Smooth muscle energetics: testing theories of crossbridge regulation.

Our results indicate that the kinetic "latch" model of Hai & Murphy is not very sensitive to the proportion of ATP assigned to crossbridges relative to that ascribed to MLC phosphorylation/dephosphorylation. Thus the basis for the relatively low efficiency of smooth muscle, attributed to high MLC phosphorylation/dephosphorylation in this model, remains open to question. Moreover, this model, or any model with mixed populations of crossbridges with differing cycle rates and/or high MLC phosphorylation/dephosphorylation rates is unlikely to account for the observed linearity of JATP and stress reported for many smooth muscles. Our studies comparing the heat production of intact and skinned smooth muscle indicate that the ATPase associated with myosin phosphorylation/dephosphorylation is unlikely to be a major factor in the tension cost of intact smooth muscle. Thus it would appear that energetics places considerable constraints on current theories of crossbridge regulation. Our modelling (Paul, 1989) suggests that it may be time to reevaluate Bozler's original hypothesis that a high attachment:detachment rate ratio for smooth muscle actin-myosin interaction may be sufficient to explain the energetics of smooth muscle.