Regulation of Vascular smooth muscle Contraction by Calponin Phosphorylation and Dephosphorylation

Calponin is a thin filament-associated smooth muscle protein that has been implicated to play a role in the regulation of smooth muscle. It inhibits actomyosin ATPase but the capacity for such inhibition is lost upon phosphorylation of calponin by protein kinase C. Thus, it seems possible that phosphorylation of calponin might modulate the contraction of smooth muscle. The objective of the present investigation was to determine whether calponin is phosphorylated by protein kinase C in intact smooth muscle in response to a vasoconstrictor and whether such phosphorylation is of functional significance with respect to the contraction of smooth muscle. Endothelin-1 and phorbol 12, 13-dibutyrate (PDBu) caused 2.3-fold and 2.6-fold increases, respectively, in tthe extent of phosphorylation of calponin during contraction of porcine coronary artery. However, high levels of KCl were ineffective despite development of an identical contractile force. These results suggest that the phosphorylation of calponin in vivo by protein kinase C might play an important role in the contraction of smooth muscle that occurs in response to endothelin-1 and PDBu.