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Eukaryotic cells control their intracellular pH using ion-transporting systems that are situated in the plasma membrane. This paper describes the different mechanisms that are involved and how their activity is regulated.The distribution of H + ions across the plasma membrane of most cells is such that the internal pH @Hi) is much higher than that predicted if H + were passively distributed [l]. For an average membrane potential of -60 mV and for an external pH of 7.4, a pHi value of 6.4 would be expected if H + were in electrochemical equilibrium across the membrane. Therefore, all cells have mechanisms for H + extrusion that maintain pHi at a value which is well above equilibrium and is compatible with the necessities of cytoplasmic reactions. Numerous studies performed during the last decade have demonstrated that changes in pHi occur during metabolic and developmental transitions in a large variety of cells. Acidic cytoplasmic conditions are usually associated with a quiescent or dormant cellular state while an increase in pHi often accompanies cellular activation [I, 21. These observations lead to a series of questions: (a) how is pHi regulated and how are pHi variations achieved? (b) How important is the steadystate pHi value for cell function?This paper reviews the most recent literature and focusses on two important aspects of internal pH regulation. The first aspect concerns the description of biochemically identifiable membrane ion-transport mechanisms that control pHi. One well-known mechanism is the Na+/Ht antiporter but other mechanisms also exist and they certainly play a role which has been relatively underestimated until now. The second aspect that will be discussed concerns the regulation of pHicontrolling systems.

The human P2Y1 receptor heterologously expressed in Jurkat cells behaves as a specific adenosine 5′-diphosphate (ADP) receptor at which purified adenosine triphosphate (ATP) is an ineffective agonist, but competitively antagonizes the action of ADP. This receptor is thus a good candidate to be the elusive platelet P2T receptor for ADP. In the present work, we examined the effects on ADP-induced platelet responses of two selective and competitive P2Y1 antagonists, adenosine-2′-phosphate-5′-phosphate (A2P5P) and adenosine-3′-phosphate-5′-phosphate (A3P5P). Results were compared with those for the native P2Y1 receptor expressed on the B10 clone of rat brain capillary endothelial cells (BCEC) and for the cloned human P2Y1 receptor expressed on Jurkat cells. A2P5P and A3P5P inhibited ADP-induced platelet shape change and aggregation (pA2 = 5) and competitively antagonized calcium movements in response to ADP in fura-2–loaded platelets, B10 cells, and P2Y1-Jurkat cells. In contrast, these compounds had no effect on ADP-induced inhibition of adenylyl cyclase in platelets or B10 cells, whereas known antagonists of platelet activation by ADP such as Sp-ATPαS were effective. These identical signaling responses and pharmacologic properties suggest that platelets and BCEC share a common P2Y1 receptor involved in ADP-induced aggregation and vasodilation, respectively. This P2Y1 receptor coupled to the mobilization of intracellular calcium stores was found to be necessary to trigger ADP-induced platelet aggregation. The present results, together with data from the literature, also point to the existence of another as yet unidentified ADP receptor, coupled to adenylyl cyclase and responsible for completion of the aggregation response. Thus, the term, P2T, should no longer be used to designate a specific molecular entity.