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 guan osine 3',5'-monophosphate (cGMP) and subsequent dephosphorylation of t he 20-kilodalton myosin light chain (MLC). We investigated this hypoth esis in procine coronary arterial smooth muscle stimulated with histam ine (3 mu M) or K+ (30 mM). Stimulation of [P-32]Pi-labeled muscle wit h histamine or K+ for 2 min resulted in a four- or 6.2-fold increase, respectively, in the incorporation of P-32 into MLC. After 48 min of e xposure 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 sustai ned increase of MLC phosphorylation but had no effect on the phosphory lation of desmin, synemin, or the three unidentified cytosolic protein s. Application of nitroglycerin (1 mu M) 48 min after histamine stimul ation inhibited the phosphorylation of desmin, synemin, and the three cytosolic proteins. The sustained phase of histamine-induced contracti on was also inhibited to a greater extent then the acute phase of hist amine-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 phospho rylation of intermediate filament proteins is required for the sustain ed phase of histamine-induced contraction. Intermediate filament prote ins, rather than MLC, may also be the target for the relaxant action o f nitroglycerin during histamine-induced sustained contraction.