Characterisation of the serotonin efflux induced by cytosolic Ca2+ and Na+ concentration increase in human platelets.
OBJECTIVE
The present study aimed at elucidating the mechanism(s) of serotonin (5-HT) efflux induced by thapsigargin from human platelets in the absence of extra-cellular Ca2+.


METHODS
Efflux of pre-loaded radiolabeled serotonin was generally determined by filtration techniques. Cytosolic concentrations of Ca2+, Na+ and H+ were measured with appropriate fluorescent probes.


RESULTS
5-HT efflux from control or reserpine-treated platelets--where reserpine prevents 5-HT transport into the dense granules--was proportional to thapsigargin evoked cytosolic [Ca2+]c increase. Accordingly factors as prostacyclin, aspirin and calyculin which reduced [Ca2+]c-increase also inhibited the 5-HT efflux. Thapsigargin, which also caused a remarkable increase in cytosolic [Na+]c, promoted less 5-HT release, in parallel to lower [Na+]c and [Ca2+]c increase, when added to platelet suspensions containing low [Na+]. The Na+/H+ exchanger monensin increased the [Na+]c and induced 5-HT efflux without affecting the Ca2+ level. The 5-HT efflux induced by both [Ca2+] or [Na+]c increase did not depend on pH or membrane potential changes, whereas it decreased in the absence of extra-cellular K+, and increased in the absence of Cl- or Na+.


CONCLUSIONS
Increases in [Ca2+]c and [Na+]c independently induce serotonin efflux through the outward directed plasma membrane serotonin transporter SERT. This event might be physiologically important at the level of capillaries or narrowed arteries where platelets are subjected to high shear stress which causes [Ca2+]c increase followed by 5-HT release which might exert vasodilatation.
Turetta
Loris
L
Department of Biological Chemistry and Institute of Neurosciences of Italian Research National Centre (CNR), University of Padova.
Donella-Deana
Arianna
A
Folda
Allessandra
A
Bulato
Cristiana
C
Deana
Renzo
R
D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D003600:Cytosol / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D008562:Membrane Glycoproteins / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D026901:Membrane Transport Proteins / Q000502:physiology; D008985:Monensin / Q000494:pharmacology; D009419:Nerve Tissue Proteins / Q000502:physiology; D012110:Reserpine / Q000494:pharmacology; D012701:Serotonin / Q000378:metabolism*; D050486:Serotonin Plasma Membrane Transport Proteins; D012964:Sodium / Q000032:analysis / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; C493230:SLC6A4 protein, human; D050486:Serotonin Plasma Membrane Transport Proteins; D012701:Serotonin; D019284:Thapsigargin; D012110:Reserpine; D008985:Monensin; D012964:Sodium; D002118:Calcium