The creatine kinase equilibrium, free [ADP] and myosin ATPase in vascular smooth muscle cross-bridges

MgADP has a more pronounced effect on the relaxation behaviour in tonic vascular smooth muscle compared to phasic smooth muscle. An apparent dissociation constant of 1.3 μM has been reported for a high affinity binding site of vascular smooth muscle cross-bridges. For this high affinity to have an effect on the low energy costs of tension maintenance (latch) it would require that free [ADP] in the region of the contractile proteins (at least sometimes) be as low as 1.3 μM. We ask, in this report, whether [ADP] could be as low as 1.3 μM in vascular smooth muscle. If creatine kinase (CK) is at equilibrium, then micromolar [ADP] is incompatible with measured concentrations of phosphocreatine (PCr), free creatine (Cr) and ATP, which entail a mean equilibrium [ADP] of around 18 μM. But CK may not be quite at equilibrium: if there is net PCr synthesis at the mitochondrion, then maintenance of the steady-state requires that there be net PCr hydrolysis in the region of the contractile proteins up to or equal to the rate of myosin ATPase. We derive a simple relationship between net flux and displacement from equilibrium which we use to argue that an [ADP] of 1.3 μM at the contractile proteins would drive significant net PCr synthesis, incompatible with normal contractile function. Thus the CK system holds [ADP] at about 18 μM near the contractile proteins in vascular smooth muscle. We conclude that smooth muscle [ADP] cannot be far from equilibrium and that a role for ADP (at the low micromolar level) in controlling smooth muscle relaxation is unlikely.