The effects of the inhibition of the renal carbonic anhydrase on the blood acid-base status in hypercapnic rats

Arterial blood acid-base status was measured in unanesthetized rats treated with benzolamide (a selective renal carbonic anhydrase inhibitor). These measurements were carried out in rats exposed to different levels of CO2 in air (0-10% CO2) for periods of up to 6 hr. In untreated rats the whole body buffer value showed a continuous increase and after 6 hr of exposure to hypercapnia its value was twice that measured initially. On the other hand, the whole body buffer value of benzolamide treated rats did not change during the 6 hr of exposure to hypercapnia. The whole body buffer value of normal rats, measured after 6 hr of hypercapnia is similar to that reported for chronic (3-5 days) hypercapnia in the normal dog. The whole body buffer value in benzolamide treated rats was similar to that reported for the normal dog and man, during acute CO2 exposures. It is suggested that mechanisms involving the renal carbonic anhydrase are responsible for the significant, rapid changes in the whole body buffer value that take place during the initial phase of acute exposure to CO2 in the rat. This may represent a mechanism of adaptation to burrow hypercapnic conditions.     

Adaptations to hypercapnic conditions in the nutria (Myocastor coypus)--in vivo and in vitro CO2 titration curves

Arterial blood acid-base status of unanesthetized, unrestrained nutria was studied during exposure to 5, 10 and 14.5% CO2 for 6 hr. Control values, pH = 7.426 +/- 0.037, PaCo2 = 36.5 +/- 3.1 mmHg and [HCO-3] = 24.3 +/- 2.5 mM/1 (n = 24), are within the normal range reported for other mammals. Values after 6 hr of exposure to 10% CO2 were: pH = 7.355 +/- 0.043, PaCO2 = 71.0 +/- 3.6 mmHg and [HCO-3] = 38.0 +/- 4.1 mM/l (n = 5). Arterial blood buffer slopes, obtained from the in vitro titration curve, did not show any pattern of adaptation to hypercapnia. Whole body buffer slopes, calculated from the in vivo CO2 titration curve, showed significantly higher values for the nutria than for the rat, dog and man, under comparable conditions [beta(delta HCO-3/delta pH)] = 57.0 slykes for nutria, 32.6 for rat and 11.8 for man. delta H+/delta PaCO2 = 0.38. mM/l/mmHg for nutria, 0.55 for rat and 0.76 for man. The results suggest that the nutria possesses an efficient metabolic mechanism for regulation of pH level during exposure to hypercapnic conditions.     

Blood acid-base status, whole blood and whole body buffer values in sand rats (Psammomys obesus) exposed to hypercapnia

Arterial blood acid-base status of unanesthetized sand rats (Psammomys obesus) were studied under normocapnic and hypercapnic conditions, and compared to those obtained for the albino rat (Rattus norvegicus). The average control blood pH: 7.396 +/- 0.034; PaCO2: 30.5 +/- 2.9 mmHg; HCO-3: 18.8 +/- 2.5 mM/l; and HCO-3 std: 20.9 +/- 2.1 (N = 15) obtained here for the sand rat are in the lower range of values found in other mammals and indicate a status of partially compensated metabolic acidosis. The blood buffer values of the sand rat, delta log PCO2/delta pH = -2.32 +/- 0.35 (N = 25) are significantly higher than those found here for the rat, delta log PCO2/delta pH = -1.51 +/- 0.10 (N = 39), and those reported for other mammals. This high blood buffer value may be related to the natural high mineral diet of the sand rat. The in vivo (whole body) buffer value delta log PaCO2/delta pH = -1.41 and -1.65 for the sand rat and the rat found here are higher than those reported for the man and dog and may represent a physiological adaptation to the hypercapnic conditions prevailing in underground burrows.     

Blood osmolality during in vivo changes of CO2 pressure

In 16 experiments male subjects, age 22.4 +/- 0.5 (SE) yr, inspired CO2 for 15 min (8% end-tidal CO2) or hyperventilated for 30 min (2.5% end-tidal CO2). Osmolality (Osm) and acid-base status of arterialized venous blood were determined at short intervals until 30 min after hypo- and hypercapnia, respectively. During hypocapnia [CO2 partial pressure (PCO2) -2.31 +/- 0.32 kPa (-17.4 Torr), pH + 0.19 units], Osm decreased by 3.9 +/- 0.3 mosmol/kg H2O; during hypercapnia [PCO2 + 2.10 +/- 0.28 kPa (+15.8 Torr), pH -0.12 units], Osm increased by 5.8 +/- 0.7 mosmol/kg H2O. Presentation of the data in Osm-PCO2 or Osm-pH diagrams yields hysteresis loops probably caused by exchange between blood and tissues. The dependence of Osm on PCO2 must result mainly from CO2 buffering and therefore from the formation of bicarbonate. In spite of the different buffer capacities in various body compartments, water exchange allows rapid restoration of osmotic equilibrium throughout the organism. Thus delta Osm/delta pH during a PCO2 jump largely depends on the mean buffer capacity of the whole body. The high estimated buffer value during hypercapnia (38 mmol/kg H2O) compared with hypocapnia (19 mmol/kg H2O) seems to result from very strong muscle buffering during moderate acidosis.     

Effect of chronic hypercapnia on body temperature regulation.

Guinea pigs and rats exposed to 15% CO2 for 7 days showed a parallel time course of changes in pH, body temperature (TB), and oxygen consumption (VO2). Between 1 and 6 h of exposure the maximal drop in actual pH occurred in guinea pigs simultaneously with the maximal fall in TB and VO2. During the subsequent period pH TB, VO2 rose again. Skin blood content (heat loss) also exhibited a biphasic pH-dependent time course. Animals showing no partial compensation of respiratory acidosis during 3 days exposure also failed in raising their TB back to normal in this time. The behavior of TB was found to be a good indicator of the acid-base status and adaptive potential of the animals to hypercapnia. Similar results were obtained in rats. Thermo-regulatory processes in the hypothalamus were affected during exposure to 15% CO2. Both guinea pigs and rats showed a decrease in norepinephrine content of the hypothalamus during the first part of exposure reaching a maximal fall at the end of 24 h. The serotonin content increased slightly during this period. During prolonged exposure to 3% CO2 for 7 days, TB showed a transient rise, and VO2 was slightly elevated.     

Effects of temperature change on acid-base regulation in skipjack tuna (Katsuwonus pelamis) blood

The effects of temperature change (in vitro) on acid-base balance of skipjack tuna blood were investigated. By examining the relationship between blood pH and temperature (in vitro) under conditions of constant CO2 tension (open system), it was observed that dpH/dT = -0.013 U/degrees C. This value falls well within the range of in vivo values reported for other ectothermic vertebrates, and is only slightly different than results obtained in vitro under conditions of constant CO2 content (closed system; dpH/dT = -0.0165 U/degrees C). It is concluded that changes in pH following temperature changes can be accounted for solely by the passive, in vitro behaviour of the chemical buffer system found in the blood, so that active regulatory mechanisms of pH adjustment need not be postulated for skipjack tuna.     

Impacts of ocean acidification on respiratory gas exchange and acid–base balance in a marine teleost, Opsanus beta

The oceanic carbonate system is changing rapidly due to rising atmospheric CO(2), with current levels expected to rise to between 750 and 1,000?μatm by 2100, and over 1,900?μatm by year 2300. The effects of elevated CO(2) on marine calcifying organisms have been extensively studied; however, effects of imminent CO(2) levels on teleost acid-base and respiratory physiology have yet to be examined. Examination of these physiological processes, using a paired experimental design, showed that 24?h exposure to 1,000 and 1,900?μatm CO(2) resulted in a characteristic compensated respiratory acidosis response in the gulf toadfish (Opsanus beta). Time course experiments showed the onset of acidosis occurred after 15?min of exposure to 1,900 and 1,000?μatm CO(2), with full compensation by 2 and 4?h, respectively. 1,900-μatm exposure also resulted in significantly increased intracellular white muscle pH after 24?h. No effect of 1,900?μatm was observed on branchial acid flux; however, exposure to hypercapnia and HCO(3) (-) free seawater compromised compensation. This suggests branchial HCO(3) (-) uptake rather than acid extrusion is part of the compensatory response to low-level hypercapnia. Exposure to 1,900 μatm resulted in downregulation in branchial carbonic anhydrase and slc4a2 expression, as well as decreased Na(+)/K(+) ATPase activity after 24?h of exposure. Infusion of bovine carbonic anhydrase had no effect on blood acid-base status during 1,900?μatm exposures, but eliminated the respiratory impacts of 1,000 μatm CO(2). The results of the current study clearly show that predicted near-future CO(2) levels impact respiratory gas transport and acid-base balance. While the full physiological impacts of increased blood HCO(3) (-) are not known, it seems likely that chronically elevated blood HCO(3) (-) levels could compromise several physiological systems and furthermore may explain recent reports of increased otolith growth during exposure to elevated CO(2).     

CO2 dissociation curves of oxygenated whole blood obtained at rest and in exercise

In vitro CO2 dissociation curves for oxygenated whole blood were determined in 19 healthy male subjects at rest and during submaximal and maximal bicycle work. Hemoglobin concentration and blood lactate increased with increasing work load and accordingly buffer value of the whole blood increased while bicarbonate and Base Excess (BE) decreased, resulting in a downward shift of the CO2 dissociation curve during exercise. Despite the marked increase in buffer values of the blood, the slopes of the CO2 dissociation curves during exercise were found to be about the same as those obtained at rest. It was inferred that the increasing effect of increased buffer value, on the dissociation slope, was essentially compensated by the decreasing effect of diminished bicarbonate content. The advantages of this relatively constant CO2 dissociation slope for the indirect measurement of cardiac output by the Fick principle are discussed.     

Effect of water alkalinity on gill CO2 exchange and internal PCO2 in aquatic animals

In addition to metabolic CO2 production and gill ventilatory flow rate, expired water PCO2 is very dependent on water acid-base balance in a complex way. This is particularly true in carbonated waters at low ambient PCO2 and high pH, where CO2 excreted in the gill water may be buffered by carbonate ions, leading to an increased CO2 capacitance coefficient. The higher the carbonate alkalinity (CA) and the lower the inspired PCO2 (i.e., the higher the inspired water pH), the stronger the carbonate buffering and the smaller the increase of PCO2 in the gill water during respiratory CO2 exchanges. As a consequence, as shown by a number of reported data, increasing the CA leads to blood hypocapnia and respiratory alkalosis at constant low, but not at high, inspired PCO2. In the low range of inspired PCO2, internal PCO2 becomes very sensitive to even small changes of water PCO2, which may explain at least in part the large variability of reported blood PCO2 values in gill breathers. Water CA also influences the amplitude of respiratory acid-base disturbances caused by changes of the gill ventilatory flow rate. Carbonate buffering of excreted CO2 and thus dependence of blood PCO2 on water alkalinity requires catalysis of CO2 hydration by carbonic anhydrase, that must be available from the water side of the gill epithelium.     

Arterial blood gas tensions and acid-base status of Wistar rats during thiopental and halothane anesthesia.

Arterial blood gas tensions and acid-base status of spontaneously-breathing, unanesthetized Wister rats were compared with values obtained during 4 hr of thiopental and 6 hr of halothane (1%) anesthesia. During thiopental anesthesia, marked respiratory depression occurred (PaCO-2:57.0 plus or minus 10.0 MM Hg, PaO-2:70.4 plus or minus 11.2 MM Hg). Thirty-six percent of the rats died. During inhalation of room air and 1% halothane, PaO-2 decreased also, whereas PaO-2 did not change. Twenty-seven percent of the original number of rats died. Lowered arterial oxygen tension may have caused death; no rats died during inhalation of oxygen and 1% halothane. This technic insured sufficient analgesia for surgical procedures without marked alterations of the acid base status and is recommended for long-term anesthesia of small laboratory animals like rats.     

Acid-base balance during social interactions in rainbow trout (Oncorhynchus mykiss)

Socially subordinate rainbow trout (Oncorhynchus mykiss) experience chronic stress that impacts upon a variety of physiological functions, including Na(+) regulation. Owing to the tight coupling between Na(+) and Cl(-) uptake and, respectively, H(+) and HCO(3)(-) loss at the gill, ionoregulatory changes associated with social status may affect acid-base regulation. The present study assessed the responses of dominant, subordinate and control trout to hypercapnia (1% CO(2)) to test this hypothesis. Social status appeared to impact net acid excretion (J(net)H(+)) as subordinate individuals failed to increase net acid flux in response to hypercapnia. However, blood acid-base status was found to be unaffected by social status before or during hypercapnic exposure, indicating that subordinate fish were as effective as dominant or control trout in achieving compensation for the acid-base disturbance induced by hypercapnic exposure. Compensation in all groups involved decreasing Cl(-) uptake in response to hypercapnia. The branchial activities of both Na(+),K(+)-ATPase (NKA) and V-type H(+)-ATPase were affected by social interactions and/or exposure to hypercapnia. Branchial NKA activity was higher but V-ATPase activity was lower in control fish than in dominant or subordinate trout. In addition, control and subordinate but not dominant trout exposed to 24h of hypercapnia exhibited significantly higher branchial V-ATPase activity than fish maintained in normocapnia. Collectively, the data suggest that subordinate trout are able to regulate blood pH during a respiratory acidosis.     

Seasonal changes in the cardiorespiratory responses to hypercarbia and temperature in the bullfrog, Rana catesbeiana

We assessed the seasonal variations in the effects of hypercarbia (3 or 5% inspired CO2) on cardiorespiratory responses in the bullfrog Rana catesbeiana at different temperatures (10, 20 and 30 degrees C). We measured breathing frequency, blood gases, acid-base status, hematocrit, heart rate, blood pressure and oxygen consumption. At 20 and 30 degrees C, the rate of oxygen consumption had a tendency to be lowest during winter and highest during summer. Hypercarbia-induced changes in breathing frequency were proportional to body temperature during summer and spring, but not during winter (20 and 30 degrees C). Moreover, during winter, the effects of CO2 on breathing frequency at 30 degrees C were smaller than during summer and spring. These facts indicate a decreased ventilatory sensitivity during winter. PaO2 and pHa showed no significant change during the year, but PaCO2 was almost twice as high during winter than in summer and spring, indicating increased plasma bicarbonate levels. The hematocrit values showed no significant changes induced by temperature, hypercarbia or season, indicating that the oxygen carrying capacity of blood is kept constant throughout the year. Decreased body temperature was accompanied by a reduction in heart rate during all four seasons, and a reduction in blood pressure during summer and spring. Blood pressure was higher during winter than during any other seasons whereas no seasonal change was observed in heart rate. This may indicate that peripheral resistance and/or stroke volume may be elevated during this season. Taken together, these results suggest that the decreased ventilatory sensitivity to hypercarbia during winter occurs while cardiovascular parameters are kept constant.     

Blood oxygen levels and acid-base status following air exposure in an air-breathing fish, Channa argus: the role of air ventilation

1. Acute air exposure of an air-breathing fish Channa argus immediately induced hypercapnic acidemia while total CO2 content of blood remained unchanged. Upon reimmersion, paco2 and pHa quickly restored to pre-exposure levels followed by gradual rise of [HCO-3]. 2. Artificial air ventilation of air-exposed fish restored acid-base status and greatly depressed voluntary air ventilatory movements. We conclude that the major cause of acid-base disturbances occurring during air exposure is the reduced air convection.     

The apparent buffer value of human blood for CO2 as a function of temperature

The influence of temperature on the acid-base status of normal human deoxygenated whole blood was studied in open systems (variable total CO2 content). (1) When the temperature was raised from 26 degrees C to 42 degrees C, the apparent buffering value of deoxygenated whole blood for CO2 increased by 7% of its value at 26 degrees C; this increase was not statistically significant. (2) Comparing the present data with those obtained previously from oxygenated whole blood in the same temperature range (Castaing & Pocidalo, 1979) indicates that arterial and venous blood have slightly different buffering capacities for CO2 in the 26 to 42 degrees C temperature range. It also suggests that, at physiological SO2 levels (SO2 greater than or equal to 30%), the apparent buffering value of venous blood for CO2 would be increased by at least 10% of its value at 26 degrees C when the temperature is raised to 42 degrees C. (3) It is concluded that pH stability would be reduced upon CO2 uptake within tissues with a high metabolism and therefore a high temperature.     

Serum electrolyte and enzyme responses to heat stress and dehydration in the fowl (Gallus domesticus)

1. Serum electrolytes, enzymes and various metabolites were determined in the hyperthermic and dehydrated fowl. 2. In normally-hydrated fowls, heat stress did not significantly affect blood constituents. 3. Water deprivation for 48 hr (dehydration) significantly (p less than 0.05) increased Na+, osmolality, SGPT and T3-retention. 4. During hyperthermic dehydration, Na+, Cl-, osmolality (p less than 0.01), BUN, glucose, T3-retention (p less than 0.02) and uric acid (p less than 0.001) significantly increased. 5. The present findings are consistent with the suggestion that changes in Na+/Ca2+ ratio might raise the hypothalamic thermoregulatory set-point and support our previous findings that acclimated fowls could efficiently regulate body temperature and acid-base status while avoiding extreme metabolic and enzymatic changes during heat exposure and dehydration.     

Experimental determination of net protein charge, [A]tot, and Ka of nonvolatile buffers in bird plasma.

The quantitative mechanistic acid-base approach to clinical assessment of acid-base status requires species-specific values for [A]tot (the total concentration of nonvolatile buffers in plasma) and Ka (the effective dissociation constant for weak acids in plasma). The aim of this study was to determine [A]tot and Ka values for plasma in domestic pigeons. Plasma from 12 healthy commercial domestic pigeons was tonometered with 20% CO2 at 37 degrees C. Plasma pH, Pco2, and plasma concentrations of strong cations (Na, K, Ca), strong anions (Cl, L-lactate), and nonvolatile buffer ions (total protein, albumin, phosphate) were measured over a pH range of 6.8-7.7. Strong ion difference (SID) (SID5=Na+K+Ca-Cl-lactate) was used to calculate [A]tot and Ka from the measured pH and Pco2 and SID5. Mean (+/-SD) values for bird plasma were as follows: [A]tot=7.76+/-2.15 mmol/l (equivalent to 0.32 mmol/g of total protein, 0.51 mmol/g of albumin, 0.23 mmol/g of total solids); Ka=2.15+/-1.15x10(-7); and pKa=6.67. The net protein charge at normal pH (7.43) was estimated to be 6 meq/l; this value indicates that pigeon plasma has a much lower anion gap value than mammals after adjusting for high mean L-lactate concentrations induced by restraint during blood sampling. This finding indicates that plasma proteins in pigeons have a much lower net anion charge than mammalian plasma protein. An incidental finding was that total protein concentration measured by a multianalyzer system was consistently lower than the value for total solids measured by refractometer.     

Effect of fasting and acute heat stress on body temperature, blood acid-base and electrolyte status in chickens

1. The tolerance of chickens to acute heat stress, evaluated by the time required to reach the critical body temperature (Tr) of 44.5 degrees C, was markedly enhanced as the period of fasting was extended. 2. Fasting reduced the rates of heat-induced changes in blood acid-base and electrolyte status. 3. Changes in Tr were correlated with changes in blood pH, pCO2, [Cl-] and [Pi] but not with changes in [Na+] or [K+]. 4. Blood acid-base and electrolyte status were related to Tr rather than time of exposure to heat stress.     

Effects of lethal levels of environmental hypercapnia on cardiovascular and blood-gas status in yellowtail,Seriola quinqueradiata

The cardiorespiratory responses were examined in yellowtail, Seriola quinqueradiata exposed to two levels of hypercapnia (seawater equilibrated with a gas mixture containing 1% CO(2) (water PCO(2) = 7 mmHg) or 5% CO(2) (38 mmHg)) for 72 hr at 20 degrees C. Mortality was 100% within 8 hr at 5% CO(2), while no fish died at 1% CO(2). No cardiovascular variables (cardiac output, Q; heart rate, HR; stroke volume, SV and arterial blood pressure, BP) significantly changed from pre-exposure values during exposure to 1% CO(2). Arterial CO(2) partial pressure (PaCO(2)) significantly increased (P < 0.05), reaching a new steady-state level after 3 hr. Arterial blood pH (pHa) decreased initially (P < 0.05), but was subsequently restored by elevation of plasma bicarbonate ([HCO(3)(-)]). Arterial O(2) partial pressure (PaO(2)), oxygen content (CaO(2)), and hematocrit (Hct) were maintained throughout the exposure period. In contrast, exposure to 5% CO(2) dramatically reduced Q (P < 0.05) through decreasing SV (P < 0.05), although HR did not change. BP was transiently elevated (P < 0.05), followed by a precipitous fall before death. The pHa was restored incompletely despite a significant increase in [HCO(3)(-)]. PaO(2) decreased only shortly before death, whereas CaO(2) kept elevated due to a large increase in Hct (P < 0.05). We tentatively conclude that cardiac failure is a primary physiological disorder that would lead to death of fish subjected to high environmental CO(2) pressures.     

Metabolic component of intestinal PCO(2) during dysoxia.

The adequacy of intestinal perfusion during shock and resuscitation might be estimated from intestinal tissue acid-base balance. We examined this idea from the perspective of conventional blood acid-base physicochemistry. As the O(2) supply diminishes with failing blood flow, tissue acid-base changes are first "respiratory, " with CO(2) coming from combustion of fuel and stagnating in the decreasing blood flow. When the O(2) supply decreases to critical, the changes become "metabolic" due to lactic acid. In blood, the respiratory vs. metabolic distinction is conventionally made using the buffer base principle, in which buffer base is the sum of HCO(3)(-) and noncarbonate buffer anion (A(-)). During purely respiratory acidosis, buffer base stays constant because HCO(3)(-) cannot buffer its own progenitor, carbonic acid, so that the rise of HCO(3)(-) equals the fall of A(-). During anaerobic "metabolism," however, lactate's H(+) is buffered by both A(-) and HCO(3)(-), causing buffer base to decrease. We quantified the partitioning of lactate's H(+) between HCO(3)(-) and A(-) buffer in anoxic intestine by compressing intestinal segments of anesthetized swine into a steel pipe and measuring PCO(2) and lactate at 5- to 10-min intervals. Their rises followed first-order kinetics, yielding k = 0. 031 min(-1) and half time = approximately 22 min. PCO(2) vs. lactate relations were linear. Over 3 h, lactate increased by 31 +/- 3 mmol/l tissue fluid (mM) and PCO(2) by approximately 17 mM, meaning that one-half of lactate's H(+) was buffered by tissue HCO(3)(-) and one-half by A(-). The data were consistent with a lumped pK(a) value near 6.1 and total A(-) concentration of approximately 30 mmol/kg. We conclude that the respiratory vs. metabolic distinction could be made in tissue by estimating tissue buffer base from measured pH and PCO(2).     

Thiopental anaesthesia and the acid-base balance in the blood of experimental dogs.

An analysis of basic parameters representative of the acid-base balance was made in arterial blood samples from 140 clinically healthy dogs, under a general intravenous Thiopental anaesthesia. The following mean values +/- S.E.M. were obtained: pH = 7.33 +/- 0.01; pCO2 = 47.16 +/- 0.95; base excess = -2.12 +/- 0.27; buffer base = 46.63 +/- 0.37. The results showed a prevalent trend of lower values of pH, base excess and buffer base and higher values of pCO2 than those found commonly in human clinical practice. Special attention was paid to the respiratory component of the acid-base balance (ABB) revealing certain undesirable side effect of Thiopental anaesthesia.