Inositol phosphates modulate human red blood cell Ca²⁺—adenosine triphosphatase activity in vitro by a guanine nucleotide regulatory protein☆

Abstract 

d-myo-inositol 1,4,5-trisphosphate [lns(1,4,5)P₃) 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 lns(1,4,5)P₃ effect. Guanosine 5′-O-(3-thiotrisphosphate) (GTP_γyS) 10⁻⁴ mol/L also inhibited RBC Ca²⁺-ATPase activity. Pertussis toxin 200 ng/mL blocked the effects of both lns(1,4,5)P₃ and GTP_γS 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 lns(1,4,5)P₃ 10⁻⁷ mol/L and GTP_γS 10⁻⁶ mol/L were added to membranes concurrently, their inhibitory actions on the enzyme were additive. At greater concentrations of lns(1,4,5)P₃ (10⁻⁶ to 10⁻⁵ mol/L) and GTP_γS (10⁻⁴ mol/L), the inositol phosphate reversed the inhibitory effect of GTP_γS. These observations indicate that the novel effect of lns(1,4,5)P₃ on the activity of a plasma membrane Ca²⁺-ATPase depends at least in part on the action of a pertussis toxin-susceptible G protein.

No full text is available. To read the body of this article, please view the PDF online.