Substrate and product dependence of force and shortening in fast and slow smooth muscle

Abstract

To explore the molecular mechanisms responsible for the variation in smooth muscle contractile kinetics, the influence of MgATP, MgADP, and inorganic phosphate (Pi) on force and shortening velocity in thiophosphorylated "fast" (taenia coli: maximal shortening velocity Vmax = 0.11 ML/s) and "slow" (aorta: Vmax = 0.015 ML/s) smooth muscle from the guinea pig were compared. Pi inhibited active force with minor effects on the Vmax. In the taenia coli, 20 mM Pi 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 Pi or in the relative population of Pi binding states during cycling. Lowering of MgATP reduced force and Vmax. The aorta was less sensitive to reduction in MgATP (Km for Vmax: 80 μM) than the taenia coli (Km for Vmax: 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 Vmax in the slow muscle. MgADP inhibited force and Vmax. Saturating concentrations of ADP did not completely inhibit maximal shortening velocity. The effect of ADP on Vmax. was observed at lower concentrations in the aorta compared with the taenia coli, suggesting that the ADP binding to phosphorylated and cycling crossbridges is stronger in slow compared with fast smooth muscle.