Force-velocity relation in chemically skinned rat portal vein. Effects of Ca2+ and Mg2+

Rat portal veins were chemically skinned using Triton X-100 and mounted for isometric and quick release experiments at 20 degrees C. The skinned preparations were activated by Ca2+ (EGTA-buffered) in solutions containing 2mM free-Mg2+ and 1 microM calmodulin. Half maximal isometric force was obtained at pCa = 6.2. Maximal force of the skinned preparations, at pCa = 4.5, was 8.2 +/- 0.8 mN/mm2 (n = 6). Force-velocity relations were determined at varied Ca2+-concentrations. Maximal shortening velocity (Vmax) was 0.10 +/- 0.01 lengths/s at pCa = 4.5. At decreasing Ca2+-levels Vmax decreased (at pCa = 6.25, Vmax = 0.051/s). At pCa = 9 an increased level of free-Mg2+ (15 mM) induces a slow and submaximal increase in tension. Force velocity relations of Mg2+-induced contractures were not different from those of Ca2+-contractures of similar magnitude (pCa = 6.3). The results indicate that the degree of activation of the contractile system, as regulated by Ca2+ and Mg2+, influences the kinetic properties of the actomyosin interaction as well as the force development.