Purpose: Endothelial-derived vasorelaxants such as prostacyclin and nitric oxide (NO) induce vascular smooth muscle relaxation through activation of cyclic nucleotide-dependent cellular signalling pathways. However, the specific events that lead to dissociation of actin and myosin and relaxation are not known. The purpose of this investigation was to determine the late phase signaling events that modulate vascular smooth muscle relaxation.
Methods: Fresh bovine carotid artery smooth muscle (BCASM) contractile responses were determined in a muscle bath under Ca(2+)-containing and Ca(2+)-free conditions. Physiologic responses were correlated with phosphorylation events using whole cell phosphorylation and two-dimensional gel electrophoresis.
Results: Cyclic nucleotide-dependent vasorelaxation can occur without detectable changes in intracellular Ca2+ concentrations. However, vascular smooth muscles that had been precontracted with the phosphatase inhibitor calyculin were refractory to relaxation. Vascular smooth muscle relaxation was associated with an increase in the phosphorylation of two 20 kDa proteins under Ca(2+)-containing and Ca(2+)-free conditions.
Conclusions: These results suggest that Ca(2+)-independent mechanisms may also modulate vascular smooth muscle relaxation. Two possible late phase signaling mechanisms include phosphatase activation and an increase in the phosphorylation of two 20 kDa phosphoproteins.