A Novel Role of Caldesmon during Smooth Muscle Relaxation

Abstract

The effect of h-caldesmon (h-CaD) on smooth muscle contractility was examined by force measurements on tissues isolated from wild-type (WT) and h-CaD-null (KO) mice. Both aorta and tail arteries from KO mice relaxed more slowly than that from WT animals. The force decay of beta-escin-permeabilized, kinase-inhibited tissues under pCa>9 was best fitted with a two-exponential process. We found that the apparently slower relaxation for the KO samples was not due to lower rate constants, but rather, was because of increased amplitude of the slower component of the decay. To search for an explanation for the observed relaxation kinetics, we drew some lessons from the work done on striated muscles. Muscle relaxation involves two major events: crossbridge detachment and change of the thin filament from active to inactive state. The latter occurs first, corresponding to the initial lag phase, followed by the rapid force decay, the crossbridge detachment. In smooth muscle, the lag phase is ascribed to myosin dephosphorylation, but it may also include the filament state change, i.e., CaD-tropomyosin moving to a position on actin filaments that hinders myosin binding. This “off-position” is under strain, because the dephosphorylated crossbridges are still attached. Assuming that there are two possible positions for CaD-tropomyosin to occupy, one displacing the bound myosin more promptly than the other, the crossbridge would detach at two different rates, resulting in the observed biphasic decay. Our data then suggest that when h-CaD is absent, fewer tropomyosin molecules are at the blocking position. Thus, the return of smooth muscle tropomyosin to the proper inhibitory position upon relaxation needs help from h-CaD. In striated muscle the movement of tropomyosin was shown to be facilitated by troponin subunits. CaD may play a troponin-like role in smooth muscle both structurally and functionally.