Substrate and Product Dependence of Force and Shortening in Fast and Slow Smooth Muscle

To explore the molecular mechanisms responsible for the variation in smooth muscle contractile kinetics, the influence of MgATP, MgADP, and inorganic phosphate (P_i) on force and shortening velocity in thiophosphorylated “fast” (taenia coli: maximal shortening velocity V_max = 0.11 ML/s) and “slow” (aorta: V_max = 0.015 ML/s) smooth muscle from the guinea pig were compared. P_i inhibited active force with minor effects on the V_max. In the taenia coli, 20 mM P_i inhibited force by 25%. In the aorta, the effect was markedly less (<10%), suggesting differences between fast and slow smooth muscles in the binding of P_i or in the relative population of P_i binding states during cycling. Lowering of MgATP reduced force and V_max. The aorta was less sensitive to reduction in MgATP (K_m for V_max: 80 μM) than the taenia coli (K_m for V_max: 350 μM). Thus, velocity is controlled by steps preceding the ATP binding and cross-bridge dissociation, and a weaker binding of ATP is not responsible for the lower V_max in the slow muscle. MgADP inhibited force and V_max. Saturating concentrations of ADP did not completely inhibit maximal shortening velocity. The effect of ADP on V_max was observed at lower concentrations in the aorta compared with the taenia coli, suggesting that the ADP binding to phosphorylated and cycling cross-bridges is stronger in slow compared with fast smooth muscle.