['The purpose of the present study was to clarify the role of Rho-kinase and/or protein kinase C in the resting tension of the isolated pig iris sphincter muscle.', 'The motor activity of the isolated pig iris sphincter muscle was measured isometrically.', 'EGTA, a chelator of extracellular Ca(2+), significantly reduced the resting tension. Y27632, a Rho-kinase inhibitor, significantly reduced the resting tension in a concentration-dependent manner. The resting tension diminished by Y27632 was significantly recovered by the addition of calyculin A, a myosin light chain phosphatase (MLCP) inhibitor, in a concentration-dependent manner. GF109203X, a protein kinase C inhibitor, had no effect on the resting tension.'] Is rho-kinase , but not protein kinase C , involved in generation of the spontaneous tone in the resting phase of the isolated pig iris sphincter muscle?