Effect of complement proteins C5b-9 on blood platelets. Evidence for reversible depolarization of membrane potential.
The carbocyanine dye 3,3'-dipropylthiodicarbocyanine iodide has been used to investigate changes in membrane potential (Em) which occur upon binding of complement proteins C5b-9 to the plasma membrane of blood platelets. Gel-filtered platelets exposed to C5b6 and C7 in serum-free medium show no change in Em from that of controls, as indicated by either 3,3,'-dipropylthiodicarbocyanine iodide fluorescence or by the distribution of [14C]tetraphenylphosphonium bromide. Addition of complement proteins C8 and C9 to the C5b67 platelets results in partial depolarization of Em, which spontaneously repolarizes to basal levels within 15-20 min at 37 degrees C. Under these conditions, C5b-9-treated platelets show no increase in lysis over complement-free controls. Isotonic replacement of external sodium by either potassium or choline alters both the rate and extent of membrane depolarization and inhibits the platelets' capacity to repolarize after C5b-9 assembly. Repolarization of Em to basal levels is also completely blocked by addition of ouabain, confirming that this recovery is mediated by the plasma membrane Na+/K+ pump. These results demonstrate that membrane binding of the C5b-9 proteins can induce a transient change in Em when bound to the plasma membrane at a sublytic concentration, providing a mechanism for target cell activation by these potentially cytolytic proteins.
D000328:Adult; D001792:Blood Platelets / Q000187:drug effects*; D002794:Choline / Q000378:metabolism; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins / Q000494:pharmacology*; D006801:Humans; D008433:Mathematics; D008564:Membrane Potentials / Q000187:drug effects; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism
D015938:Complement Membrane Attack Complex; D010042:Ouabain; D003165:Complement System Proteins; D012964:Sodium; D002794:Choline; D011188:Potassium