PHORBOL ESTER-INDUCED CONTRACTIONS OF SWINE CAROTID-ARTERY ARE SUPPORTED BY SLOWLY CYCLING CROSSBRIDGES WHICH ARE NOT DEPENDENT ON CALCIUM OR MYOSIN LIGHT-CHAIN PHOSPHORYLATION

This study determined if phorbol ester-induced contraction of vascular smooth muscle requires calcium-dependent myosin light chain (MLC) phosphorylation and, if not, whether the mechanical characteristics of the contraction in terms of stiffness and crossbridge cycling are similar to those during a calcium- and MLC phosphorylation-dependent contraction. Carotid arterial strips were exposed to 1.0 muM phorbol 12,13-dibutyrate (PDBu) in the presence of normal physiological salt solution (PSS) or after calcium depletion in calcium-free PSS and compared with contraction elicited by calcium-containing 110 mM KCl-PSS. PDBu induced maximal stress in both the presence and absence of calcium. While there was a temporal correlation between MLC phosphorylation and shortening velocity during KCl depolarization, shortening velocity was dissociated from MLC phosphorylation during PDBu stimulation. The stress-stiffness relationship was not different during KCl and PDBu stimulation, suggesting similar crossbridge interactions even though MLC phosphorylation levels were significantly different. These results demonstrate that PDBu-induced contraction of the swine carotid artery is not dependent on calcium or MLC phosphorylation. We suggest the possibility that activation of a calcium-independent PKC isoform may result in the expression of an inherent level of actin-activated myosin ATPase activity resulting in the slow development of stress.