The Na+/H+ exchanger role in the intracellular pH regulation of human platelets

Thrombin stimulation of Cl-/HCO3- exchange in human platelets

The presence of one acidifying Cl-/HCO3- exchange mechanism in human platelets has not been previously reported. This paper demonstrates that this mechanism does function and that it increases its activity after stimulation with thrombin. On resuspension of BCECF-loaded platelets in a chloride-free medium (gluconate replaced) that contains bicarbonate, cytosolic pH (pHi) increased and stabilized after 10 min at an alkaline value. After addition of 50 mM NaCl, pHi fell rapidly reaching steady state in the succeeding 5 min. The stilbene derivative 4-acetamido-4'-isothiocyanato stilbene-2,2' disulfonic acid (SITS) inhibited both, the alkalization in chloride-poor solution and the recovery from the alkaline load after chloride enrichment. The decline in pHi was observed whether chloride was delivered to the solution in the form of LiCl or NaCl, or when the later was applied after blockage of the Na+/H+ exchanger. The recovery in chloride-containing solution was in contrast to the effect of a similar change in osmolarity by addition of 50 mM sodium gluconate that did not produced a significant variation of pHi. Posterior addition of NaCl after 5 min in high gluconate reproduced the pHi fall of the control experiment. Alkali loads produced by 25 mM trimethylamine hydrochloride (TMA) were also counteracted by HCO(3-)-equivalent efflux via Cl-/HCO3- exchange. One of the major observations of the present study is that HCO3- equivalent efflux was twice as high when the platelets were previously stimulated with 0.1 IU of thrombin, but thrombin did not produce significant changes of the pHi recovery rate in a bicarbonate-free solution. The increase of the decline in pHi elicited by preexposure to thrombin was still observed in the presence of an inhibitor of the Na+/H+ exchange or in sodium-free solutions. It is concluded that a Na-independent Cl-/HCO3- exchange mechanism mediates the recovery of pHi from alkalosis in platelets and that thrombin activates this exchanger by a direct regulatory pathway.

Sodium-hydrogen exchange and platelet function

On stimulation of platelets with agonists, for example, thrombin, a rapid rise in intracellular pH is observed. This alkalinization is mediated by an increase in transport activity of the Na(+)/H(+) exchanger isoform NHE1. In addition to this Na(+)/H(+) exchange mechanism, platelets express bicarbonate/chloride exchangers, which also contribute to pH(i) homeostasis. The main functions of NHE1 in platelets include pH(i) control, volume regulation, and participation in cell signaling. The isoform NHE1 is highly sensitive toward inhibition by EIPA, Hoe694, and Hoe642. The regulation of NHE1 activity is complex and is not completely understood. It includes the MAP kinase cascade, the Ca/calmodulin system, several heterotrimeric G proteins (Galpha12, Galpha13, Galphaq, and Galphai), small G proteins (ras, cdc42, rhoA), and downstream kinases (e.g., p160ROCK). Volume challenges stimulate tyrosine phosphorylation of cytoplasmic proteins, which ultimately activate NHE1. Thrombin, thromboxane, platelet-activating factor, angiotensin II, endothelin, phorbol ester, and Ca(2+) ionophors stimulate NHE1 activity in platelets. Blockade of platelet NHE1 can inhibit platelet activation. With the development of highly specific NHE1 inhibitors, detailed investigation of the relationships between NHE1 activity and platelet activation now becomes feasible.

Endothelin influences pHi of human platelets through protein kinase C mediated pathways

Stimulation of human platelets with endothelin raises cytosolic pH (pHi). In order to determine whether this effect is mediated via protein kinase C and Na(+)-H+ linked pathways, the effects of staurosporine and calphostin C (protein kinase C inhibitors) and 5-(N,N-hexamethylene) amiloride (Na(+)-H+ exchange blocker) on endothelin-induced pHi responses in human platelets were assessed. In addition, platelet endothelin receptor subtypes were determined pharmacologically using the selective ETA receptor antagonist BQ-123 and the ETB receptor agonist sarafotoxin S6c. pHi was measured spectrofluorometrically using the fluorescent probe BCECF-AM in platelets obtained from 15 healthy subjects. Endothelin-1 at a fixed concentration of 10(-9) M significantly increased pHi from 7.11 +/- 0.01 ([H+]i = 77 +/- 0.9 nM) to 7.19 +/- 0.04 ([H+]i = 64 +/- 0.9 nM) (p < 0.01). The pHi-stimulating effect of endothelin-1 was inhibited by 10(-7) M staurosporine, calphostin C and 5-(N,N-hexamethylene) amiloride. BQ-123 (10(-7) M) abolished the pHi responses to endothelin-1, whereas sarafotoxin S6c had no effect on platelet pHi. These data suggest that in vitro effects of endothelin-1 on platelet pHi are receptor-mediated via a pathway involving protein kinase C. Platelet endothelin receptors appear to be of the ETA subtype.