EFFECTS OF CALCIUM AND SUBSTRATE ON FORCE VELOCITY RELATION AND ENERGY TURNOVER IN SKINNED SMOOTH-MUSCLE OF THE GUINEA-PIG

Mechanical properties and rate of ATP breakdown (JATP) were determined in the chemically skinned guinea pig taenia coli at 22.degree. C. The influence of varied [Ca2+], [MgATP] 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 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 .+-. SE 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 .mu.M-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 .apprx. 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 forece 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 .apprx. 30%. At 2 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 [MgATP]. 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.