Nitroglycerin inhibits the phosphorylation of intermediate filament proteins rather than myosin light chain on porcine coronary artery sustained contraction

The smooth muscle relaxation induced by nitroglycerin is hypothesized to be mediated by an increase in the cytoplasmic concentration of guanosine 3′,5′-monophosphate (cGMP) and subsequent dephosphorylation of the 20-kilodalton myosin light chain (MLC). We investigated this hypothesis in procine coronary arterial smooth muscle stimulated with histamine (3 μM) or K⁺ (30 mM). Stimulation of ³²P]Pi-labeled muscle with histamine or K⁺ for 2 min resulted in a four- or 6.2-fold increase, respectively, in the incorporation of³²P into MLC. After 48 min of exposure to histamine. MLC phosphorylation decreased to the basal level and the phosphorylation of desmin, synemin, and of three unidentified cytosolic proteins was increased. K⁺ stimulation resulted in a sustained increase of MLC phosphorylation but had no effect on the phosphorylation of desmin, synemin, or the three unidentified cytosolic proteins. Application of nitroglycerin (1 μM) 48 min after histamine stimulation inhibited the phosphorylation of desmin, synemin, and the three cytosolic proteins. The sustained phase of histamine-induced contraction was also inhibited to a greater extent then the acute phase of histamine-induced contraction and both the acute and sustained phases of K⁺-induced contraction. These results suggest that MLC phosphorylation is required for both phases of K⁺-induced contraction, whereas phosphorylation of intermediate filament proteins is required for the sustained phase of histamine-induced contraction. Intermediate filament proteins, rather than MLC, may also be the target for the relaxant action of nitroglycerin during histamine-induced sustained contraction.