Effects of exogenously applied calponin on Ca²⁺-regulated force in skinned smooth muscle of the rabbit mesenteric artery

To help elucidate the physiological role of calponin (a thin-filament-linked regulatory protein) in smooth muscle contraction, the effects of its exogenous application were investigated on actin-activated MgATPase activity in crude actomyosin from chicken gizzard, and on contraction induced by Ca²⁺-dependent and -independent means in arterial smooth muscle strips skinned by saponin or β-escin. Calponin concentration dependently inhibited actin-activated MgATPase activity with a proportional increase in its binding to actomyosin and also attenuated Ca²⁺-induced contractions, in the presence or absence of calmodulin, in skinned arterial strips. Calponin, when phosphorylated by protein kinase C, reduced both its ability to bind to actomyosin and its inhibitory action on actomyosin MgATPase. The phosphorylated calponin also had no effect on the maximum Ca²⁺-induced contraction in skinned smooth muscle, suggesting that these actions of calponin are not non-specific. Calponin attenuated the Ca²⁺-independent contraction observed in myosin light chain thio-phosphorylated strips, or on application of trypsin-treated myosin light chain kinase. However, calponin had no effect on maintained rigor contraction. These results suggest that in vascular smooth muscle, calponin may play a physiological role in the inhibition of Ca²⁺-regulated force, possibly through a direct action on active actin-myosin interactions.