Vmax and JATP were all dependent on [Mg ATP]. Half‐maximal response was obtained at 0.1 mM for force and Vmax, and at 0.5 mM for JATP. The results are discussed in relation to a possible influence of both Ca2+ and Mg ATP on kinetic properties of the cross‐bridge cycle.