ROLE OF CALCIUM-ION IN PLATELET SEROTONIN UPTAKE REGULATION

It is generally accepted that intracellular Ca2+ is a key substance in the intracellular signal transducing mechanism of platelets. We inves tigated the possibility that extracellular and/or intracellular Ca2+ m ight regulate the transport activity of serotonin (5-HT) into platelet s. We found that extracellular Ca2+ chelation with EGTA caused inhibit ion of 5HT uptake activity, which was recovered by extracellulary appl ied excess Ca2+. Intracellular Ca2+ chelation with acetoxymethyl bis(O -aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA-AM) did not, howeve r, have any inhibitory effect on 5HT uptake activity in the presence o f extracellular Ca2+. In the absence of extracellular Ca2+, BAPTA-AM s ignificantly inhibited 5-HT uptake. The restorative effect of Ca2+ on 5-HT transport into EGTA-treated platelets was mimicked by Ba2+, but n ot by Sr2+. It was antagonised by inorganic Ca2+ channel antagonist in cluding Ni2+, La3+ and Gd3+, but not by organic Ca2+ channel blockers including verapamil, nifedipine, diltiazem, omega-conotoxin GVIA and o mega-agatoxin IVA. Furthermore, 3,4,5-trimethoxybenzoic acid 8-(diethy lamino)octyl ester hydrochloride (TMB-8), an intracellular Ca2+ antago nist, was found to inhibit the restorative effect of Ca2+. These resul ts have led to the suggestion that depletion of intracellular Ca2+ poo l(s) by EGTA might result in a reduction of 5-HT uptake activity. Thus , the intracellular Ca2+ pool(s) susceptible to EGTA might have a regu latory role in maintaining 5-HT transport into blood platelets.