Relationship between plasma pH and pancreatic HCO3- secretion at different intravenous secretin infusion rates

The relationship between pancreatic HCO₃^- secretion and plasma pH during acute systemic acid-base changes was investigated in 6 anesthetized, artificially ventilated pigs (20–25 kg) at 2 different, i.v. secretin infusion rates. At 0.45 C.U./kg b.wt. h^-1 secretin infusion and plasma pH 7.40±0.01 pancreatic HCO₃^- secretion averaged 61±12 μmol/min. Stepwise lowering of plasma pH through i.v. infusion of HCI and CO₂ administration to inspired air proportionately reduced secretion rate; estimated zero HCO₃^- secretion occurring at plasma pH 7.01. Subsequent i.v. secretin infusion at 2.70 C.U./kg b.wt. h^-1 increased HCO₃^- secretion to 249±42 μmol/min at plasma pH 7.33+0.04; stepwise lowering of plasma pH proportionately reduced HCO₃^- secretion to estimated zero at plasma pH 6.71. A reduction of plasma pH by 0.1 pH unit reduced HCO₃^- secretion during low and high rate of i.v. secretin infusion by 18±3 μmol/min and 35±8 μmol/min, respectively. Secretin infusion rate did not affect pancreatic chloride excretion. These findings support the view that secretin increases HCO₃^- secretion, and hence proton transport to the interstitial fluid, by augmenting the proton motive force developed by HCO₃^- secreting cells.     

Plasma potassium concentration as a determinant of proximal tubular NaCl and NaHCO3 reabsorption in dog kidneys

To examine whether an acute increase in plasma potassium concentration ([K]_p) may inhibit proximal tubular transport, clearance studies were performed in seven anaesthetized, volume expanded dogs treated with amiloride (1 mg kg^-1body wt) to block tubular potassium secretion, and with bumetanide (30 μg kg^-1body wt) to inhibit NaCl reabsorption in Henle's loop. As [K]_p was raised in steps from 2.6 ± 0.2 to 7.9 ± 0.2 mm , bicarbonate, chloride, and sodium reabsorption decreased by 232 ± 56, 520 ± 59 and 958 ± 112 μmol min^-1, respectively, at constant glomerular filtration rate (GFR). On average, the molar ratio between the inhibitory effects on bicarbonate and chloride reabsorption were 1:2.2–2.4. Reabsorption was calculated at GFR 100 ml min^-1: (reabsorption 100/GFR (mmol min^-1). It was inversely correlated to In [K]_p with r=–0.82 for bicarbonate and with r =–0.89 for chloride. Fractional potassium reabsorption remained constant at 0.31 ± 0.03. Administration of acetazolamide (100 mg kg^-1body wt) in eight dogs at [K]_p 8 mm reduced fractional reabsorption of bicarbonate, chloride and sodium as much as in previous studies on normokalaemic dogs. We conclude that acute elevation of [K]_p inhibits NaHCO₃ transport and passive proximal tubular NaCl reabsorption. This inhibition is not related to changes in potassium secretion and carbonic anhydrase activity, but may be secondary to depolarization of the basolateral membrane.     

Colchicine inhibits the effects of secretin on pancreatic duct cell tubulovesicles and HC03 secretion in the pig

Secretin stimulation clears the cytoplasm of intralobular pancreatic duct cells in pigs of tubulovesicles and causes these cells to secrete HCO₃^- into the pancreatic juice. To determine whether the clearance of cytoplasmic tubulovesicles involves the microtubule system and is important for initiation of HCO₃^- secretion, the effect of the microtubule poison colchicine on duct cell morphology and pancreatic HCO₃^- secretion was measured in anaesthetized pigs. Before colchicine, secretin reduced the density of tubulovesicles in the cytoplasm of pancreatic duct cells from 92 ± 8 U to 8 ± 2 U and initiated pancreatic secretion of 176 ± 21 μmol min^-1 HCO₃^-. After colchicine, secretin failed to lower duct cell tubulovesicle density and caused the secretion of only 77 ± 14μmol min^-1 HCO₃^-. By contrast, lumicolchicine, an isomer of colchicine that does not affect microtubules, did not inhibit pancreatic HCO3 secretion. Colchicine did not reduce carbonic anhydrase or Na,K-ATPase activities in in-vitro assays. The clearance of tubulovesicles from the cytoplasm of pancreatic duct cells therefore seems to be microtubule-dependent and important for the pancreatic HCO₃^- secretion.     

Abolished relationship between pancreatic HCO-3 secretion and arterial pH during carbonic anhydrase inhibition

After acetazolamide administration, CO₂ hydration in pancreatic cells would be slow and might become a rate-limiting factor to pancreatic HCO^-₃ secretion. Correspondingly, pancreatic HCO^-₃ secretion—normally pH dependent—would become slow and pH-independent. However, acetazolamide would not be expected to interfere with the capacity of the secretory mechanism to generate a proton potential gradient between pancreatic cells and interstitial fluid. These predictions were examined in 5 anesthetized, secretin infused (2.7 C. U./kg b. wt. h^-1) pigs. Pancreatic juice was collected from a catheter in the pancreatic duct. Arterial pH was varied through i.v. HCI and NaHCO₃ infusions and CO₃ addition to inspired air. Before acetazolamide, HCO^-₃ secretion varied with plasma pH and averaged 298±30 μmol/min at control arterial pH. Acetazolamide (150 mg/kg, i.v.) reduced HCO^-₃ secretion to 84±12 μmol/min and rendered secretion independent of arterial pH between pH 7.6 and pH 7.0. It is concluded that acetazolamide imposes a pH-independent transport maximum on pancreatic HCO^-₃ secretion, but does not reduce the capacity of the secretory mechanism to sustain a proton potential gradient between cells and interstitial fluid.     

Brain carbonic acid acidosis after acetazolamide.

In cats in barbiturate anesthesia extracellular pH and potassium were continously recorded from brian cortex by implanted microelectrodes. Implantation of the electrodes preserved the low permeability of the blood-brain-barrier to HCO3-minus and H+ions as indicated by the development of brain acidosis by I.V. injection of HCO3-minus. Acetazolamide (25 mg/kg) i.v. was followed by a marked brain acidosis which after 10 min had progressed to a drop in pH of 0.203 plus or minus 0.046 (x bar plus or minus S.D., n equals 8). The slowness ofthe development of acidosis points to a direct effect of the carbonic anhydrase inhibition on the brain tissue. As a further support for this conclusion was considered the finding of a prolonged response time of brain pH to HCO3-minus i.v. to CO2-minus inhalation, and to hyperventilation after the acetazolamide inhibtion. No changes in brain extracelllular potassium were found.     

NN′-dicyclohexylcarbodiimide (DCCD) reduces pancreatic NaHCO3 secretion without changing pancreatic tissue ATP levels

To study the mechanism responsible for pancreatic NaHCO₃ secretion, the inhibitor NN'-dicyclohexylcarbodiimide (DCCD) was administered to six secretin-infused, anaesthetized pigs. Pancreatic juice was collected from a catheter in the main pancreatic duct. Secretion rate was measured at several arterial pH values in each animal, both before and after DCCD. 15 (14–30) μmol kg^-1 body wt DCCD, intra-arterially, reduced pancreatic NaHCO₃ secretion from 296 (234–398) to 181 (134–237) μmol min^-1 at arterial pH 7.43 (7.42-7.47). Similar fractional reductions of secretion occurred at lower arterial pH. Pancreatic tissue ATP concentration, 1.8 (i.4-2.o)μmol g^-1 wet wt, was not changed by DCCD. DCCD, ≤ 10^-4 mol l^-1, did not change Na,K-ATPase nor carbonic anhydrase activities in separate in vitro assay systems. It is concluded that DCCD reduced pancreatic NaHCO₃ secretion by a mechanism not involving ATP depletion nor inhibition of Na,K-ATPase nor carbonic anhydrase activities in pancreatic cells. Because DCCD inhibits proton pumps, DCCD may have reduced NaHCO₃ secretion through interfering with a proton pump involved in extruding H⁺ from HCO₃^- secreting cells to interstitial fluid in the pancreas.     

Renal handling and plasma elimination kinetics of carbonic anhydrase isoenzymes I,II and III in the rat

Using a radio-immunosorbent technique, the levels of the carbonic anhydrase (CA) isoenzymes CA I, II and III in plasma (1–3 μ ml^-1), lymph (0.5-1.6 μg ml^-1) and urine (0.03-0.06 ± 3, 42 ± 11 and 35 ± 4 nl min^-1 (g kidney wet wt)^-1 (n = 4–5), respectively. After a single i.v. injection of purified native or ¹²⁵I-labelled isoenzymes, the elimination of CA I, and CA III from plasma followed a bi-exponential decline, with half-times of 7 and 9 min for the rapid phase and 112 min for the slow phase, respectively. Nephrectomy decreased the rapid phase and the build-up of catabolites. Therefore, the rapid phase of CA I and III elimination is probably explained by filtration of unbound isoenzyme at the glomeruli and subsequent degradation by the proximal tubules. The plasma elimination curve for CA II was different and followed a mono-exponential decline, with a half-time of 210 min both in normal and nephrectomized animals. This indicates that CA II is not filtered at the glomeruli. However, in acute renal failure, with leaking tubular cells, CA II was excreted into the urine. The slow elimination of the major part of the isoenzymes from plasma is explained by the binding of CA I, II and III to a plasma protein, immunochemically similar to transferrin, forming a macromolecular complex with a mol wt of 114 ± 2 kDa.     

Effect of plasma H+-ion concentration on pancreatic HCO-3 secretion

The relationship between the rate of pancreatic HCO^-₃ secretion and plasma H⁺-ion concentration was investigated in 15 pentothal anesthetized, secretin infused pigs (1.8 C.U./kg b.w. h^-1 intravenously) during acute metabolic and respiratory acid-base disturbances. Pancreatic HCO^-₃ secretion increased to 196 ± 10% of control during alkalosis and fell to 41 ± 4% of control during acidosis. Partial metabolic compensation of respiratory acidosis restored HCO^-₃ secretion to 87 ± 6% of control. A proportional relationship was found between HCO^-₃ secretion and plasma pH. Different, proportional relationships were found between HCO^-₃ secretion and plasma HCO^-₃ concentration during metabolic and respiratory acid-base changes. HCO^-₃ secretion was independent of H⁺-ion concentration in pancreatic juice. Plasma H⁺-ion concentration, therefore, seems to determine the rate of pancreatic HCO^-₃ secretion. This finding supports the hypothesis that a proton pump is responsible for pancreatic HCO^-₃ secretion.     

Extracellular bicarbonate and non-bicarbonate buffering against lactic acid during and after exercise

Defense of extracellular pH constancy against lactic acidosis can be estimated from changes (Δ) in lactic acid ([La]), [HCO₃⁻], pH and PCO₂ in blood plasma because it is equilibrated with the interstitial fluid. These quantities were measured in earlobe blood during and after incremental bicycle exercise in 13 untrained (UT) and 21 endurance-trained (TR) males to find out if acute and chronic exercise influence the defense. During exercise the capacity of non-bicarbonate buffers (β_nbi = −Δ[La] · ΔpH⁻¹ − Δ[HCO₃⁻] · ΔpH⁻¹) available for the extracellular fluid (mainly hemoglobin, dissolved proteins and phosphates) amounted to 32 ± 2(SEM) and 20 ± 2 mmol l⁻¹ in UT and TR, respectively (P < 0.02). During recovery β_nbi decreased to 14 (UT) and 12 (TR) mmol l⁻¹ (both P < 0.001) corresponding to values previously found at rest by in vivo CO₂ titration. Bicarbonate buffering (β_bi) amounted to 44–48 mmol l⁻¹ during and after exercise. The large exercise β_nbi seems to be mainly caused by an increasing concentration of all buffers due to shrinking of the extracellular volume, exchange of small amounts of HCO₃⁻ or H⁺ with cells and delayed HCO₃⁻ equilibration between plasma and interstitial fluid. Increase of [HCO₃⁻] during titration by these mechanisms augments total β and thus the calculated β_nbi more than β_bi because it reduces ΔpH and Δ[HCO₃⁻] at constant Δ[La]. The smaller rise in exercise β_nbi in TR than UT may be caused by an increased extracellular volume and an improved exchange of La⁻, HCO₃⁻ and H⁺ between trained muscles and blood.     

Effect of intravenous infusion of hypertonic glucose solutions on pancreatic HCO3 secretion

To examine why intravenous infusion of hypertonic non-electrolyte solutions inhibit pancreatic HCO₃^- secretion, the relationship between pancreatic HCO₃^- secretion and plasma pH was examined before and following intravenous infusion of hypertonic glucose to 5 anesthetized, secretin infused (2.7 C.U./kg b. wt. h^-1) pigs. Hyperglycemia (plasma glucose 103±6 mmol/l) did not significantly change plasma pH, NaM⁺, K⁺, Cl^- and HCO₃^- concentrations. Hyperglycemia reduced pancreatic water flux by 48±5% and raised pancreatic juice HCO₃^- concentration by 43±4 mmol/l. Concurrently, HCO₃^- secretion fell by 34±5%. Acidosis, produced through intravenous HC1 infusion and CO₂ addition to inspired air, reduced HCO₃^- secretion by 40±6 ±mol/min and 30±5 ±mol/min per 0.1 pH unit reduction in plasma pH before and during hyperglycemia, respectively, and abolished HCO₃ secretion at an estimated plasma pH of 6.51 ±0.06 before and a pH of 6.63±0.05 during hyperglycemia. We conclude that hypertonic glucose infusions inhibit pancreatic water flux and cause an increase in pancreatic juice HCO₃^- concentration which may inhibit HCO₃^- secretion through an effect on acid-base balance in secretory cells.     

The Effect of Iso-Carbic Metabolic Acidosis in Blood on [H+] and [HCO3-] in CSF with Deductions about the Regulation of an Active Transport of H+/HCO3 - between Blood and CSF

The changes in pH and [HCO₃] in csf of anesthetized, paralyzed dogs during metabolic acidosis were compared with those changes when the blood acid-base parameters were tept at a normal level. Arterial and csf Pco₂ were kept constant at the same level during both kinds of experiments. Arterial [HCO₃^-_-] was kept cmstant at about 25 meq/l during normal acid-base experiments and at about 19 meq/l during acidosis experiments. Arterial and csf samples were simultaneously withdrawn after 1 hr, 3 hrs, and 6 hrs. The changes in csf pH and [HCO₃-_-] were expressed as the changes in the electrochemical potential differences (csf-blood) for H⁺‘ and HCO₃^- During metabolic acidosis the electrochemical potential differences for H⁺ and HCO₃^- decreased which could be explained by a decrease in the rate of an active transport of H+/HCO₃^-_- between blood and csf during metabolic acidosis at a constant Pco₂. The findings are compared with previous findings and we conclude that the rate of active transport of H⁺/HCO₃^- between blood and csf if affected by changes in plasma [HCO₃^-]. This mechanism provides pH stability in the brain environment during metabolic blood acid-base changes, whereas the effect is opposite during respiratory acid-base changes. The mechanism is however “useful” during respiratory acidosis because it provides a negative feed-back mechanism for the increase in Pco₂ during respiratory acidosis.     

Potassium kinetics and its relationship with ventilation during repeated bouts of exercise in women

The purpose of this study was to determine the electrolyte concentration changes in arterial plasma from high-intensity repeated bouts of cycling exercise in well-trained females and to determine the relationships between arterial plasma lactate, potassium (K⁺), bicarbonate (HCO₃⁻), and pH with minute ventilation. Fourteen female subjects (mean age = 27 ± 4 years; mean height = 170 ± 7 cm; mean weight = 62 ± 7 kg; maximal oxygen uptake = 50 ± 6 ml/kg/min) were recruited to perform 3 × 5 min bouts of exercise at 236 ± 27 W with 10 min recovery between each set. Minute ventilation, arterial plasma lactate, potassium, calcium, chloride, and sodium ion concentrations were measured a minute 0, 1, 2, 3, 4, 5 of each set and midway through recovery (21 sampling points total per subject). The results showed that the strongest relationship was between arterial plasma K⁺ concentration and minute ventilation (r ² = 0.91), and, that arterial plasma lactate mirrored both arterial plasma HCO₃⁻ and pH. In conclusion, this study demonstrates that women exhibit similar electrolyte responses as reported elsewhere in men, and support the idea that K⁺ may partly contribute to controlling ventilation during high-intensity exercise and recovery.     

The CSF/Blood Potential in Sustained Acid-Base Changes in the Rat. With Calculations of Electrochemical Potential Differences for H+and HCO-3

The electrical D.C. potential difference between cisternal cerebrospinal fluid and external jugular blood was measured in rats during sustained acid-base changes. One group, serving as a control series, was given an artificial extracellular fluid i.p., while two other groups were made acidotic or alkalotic by means of i.p. injections of NH₄Cl or NaHCO₃ solutions. In the last two groups NH₄Cl or NaHCO₃ was given, but Con was simultaneously administered so as to keep either the plasma bicarbonate or the plasma pH constant. In all groups the CSF/ plasma potential was permanently changed in relation to the arterial plasma pH, but there was a larger potential change in alkalosis (about 50 mV/pH unit) than in acidosis (about 30 mV/pH unit). Calculations of electrochemical potential differences for H+and HCO₃^-between CSF and plasma showed no significant differences in nonrespiratory acidosis and alkalosis but significant increases in hypercapnia. The results do not support the theory that an active H+transport regulates the CSF pH to constancy.     

Extracellular pH defense against lactic acid in untrained and trained altitude residents

The assumption that buffering at altitude is deteriorated by bicarbonate (bi) reduction was investigated. Extracellular pH defense against lactic acidosis was estimated from changes (Δ) in lactic acid ([La]), [HCO₃ ⁻], pH and PCO₂ in plasma, which equilibrates with interstitial fluid. These quantities were measured in earlobe blood during and after incremental bicycle exercise in 10 untrained (UT) and 11 endurance-trained (TR) highlanders (2,600 m). During exercise the capacity of non-bicarbonate buffers (β _nbi = −Δ[La] · ΔpH⁻¹ − Δ[HCO₃ ⁻] · ΔpH⁻¹) amounted to 40 ± 2 (SEM) and 28 ± 2 mmol l⁻¹ in UT and TR, respectively (P < 0.01). During recovery β _nbi decreased to 20 (UT) and 16 (TR) mmol l⁻¹ (P < 0.001) corresponding to values expected from hemoglobin, dissolved protein and phosphate concentrations related to extracellular fluid (ecf). This was accompanied by a larger decrease of base excess after than during exercise for a given Δ[La]. β _bi amounted to 37–41 mmol l⁻¹ being lower than at sea level. The large exercise β _nbi was mainly caused by increasing concentrations of buffers due to temporary shrinking of ecf. Tr has lower β _nbi in spite of an increased Hb mass mainly because of an expanded ecf compared to UT. In highlanders β _nbi is higher than in lowlanders because of larger Hb mass and reduced ecf and counteracts the decrease in [HCO₃ ⁻]. The amount of bicarbonate is probably reduced by reduction of the ecf at altitude but this is compensated by lower maximal [La] and more effective hyperventilation resulting in attenuated exercise acidosis at exhaustion.     

The Intracellular pH‘ in the Brain in Acute and Sustained Hypercapnia

The regulation of intracellular pH′ in the brain was studied in rats exposed to about 11% CO₂ for periods of 15 min to 72 h. At the end of each exposure period cisternal cerebrospinal fluid, as well as the supratentorial parts of the brain, were sampled and analysed for the CO₂ contents. The intracellular HCO₃^- concentration was calculated assuming extracellular volumes of 12, 15 and 20%, respectively, and the intracellular pH′ was derived from the HCO₃^- concentration and the mean tissue CO₂ tension. Acute hypercapnia (15 min) was associated with an increase in the intracellular HCO₃^- concentration of about 6 meq/kg of i.e. water. With an extracellular volume of 15% the corresponding decrease in pH′_i was from 7.06 to 6.93. When the hypercapnia was upheld there was a further increase in the intracellular HCO₃^- concentration of about 5 meq/kg and an increase in pH′_i to about 7.03. Most, if not all, of this regulation of pH′_i occurred during the first 3 h. An analysis of the regulating mechanisms suggests that physicochemical buffering, metabolic consumption of acids, and transmembrane fluxes of H⁺ or HCO₃^- each contributed about a third of the total accumulation of bicarbonate in the cell during hypercapnia.     

Na+-H+ exchange is not important for pancreatic HCO-3 secretion in the pig

Veel , T., Villanger , O., Holthe , M.R., Cragoe jr ., E.J. & Reder , M.G. 1992. Na⁺-H exchange is not important for pancreatic HCO^-₃ secretion in the pig. Acta Physiol Scand 144 , 239–246. Received 13 September 1991, accepted 14 November 1991. ISSN 0001–6772. University of Oslo, Institute for Experimental Medical Research and Surgical Department, Ullevaal Hospital, Oslo, Norway. Pancreatic inter- and intralobular duct cells extrude H⁺-ions to interstitial fluid when they secrete HCO^-₃ to pancreatic juice. This study assesses the potential importance of Na⁺-H⁺-ion exchange for H⁺-ion extrusion and secretion of HCO^-₃ using the Na⁺-H⁺ exchange blockers amiloride and hexamethylene-amiloride. Intracellular pH (pH,) in inter- and intralobular pancreatic duct epithelium was measured using BCECF fluorescence. H⁺-ion efflux was measured using a NH₄Cl prepulse, acid-loading technique. In HCO^-₃-free media, pH₁ recovery following acid loading was blocked by amiloride (10^-4 m) and hexamethylene-amiloride (10^-6 m) , demonstrating amiloride-and hexamethylene-amiloride-sensitive Na⁺-H⁺ exchange. However, 5 × 10^-6 M hexamethylene-amiloride did not reduce secretin-dependent pancreatic HCO, secretion in vivo. Maximal H⁺-efflux through Na⁺-H⁺ exchange was 1.5 ± 0.2μmol min^-1 ml cell volume^-l, i.e. less than 1 % of estimated net H⁺-ion efflux during HCO^-₃ secretion. Conclusion: amiloride- and hexamethylene amiloride sensitive Na⁺-H⁺ exchange is not important for secretin-dependent pancreatic HCO^-₃ secretion in the pig. Other mechanisms for H⁺ extrusion dominate.     

Influence of Adipose Tissue Blood Flow on the Lipolytic Response to Circulating Noradrenaline at Normal and Reduced pH

Hypercapnic acidosis (pH 7.0) inhibits the lipolytic response of canine subcutaneous adipose tissue to i.v. infused noradrenaline (NA) by 80 per cent or more. The response to sympathetic nerve stimulation, on the other hand, is only reduced by 1040 per cent during acidosis. The fate of intravenously infused ³H-labelled NA (0.35 ug × kg^-1× min^-1 for 30 min) was not significantly altered by acidosis. The rate of disappearance of unmetabolized NA from the arterial plasma after an infusion was the same at pH 7.4 and 7.0 and the calculated increase in circulating NA during infusions was 4 ng/ml at both pH:s. I.v. infusion of Na increases adipose tissue blood flow, an effect which is attenuated by acidosis. There was a significant correlation (p< 0.001) between adipose tissue blood flow and the lipolytic response at normal pH. Preventing the NA-induced increase in blood flow by constant flow perfusion reduced the lipolytic response at normal pH. The degree of inhibition by acidosis of the lipolytic response to i.v. NA was significantly reduced (from 79 to 56 per cent, p < 0.05) when the adipose tissue was perfused at constant flow. These data suggest that adipose tissue blood flow is important in determining the lipolytic response to i.v. NA, probably by influencing the delivery of NA to the tissue. The marked inhibition by acidosis of lipolysis due to i.v. infused NA therefore appears to be the combined effect of a direct antilipolytic effect of acidosis and a decreased delivery of N A to the adipose tissue due to the attenuated blood flow response.     

Fast bicarbonate-chloride exchange between brain cells and brain extracellular fluid in respiratory acidosis

The extracellular pH, \(P_{CO_2 } \) , and [Cl^?] at the surface of the brain cortex, expiratory \(P_{CO_2 } \) and arterial blood pressure were continuously recorded in anaesthetized and artificially ventilated cats. The observations from such a preparation were:

1. In response to a nearly step increase in end-tidal \(P_{CO_2 } \) , the brain ECF pH, \(P_{CO_2 } \) , [Cl^?] and calculated [HCO ₃ ^? ] changed in the form of a nearly mono-exponential time function after a delay of 5–7 s.
2. The time constants of the changes in the extracellular pH, \(P_{CO_2 } \) , [Cl^?] and [HCO ₃ ^? ] were in the range of 30–40 s.
3. The extracellular [HCO ₃ ^? ] increased markedly at an initial rate of 4.22 mmol·l^?1·min^?1 after 36 s.
4. This increase occurred almost simultaneously with a decrease in the extracellular [Cl^?]. An [HCO ₃ ^? ]?[Cl^?] exchange ratio was determined which very closely approached one.

It is concluded that the brain extracellular bicarbonate concentration in respiratory acidosis increases because the H⁺ formed from the hydrated CO₂ reacts with the intracellular buffers of brain cells, mainly glial cells, and HCO ₃ ^? inside the cell is formed and exchanged for Cl^? outside the cell similar to the HCO ₃ ^? /Cl^? exchange which occurs between red cells and blood plasma during CO₂ loading. The described time constants of the anion exchange represent thewash in orwash out time of CO₂ in a tissue containing intracellular buffer.     

Role of physiological HCO3 –buffer on intracellular pH and histamine release in rat peritoneal mast cells

 The purpose of this study was to examine how intracellular pH (pH_i) regulation and histamine release are affected by HCO₃ ^–in rat peritoneal mast cells. The pH_i was measured using the pH-sensitive dye 2′, 7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). We observed a pH_i of 6.88±0.012 (n=24) in resting mast cells exposed to a HEPES buffer (pH 7.4), but a sustained drop of 0.21 pH units to 6.67±0.015 (n=23) when we exposed the mast cells to a HEPES/HCO₃ ^–buffer equilibrated at all time with 5% CO₂ (pH 7.4). This fall in pH_i is inhibited by the carbonic anhydrase inhibitor dichlorphenamide and is Na⁺-independent, indicating the involvement of Na⁺-independent Cl^–/HCO₃ ^–exchange activity. Furthermore removal of external Cl^–in the presence but not in the absence of HCO₃ ^–reversed the Cl^–/HCO₃ ^–exchange and induced an alkaline load. The recovery from this alkaline load was dependent on external Cl^–but independent of Na⁺. Both the alkalinization and the recovery were inhibited by the anion transport inhibitor 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS). In addition, ³⁶Cl^–uptake measurements confirm the presence of a Cl^–/HCO₃ ^–exchanger. Histamine release stimulated by antigen and compound 48/80 was substantially reduced in the presence of HEPES/ HCO₃ ^–buffer (pH_o 7.4, pH_i 6.66). Histamine release was increased, however, when pH_i was clamped to 6.66 in HCO₃ ^–-free media (pH_o 6.9). We conclude that: (1) Na⁺-independent Cl^–/HCO₃ ^–exchange determines steady-state pH_i in rat peritoneal mast cells; and (2) the reduction in histamine release observed in the presence of HCO₃ ^–is not due to its effect on pH_i per se, but rather on other changes in ion transport. Received: 29 January 1998 / Received after revision and accepted: 3 April 1998