Chloride increases adrenergic receptor-mediated platelet and vascular responses.
BACKGROUND
We postulated that increasing intracellular chloride concentration ([Cl-]i) in human platelets would potentiate alpha2 adrenergic receptor (A2AR)-mediated platelet aggregation, and that vascular reactivity would also be increased by raising [Cl-]i in blood vessels. We further hypothesized that ligands binding to the A2AR would increase [Cl-]i by stimulating carbonic anhydrase-dependent chloride/bicarbonate exchange. Because diuretics are potent inhibitors of carbonic anhydrase, we speculated that these agents inhibit platelet aggregation and vascular contractility through inhibition of chloride influx by decreasing carbonic anhydrase activity, and subsequently, chloride/bicarbonate exchange. The aim of this study was to test these hypotheses.


METHODS
Platelet aggregation was measured by determining changes in optical density of platelet-rich plasma. Contractile responses to A2AR agonists were recorded in isolated vascular smooth muscle. The substances [Cl-]i and intracellular pH (pHi) were measured using microfluorometric methods. Carbonic anhydrase activity and chloride/bicarbonate exchange were determined by an in vitro assay based on the Stewart cycle.


RESULTS
Increasing [Cl-]i potentiated platelet aggregation and vascular contractility, and epinephrine raised [Cl-]i by stimulating carbonic anhydrase-dependent chloride/bicarbonate exchange. Furthermore, diuretic-dependent inhibition of carbonic anhydrase activity decreased chloride/bicarbonate exchange.


CONCLUSIONS
Our data support the concept that diuretics inhibit carbonic anhydrase activity and chloride/bicarbonate exchange in platelets and vascular smooth muscle. The ensuing reduction in [Cl-]i that is induced by diuretics in these tissues could play a role in reducing the effect of catecholamines on precipitating thrombotic stroke or myocardial infarction.
Vaitkevicius
Henrikas
H
Wayne State University School of Medicine, Detroit, Michigan, USA.
Turner
Immanuel
I
Spalding
Aaron
A
Lockette
Warren
W
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D001581:Benzothiadiazines; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology; D002256:Carbonic Anhydrases / Q000187:drug effects* / Q000502:physiology; D027963:Chloride-Bicarbonate Antiporters / Q000187:drug effects* / Q000502:physiology; D003000:Clonidine / Q000494:pharmacology*; D004232:Diuretics; D004837:Epinephrine / Q000494:pharmacology; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D010974:Platelet Aggregation / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects* / Q000502:physiology; D049993:Sodium Chloride Symporter Inhibitors / Q000494:pharmacology; D013050:Spectrometry, Fluorescence
D000316:Adrenergic alpha-Agonists; D001581:Benzothiadiazines; D027963:Chloride-Bicarbonate Antiporters; D004232:Diuretics; D011942:Receptors, Adrenergic, alpha; D049993:Sodium Chloride Symporter Inhibitors; D002256:Carbonic Anhydrases; D003000:Clonidine; D004837:Epinephrine