Platelet glycoproteins IIb and IIIa as a calcium channel in liposomes
ME Rybak, LA Renzulli, MJ Bruns and DP Cahaly
University of Massachusetts Medical Center, Worcester 01655.
Human platelet membrane glycoproteins IIb and IIIa (GPIIb and IIIa) were
incorporated into phospholipid vesicles by the reverse-phase technique to
assess the ability of GPIIb and IIIa to function as a Ca2+ channel.
Movement of Ca2+ across the lipid bilayer was quantitated by injection of
proteoliposomes with encapsulated Fura-2 into Ca2+ buffers and measurement
of Fura-2 fluorescence as an indicator of Ca2+ influx. Reciprocally, to
assess the function of proteins in an inside-out orientation, Ca2+-loaded
vesicles were injected into Ca2+-free buffer and Ca2+ efflux monitored by a
calcium electrode. Incorporation of the IIb-IIIa complex produced
significant facilitation of Ca2+ movement across the lipid bilayer. No net
transmembrane Ca2+ movement was seen with dissociated IIb and IIIa.
Movement of Ca2+ was proportional to the transmembrane Ca2+ gradient. Ca2+
movement into the vesicles was inversely proportional to extravesicular
NaCl from 25 to 150 mmol/L, analogous to several studies in the intact
platelet. Adenosine triphosphate had no effect on Ca2+ movement into or out
of the vesicles. Specific inhibition of a Ca2+ shift into the vesicles was
seen with M148, a monoclonal antibody to IIb/IIIa, while no inhibition was
observed with a panel of other anti-IIb/IIIa monoclonal antibodies. This
suggests that a specific site on the complex or orientation of the complex
is essential for calcium channel function. These data demonstrate that the
GPIIb/IIIa complex can serve as a passive Ca2+ channel across a
phospholipid bilayer and has the potential to play a role in Ca2+ flux
across the platelet plasma membrane.
Volume 72,
Issue 2,
pp. 714-720,
08/01/1988
Copyright © 1988 by The American Society of Hematology
