Extracellular Bicarbonate Concentration Research Articles

Upregulation of carbonic anhydrase 1 beneficial for depressive disorder

Carbonic Anhydrase 1 (CAR1) is a zinc-metalloenzyme that catalyzes the hydration of carbon dioxide, and the alteration of CAR1 has been implicated in neuropsychiatric disorders. However, the mechanism underlying the role of CAR1 in major depressive disorder (MDD) remains largely unknown. In this study, we report the decreased level of CAR1 in MDD patients and depression-like model rodents. We found that CAR1 is expressed in hippocampal astrocytes and CAR1 regulates extracellular bicarbonate concentration and pH value in the partial hilus. Ablation of the CAR1 gene increased the activity of granule cells via decreasing their miniature inhibitory postsynaptic currents (mIPSC), and caused depression-like behaviors in CAR1-knockout mice. Astrocytic CAR1 expression rescued the deficits in mIPSCs of granule cells and reduced depression-like behaviors in CAR1 deficient mice. Furthermore, pharmacological activation of CAR1 and overexpression of CAR1 in the ventral hippocampus of mice improved depressive behaviors. These findings uncover a critical role of CAR1 in the MDD pathogenesis and its therapeutic potential.

High bicarbonate concentration increases glucose-induced insulin secretion in pancreatic β-cells

Low serum bicarbonate is closely related to type 2 diabetes mellitus. However, the precise role of bicarbonate on glucose homeostasis and insulin secretion remains unknown. In this study, we investigated the effects of bicarbonate concentration on pancreatic β-cells. It was observed that the high bicarbonate concentration of the cell culture medium significantly increased the glucose-induced insulin secretion (GSIS) levels in mouse islets, MIN6, and the INS-1E β cells. MIN6 cells presented an impaired GSIS; the cells produced a lower bicarbonate concentration when co-cultured with Capan-1 than when with CFPAC-1. NBCe1, a major bicarbonate transporter was observed to block the increasing insulin secretions, which were promoted by a high concentration of bicarbonate. In addition, higher extracellular bicarbonate concentration significantly increased the intracellular cAMP level, pHi, and calcium concentration with a 16.7 mM of glucose stimulation. Further study demonstrated that a low concentration of extracellular bicarbonate significantly impaired the functioning of pancreatic β cells by reducing coupling Ca2+ influx, whose process may be modulated by NBCe1. Taken together, our results conclude that bicarbonate may serve as a novel target in diabetes prevention-related research.

STAT3-dependent regulation of the electrogenic Na+/ HCO3- cotransporter 1 (NBCe1) functional expression in cortical astrocytes.

The electrogenic Na+ /HCO3- cotransporter (NBCe1) in astrocytes is crucial in regulation of acid-base homeostasis in the brain. Since many pathophysiological conditions in the brain have been associated with pH shifts we exposed primary mouse cortical and hippocampal astrocytes to prolonged low or high extracellular pH (pHo ) at constant extracellular bicarbonate concentration and investigated activation of astrocytes and regulation of NBCe1 by immunoblotting, biotinylation of surface proteins, and intracellular H+ recordings. High pHo at constant extracellular bicarbonate caused upregulation of NBCe1 protein, surface expression and activity via upregulation of the astrocytic activation markers signal transducer and activator of transcription 3 (STAT3) signaling and glial fibrillary acidic protein expression. High pHo -induced increased NBCe1 protein expression was prevented in astrocytes from Stat3flox/flox ::GfapCre/+ mice. In vitro, basal and high pHo -induced increased NBCe1 functional expression was impaired following inhibition of STAT3 phosphorylation. These results provide a novel regulation mode of NBCe1 protein and activity, highlight the importance of astrocyte reactivity on regulation of NBCe1 and implicate roles for NBCe1 in altering/modulating extracellular pH during development as well as of the microenvironment at sites of brain injuries and other pathophysiological conditions.

Sodium Fate after Sodium Bicarbonate Infusion: Influence of Altered Acid-Base Status

Background: We have previously investigated the fate of administered bicarbonate infused as a hypertonic solution in animals with each of the 4 chronic acid-base disorders. Those studies did not address the fate of sodium, the coadministered cation. Methods: We examined baseline total body water (TBW), Na⁺ space, HCO₃^– space, and urinary sodium and bicarbonate excretion after acute hypertonic NaHCO₃ infusion (1-N solution, 5 mmol/kg body weight) in dogs with each of the 4 chronic acid-base disorders. Observations were made at 30, 60, and 90 min postinfusion. Retained sodium that remains osmotically active distributes in an apparent space that approximates TBW. Na⁺ space that exceeds TBW uncovers nonosmotic sodium storage. Results: Na⁺ space approximated TBW at all times in normal and hyperbicarbonatemic animals (metabolic alkalosis and respiratory acidosis), but exceeded TBW by ~30% in hypobicarbonatemic animals (metabolic acidosis and respiratory alkalosis). Such osmotic inactivation was detected at 30 min and remained stable. The pooled data revealed that Na⁺ space corrected for TBW was independent of the initial blood pH but correlated with initial extracellular bicarbonate concentration (y = –0.01x + 1.4, p= 0.002). The fate of administered sodium and bicarbonate (internal distribution and urinary excretion) was closely linked. Conclusions: This study demonstrates that hypobicarbonatemic animals have a Na⁺ space that exceeds TBW after an acute infusion of hypertonic NaHCO₃ indicating osmotic inactivation of a fraction of retained sodium. In addition to an expanded Na⁺ space, these animals have a larger HCO₃^– space compared with hyperbicarbonatemic animals. Both phenomena appear to reflect the wider range of titration of nonbicarbonate buffers (Δ pH) occurring during NaHCO₃^– loading whenever initial [HCO₃^–]_e is low. The data indicate that the fate of administered bicarbonate drives the internal distribution and the external disposal of sodium, the co-administered cation, and is responsible for the early, but non-progressive, osmotic inactivation of a fraction of the retained sodium.

Telmisartan, an Angiotensin II receptor blocker, abrogates troglitazone‐induced cell signaling coupled to ammoniagenesis in proximal tubule‐like LLC‐PK1 cells

Troglitazone(TRO) induces a large increase in ammonium production from glutamine via the mitochondrial pathway in proximal tubule‐like LLC‐PK1 cells. To elucidate the upstream site of tro action we measured the acute effects on cell signaling in LLC‐PK1 cells grown in chambers designed for fluorometric ratiometric imagining (BCECF and Fura 2) and in 24 well plates for 18h metabolic studies. TRO (25uM) induces a large fall (p<0.01) in cytosolic pH (7.19±0.09 to 6.85±0.08) and a doubling of cell calcium occurring within 8minutes. Associated with these responses was a similarly rapid and large increase (p<0.05) in phospho‐ERK1/2(p‐ERK to total ERK ratio=1±0.2 for control vs 10.1±3 fold for TRO). Treating the cells with 30uM telmisartan (pre‐incubation for 4minutes and during 8minutes with TRO) markedly reduced both the rise in p‐ERK1/2(10.1±3 to 2.4±0.5) and acidity (6.85±0.08 to 7.11± 0.07) suggesting that telmisartan acutely blocks TRO's upstream site of action. At the downstream site, TRO (25uM) enhanced (p<0.01) mitochondrial ammoniagenesis (NH4+ production 1011±110 to 1678±84nmol/mg/18h) despite a normal extracellular bicarbonate concentration and pH; addition of telmisartan (30uM) to the TRO‐media prevented (p<0.05) the enhanced ammonium production (1193±142 vs 1678±84 nmol/mg/18h for TRO+telmisartan vs TRO). These findings are consonant with angiotensin II‐like receptor involvement in TRO's initial activation of cell signaling pathways underlying the ammoniagenic response in these model porcine proximal tubule‐like cells.

Basis for presentation of acid-base in two dimensions.

Buffering of "metabolic" acid in tissues other than blood correlates closely with a change in extracellular bicarbonate concentration rather than with a change in extracellular pH. Of particular importance is the evidence for an absence of relation to change in pH. Questions are raised with respect to buffering mechanism, but simplification is offered for diagnosis. A clearer focus can be given to the guidepost changes in PCO2 and bicarbonate concentration. Basic relationships of buffering in the whole body are reviewed, and a modified diagnostic rationale is offered, based on a two-rather than a three-dimensional analysis.

Carbon dioxide and the cerebral circulation.

Carbon dioxide and the cerebral circulation.

Effects of extracellular Ca 2+ and HCO 3− on epidermal growth factor-induced DNA synthesis in cultured rat hepatocytes

Background/Aims: The elevation of cytosolic free calcium concentration ([Ca 2+] i) and intracellular pH mediate the growth factor-initiated proliferation of many cells, but it is not known if they trigger mitosis in resting hepatocytes. The maintenance of [Ca 2+] i and intracellular pH depends partly on extracellular calcium concentration ([Ca 2+] e) and extracellular bicarbonate concentration ([HCO 3 −] e). Therefore, the effects of [Ca 2+] e and [HCO 3 −] e on hepatocyte proliferation were examined. Methods: Epidermal growth factor induced proliferation in primary cultures of rat hepatocytes. [ 3H]thymidine incorporation into DNA and nuclear labeling indices were measured. Results: Between 0.2 and 0.9 mmol/L of [Ca 2+] e, the proliferative response to epidermal growth factor increased, and total hepatocellular Ca 2+ content was increased. Increasing [HCO 3 −] e also stimulated DNA synthesis in a concentration-dependent manner, maximal at 35 mmol/L. Using optimal [Ca 2+] e (0.9 mmol/L) and [HCO 3 −] e (35 mmol/L), a synergistic stimulation of hepatocellular DNA synthesis was shown. Voltage-dependent Ca 2+ channel blockers failed to inhibit hepatocyte proliferation when administered in concentrations that inhibit proliferation in other cell types. Conclusions: [Ca 2+] e and [HCO 3 −] e are both essential for hepatocyte proliferation, and their effects are synergistic. The entry of extracellular Ca 2+ is critical for epidermal growth factor-induced DNA synthesis in hepatocytes, but this is not mediated by voltage-dependent Ca 2+ channels.

Regulation of Proximal Renal Tubular K<sup>+</sup> Conductance by Intracellular pH

Conventional electrophysiology and 2', 7'-bis-(2-carboxyethyl)-5-(and 6)-carboxyfluorescein fluorescence have been applied to elucidate the effects of metabolic acidosis on straight proximal tubules of the mouse kidney. Reduction of extracellular bicarbonate concentration from 20 to 10 mmol/l leads to a decline of intracellular pH from 7.00 +/- 0.06 to 6.85 +/- 0.05, a depolarization of the cell membrane (PDbl) from -72 +/- 1 to -59 +/- 2 mV, a decrease of the basolateral transference number for potassium (tK) from 0.80 +/- 0.01 to 0.54 +/- 0.03, an increase of the basolateral transference number for bicarbonate (tb) from 0.16 +/- 0.02 to 0.42 +/- 0.03 and an increase of the fractional resistance of the basolateral over the luminal cell membrane (Rb/Ra) by 64 +/- 8%. Upon return to 20 mmol/l bicarbonate after a 5-min exposure to 10 mmol/l bicarbonate, the intracellular pH approached a more alkaline value (7.28 +/- 0.08) than before exposure to acidosis. Despite the intracellular alkalosis, PDbl (-67 +/- 1 mV) and tK (0.73 +/- 0.02) remained significantly below, and tb (0.26 +/- 0.02) and Rb/Ra (32 +/- 8%) significantly above the respective values before induction of acidosis. Even transient exposure of the tubules to 40 mmol/l extracellular bicarbonate did not restore the original electrophysiological properties of the tubule cells. It is concluded that both a rapidly reversible and a long-lasting decrease of proximal tubular K+ conductance follows cellular acidosis.

Effect of bicarbonate administration on skeletal muscle intracellular pH in the rat: implications for acute administration of bicarbonate in man.

1. The effect of bicarbonate administration on the intracellular pH of rat skeletal muscle was examined by using 31P n.m.r. 2. Bicarbonate administered intraperitoneally caused a significant intracellular acidosis in rat skeletal muscle in vivo. When the bicarbonate was administered intravenously there was no such change in the pH of the skeletal muscle. 3. Bicarbonate administration by either route resulted in an elevated mixed venous partial pressure of carbon dioxide and an elevated arterial pH, but no significant change in the arterial partial pressure of carbon dioxide. The increase in arterial bicarbonate concentration after intraperitoneal injection of bicarbonate was delayed when compared with that after intravenous injection. 4. The administration of hypertonic solutions intravenously caused a transient 40-50% fall in blood pressure, which had resolved within 1 min. 5. The data suggest that the effect of bicarbonate administration on intracellular pH in vivo is related not only to carbon dioxide loading of the cell but also to the rate of change in the extracellular bicarbonate concentration.

High blood CO2 levels in rainbow trout exposed to hypercapnia in bicarbonate-rich hard fresh water--a methodological verification.

ABSTRACT Heisler (1984, 1986) suggested that fish have a maximum extracellular bicarbonate concentration of about 30 mmol l−1, which can be attained and used for the compensation of blood acidosis induced during environmental hypercapnia. In contrast, Börjeson (1977) and Dimberg & Höglund (1987) reported HCO3−concentrations, analysed using Astrup’s tonometric method (Astrup, 1956), of 40–50 mmol l−1 in venous blood drawn by a syringe from captured fish. However, blood pH in rainbow trout is greatly affected by sampling stress induced by venous puncture (Railo, Nikinmaa & Soivio, 1985). This may introduce error into the determination of blood [HCO3−] when using a tonometric technique based on pH measurements. Consequently, the present study was performed to determine whether the maximal blood HCO3− level in hypercapnic fish is limited to 30 mmol l−1, and if the higher reported blood bicarbonate values could be the result of sampling and/or analytical errors. Rainbow trout (Salmo gairdneri) were exposed to different levels of hypercapnia and acid-base variables were analysed either in venous blood taken with a syringe in the ductus Cuvieri from captured fish or in blood sampled via a dorsal aortic catheter from undisturbed fish. In addition, tonometry and gasometry were carried out separately to determine total blood CO2 concentration.

Interaction of CO2 and ammonia on cerebral blood flow and O2 consumption in dogs.

Studies of acutely induced hyperammonemia and chronic hyperammonemia associated with liver dysfunction suggest that cerebral blood flow (CBF) and O2 consumption (CMRO2) become uncoupled and that CMRo2 may depend on arterial CO2 tension (PaCO2). We examined CBF (radiolabeled microspheres) and CMRO2 during hypercapnia (PaCO2 congruent to 74 Torr) and hypocapnia (PaCO2 congruent to 21 Torr) both before and during intravenous ammonium acetate infusion in pentobarbital-anesthetized dogs. Continuous infusion over 120 min produced stable increases of arterial ammonia levels (1,400 mumol/l) by 30 min, whereas CBF, CMRO2, and O2 extraction (measured at sagittal sinus) remained unchanged when PaCO2 was held constant (congruent to 35 Torr). Acute hyperammonemia attenuated the increase in CBF during hypercapnia by 44% and abolished the decrease in CBF during hypercapnia. Regional blood flow to pons and midbrain increased under normocapnic conditions, and midbrain blood flow increased further during hypocapnia. Sodium acetate infusion did not affect CBF responses to CO2. Thus we failed to observe an uncoupling of global CBF and CMRO2 during normocapnic hyperammonemia, or an interaction of CO2 and ammonia on CMRO2, although the increased pons and midbrain blood flow may reflect regional effects of ammonia on reticular activating system metabolism. On the basis of the literature, we suggest that the attenuated hypercapnic CBF response may arise from impaired glial regulation of extracellular potassium and bicarbonate concentrations and that lactic acid production, enhanced by combined alkalosis and hyperammonemia, may contribute to the abolition of hypocapnic vasoconstriction.

Determinants of deoxyglucose uptake in cultured astrocytes: the role of the sodium pump.

Glucose utilization in primary cell cultures of mouse cerebral astrocytes was studied by measuring uptake of tracer concentrations of [3H]2-deoxyglucose ([3H]2-DG). The resting rate of glucose utilization, estimated at an extracellular K+ concentration ([K+]o) of 5.4 mM, was high (7.5 nmol glucose/mg protein/min) and was similar in morphologically undifferentiated and "differentiated" (dibutyryl cyclic AMP-pretreated) cultures. Resting uptake of [3H]2-DG was depressed by ouabain, by reducing [K+]o, and by cooling. These observations suggest that resting glucose utilization in astrocytes was dependent on sodium pump activity. Sodium pump-dependent uptake in 2-3-week-old cultures was about 50% of total [3H]2-DG uptake but this fraction declined with culture age from 1 to 5 weeks. Uptake was not affected by changes in extracellular bicarbonate concentration ([HCO3-]o) in the range of 5-50 mM but was significantly reduced in bicarbonate-free solution. At high [HCO3-]o (50 mM) uptake was insensitive to pH (pH 6-8), whereas at low [HCO3-]o (less than 5 mM) uptake was markedly pH-dependent. Elevation of [K+]o from 2.3 mM to 14.2-20 mM (corresponding to extremes of the physiological range of [K+]o) resulted in a 35-43% increase in [3H]2-DG uptake that was not affected by culture age or by morphological differentiation. Our results indicate a high apparent rate of glucose utilization in astrocytes. This rate is dynamically responsive to changes in extracellular K+ concentration in the physiological range and is partially dependent on sodium pump activity.

Skeletal carbon dioxide stores during metabolic acidosis.

These experiments were performed to examine the buffering functions of skeletal carbon dioxide during metabolic acidosis. Acidosis of several days duration was produced in rats either by inclusion of acidogenic substances in the diet or by chemical induction of diabetic ketoacidosis. Titrimetric methods were used to measure the carbon dioxide content of bone, which was divided into readily exchangeable and slowly exchangeable pools according to a model described in the text. Acid feeding resulted in a mild acidemia (blood pH greater than 7.25), whereas profound metabolic acidemia occurred during diabetic ketoacidosis (blood pH less than 7.00). Total bone carbon dioxide was reduced during both forms of metabolic acidosis. This reduction in skeletal carbon dioxide occurred within the first 24 h after the onset of metabolic acidosis, was associated with a decline in the readily exchangeable fraction of skeletal carbon dioxide, and was directly proportional to the declines in extracellular bicarbonate concentration and plasma carbon dioxide tension.

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

The extracellular pH, PCO2, and [Cl-] at the surface of the brain cortex, expiratory PCO2 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 PCO2, the brain ECF pH, PCO2, [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, PCO2, [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.1(-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 the wash in or wash out time of CO2 in a tissue containing intracellular buffer.

Effect of bicarbonate on the arginine-induced insulin and glucagon secretion in vitro.

The isolated perfused rat pancreas was used to investigate the effect of extracellular bicarbonate concentration (25 and 40 mmol/l) on the arginine-induced insulin and glucagon release with or without 5.5 mmol/l glucose in the perfusate. In the absence of glucose, the insulin response to arginine was increased at 40 mmol/l bicarbonate, while no potentiation by glucose of the arginine-induced insulin release, at this bicarbonate concentration, was observed. At 40 mmol/l bicarbonate, glucose inhibition of basal glucagon release was abolished. In contrast, the glucagon response to arginine was not affected by the bicarbonate concentration in the perfusion medium. These results confirm that the bicarbonate concentration in the medium markedly influences the insulin release from the isolated perfused rat pancreas, while the glucagon response to arginine seems to be less sensitive to changes in extracellular bicarbonate concentration.

Effects of diphosphonate and colchicine administration upon acid-base changes induced in rats by bilateral nephrectomy.

1. The effects of disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) and colchicine on acid--base balance were examined in intact and nephrectomized rats. 2. Both drugs increased extracellular hydrogen ion concentrations and depressed extracellular bicarbonate concentrations in nephrectomized rats compared with controls but did not alter these parameters in intact animals. 3. Intracellular hydrogen ion concentrations in the skeletal muscle of nephrectomized rats given EHDP were higher than those of control animals. 4. It is postulated that colchicine and EHDP inhibit skeletal buffering of non-volatile acids produced endogenously in nephrectomized rats.

Extracellular and intracellular pH with changes of temperature in the Dogfish Scyliorhinus stellaris

Larger Spotted Dogfish, Scyliorhnmus stellaris, were exposed to varied ambient temperature (t) in order to determine the behavior of extracellular pH (pHe), P CO 2 and bicarbonate concentration as well as intracellular pH (pHi) in three muscle types. pHe was found to vary with temperature slightly less than expected on the basis of the rule of constant relative alkalinity in juvenile ( Δ pH Δ t = −O.0148 per degree centigrade) as well as in adult ( solΔ pH Δ t = −O.0136 ) fish. The absolute pHe values of adult fish were about O.08 pH units higher than in juvenile fish. Arterial P CO 2 increased with rising temperature, the increase being much more marked in adult than in juvenile fish. Extracellular bicarbonate concentration (calculated from the pH and P CO 2 values measured in arterial blood) was not maintained constant, but diminished in juvenile and increased in adult fish with increasing temperature, indicating that extracellular pH in dogfish is regulated by variations of both P CO 2 and bicarbonate concentration. Variations of intracellular pH with temperature ( Δ pHi Δ t ), −O.0178 for white muscle, −O.0334 for red muscle, and −O.0098 for heart muscle, were significantly different from the values of the extracellular compartment and, except for white muscle, significantly different from the condition for constant relative alkalinity ( Δ pH Δ t = −O.0183 ). These results are in agreement with the rule of constant relative alkalinity with respect to extracellular pH and possibly also with respect to an overall mean intracellular pH, but the rule is not quantitatively followed by the individual body compartments and tissues.

Bicarbonate and the regulation of ventilation

The regulation of ventilation involves a multifactorial control system with several feedback loops transmitting deviations from normal in pH, carbon dioxide tension (pCO 2) and oxygen tension (pO 2) to the control area. Variations in the size of the bicarbonate pool, caused by ventilatory or metabolic disturbances, can be expected to modify resting ventilation if hydrogen ion activity is the ultimate stimulus of the regulation of ventilation. A relationship between serum bicarbonate and resting ventilation can be identified in patients with stable acid-base disturbances including those in whom correction of the arterial blood pH was not achieved by respiratory adaptation. Why the pH in arterial blood is rarely returned to the normal range is not well understood. It may be an inadequacy of the control system, a “compromise” solution avoiding hypoxia in metabolic alkalosis or increasing work of breathing in metabolic acidosis, or a consequence of discrepancies in hydrogen ion activity in body fluids adjacent to and remote from the control site. Additional information about the role of bicarbonate in the control of ventilation may be obtained by measuring the response to carbon dioxide inhalation at varying extracellular bicarbonate concentrations. The increments in ventilation during inhalation of carbon dioxide are within individual limitations, inversely and exponentially related to the bicarbonate concentrations in blood. These observations are in accord with the concept that the extracellular bicarbonate concentration modulates resting ventilation and the ventilatory response to inhalation of fixed concentrations of carbon dioxide by acting as a determinant for the hydrogen ion activity within or adjacent to the central chemosensitive control area.

Respiratory-renal adjustments in chronic hypercapnia in man: Extracellular bicarbonate concentration and the regulation of ventilation

In patients with stable mechanical ventilatory limitation and chronic hypercapnia, chloride depletion by administration of diuretics leads to elevations in extracellular bicarbonate levels which are inordinate for the carbon dioxide tension (PaCO 2). Chloride replacement returns this bicarbonate-PaCo 2 relationship o a predicted level. Ammonium chloride induces an inordinate reduction in extracellular bicarbonate concentrations for the levels of PaCO 2 in arterial blood. The ventilatory responsiveness to inhalation of 5 per cent carbon dioxide following these interventions is related exponentially to the extracellular bicarbonate concentration and not to the level of extracellular hydrogen ion activity measured prior to carbon dioxide inhalation. However, the increase in ventilation per unit increment in extracellular hydrogen ion activity induced by carbon dioxide inhalation ( ΔV E ΔH+ ) is constant. This suggests that the extracellular bicarbonate concentration acts to modulate the ventilatory response to stimulation with carbon dioxide whereas the changes in hydrogen ion activity during carbon dioxide inhalation act as the determinants of the ventilatory response to inhaled carbon dioxide.