Carbon dioxide excretion in fishes

Thomas

Esbaugh

et al. 2009

Self Cite

SUMMARYCarbonic anhydrase (CA) is critical for CO 2 excretion in adult fish, but little is known of the expression or function of CA during early development. The present study examined the hypothesis that, as rates of CO 2 production increased during early development in zebrafish (Danio rerio), CA would become necessary for effective CO 2 excretion, and that the pattern of CA expression during early development would reflect this transition. Real-time RT-PCR was used to examine the mRNA expression of the two main intracellular CA isoforms over a time course of early development ranging from 0 to 120h post fertilization (h.p.f.). The mRNA expression of zCAb was generally higher than that of zCAc, particularly during the earliest stages of development. Rates of CO 2 excretion increased approximately 15-fold from 24 to 48h.p.f. whereas rates of O 2 uptake increased only 6.7-fold over the same period, indicating a relative stimulation of CO 2 excretion over O 2 uptake. Treatment of 48h.p.f. larvae with the CA inhibitor acetazolamide resulted in CO 2 excretion rates that were 52% of the value in control larvae, a significant difference that occurred in the absence of any effect on O 2 uptake. Antisense morpholino oligonucleotides were used to selectively knock down one or both of the main intracellular CA isoforms. Subsequent measurement of gas transfer rates at 48h.p.f. indicated that CA knockdown caused a significant relative inhibition of CO 2 excretion over O 2 uptake, regardless of which cytosolic CA isoform was targeted for knockdown. These results suggest that between 24h.p.f. and 48h.p.f., developing zebrafish begin to rely on CA to meet requirements for increased CO 2 excretion.

Section: Hmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Carbonic anhydrase expression and CO2 excretion during early development in zebrafishDanio rerio

Thomas

Esbaugh

et al. 2009

Self Cite

“…As is the case in dogfish (Wood et al, 1994;Henry et al, 1997;Gilmour et al, 2001;Perry and Gilmour, 2002), it was hypothesized that a membrane-associated plasma-facing CA isoform could participate in the dehydration of plasma HCO 3 -, thereby aiding CO 2 excretion. Such a scheme might be particularly beneficial to Protopterus owing to its low rate constant for RBC Cl -/HCO 3 -exchange (Jensen et al, 2003), the step normally considered to be rate limiting in CO 2 excretion (Perry, 1986;Tufts and Perry, 1998). In light of the results of the initial respirometry experiments, which showed the gill to be a minor route for CO 2 elimination, clearly the original hypothesis was no longer appropriate.…”

Section: Carbonic Anhydrase and Co 2 Excretionmentioning

confidence: 99%

“…First, we hypothesised that, as in dogfish, extracellular branchial membrane-associated carbonic anhydrase (CA) aids gill CO 2 excretion by allowing the catalysed dehydration of HCO 3 -within the plasma . Assuming a high enough buffer capacity in the plasma (Desforges et al, 2001;, this would allow CO 2 excretion to occur without a requirement for red blood cell (RBC) Cl -/HCO 3 -exchange, which is the rate-limiting step in CO 2 excretion (Perry, 1986) and the origin of chemical equilibrium limitations . To test this hypothesis, aerial and aquatic gas transfers were measured before and after inhibition of extracellular CA using a relatively impermeant CA inhibitor, benzolamide (BZ).…”

mentioning

confidence: 99%

An investigation of the role of carbonic anhydrase in aquatic and aerial gas transfer in the African lungfishProtopterus dolloi

Perry

Swenson

et al. 2005

SUMMARY Experiments were performed on bimodally breathing African lungfish Protopterus dolloi to examine the effects of inhibition of extracellular vs total (extracellular and intracellular) carbonic anhydrase (CA) activity on pulmonary and branchial/cutaneous gas transfer. In contrast to previous studies on Protopterus, which showed that the vast majority of CO2 is excreted into the water through the gill and/or skin whereas O2 uptake largely occurs via the lung, P. dolloi appeared to use the lung for the bulk of both O2uptake (91.0±2.9%) and CO2 excretion (76.0±6.6%). In support of the lung as the more important site of CO2 transfer,aerial hypercapnia (PCO2=40 mmHg) caused a significant rise in partial pressure of arterial blood CO2(PaCO2) whereas a similar degree of aquatic hypercapnia was without effect on PaCO2. Intravascular injection of low levels (1.2 mg kg-1) of the slowly permanent CA inhibitor, benzolamide, was without effect on red blood cell CA activity after 30 min, thus confirming its suitability as a short-term selective inhibitor of extracellular CA. Benzolamide treatment did not affect CO2 excretion, blood acid–base status or any other measured variable within the 30 min measurement period. Injection of the permeant CA inhibitor acetazolamide (30 mg kg-1) resulted in the complete inhibition of red cell CA activity within 10 min. However, CO2excretion (measured for 2 h after injection) and arterial blood acid–base status (assessed for 24 h after injection) were unaffected by acetazolamide treatment. Intra-arterial injection of bovine CA (2 mg kg-1) caused a significant increase in overall CO2excretion (from 0.41±0.03 to 0.58±0.03 mmol kg-1h-1) and an increase in air breathing frequency (from 19.0±1.3 to 24.7±1.8 breaths min-1) that was accompanied by a slight, but significant, reduction in PaCO2 (from 21.6±1.6 to 19.6±1.8 mmHg). The findings of this study are significant because they (i) demonstrate that, unlike in other species of African lungfish that have been examined, the gill/skin is not the major route of CO2 excretion in P. dolloi, and (ii) suggest that CO2 excretion in Protopterus may be less reliant on carbonic anhydrase than in most other fish species.

“…Assuming that CO 2 excretion in tambaqui follows the pathway mapped out for teleost fish in general (e.g. Perry, 1986;Tufts and Perry, 1998), carbonic anhydrase will catalyze the interconversion of CO 2 and HCO 3 -within the red blood cell, a step that is critical to the transfer of CO 2 from tissues to blood, and from blood to ventilatory water. In acetazolamide-treated fish, this step would be slowed to the uncatalyzed rate, thereby causing CO 2 retention (Henry and Heming, 1998).…”

Section: Table·1 the Effects In Tambaquimentioning

confidence: 99%

Cardiorespiratory responses to hypercarbia in tambaquiColossoma macropomum: chemoreceptor orientation and specificity

Milsom

Rantin

et al. 2005

Self Cite

SUMMARY Experiments were carried out to test the hypothesis that ventilatory and cardiovascular responses to hypercarbia (elevated water PCO2) in the tambaqui Colossoma macropomum are stimulated by externally oriented receptors that are sensitive to water CO2 tension as opposed to water pH. Cardiorespiratory responses to acute hypercarbia were evaluated in both the absence and presence of internal hypercarbia (elevated blood PCO2), achieved by treating fish with the carbonic anhydrase inhibitor acetazolamide. Exposure to acute hypercarbia (15 min at each level, final water CO2 tensions of 7.2,15.5 and 26.3 mmHg) elicited significant increases in ventilation frequency(at 26.3 mmHg, a 42% increase over the normocarbic value) and amplitude(128%), together with a fall in heart rate (35%) and an increase in cardiac stroke volume (62%). Rapid washout of CO2 from the water reversed these effects, and the timing of the changes in cardiorespiratory variables corresponded more closely to the fall in water PCO2(PwCO2) than to that in blood PCO2(PaCO2). Similar responses to acute hypercarbia (15 min,final PwCO2 of 13.6 mmHg) were observed in acetazolamide-treated (30 mg kg-1) tambaqui. Acetazolamide treatment itself, however, increased PaCO2 (from 4.81±0.58 to 13.83±0.91 mmHg, mean ± s.e.m.; N=8) in the absence of significant change in ventilation, heart rate or cardiac stroke volume. The lack of response to changes in blood PCO2 and/or pH were confirmed by comparing responses to the bolus injection of hypercarbic saline(5% or 10% CO2; 2 ml kg-1) into the caudal vein with those to the injection of CO2-enriched water (1%, 3%, 5% or 10%CO2; 50 ml kg-1) into the buccal cavity. Whereas injections of hypercarbic saline were ineffective in eliciting cardiorespiratory responses, changes in ventilation and cardiovascular parameters accompanied injection of CO2-laden water into the mouth. Similar injections of CO2-free water acidified to the corresponding pH of the hypercarbic water (pH 6.3, 5.6, 5.3 or 4.9, respectively) generally did not stimulate cardiorespiratory responses. These results are in agreement with the hypothesis that in tambaqui, externally oriented chemoreceptors that are predominantly activated by increases in water PCO2,rather than by accompanying decreases in water pH, are linked to the initiation of cardiorespiratory responses to hypercarbia.