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Am J Physiol Cell Physiol 273: C671-C678, 1997;
0363-6143/97 $5.00
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AJP - Cell Physiology, Vol 273, Issue 2 C671-C678, Copyright © 1997 by American Physiological Society

ARTICLES

Protein kinase and Ca2+ modulation of myo-inositol transport in cultured retinal pigment epithelial cells

A. Karihaloo, K. Kato, D. A. Greene and T. P. Thomas
Department of Internal Medicine, University of Michigan, Ann Arbor 48109-0678, USA.

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.


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