O-2 AVAILABILITY MODULATES TRANSMEMBRANE CA2-MESSENGER PATHWAYS IN ANOXIA-TOLERANT HEPATOCYTES( FLUX VIA 2ND)

Transmembrane Ca2+-flux was studied from single isolated turtle hepato cytes by using a noninvasive Ca2+-selective self-referencing microelec trode. Cells in Ca2+-reduced culture medium demonstrated a vanadate-an d lanthanum-inhibitable Ca2+-efflux of 4 x 10(-17) mol Ca2+.mu m(-2). s(-1) continuously over 170 h. This flux diminished with 50 nM phorbol 12-myristate 13-acetate, a protein kinase C (PKC) activator, and was reinstated on PKC deactivation with sphingosine. Progressive hypoxia r esulted in a reversible suppression of Ca2+ efflux to 90% of normoxic controls with an apparent Michaelis constant for oxygen of 145 mu M. P KC activation was critical in this suppression, as anaerobic administr ation of sphingosine caused a Ca2+ influx and cell rupture. Hypoxia wa s also associated with an altered pattern of adenosine-mediated contro l over Ca2+ efflux. Adenosine (100 mu M) elevated Ca2+ efflux twofold in normoxia, but neither adenosine nor the A(1)-purinoreceptor antagon ist 8-phenyltheophylline altered the observed anaerobic suppression. A erobic administration of 2-10 mM KCN failed to reproduce the anaerobic suppression; however, in conjunction with 10 mM iodoacetate, complete metabolic blockade caused a Ca2+ influx and cell rupture. These obser vations suggest modulatory control by oxygen over transmembrane Ca2+ e fflux involving second-messenger systems in the hypoxic transition.