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 "fas
t" (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 marke
dly less (<10%), suggesting differences between fast and slow smooth muscle
s 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 l
ess sensitive to reduction in MgATP (K-m for V-max: 80 <mu>M) than the taen
ia coli (K-m for V-max: 350 muM). Thus, velocity is controlled by steps pre
ceding 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 inh
ibited force and V-max. Saturating concentrations of ADP did not completely
inhibit maximal shortening velocity. The effect of ADP on V-max was observ
ed at lower concentrations in the aorta compared with the taenia coli, sugg
esting that the ADP binding to phosphorylated and cycling cross-bridges is
stonger in slow compared with fast smooth muscle.