Force velocity relations in vascular smooth muscle: the influence of pH, pCa, and noradrenaline

The kinetics of vascular smooth musclw activity was studied by means of afterloaded isotonic contractions of the tetanized rat portal vein at varied pH (8.0-5.9), pCa (3.4-2.1), and during noradrenaline incubation (0.4 mug/ml). Under control conditions (pH 7.3, pCa 2.6) the following parameters of the force velocity relation were calculated: a of Hill's equation (relating to the isometric peak tension) = 0.36; b (relating to the actual muscle length) = 0.19 ML/s; VM Trelating to the actual muscle length) = 0.56 ML/s. Within the range of pCa between 2.0 and 3.2 the amount of force generation (= delta P) depended on the extracellular calcium level whereas the extrapolated velocity of shortening of the unloaded preparation (= VM) did not. Also pH changes between 8.0 and 6.8 as well as noradrenaline incubation at a pH of 5.9 affected delta P quite considerably, but VM only scarcely. At a pH of 6.3, however, VM was distinctly diminished, and a reduced calcium sensitivity of the ATPase was inferred from the shift of ED50 of extracellular calcium from 0.66 mM Ca at a pH of 7.3 to 1.56 mM Ca at a pH of 6.3 (P less than 0.0005). It is concluded from these results that the experimental conditions-pCa between 2.0 and 3.2, pH between 8.0 and 6.8, and noradrenaline added at a pH of 5.9-obviously change the intracellular calcium concentration which influences the number of activated interaction sites rather than the velocity of crossbridge movement.