The acute regulation of inwardly directed Na(+)-myo-inositol (MI) cotransporter activity and basal and volume-sensitive MI efflux by protein kinases C (PKC) and A (PKA), cytosolic Ca2+, and phosphoinositide (PI) turnover were characterized in cultured human retinal pigment epithelial cells using 2-[3H]MI and liquid scintillation spectrometry. Kinetic analysis revealed two distinct Na(+)-MI cotransporter components differing in apparent Michaelis constant and maximal velocity. Composite Na(+)-MI cotransport activity was stimulated by PKA activation, the muscarinic agonist carbachol, and the Ca2+ ionophore A-23187 and was inhibited by PKC activation. PKC activation also increased MI efflux, but only the volume-sensitive component, whereas PKA activation increased both basal and volume-sensitive MI efflux. These studies implicate PKC as a negative modulator of MI content through Na(+)-MI cotransport inhibition and potentiation of volume-sensitive MI efflux. PKA is a positive modulator of both Na(+)-MI cotransport and basal and volume-sensitive MI efflux. Cytosolic Ca2+ release through receptor-mediated PI hydrolysis may facilitate Na(+)-MI cotransport activity.