Interactive Role of Protein Phosphatase 2A and Protein Kinase Cα in the Stretch-Induced Triphosphorylation of Myosin Light Chain in Canine Cerebral Artery

The interactive role of protein kinase C (PKC) isoforms and protein phosphatase 2A (PP2A) in the mechanisms underlying the gradual reduction in stretch-induced contraction through triphosphorylation of 20-kDa myosin light chain (MLC(20)) was investigated in the canine basilar artery. In the presence of 5 mM tetraethylammonium, stretching at a rate of 1 mm/s from the initial length (L(i)) to 1.5 L(i) produced a contraction. Maintaining the stretched state for 15 min (15-min stretch) produced triphosphorylation of MLC(20 )at Ser-19, Thr-18 and Thr-9, and a gradual reduction in the contraction, both of which were reversed by Gö6976 (1 microM), an inhibitor of conventional PKC. The 15-min stretch increased PKCalpha activity whereas it decreased PP2A activity, both of which were blocked by Y-27632, an inhibitor of rho kinase. Okadaic acid (OA; 1 microM), a PP2A inhibitor, also produced triphosphorylation of MLC(20) at the same amino acid residues and activated PKCalpha, which was inhibited by Gö6976. Stretching and OA increased phosphorylation of 17-kDa PKC-potentiated inhibitory phosphoprotein (CPI-17), and this phosphorylation was inhibited by Gö6976. The present results suggest that activation of PKCalpha mediated by an inhibitor of PP2A is involved in the stretch-induced triphosphorylation of MLC(20), and that this triphosphorylation counteracts the stretch-induced contraction.