The techniques and uses of intracellular pH measurements

Ketone bodies promote a rapid rise in glutamate efflux from the isolated perfused rat liver without altering the rate of glutamine production

Livers of starved (48 hr) male Wistar rats were perfused in a non recirculating manner with a near physiological mix of ammonium, lactate, ornithine and pyruvate in Krebs buffer. The addition of ketone bodies (3-DL-hydroxybutyrate [B OHB] 2-30 mM or lithium-acetoacetate (15 mM) to the perfusate resulted in a rapid rise in the efflux of glutamate from the liver (five times above basal). This was not seen with control solutions (sodium chloride or lithium chloride). The increased efflux was sustained for the duration of the addition of the ketone bodies (7 min), was rapidly reversible and dose dependant. Glutamine export rates were not altered, suggesting that either the glutamate originated from cells not responsible for glutamine synthesis or that this glutamate was superfulous to the requirement of glutamine synthesis. There was no evidence that the lactate transporter was involved in the entry of lactate into perivenous hepatocytes for glutamine synthesis; lactate presumably entering the hepatocyte by an alternative pathway, probably nonionic diffusion.

Lactate supply as a determinant of the distribution of intracellular pH within the hepatic lobule

When isolated livers from starved rats are perfused with lactate at constant perfusate pH and P(co(2)), there is a marked gradient of cell pH (pH(i)) along the length of the lobular radius, with periportal cells being substantially more alkaline than perivenous cells. In the present studies, the perivenous 21% of the lobular volume was destroyed by retrograde digitonin perfusion, and antegrade perfusion restored. pH(i) was determined by (31)P-NMR. The remaining periportal cells, the site of gluconeogenesis from lactate, had a substantially higher mean pH(i) (7.42) than did the intact liver (7.23). When lactate was removed from the perfusate, mean pH(i) decreased to 7.25. The corresponding concentration of cell bicarbonate fell with a half-time of approximately 5 min. When lactate was re-introduced mean pH(i) rose to 7.34. We conclude that a major contributor to periportal alkalinity under these conditions is proton consumption during gluconeogenesis from lactate ions.

Effects of D-3-hydroxybutyrate and acetoacetate on lactate removal in isolated perfused livers from starved and fed rats

We examined the influence of nutritional state on the role of the hepatic plasma membrane lactate transporter in determining overall hepatic lactate disposal. The effects of infusion of sodium D-3-hydroxybutyrate (DOHB) on lactate uptake were studied in perfused livers from fed or starved rats. In livers from starved rats, DOHB (15 to 20 mmol/L) inhibited lactate removal by approximately 45%. This effect was associated with a decrease in intracellular lactate concentration, with cell pH remaining unchanged. Inhibition was maximal when perfusate lactate was less than 1.6 mmol/L, and was undetectable at concentrations exceeding 2.5 mmol/L. A similar degree of inhibition was observed with infusion of acetoacetate. These observations add to the evidence that the inhibition of lactate removal by DOHB seen in livers from starved animals is mediated through an effect on the hepatocyte lactate transporter. At similar low levels of perfusate lactate, DOHB infusion produced a decrease in output of lactate from livers obtained from fed animals. When such livers were subjected to prolonged preperfusion, lactate removal, rather than output, was observed; in these livers DOHB stimulated lactate removal, an effect directionally opposite to that observed in livers from starved animals. These data confirm that hepatic lactate transport is a limiting factor for lactate utilization in intact livers from starved rats; in contrast, lactate utilization in livers from fed animals is limited at a step subsequent to plasma membrane transport, ie, possibly pyruvate transport into mitochondria.

Gluconeogenesis and the protection of hepatic intracellular pH during diabetic ketoacidosis in rats

Studies were made of the mechanism whereby hepatic gluconeogenesis is increased in diabetic ketoacidosis (DKA) despite evidence in vitro of inhibition of gluconeogenesis by systemic acidosis. In perfused livers taken from normal rats, marked inhibition of lactate uptake and glucose output was achieved by simulation of metabolic acidosis in the perfusate. In perfused livers obtained from animals with DKA, lactate uptake and glucose output were greater than in normal perfused liver at all values of perfusate pH, and it was not possible to demonstrate significant inhibition of gluconeogenesis from lactate by perfusate acidosis. Varying severity of acidosis was induced in rats by (a) HCl infusion, (b) NH4Cl ingestion or (c) experimental DKA. Hepatic intracellular pH (pHi) was measured in vivo by 31P-n.m.r. spectroscopy. Whereas at the severer degrees of systemic acidosis marked falls in hepatic pHi were seen in the HCl- and NH4Cl-treated animals, little fall was seen in rats with DKA. The protection of hepatic pHi in rats with DKA was not due to differences in respiratory compensation compared with the other groups. It is suggested that this protection of hepatic pHi in DKA may be responsible for the failure of acidotic inhibition of gluconeogenesis from lactate. Possible reasons for pHi protection in DKA are considered. There is no difference in hepatic energy status as assessed in vivo by ATP/Pi ratios between control, DKA and NH4Cl-treated rats. DKA rats show a striking decrease in hepatic glycerophosphoethanolamine content.

Phosphorus-31 nuclear-magnetic-resonance study of phosphorylated metabolites compartmentation, intracellular pH and phosphorylation state during normoxia, hypoxia and ethanol perfusion, in the perfused rat liver

A quantitative analysis of the phosphorus-31 NMR spectra of excised perfused rat liver has been carried out at 80.9 MHz using a 30-mm sample cell. The results indicate that in liver from fed rats, all intracellular ATP is detected by NMR. In contrast, only the cytosolic fractions of Pi and ADP can be observed as indicated by careful analysis of spectra obtained from perchloric acid liver extracts and intact liver under valinomycin perfusion. In well-oxygenated perfused liver the ATP concentration is 7.4 mM. Values of 5.3 mM and 0.9 mM are found respectively for Pi and ADP concentrations in the cytosolic compartment. Cytosolic pH value (pHi) is 7.25 +/- 0.05 and free magnesium concentration 0.5 mM. Addition of 70 mM (0.4%) ethanol to the perfusate of a fed rat liver induces 25% and 38% reduction of ATP and Pi levels, respectively. A large amount of sn-glycerol 3-phosphate is synthesized (up to 11 mM) in the cytosol. After ethanol withdrawal, a large overshoot in cytosolic Pi is observed, which is indicative of a net uptake of Pi across the plasma membrane that occurred during ethanol oxidation. No significant pH variation is observed during ethanol infusion. In perfused liver of rats subjected to 48-h fasts, the concentrations of cytosolic phosphorylated metabolites are 5.3 mM, 0.8 mM and 11.5 mM for ATP, ADP and Pi, respectively. The perfusion of the liver with 70 mM ethanol does not change the adenine nucleotide levels, while the Pi content is decreased by 10%. During a 4-min hypoxia, induced by reducing the perfusion flow rate from 12 ml to 3 ml min-1 (100 g body weight)-1, ATP concentration decreases to 5.8 mM in the fed rat liver. Cytosolic Pi and ADP increase to 8.7 mM and 1.6 mM, respectively. The cytosolic pH evolves to more acidic values and reaches 7.02 +/- 0.05 at the end of the 4-min hypoxic period.

Inhibition of lactate removal by ketone bodies in rat liver. Evidence for a quantitatively important role of the plasma membrane lactate transporter in lactate metabolism

We studied the effect of DL-3-hydroxybutyrate and acetoacetate on lactate transport into isolated hepatocytes and on lactate removal in the isolated perfused rat liver. Ketone bodies inhibited lactate transport into isolated hepatocytes (maximum, 35% at concentrations of 10-20 mM). Lactate removal and glucose production by perfused livers were examined before and after the introduction of a constant infusion of hydroxybutyrate, acetoacetate, or appropriate control into the portal venous limb. Lactate removal was significantly inhibited within 10 s of the appearance of increasing concentrations of ketone bodies in the effluent. Corresponding decreases in glucose production were observed. The dependence of inhibition on D-3-hydroxybutyrate concentration was documented in isolated perfused livers (maximum inhibition of lactate removal, 58% at 14 mM). This phenomenon could be a factor in the development of lactic acidosis accompanying ketoacidosis, and indicates that plasma membrane lactate transport may determine the rate of hepatic lactate removal.

Metabolic inter-relationships of intracellular pH measured by double-barrelled micro-electrodes in perfused rat liver

1. Intracellular pH (pHi) was measured under differing conditions in the functionally isolated, perfused rat liver using double-barrelled recessed-tip micro-electrodes. 2. Under normal acid-base conditions mean hepatic pHi was 7.18. 3. Over the range studied, hepatic pHi was negatively and linearly related to the logarithm of perfusate PCO2. 4. Measurement of pHi together with measurement of pHe and intracellular and extracellular lactate concentrations suggested a non-equilibrium distribution of lactate and lactic acid across the hepatic cell membrane requiring an active transport process for entry of lactate to the cell. 5. Upon addition of D-fructose (10 mM) to the perfusate there was a rapid fall in pHi of between 0.2 and 0.54 units, which was substantially greater than observed in earlier similar experiments in which 31P-nuclear magnetic resonance was used to measure pHi. Reasons for this discrepancy are considered.