CA2+ INFLUX IN PLATELETS - ACTIVATION BY THROMBIN AND BY THE DEPLETION OF THE STORES - EFFECT OF CYCLIC-NUCLEOTIDES

In aspirin-treated platelets the thrombin-induced increase of cytosolic Ca2+ ([Ca2+](i)) associated with the release from the intracellular stores is followed by a decrease to the baseline which is largely dependent on the re-uptake into the stores. This:rs shown by the further increase of [Ca2+](i) upon inhibition of the endomembrane Ca2+-ATPase with thapsigargin. The re-uptake of Ca2+ into the stores is accelerated by sodium nitroprusside (SNP) or prostacyclin (PGI(2)). In all cases, after store depletion with thapsigargin the influx of external Ca2+ is maximal. After a thrombin-induced cycle of Ca2+-release re-uptake the stores are partly full: in these conditions the addition of external Ca2+ elicits a significant increment of [Ca2+](i) and a further filling of the stores. Both are strongly reduced if Ca2+ addition is preceded by SNP or PGI(2). Similar results are obtained also if (by supplementing and then cheleting Ca2+) the stores are as full as in native platelets at the moment of adding Ca2+. The thrombin-activated Ca2+ influx is reversed by hirudin. A PGI(2)- and SNP-sensitive Mn2+ influx is observed if Mn2+ is added in place of Ca2+. It is concluded that thrombin activates a cyclic nucleotide-sensitive Ca2+ (and Mn2+) influx pathway dependent on the occupancy of the thrombin receptor and independent of the filling state of the stores. In the absence of thrombin, thapsigargin releases Ca2+ relatively rapidly from a fraction of the stores; the remaining deposits are discharged much more slowly. This may indicate that platelets contain two distinct classes of agonist-sensitive stores. The addition of external Ca2+ (or Mn2+) at short or long incubation times with thapsigargin monitors the influx of Ca2+ activated by the depletion of one or both types of stores. The depletion of each type of store activates Ca2+ (Mn2+) influx. This type of cation influx is not inhibited by the cyclic nucleotides.