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Original Article

^a Department of Physiological Sciences, Lund University, Tornavägen 10, BMC F11, S-22184 Lund, Sweden
Department of Physiological Sciences, Lund University, Tornavägen 10, BMC F11, S-22184 Lund, Sweden.46-46-222- 7765

Anders.Arner{at}mphy.lu.se

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.

Key Words: myosin isoforms • phosphate • ATP • ADP • force-velocity relation

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