The intracellular pH of the human platelet

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.

Stereoselective accumulation of the beta-receptor blocking drug atenolol by human platelets

We have studied the stereochemistry of accumulation of the hydrophilic beta-adrenoceptor antagonist rac-atenolol by human platelets in vitro. The accumulation was slow, not reaching equilibrium until 90 min at 37 degrees C. The uptake was temperature dependent with the accumulation at 37 degrees C being 3-4 times greater than at 4 degrees C. The accumulation was also stereoselective at 37 degrees C, favoring the active (-)-enantiomer over the (+)-enantiomer by 2.3-fold. Reserpine, but not desipramine, inhibited the platelet accumulation of rac-atenolol enantiospecifically. This uptake profile is different from the platelet uptake of lipophilic beta-blockers, which is characterized by nonspecific membrane binding, but similar to the carrier-mediated accumulation of the neurotransmitter norepinephrine by storage granules within the platelet.

Thrombin-induced cytosolic alkalinization in human platelets occurs without an apparent involvement of HCO3-/Cl- exchange

We have estimated the changes in cytosolic pH (pHi) that occur when human platelets are stimulated by thrombin. Changes in pHi were estimated (i) from the H+ efflux across the plasma membrane using an extracellular pH electrode and (ii) using an intracellular pH-sensitive fluorescent dye (BCECF). Stimulation of platelets with thrombin (0.5 unit/ml) resulted in an H+ efflux that averaged 7.7 +/- 1.6 mumol/10(11) platelets (means +/- SD) leading to an increase in pHi, from 7.05 +/- 0.04 to 7.45 +/- 0.05. Both H+ efflux and pHi changes were unaffected by 0.1 mM 4,4-diisothiocyanostilbene-2,2 disulphonate (DIDS), 0.1 mM 4'-acetamido 4'-isothiostilbene-2,2'-disulphonic acid (SITS), or 0.5 mM bumetanide, suggesting no involvement of anion transport systems, e.g. an HCO3-/Cl- exchange. Removal of HCO3- or Cl- from the suspending buffer had no effect on the extent of the rise in pHi. After blockade of Na+/H+ exchange by 100 microM ethylisopropylamiloride (EIPA), thrombin induced a decrease in pHi the rate of which averaged 0.39 unit/min in HCO3(-)-containing medium, and 0.57 unit/min in HCO3(-)-free medium. The cytosolic buffer capacity for H+ was determined by the nigericin/NH4Cl technique in BCECF-loaded platelets and averaged 25.3 mmol/(1xpH) in buffer containing 8 mM HCO3-, but only 17.2 mmol/(1xpH) in HCO3(-)-free buffer. The total amount of H+ transferred by Na+/H+ exchange can be estimated from our measurements at 10 mmol/l platelet cytosol in the absence of HCO3- and to 14 mmol/l platelet cytosol in the presence of HCO3-, and is in good agreement with the estimated amount of Na+ uptake by ADP-stimulated platelets.(ABSTRACT TRUNCATED AT 250 WORDS)

Lowering pH in blood platelets dissociates myosin phosphorylation from shape change and myosin association with the cytoskeleton

Platelet shape change induced by ADP is relatively independent of external pH over the range 6-7. If the chloride ion in the buffer is replaced by weak acids, however, shape change is rapidly and reversibly inhibited as a function of lowered pH (92% at pH 6.0). This inhibition is correlated with lowered internal pH caused by the weak acids, as measured by the 5,5-dimethyloxazolidine 2,4-dione technique. Shape change was 50% inhibited at internal pH 6.4 when 50 mM NaCl was replaced by propionate (PR). When platelets were stimulated with ADP 10-20 s after addition of PR to a final pH of 6 (PR6), both myosin light chain (MLC) phosphorylation and myosin and actin association with the cytoskeleton were reduced in correlation with the inhibition of shape change. But when ADP was added 30 s after PR6, the MLC phosphorylation was essentially the same in PR or in chloride, although shape change and myosin and actin association with the cytoskeleton remained inhibited. This was shown to be due mainly to endogenous phosphorylation of MLC. On return to neutral pH, platelets in PR immediately changed shape and myosin and actin became associated with the cytoskeleton. Two-dimensional tryptic peptides of MLC showed two major spots after PR6 treatment, indicating that both the MLC kinase site and the protein kinase C sites were phosphorylated. The results show that increased internal pH is not required for shape change, although it may affect the rate. In PR6, as after phorbol esters, MLC phosphorylation can be uncoupled from shape change. The association of myosin and actin with the cytoskeleton is closely correlated with shape change, suggesting that shape change requires the active interaction of these contractile proteins.

Regulation of intracytoplasmic pH and "apparent" intracellular pH in alveolar macrophages

An approach for determining the permeability of cell and subcell membranes to H+/HCO3- has been applied to alveolar macrophages. The approach uses analysis of substrate ratios which are compartment specific and involved in pH-dependent reactions. By determining the changes in substrate pair ratios produced by perturbations of CO2, the effects of intracellular acidosis and alkalosis can be determined. By measuring substrate pair ratios at a constant PCO2 and variable H+/HCO3- it is possible to determine the permeability of the macrophage plasma membrane to these ions qualitatively. Measurements of intracellular lactate/pyruvate ratios showed that the alveolar macrophage plasma membrane is permeable to H+/HCO3-. Previous studies have shown that alveolar macrophages are freely permeable to Cl-. pHi was determined from measurements of pHe and Cli-/Cle- following changes of external PCO2 at a constant HCO3- concentration and following changes in pHe produced by changes of external HCO3- concentration at a constant PCO2. The data show that the relationship between pHi and Phe is: pHi = 0.973 pHe - 0.027 (r = 0.99, P less than 0.001). Thus at pHe = 7.40, macrophage intracellular pHi = 7.17. Alveolar macrophages appear to be permeable to H+/HCO3- and thus resemble erythrocytes, platelets, and lymphocytes and not brain and muscle cells.

The sensitivity of a malignant cell line to hyperthermia (42 degrees C) at low intracellular pH

The postulate that low intracellular pH acts as a preconditioner for the destructuve effects of hyperthermia (42 degrees C) was examined, using a heat-sensitive line of malignant cells derived from rat mammary gland (SDB). Intracellular pH (pHi) was measured indirectly, from the distribution of the weak, non-metabolizable organic acid 5,5-dimethyl-2,4-oxazolidinedione (DMO) between intra- and extra-cellular water. Respiration, aerobic and anaerobic and anaerobic glycolysis of the cells were studied at normal pHi (pH 7-0-7-4) or at low pHi (pH 6-2-6-6) and at 38 degrees C or 42 degrees C over 6 h in Warburg manometers; the ability of the cells to replicate in culture was examined after 3 h or 6 h incubation in the flasks. The relationship between pHi and extracellular pH (pHe) depended upon the buffer system used and the exact pH in question; no assumption regarding pHi based only on pHe measurement could be made. At 38 degrees C and low pHi, the Pasteur effect became negative due to a relatively greater inhibition of anaerobic than aerobic glycolysis. Respiration was unaffected and cell replicative ability unimpaired. At 42 degrees C and normal pHi, respiration was totally inhibited after 4 h and the Pasteur effect was decreased, in this case due to a compensatory increase in aerobic glycolysis without alteration in anaerobic CO2 production. Low pHi in the presence of hyperthermia enabled cell respiration to continue at a reduced level with no further change in glycolysis. There was delayed cell replication after 3 h at 42 degrees C and inability to multiply following 6 h hyperthermia: low pHi did not influence these results. It is concluded that with these cancer cells, pHi values maintained in the region of 1-0 pH unit below normal for 6 h had no deleterious effect on the cells. No sensitizing effect of the low pHi for the destructive effect of hyperthermia on the cells was observed.

Intracellular pH: measurement, control, and metabolic interrelationships

Recent work in the field of cell pH has been characterized by many developments in techniques of measurement, by increasing knowledge of the mechanism of control of cell pH, and by progress in the establishment of relationships between cell pHi and certain areas of intermediary metabolism. Though the weight of evidence is much in favor of control cell pH by active transport of H+, the situation remains somewhat unsatisfactory due to lack of a completely adequate explanation of the work of Carter's group. The heterogeneity of cell pH raises problems in the intrepretation of hydrogen ion equilibria across cell membranes and serious difficulties in correlating changes with alterations in metabolism. To put this into perspective, however, the later difficulties are no greater than many experienced with cell constituents other than hydrogen ions.90 As in other fields, knowledge must advance by making the best of the currently available methods until such time as better techniques become available.

Uptake, metabolism and release of (3H)-adrenaline by human platelets

1. Measurements were made of the uptake, metabolism and release of [(3)H]-adrenaline by human platelets in citrated plasma or in an artificial medium.2. Radioactive adrenaline was not taken up at 0-2 degrees C. At 37 degrees C there was a slow uptake which continued for at least 5 hours.3. About half of the radioactivity in the platelets was intact adrenaline. The other half was an acidic metabolite from which adrenaline was released by acid hydrolysis.4. The immediate uptake of adrenaline was proportional to its concentration in the plasma up to at least 1 x 10(-5)M. Uptake measured after 1 h also increased linearly with concentration up to about 1 x 10(-4)M but less with higher concentrations. The highest concentration ratio was about 12.5. The concentration of metabolite in the platelets increased with the concentration of added adrenaline only up to about 2 x 10(-4)M.6. The immediate uptake of adrenaline was partially inhibited by phentolamine and dihydroergotamine. Measurement of uptake both immediately and after 1 h showed that the inhibition produced was not increased beyond about 50% by these drugs or by (+/-)-propranolol, chlorpromazine or amitriptyline up to 1 x 10(-4)M.7. Formation of the metabolite was inhibited by pyrogallol, 8-hydroxyquinoline, or tropolone. This inhibition was associated with a corresponding increase in the adrenaline accumulated intact. Formation of the metabolite was not inhibited by monoamine oxidase inhibitors.8. Reserpine caused a small decrease in the uptake of adrenaline radioactivity in 1 h and a great increase in the proportion recovered as metabolite.9. Thrombin caused the release from platelets of intact adrenaline but not of the metabolite.10. Platelets of albino patients with spontaneous haemorrhages accumulated adrenaline radioactivity at the normal rate but this radioactivity was wholly accounted for by metabolite and not released by thrombin.11. After taking up adrenaline, platelets resuspended in artificial medium at 37 degrees C slowly released both adrenaline and its metabolite. At the same time, the intracellular adrenaline was slowly metabolized.12. The Result suggest that human platelets take up adrenaline by two processes, one of which is inhibited by both alpha- and beta-adrenoceptor blocking agents ad well as by phenothiazines; and that in the platelets adrenaline is partly stored in organelles from which, like 5-hydroxytryptamine, it can be specifically released.