Long-term Hypercapnia Research Articles

Long‐term hypercapnia does not alter Galanin immunoreactivity in mouse brain respiratory and chemoreceptor regions

Long‐term hypercapnia has been implicated in respiratory disorders such as sleep apnea and chronic obstructive pulmonary disease (COPD). However, the mechanisms by which long‐term hypercapnia contributes to central respiratory dysfunction in these conditions are unclear. The brainstem respiratory circuit, including the ventral respiratory column (VRC), comprises distinct neuronal populations that govern expiratory and inspiratory drive. The neuropeptide galanin is extensively distributed in brainstem chemoreceptor regions and the VRC, and microinjection of galanin into the VRC inhibits breathing and attenuates ventilatory reflex responses to acute hypercapnia. Nevertheless, the normal pattern of regional expression of galanin in the mouse brain is not known and the impact of long‐term hypercapnia on galanin immunoreactivity (Gal‐ir) in the vicinity of brainstem respiratory and chemoreceptor regions has not yet been studied. Therefore, we aimed to determine Gal‐ir across the mouse brain and investigate the impact of prolonged hypercapnia exposure on Gal‐ir in brainstem respiratory and chemoreceptor regions (pons and medulla). Adult male Phox2b‐EGFP (MGI ID: 5776545) mice (31±2 g, n=5 mice/group) were exposed to hypercapnia (8% CO2; balance room air) or room air (control) for 10 or 14 days. To determine Gal‐ir, brain slices were collected and processed for immunohistochemistry. Quantitative measurement of Gal‐ir was performed using ImageJ 1.53a (a QuPath‐integrated version of ImageJ) where a positive area fraction, reflecting the proportion of positively labeled pixels, was calculated for each region of interest [locus coeruleus, the retrotrapezoid nucleus, caudal‐nucleus of the solitary tract (cNTS), rostral‐NTS (rNTS), VRC, lateral reticular nucleus, and caudoventrolateral reticular nucleus]. Under control conditions, we found that Gal‐ir was broadly present across the medulla oblongata, and abundant in the caudal aspects when compared with the rostral levels. The cNTS had a higher average percentage of Gal‐ir area (p = 0.043, p = 0.05, 2‐way ANOVA) compared to rNTS following 10‐ and 14‐days of room air exposure, respectively. A significant difference in Gal‐ir (p = 0.011, 2‐way ANOVA) was observed between the cNTS and rNTS following 14‐days of hypercapnia exposure. Overall, there was no significant difference of Gal‐ir in all regions assessed after long‐term hypercapnia compared to controls. Collectively, we found that long‐term hypercapnia does not induce changes in galanin neuropeptide levels within respiratory and chemoreceptor regions in the mouse brain. Under control and hypercapnia conditions, cNTS contains higher Gal‐ir compared to rNTS. Further investigation is required to elucidate the involvement of NTS‐galaninergic subsets in respiratory homeostasis.

Hypercapnia‐evoked Chronic Stress Alters IL‐1β Levels in Brain Respiratory Chemoreceptor Regions

Chronic stress often elicits neuronal, hormonal, and immune responses. Long term hypercapnia is a respiratory stressor linked with sleep apnea, sudden infant death syndrome (SIDS) and chronic obstructive pulmonary disease (COPD), however, mechanisms of central respiratory dysfunction in these conditions are unclear. We aimed to explore the neuroinflammatory responses to hypercapnia in key brain respiratory regions. We tested the hypothesis that long-term exposure to 8% CO2 activates the releaseof brain stress hormone corticosterone (CORT) in the circulation which may contribute to inflammation in brainstem circuits that regulate CO2-stimulated breathing. Age-matched adult male C57BL/6J mice (n=10 mice/group) were exposed to hypercapnia (8% CO2; balance room air) in the short-term (6 hours) or long-term (10 or 14 days), or room air (control) conditions.Following anesthesia, plasma samples were collected for corticosterone (CORT) assay and brain tissues including cerebellum, hypothalamus, the nucleus of the solitary tract (NTS), rostral and caudal retrotrapezoid nucleus (rRTN, cRTN) were micro-dissected and processed for cytokine assays. Plasma CORT levels were not different between hypercapnia and room air groups in the short-term, but a significantly higher CORT level (p < 0.01, 2 way ANOVA) was observed following 10 days of exposure to hypercapnia compared to control conditions. CORT level returned to baseline following 14 days of hypercapnia compared to control (14 days room air) conditions. In the brain, the pro-inflammatory cytokine interleukin-1β (IL-1β) was raised in the cerebellum (p < 0.01, 2 way ANOVA) following 6 hours but not 10 days of hypercapnia exposure, compared to control conditions. In the hypothalamus and cRTN, IL-1β was unaltered at the 6 hours time point, significantly upregulated (p < 0.05, p < 0.01, 2 way ANOVA) at 10 days of hypercapnia exposure, compared to controls. In these brain regions, IL-1β levels returned to baseline after 14 days hypercapnia exposure time point. There was no change in IL-1β in the rRTN and NTS following 6 hours, 10 days, and 14 days of hypercapnia, compared to controls. Taken together, the level of plasma CORT positively correlates to brain IL-1β during long-term hypercapnia. These results suggest that hypercapnia activates a physiological stress response, and the associated brain inflammation depends on the duration of hypercapnia exposure. We propose that the stress and inflammatory adaptive responses observed are part of compensatory mechanisms following long-term hypercapnia. The cellular source and underlying mechanisms of interactions between plasma CORT and brain IL-1β are under investigation.

Adaptation of Respiratory-Related Brain Regions to Long-Term Hypercapnia: Focus on Neuropeptides in the RTN.

Long-term hypercapnia is associated with respiratory conditions including obstructive sleep apnea, chronic obstructive pulmonary disease and obesity hypoventilation syndrome. Animal studies have demonstrated an initial (within hours) increase in ventilatory drive followed by a decrease in this response over the long-term (days–weeks) in response hypercapnia. Little is known about whether changes in the central respiratory chemoreflex are involved. Here we investigated whether central respiratory chemoreceptor neurons of the retrotrapezoid nucleus (RTN), which project to the respiratory pattern generator within the ventral respiratory column (VRC) have a role in the mechanism of neuroplasticity associated with long-term hypercapnia. Adult male C57BL/6 mice (n = 5/group) were used. Our aims were (1) to determine if galanin, neuromedin B and gastrin-releasing peptide gene expression is altered in the RTN after long-term hypercapnia. This was achieved using qPCR to measure mRNA expression changes of neuropeptides in the RTN after short-term hypercapnia (6 or 8 h, 5 or 8% CO2) or long-term hypercapnia exposure (10 day, 5 or 8% CO2), (2) in the mouse brainstem, to determine the distribution of preprogalanin in chemoreceptors, and the co-occurrence of the galanin receptor 1 (GalR1:Gi-coupled receptor) with inhibitory GlyT2 ventral respiratory column neurons using in situ hybridization (ISH) to better characterize galaninergic RTN-VRC circuitry, (3) to investigate whether long-term hypercapnia causes changes to recruitment (detected by cFos immunohistochemistry) of respiratory related neural populations including the RTN neurons and their galaninergic subset, in vivo. Collectively, we found that hypercapnia decreases neuropeptide expression in the RTN in the short-term and has the opposite effect over the long-term. Following long term hypercapnia, the number of RTN galanin neurons remains unchanged, and their responsiveness to acute chemoreflex is sustained; in contrast, we identified multiple respiratory related sites that exhibit blunted chemoreflex activation. GalR1 was distributed in 11% of preBötC and 30% of BötC glycinergic neurons. Our working hypothesis is that during long-term hypercapnia, galanin co-release from RTN neurons may counterbalance glutamatergic inputs to respiratory centers to downscale energetically wasteful hyperventilation, thereby having a role in neuroplasticity by contributing to a decrease in ventilation, through the inhibitory effects of galanin.

Are galaninergic retrotrapezoid nucleus neurons more responsive to long-term hypercapnia?

Are galaninergic retrotrapezoid nucleus neurons more responsive to long-term hypercapnia?

Interrelationships between obesity, obstructive sleep apnea syndrome and cardiovascular risk in obese adolescents.

Background/ObjectivesObstructive sleep apnea syndrome (OSAS) may be a cardiovascular disease (CVD) risk factor independently of obesity in adults. Pediatric studies have associated OSAS with endothelial dysfunction, but few studies have examined relationships between OSAS and macrovascular sequelae. Our objective was to examine OSAS’s independent contribution to macrovascular CVD risk measures in obese adolescents.Subjects/MethodsThis cross-sectional observational study was conducted at Children’s Hospital of Philadelphia Clinical Research and Academic Sleep Centers, and University of Pennsylvania Vascular Research Unit. 31 obese non-diabetic adolescents underwent anthropometric measurements, overnight polysomnography, fasting laboratory draw, and cardiovascular imaging. Cardiovascular outcome measures included maximal carotid intima-media thickness (cIMTmax), a measure of carotid structural changes, and carotid-femoral pulse wave velocity (CFPWV), an aortic stiffness measure whose relationship vis-à-vis OSAS in children has not been previously examined. Carotid diameter and augmentation index (AIx, measuring central pressure augmentation from wave reflections) were assessed. Potential confounding variables examined included blood pressure, lipoproteins, high-sensitivity C-reactive protein, insulin and glucose.ResultsThe apnea hypopnea index, a primary OSAS measure, was not associated with cIMTmax, carotid diameter, CFPWV or AIx. BMI associated positively with cIMTmax (r=0.52, p=0.006) and CFPWV (r=0.45, p=0.01). Mean asleep end-tidal CO2 was negatively associated with carotid diameter (r=−0.63, p<0.0005). Insulin levels were negatively associated with AIx (r=−0.53, p=0.02).ConclusionsOSAS did not predict carotid structural changes or arterial stiffness independently of BMI in obese adolescents. Higher insulin levels associated with lower central pressure wave augmentation. Finally, long-term hypercapnia may predispose to carotid narrowing.

Prolonged SDA and reduced digestive efficiency under elevated CO2 may explain reduced growth in Atlantic cod (Gadus morhua)

Land-based aquaculture systems expose fish to elevated dissolved CO2 levels, a factor that is correlated with reduced growth, feed conversion efficiency and body condition index. The physiological basis underlying the pathological effects of environmental hypercapnia is poorly understood, in particular for marine fish species. We investigated whether changes in energy expenditure and the specific dynamic action (SDA) of digestion and assimilation could account for the lower growth of adult Atlantic cod (Gadus morhua) under environmental hypercapnia. Fish acclimated to a partial pressure of 800μatm CO2 (0.6mmHg, 1.5mg/L) and 9200μatm CO2 (7mmHg, 18.7mg/L) exhibited no difference in maintenance metabolic rates, which concurs with previous research for this species and other fish species. At 9200μatm CO2 Atlantic cod had a significantly diminished (14%) maximum aerobic capacity. While hypercapnia did not affect the amount of oxygen required for the SDA process, it did prolong the SDA duration by 23%. The longer SDA process time may offer an explanation for the observation of lower feed intake, growth and condition factor in long-term hypercapnia studies. Comparison of aerobic scope and cardiac performance during digestion suggested that reduced oxygen delivery capacity under hypercapnia could be one mechanism by which CO2 prolongs SDA, although our results could not definitively demonstrate this effect.

The effect of carbon dioxide on growth of juvenile Atlantic cod Gadus morhua L.

A trial was undertaken to investigate how exposure to graded hypercapnia affected the growth performance of juvenile (15–80 g) Atlantic cod. Juveniles were grown at 20‰ salinity and 10 °C for 55 days under three hypercapnic regimes: low (2 ± 0.9 mg L −1 CO 2, 0.6 mm Hg, 1000 μatm), medium (8 ± 0.5 mg L −1 CO 2, 2.8 mm Hg, 3800 μatm) and high CO 2 exposure (18 ± 0.2 mg L −1 CO 2, 6.3 mm Hg, 8500 μatm). All water quality parameters were within the range of what might normally be considered acceptable for good growth, including the CO 2 levels tested. Weight gain, growth rate and condition factor were substantially reduced with increasing CO 2 dosage. The size-specific growth trajectories of fish reared under the medium and high CO 2 treatments were approximately 2.5 and 7.5 times lower (respectively) than that of fish in the low treatment. Size variance and mortality rate was not significantly different amongst treatments, indicating that there was no differential size mortality due the effects of hypercapnia, and the CO 2 levels tested were within the adaptive capacity of the fish. In addition, an analysis was carried out of the test CO 2 concentrations reported in three other long-term hypercapnia experiments using marine fish species. The test concentrations were recalculated from the reported carbonate chemistry conditions, and indicated that the CO 2 concentration effect threshold may have been overestimated in two of these studies. Our study suggests that juvenile Atlantic cod are more susceptible to the chronic effects of environmental hypercapnia than other marine fish examined to date.

Effects of long-term acclimation to environmental hypercapnia on extracellular acid–base status and metabolic capacity in Mediterranean fish Sparus aurata

In the context of future scenarios of anthropogenic CO2 accumulation in marine surface waters, the present study addresses the effects of long-term hypercapnia on a Mediterranean fish, Sparus aurata. By equilibration with elevated CO2 levels seawater pH was lowered to a value of 7.3, close to the maximum pH drop expected in marine surface waters from atmospheric CO2 accumulation. Intra- and extracellular acid–base parameters as well as changes in enzyme profiles were studied in red and white muscles and the heart under both normocapnia and hypercapnia. The activities of pyruvate kinase (PK), lactate dehydrogenase (L-LDH), citrate synthase (CS), malate dehydrogenase and and 3-hydroxyacyl CoA dehydrogenase (HOAD) reflect the pathways and capacity of oxidative processes in metabolism. Long-term hypercapnia caused a transient reduction in blood plasma pH (pHe) as well as in intracellular pH (pHi). Compensation of the acidosis occurred through increased plasma and cellular bicarbonate levels. Changes in enzymatic activities, especially the increase in the activity of L-LDH, paralleled by a drop in CS activity in white and red muscles reflect a shift from aerobic to anaerobic pathways of substrate oxidation during long-term acclimation under hypercapnia. The present results suggest that moderate environmental hypercapnia changes the metabolic profile in tissues of S. aurata. Consequences for slow processes like growth and reproduction potential as well as potential harm at population, species and ecosystem levels require further investigation.

Hemodynamic and tissue blood flow responses to long-term pneumoperitoneum and hypercapnia in the pig

Increased peritoneal blood flow may influence the ability of cancer cells to adhere to and survive on the peritoneal surface during and after laparoscopic cancer surgery. Carbon dioxide (CO2) pneumoperitoneum is associated with a marked blood flow increase in the peritoneum. However, it is not clear whether the vasodilatory effect in the peritoneum is related to a local or systemic effect of CO2. In this study, 21 pigs were exposed to pneumoperitoneum produced with either CO2 (n = 7) or helium (He) (n = 7) insufflation at 10 mmHg for 4 h, or to two consecutive levels of hypercapnia (7 and 11 kPa) (n = 7) produced by the addition of CO2 to the inhalational gas mixture. Tissue blood flow measurements were performed using the colored microsphere technique. Blood flow in peritoneal tissue increased during CO2, but not He, pneumoperitoneum, whereas it did not change at any level of hypercapnia alone. There was no change in blood flow in most organs at the partial pressure of CO2 (PaCO2) level of 7 kPa. However, at a PaCO2 of 11 kPa, blood flow was increased in the central nervous system, myocardium, and some gastrointestinal organs. The blood flow decreased markedly in all striated muscular tissues during both levels of hypercapnia. The effect of CO2 on peritoneal blood flow during laparoscopic surgery is a local effect, and not attributable to central hemodynamic effects of CO2 pneumoperitoneum or high systemic levels of CO2.

Effects of long-term moderate hypercapnia on acid-base balance and growth rate in marine mussels Mytilus galloprovincialis

In the context of future scenarios of progressive accumulation of anthropogenic CO2 in marine surface waters, the present study addresses the effects of long-term hypercapnia on a Mediterranean bivalve, Mytilus galloprovincialis. Sea-water pH was lowered to a value of 7.3 by equilibration with elevated CO2 levels. This is close to the maximum pH drop expected in marine sur- face waters during atmospheric CO2 accumulation. Intra- and extracellular acid-base parameters as well as changes in metabolic rate and growth were studied under both normocapnia and hypercap- nia. Long-term hypercapnia caused a permanent reduction in haemolymph pH. To limit the degree of acidosis, mussels increased haemolymph bicarbonate levels, which are derived mainly from the dissolution of shell CaCO3. Intracellular pH in various tissues was at least partly compensated; no deviation from control values occurred during long-term measurements in whole soft-body tissues. The rate of oxygen consumption fell significantly, indicating a lower metabolic rate. In line with pre- vious reports, a close correlation became evident between the reduction in extracellular pH and the reduction in metabolic rate of mussels during hypercapnia. Analysis of frequency histograms of growth rate revealed that hypercapnia caused a slowing of growth, possibly related to the reduction in metabolic rate and the dissolution of shell CaCO3 as a result of extracellular acidosis. In addition, increased nitrogen excretion by hypercapnic mussels indicates the net degradation of protein, thereby contributing to growth reduction. The results obtained in the present study strongly indicate that a reduction in sea-water pH to 7.3 may be fatal for the mussels. They also confirm previous obser- vations that a reduction in sea-water pH below 7.5 is harmful for shelled molluscs.

Acid-base regulation, metabolism and energetics in sipunculus nudus as a function of ambient carbon dioxide level

Changes in the rates of oxygen consumption and ammonium excretion, in intra- and extracellular acid-base status and in the rate of H+-equivalent ion transfer between animals and ambient water were measured during environmental hypercapnia in the peanut worm Sipunculus nudus. During exposure to 1 % CO2 in air, intracellular and coelomic plasma PCO2 values rose to levels above those expected from the increase in ambient CO2 tension. Simultaneously, coelomic plasma PO2 was reduced below control values. The rise in PCO2 also induced a fall in intra- and extracellular pH, but intracellular pH was rapidly and completely restored. This was achieved during the early period of hypercapnia at the expense of a non-respiratory increase in the extracellular acidosis. The pH of the extracellular space was only partially compensated (by 37 %) during long-term hypercapnia. The net release of basic equivalents under control conditions turned to a net release of protons to the ambient water before a net, albeit reduced, rate of base release was re-established after a new steady state had been achieved with respect to acid-base parameters. Hypercapnia also affected the mode and rate of metabolism. It caused the rate of oxygen consumption to fall, whereas the rate of ammonium excretion remained constant or even increased, reflecting a reduction of the O/N ratio in both cases. The transient intracellular acidosis preceded a depletion of the phosphagen phospho-l-arginine, an accumulation of free ADP and a decrease in the level of Gibbs free energy change of ATP hydrolysis, before replenishment of phosphagen and restoration of pHi and energy status occurred in parallel. In conclusion, long-term hypercapnia in vivo causes metabolic depression, a parallel shift in acid-base status and increased gas partial pressure gradients, which are related to a reduction in ventilatory activity. The steady-state rise in H+-equivalent ion transfer to the environment reflects an increased rate of production of protons by metabolism. This observation and the reduction of the O/N ratio suggest that a shift to protein/amino acid catabolism has taken place. Metabolic depression prevails, with completely compensated intracellular acidosis during long-term hypercapnia eliminating intracellular pH as a significant factor in the regulation of metabolic rate in vivo. Fluctuating levels of the phosphagen, of free ADP and in the ATP free energy change values independent of pH are interpreted as being correlated with oscillating ATP turnover rates during early hypercapnia and as reflecting a tight coupling of ATP turnover and energy status via the level of free ADP.

Laryngeal nerve paraganglia of the rat are morphologically and biochemically unchanged by long-term hypercapnia

Exposure to hypercapnia (8.2 ± 0.7% CO 2) for 3 weeks failed to change the morphometric characteristics (mean cell area, nuclear, mitochondria and vesicle volume density) of the recurrent laryngeal nerve paraganglia of the rat. Moreover, this treatment had no effect on the dopamine and noradrenaline content of the superior and recurrent larynegal nerves. The results are in contrast to what is found after exposure to hypoxia which increases the dopamine content and the cell area of the paraganglia and indicate that the mechanisms of the paraganglia in long-term hypoxia and hypercapnia differ.

31P-NMR study of resting in vitro rat diaphragm exposed to hypercapnia

We have reported previously that, when exposed to hypercapnia of various intensities, the diaphragm reduces its force of twitch and tetanic contractions in the in vitro rat preparation as well as in the in vivo dog preparation. The experiments reported here with 31P nuclear magnetic resonance (31P-NMR) spectroscopy attempt to examine cellular mechanisms that might be responsible for this deterioration in mechanical performance. Specifically they describe certain characteristics of this preparation and cautions needed to study the resting in vitro rat diaphragm with such techniques. Second, they report the response of intracellular pH (pHi), phosphocreatine (PCr), ATP, and inorganic phosphate (Pi) in the resting in vitro rat diaphragm exposed to long-term normocapnia or to long-term hypercapnia. The results show that 1) to maintain a viable preparation, it was necessary to keep the diaphragm extended to an area approximating that at functional residual capacity, 2) the diaphragm seemed quite capable of maintaining a constant pHi and constant contents of ATP and Pi during normocapnia, but there was a gradual decline in PCr, and 3) during hypercapnia there was a significant decrease in pHi, but the behavior of the phosphate metabolites was exactly as during normocapnia. The results suggest that the decrease in mechanical performance of the diaphragm is probably not due to a decrease in the availability of the high-energy phosphates, although they do not completely exclude this possibility or possibilities related to regional compartmentation.

Carbonic anhydrase inhibition in vivo in rainbow trout ( Salmo gairdneri) acclimated to water of different ionic and gaseous compositions

1. 1. The effects of acetazolamide and metazolamide on acid-base variables and plasma ions in rainbow trout acclimated to fresh water, deionized water, and salt water, as well as to hypercapnia were investigated. 2. 2. Total blood CO 2 concentration decreased in response to metazolamide treatment in fish exposed to hard fresh water. The decrease was most pronounced after long-term hypercapnia. 3. 3. Metazolamide caused an increase in plasma Na + and Cl − content in fish acclimated to both deionized water and salt water. Plasma Cl − increased after treatment with metazolamide in hypercapnic fish. 4. 4. The results indicate that carbonic anhydrase plays an important role in maintaining blood CO 2 balance in trout acclimated to hard fresh water, especially water with a high pCO 2.

31P-NMR study of resting in vitro rat diaphragm exposed to hypercapnia.

We have reported previously that, when exposed to hypercapnia of various intensities, the diaphragm reduces its force of twitch and tetanic contractions in the in vitro rat preparation as well as in the in vivo dog preparation. The experiments reported here with 31P nuclear magnetic resonance (31P-NMR) spectroscopy attempt to examine cellular mechanisms that might be responsible for this deterioration in mechanical performance. Specifically they describe certain characteristics of this preparation and cautions needed to study the resting in vitro rat diaphragm with such techniques. Second, they report the response of intracellular pH (pHi), phosphocreatine (PCr), ATP, and inorganic phosphate (Pi) in the resting in vitro rat diaphragm exposed to long-term normocapnia or to long-term hypercapnia. The results show that 1) to maintain a viable preparation, it was necessary to keep the diaphragm extended to an area approximating that at functional residual capacity, 2) the diaphragm seemed quite capable of maintaining a constant pHi and constant contents of ATP and Pi during normocapnia, but there was a gradual decline in PCr, and 3) during hypercapnia there was a significant decrease in pHi, but the behavior of the phosphate metabolites was exactly as during normocapnia. The results suggest that the decrease in mechanical performance of the diaphragm is probably not due to a decrease in the availability of the high-energy phosphates, although they do not completely exclude this possibility or possibilities related to regional compartmentation.

Carbonic anhydrase activity in the blood and the gills of rainbow trout during long-term hypercapnia in hard, bicarbonate-rich freshwater.

Carbonic anhydrase (CA) activities in gills and venous blood, acid-base balance, and haematological variables were studied during environmental hypercapnia in rainbow trout (Salmo gairdneri). Batches of 8-10 fish were exposed to about 3 or 13 mmHg Pco2 in flow-through tests of various duration from 4 h to 80 days. After initial acidosis, blood pH rose above pre-experimental values. At 3 mmHg it became normal again within 21 days, while at 13 mmHg the overshoot lasted for 80 days. In fish acclimated for 3 weeks or more to 13 mmHg Pco2, blood HCO-3 increased four to five times while plasma Cl- levels were lower and K+ higher. Na+ levels did not show any consistent trend associated with exposure to hypercapnia. After an initial acidaemia, Hct, Hb, and RBC remained relatively constant. Patterns of change in CA activity differed between gills and erythrocytes. Initially, blood CA decreased at both Pco2 levels. It then began rising after about 3 weeks and tended to reach pre-experimental values by 80 day's hypercapnia. At 13 mmHg Pco2, gill CA increased to twice the pre-experimental level. Compared with blood CA, gill CA appeared to be more specifically involved in fish acclimation to hypercapnia, which demands an increase in blood bicarbonate to provide a sufficient buffering capacity. Increased CA indicates that the gill enzyme may play a more important role than blood CA in acid-base regulation in fish during hypercapnia.

Acid-base regulation and ion transfers in the carp (Cyprinus carpio): pH compensation during graded long- and short-term environmental hypercapnia, and the effect of bicarbonate infusion.

To study both temporal and quantitative effects of hypercapnia on the extent of pH compensation in the arterial blood, specimens of carp (Cyprinus carpio) were exposed to a PCO2 of about 7.5 mmHg (1 mmHg = 133.3 Pa) (1% CO2) in the environmental water for several weeks, and a second group of animals was subjected to an environmental PCO2 of about 37 mmHg (5% CO2) for up to 96 h. A third series of experiments was designed to test the possibility that infusion of bicarbonate would increase the extent of plasma pH compensation. Dorsal aortic plasma pH, PCO2 and [HCO3-], as well as net transfer of HCO3- -equivalent ions, NH4+, Cl- and Na+, between fish and ambient water, were monitored throughout the experiments. Exposure to environmental PCO2 of 7.5 mmHg resulted in the expected respiratory acidosis with the associated drop in plasma pH, and subsequent compensatory plasma [HCO3-] increase. The compensatory increase of plasma bicarbonate during long-term hypercapnia continued during 19 days of exposure with plasma bicarbonate finally elevated from 13.0 mmoll-1 during control conditions to 25.9 mmoll-1 in hypercapnia, an increase equivalent to 80% plasma pH compensation. Exposure to 5% hypercapnia elicited much larger acid-base effects, which were compensated to a much lesser extent. Plasma pH recovered to only about 45% of the pH depression expected at constant bicarbonate concentration. At the end of the 96-h exposure period, plasma [HCO3-] was elevated by a factor of 2.5 to about 28.2 mmoll-1. The observed increase in plasma bicarbonate concentration during 5% hypercapnic exposure was attributable to net gain of bicarbonate equivalent ions from (or release of H+-equivalent ions to) the environmental water. Quantitatively, the gain of 15.6 mmol kg-1 was considerably larger than the amount required for compensation of the extracellular space, suggesting that acid-base relevant ions were transferred for compensation of the intracellular body compartments. The uptake of bicarbonate-equivalent ions from the water was accompanied by a net release of Cl-and, to a smaller extent, by a net uptake of Na+, suggesting a 75% contribution of the Cl-/HCO-3 exchange mechanism. Infusion of bicarbonate after 48 h of exposure to 7.5 mmHg PCo2 had only a transient effect on further pH compensation. The infused bicarbonate was lost to the ambient water, and pre-infusion levels of bicarbonate were reattained within 24 h. Repetition of the infusion did not result in a notable improvement of the acid-base status.(ABSTRACT TRUNCATED AT 400 WORDS)