Abstract

The degree of tonic force (F) maintenance induced in vascular smooth muscle upon K⁺ depolarization with 110 mM KCl can be greatly reduced by inhibition of rhoA kinase (ROCK). We explored the possibility that a protein kinase C (PKC) isotype may also play a role in causing KCl-induced Ca²⁺ sensitization. In isometric rings of rabbit artery, the PKC inhibitors, Go-6983 (3-[1-[3-(dimethylamino)propyl]-5-methoxy-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione), GF-109203X (2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide), and a cell-permeable (myristoylated) pseudosubstrate inhibitor of PKCζ (PI_PKCζ) inhibited KCl-induced tonic F. A myristoylated pseudosubstrate inhibitor of PKCα/β that inhibited phorbol dibutyrate-induced F slightly potentiated KCl-induced tonic F and attenuated 30 mM KCl-induced F. Although the ROCK inhibitor, H-1152 [(S)-(+)-2-methyl-1-[(4-methyl-5-isoquinolinyl)-sulfonyl]-hexahydro-1H-1,4-diazepine dihydrochloride], reduced basal phosphorylation of myosin light-chain phosphatase-targeting subunit at Thr853 (MYPT1-pT853), 3 and 10 μM GF-109203X inhibited only KCl-stimulated phosphorylation, not basal MYPT1-pT853. In fura-2-loaded tissues, GF-109203X and PI_PKCζ elevated basal [Ca²⁺]_i (calcium) and potentiated KCl-induced tonic increases in calcium while reducing KCl-induced tonic increases in F. Blockade by nifedipine of Ca²⁺ entry through voltage-operated Ca²⁺ channels reduced KCl-induced Ca²⁺ sensitization and KCl-stimulated but not basal MYPT1-pT853. These data together support a model in which ROCK and PKCζ are constitutively active and function in “resting” muscle to regulate the basal levels of MYPT1-pT853 and calcium, respectively. In this model, KCl-induced increases in calcium activate PKCζ to feed forward and cause additional MYPT1-pT853 above that induced by constitutive ROCK, permitting Ca²⁺ sensitization and strong F maintenance. Active PKCζ also feeds back to attenuate the degree of KCl-induced increases in calcium.