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 investigated the possibility that extracellular and/or intracellular Ca2+ might regulate the transport activity of serotonin (5-HT) into platelets. We found that extracellular Ca2+ chelation with EGTA caused inhibition of 5HT uptake activity, which was recovered by extracellulary applied excess Ca2+. Intracellular Ca2+ chelation with acetoxymethyl bis(O-aminophenoxy)ethane-N,N,N'-tetraacetate (BAPTA-AM) did not, however, have any inhibitory effect on 5HT uptake activity in the presence of extracellular Ca2+. In the absence of extracellular Ca2+, BAPTA-AM significantly inhibited 5-HT uptake. The restorative effect of Ca2+ on 5-HT transport into EGTA-treated platelets was mimicked by Ba2+, but not by Sr2+. It was antagonised by inorganic Ca2+ channel antagonist including Ni2+, La3+ and Gd3+, but not by organic Ca2+ channel blockers including verapamil, nifedipine, diltiazem, omega-conotoxin GVIA and omega-agatoxin IVA. Furthermore, 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester hydrochloride (TMB-8), an intracellular Ca2+ antagonist, was found to inhibit the restorative effect of Ca2+. These results have led to the suggestion that depletion of intracellular Ca2+ pool(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 regulatory role in maintaining 5-HT transport into blood platelets.
Nishio
H
H
Department of Pharmacology, Hiroshima University, School of Medicine, Japan.
Nezasa
K
K
Nakata
Y
Y
D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002132:Calcium Radioisotopes; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D066298:In Vitro Techniques; D011817:Rabbits; D012701:Serotonin / Q000097:blood*; D015398:Signal Transduction / Q000502:physiology; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D002121:Calcium Channel Blockers; D002132:Calcium Radioisotopes; D002614:Chelating Agents; D013729:Terpenes; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D012701:Serotonin; D004533:Egtazic Acid; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D002118:Calcium