Calponin decreases the rate of cross-bridge cycling and increases maximum force production by smooth muscle myosin in an in vitro motility assay

Mammalian smooth muscles, like invertebrate "catch" muscles, dynamically regulate the rate of cross-bridge cycling. During sustained contractions, cross-bridge cycling rates may decrease severalfold, whereas tension remains elevated ("latch-state"). Physiologic studies have shown that decreased cycling rate is frequently, but not always, associated with dephosphorylation of the 20,000-dalton light chain of myosin. Tropomyosin, caldesmon, and calponin are actin-binding proteins found in mammalian smooth muscles that modulate actin-activated myosin ATPase activity in vitro and may modulate cross-bridge cycling in situ. Using an in vitro motility assay in combination with a new method for estimating relative changes in the force exerted on actin filaments, the present study demonstrates that the effects of calponin on actin filament motility are: increased actin filament binding to thiophosphorylated smooth muscle myosin, decreased filament velocity from 2.0 to 0.74 microns/s, and a 3-4-fold increase in the force exerted on stationary actin filaments. Taken together, these observations suggest that calponin inhibits the rate of dissociation of the high-affinity actomyosin complex and, consequently, that it may be an integral component of the latch-state in mammalian smooth muscles.