The maximal velocity of vascular smooth muscle shortening is independent of the expression of calponin

In smooth muscle, the phosphorylation/dephosphorylation of the 20-kDa regulatory light chain of myosin (MLC₂₀) is known to regulate actomyosin interaction and force. However, a thin filament based regulatory system for actomyosin interaction has been suggested to exist in parallel to MLC₂₀ phosphorylation. Calponin is a thin filament associated protein that in vitro inhibits actomyosin interaction, and has been suggested to reduce maximal shortening velocity (υ(max)), Using antibodies to h1- and h2-calponin, we demonstrated that calponin was present in smooth muscle from Sprague Dawley (SD) rats, while calponin was not detectable in the smooth muscle from Wistar Kyoto (WKY) rats. V(max) determined from the force vs. velocity relationship at maximal Ca²⁺ activation was not different for either the aorta or the portal vein of SD vs. WKY rats. These results suggest that physiological levels of calponin do not contribute to a thin filament-based secondary regulation to inhibit smooth muscle contraction.