INOSITOL PHOSPHATES MODULATE HUMAN RED-BLOOD-CELL CA2+-ADENOSINE TRIPHOSPHATASE-ACTIVITY IN-VITRO BY A GUANINE-NUCLEOTIDE REGULATORY PROTEIN

D-myo-Inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3] inhibits human red blood cell (RBC) Ca2+-stimulable, Mg2+-dependent adenosine triphosphatase (Ca2+-ATPase) activity in vitro. Because we have previously shown that adrenergic receptors exist on the human mature RBC membrane and can modulate Ca2+-ATPase activity. we examined the possibility that a guanine nucleotide regulatory protein (G protein) mediated the Ins(1,4,5)P-3 effect. Guanosine 5'-O-(3-thiotrisphosphate) (GTP gamma S) 10(-4) mol/L also inhibited RBC Ca2+-ATPase activity. Pertussis toxin 200 ng/mL blocked the effects of both Ins(1,4,5)P-3 and GTP gamma S on Ca2+-ATPase activity. In separate studies, pertussis toxin-catalyzed adenosine diphosphate (ADP) ribosylation was shown to occur in RBC membranes under conditions in which measurements of Ca2+-ATPase activity were performed. When Ins(1,4,5)P-3 10(-7) mol/L and GTP gamma S 10(-6) mol/L were added to membranes concurrently, their inhibitory actions on the enzyme were additive. At greater concentrations of Ins(1,4,5)P-3 (10(-6) to 10(-5) mol/L) and GTP gamma S (10(-4) mol/L), the inositol phosphate reversed the inhibitory effect of GTP gamma S. These observations indicate that the novel effect of Ins(1,4,5)P-3 on the activity of a plasma membrane Ca2+-ATPase depends at least in part on the action of a pertussis toxin-susceptible G protein. Copyright (C) 1995 by W.B. Saunders Company