O2 availability modulates transmembrane Ca2+ flux via second-messenger pathways in anoxia-tolerant hepatocytes.
Transmembrane Ca(2+)-flux was studied from single isolated turtle hepatocytes by using a noninvasive Ca(2+)-selective self-referencing microelectrode. Cells in Ca(2+)-reduced culture medium demonstrated a vanadate- and lanthanum-inhibitable Ca(2+)-efflux of 4 x 10(-17) mol Ca2+. microns-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 resulted in a reversible suppression of Ca2+ efflux to 90% of normoxic controls with an apparent Michaelis constant for oxygen of 145 microM. PKC activation was critical in this suppression, as anaerobic administration of sphingosine caused a Ca2+ influx and cell rupture. Hypoxia was also associated with an altered pattern of adenosine-mediated control over Ca2+ efflux. Adenosine (100 microM) elevated Ca2+ efflux twofold in normoxia, but neither adenosine nor the A1-purinoreceptor antagonist 8-phenyltheophylline altered the observed anaerobic suppression. Aerobic 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 observations suggest modulatory control by oxygen over transmembrane Ca2+ efflux involving second-messenger systems in the hypoxic transition.
Land
S C
SC
Biocurrents Research Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA. sland@mbl.edu
Sanger
R H
RH
Smith
P J
PJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects*; D002478:Cells, Cultured; D005260:Female; D008099:Liver / Q000187:drug effects*; D010100:Oxygen / Q000494:pharmacology*; D015290:Second Messenger Systems / Q000187:drug effects*; D014426:Turtles
D010100:Oxygen; D002118:Calcium