Apparent diffusion limitations for CO(2) excretion in rainbow trout are relieved by injections of carbonic anhydrase

The physiological significance of plasma-accessible carbonic anhydrase in the respiratory systems of fishes

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work.

Effects of gill excision and food deprivation on metabolic scaling in the goldfish Carassius auratus

According to the metabolic-level boundaries hypothesis, metabolic level mediates the relative influence of surface area or volume-related metabolic processes on metabolic scaling in organisms. Therefore, variation in both metabolic level and surface area may affect metabolic scaling. Goldfish were used to determine the influence of both a surgical reduction in respiratory surface area and food deprivation on metabolic scaling exponents (b_R ). Gill excision did not change resting metabolic rate (RMR) or b_R (a common value of 0.895). However, ventilation frequency (VF) increased from 21.6 times min^-1 before gill excision to 52.8 times min^-1 after gill excision. This suggests that the acceleration of breathing after gill excision offsets the constraints of the respiratory surface area on RMR and results in no influence of surface area reduction on metabolic scaling. In the food deprivation experiment, RMR decreased; however, b_R (a common value of 0.872) did not increase. The VFs of the fish at weeks 1 and 2 were approximately 22% and 38% lower than that at Week 0, which may enhance exchange surface area limits and result in no increase in b_R with a decreasing RMR induced by food deprivation. The results suggest that food deprivation reduces metabolic level, but does not alter metabolic scaling exponent owing to variation in VF.

Conflict and Compromise: Using Reversible Remodeling to Manage Competing Physiological Demands at the Fish Gill

The structural features of the fish gill necessary for oxygen uptake also favor undesirable, passive movements of ions and water. Reversible gill remodeling is one solution to this conflict. Cell masses that limit functional surface area are lost when oxygen availability decreases in hypoxia or oxygen demand increases with exercise or high temperature. However, much remains to be learned about how widespread reversible gill remodeling is among fish species, and how and why it occurs.

The interactive effects of exercise and gill remodeling in goldfish (Carassius auratus)

Gill remodeling in goldfish (Carassius auratus) is accomplished by the appearance or retraction of a mass of cells (termed the interlamellar cell mass or ILCM) between adjacent lamellae. Given the presumed effects of gill remodeling on diffusing capacity, the goals of the current study were (1) to determine the consequences of increased aerobic O(2) demand (swimming) on gill remodelling and (2) to assess the consequences of the presence or absence of the ILCM on aerobic swimming capacity. Fish acclimated to 7 °C exhibited a marked increase in the ILCM which occupied, on average, 70.0 ± 4.1% of the total interlamellar channel area in comparison to an average ILCM area of only 28.3 ± 0.9% in fish acclimated to 25 °C. Incrementally increasing swimming velocity in fish at 7 °C to achieve a maximum aerobic swimming speed (U (CRIT)) within approximately 3 h resulted in a marked loss of the ILCM area to 44.8 ± 3.5%. Fish acclimated to 7 °C were subjected to 35 min swimming trials at 30, 60 or 80% U (CRIT) revealing that significant loss of the ILCM occurred at swimming speeds exceeding 60% U (CRIT). Prior exposure of cold water-acclimated fish to hypoxia to induce shedding of the ILCM did not affect swimming performance when assessed under normoxic conditions (control fish U (CRIT) = 2.34 ± 0.30 body lengths s(-1); previously hypoxic fish U (CRIT) = 2.99 ± 0.14 body lengths s(-1)) or the capacity to raise rates of O(2) consumption with increasing swimming speeds. Because shedding of ILCM during U (CRIT) trials complicated the interpretation of experiments designed to evaluate the impact of the ILCM on swimming performance, additional experiments using a more rapid 'ramp' protocol were performed to generate swimming scores. Neither prior hypoxia exposure nor a previous swim to U (CRIT) (both protocols are known to cause loss of the ILCM) affected swimming scores (the total distance swum during ramp U (CRIT) trials). However, partitioning all data based on the extent of ILCM coverage upon cessation of the swimming trial revealed that fish with less than 40% ILCM coverage exhibited a significantly greater swimming score (539 ± 86 m) than fish with greater than 50% ILCM coverage (285 ± 70 m). Thus, while loss of the ILCM at swimming speeds exceeding 60% U (CRIT) confounds the interpretation of experiments designed to assess the impact of the ILCM on swimming performance, we suggest that the shedding of the ILCM, in itself, coupled with improved swimming scores in fish exhibiting low ILCM coverage (<40%), provide evidence that the ILCM in goldfish acclimated to cold water (7 °C) is indeed an impediment to aerobic swimming capacity.

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).

Respiratory responses to hypoxia or hypercapnia in goldfish (Carassius auratus) experiencing gill remodelling

The presence of an interlamellar cell mass (ILCM) on the gills of goldfish significantly decreases the functional lamellar surface area and increases the diffusion distance for gas transfer and thus may impose a serious challenge for the transfer of respiratory gases (O₂ and CO₂). Here we tested the hypothesis that the presence of the ILCM in goldfish acclimated to 7°C impedes the uptake of O2 and excretion of CO₂. While Pa(O₂) remained unaltered, the baseline values of Pa(CO)₂ were significantly higher in goldfish at 7°C with ILCM present (5.55 ± 0.54 mmHg; mean ± SEM) than in goldfish at 25°C without the ILCM (3.98 ± 0.18 mmHg). Carbonic anhydrase (CA) injections relieved the apparent diffusion limitation imposed by the presence of the ILCM on CO₂ excretion (Pw(CO₂) levels dropped to 3.07 ± 0.32 mmHg). Interestingly, the exposure of fish to acute hypoxia evoked similar changes in Pa(O₂) at the two acclimation temperatures. Ethanol (EtOH) exposure was also used as a tool to further investigate the potential effects of the ILCM on branchial solute transfer. The results showed that the ILCM does not impede EtOH uptake in 7°C goldfish. Overall, the results of this study demonstrate that the remodelling of the goldfish gill associated with acclimation to 7°C water, while increasing Pw(CO₂) , has minimal impact on branchial O2 transfer.

The phylogeny of central chemoreception

Respiratory chemoreceptors responsive to changes in CO(2)/H(+) appear to be present in all vertebrates from fish to birds and mammals. They appear to have arisen first in the periphery sensitive to the external environment. Thus, in most fish CO(2)/H(+) chemoreceptors reside primarily in the gills and respond to changes in aquatic rather than arterial P(CO)₂ . In the air-breathing tetrapods (amphibians, mammals, reptiles and birds), the branchial arches regress developmentally and the derivatives of the branchial arteries are now exclusively internal. The receptors associated with these arteries now sense only arterial (not environmental) P(CO)₂/pH . Central CO(2)/H(+) chemoreception also appears to have arisen with the advent of air breathing, presumably as a second line of defense. These receptors may have arisen multiple times in association with several (but not all) of the independent origins of air breathing in fishes. There is strong evidence for multiple central sites of CO(2)/H(+) sensing, at least in amphibians and mammals, suggesting that it may not only have originated multiple times in different species but also multiple times within a single species.

Hydrogen sulfide as an oxygen sensor in trout gill chemoreceptors

O2 chemoreceptors elicit cardiorespiratory reflexes in all vertebrates, but consensus on O2-sensing signal transduction mechanism(s) is lacking. We recently proposed that hydrogen sulfide (H2S) metabolism is involved in O2 sensing in vascular smooth muscle. Here, we examined the possibility that H2S is an O2 sensor in trout chemoreceptors where the first pair of gills is a primary site of aquatic O2 sensing and the homolog of the mammalian carotid body. Intrabuccal injection of H2S in unanesthetized trout produced a dose-dependent bradycardia and increased ventilatory frequency and amplitude similar to the hypoxic response. Removal of the first, but not second, pair of gills significantly inhibited H2S-mediated bradycardia, consistent with the loss of aquatic chemoreceptors. mRNA for H2S-synthesizing enzymes, cystathionine beta-synthase and cystathionine gamma-lyase, was present in branchial tissue. Homogenized gills produced H2S enzymatically, and H2S production was inhibited by O2, whereas mitochondrial H2S consumption was O2 dependent. Ambient hypoxia did not affect plasma H2S in unanesthetized trout, but produced a PO2-dependent increase in a sulfide moiety suggestive of increased H2S production. In isolated zebrafish neuroepithelial cells, the putative chemoreceptive cells of fish, both hypoxia and H2S, produced a similar approximately 10-mV depolarization. These studies are consistent with H2S involvement in O2 sensing/signal transduction pathway(s) in chemoreceptive cells, as previously demonstrated in vascular smooth muscle. This novel mechanism, whereby H2S concentration ([H2S]) is governed by the balance between constitutive production and oxidation, tightly couples tissue [H2S] to PO2 and may provide an exquisitely sensitive, yet simple, O2 sensor in a variety of tissues.

Acid-base balance and CO2 excretion in fish: unanswered questions and emerging models

Carbon dioxide (CO(2)) excretion and acid-base regulation in fish are linked, as in other animals, though the reversible reactions of CO(2) and the acid-base equivalents H(+) and HCO(3)(-): CO(2)+H(2)O<-->H(+)+HCO(3)(-). These relationships offer two potential routes through which acid-base disturbances may be regulated. Respiratory compensation involves manipulation of ventilation so as to retain CO(2) or enhance CO(2) loss, with the concomitant readjustment of the CO(2) reaction equilibrium and the resultant changes in H(+) levels. In metabolic compensation, rates of direct H(+) and HCO(3)(-) exchange with the environment are manipulated to achieve the required regulation of pH; in this case, hydration of CO(2) yields the necessary H(+) and HCO(3)(-) for exchange. Because ventilation in fish is keyed primarily to the demands of extracting O(2) from a medium of low O(2) content, the capacity to utilize respiratory compensation of acid-base disturbances is limited and metabolic compensation across the gill is the primary mechanism for re-establishing pH balance. The contribution of branchial acid-base exchanges to pH compensation is widely recognized, but the molecular mechanisms underlying these exchanges remain unclear. The relatively recent application of molecular approaches to this question is generating data, sometimes conflicting, from which models of branchial acid-base exchange are gradually emerging. The critical importance of the gill in acid-base compensation in fish, however, has made it easy to overlook other potential contributors. Recently, attention has been focused on the role of the kidney and particularly the molecular mechanisms responsible for HCO(3)(-) reabsorption. It is becoming apparent that, at least in freshwater fish, the responses of the kidney are both flexible and essential to complement the role of the gill in metabolic compensation. Finally, while respiratory compensation in fish is usually discounted, the few studies that have thoroughly characterized ventilatory responses during acid-base disturbances in fish suggest that breathing may, in fact, be adjusted in response to pH imbalances. How this is accomplished and the role it plays in re-establishing acid-base balance are questions that remain to be answered.

Does bradycardia or hypertension enhance gas transfer in rainbow trout (Oncorhynchus mykiss)?

Experiments were conducted to test the hypothesis that branchial gas transfer is enhanced in rainbow trout during hypoxia or hypercarbia by bradycardia and systemic vasoconstriction. Gas transfer was indirectly assessed by continuous monitoring of arterial blood gases, PaO2 and PaCO2. Cardiac frequency was maximally decreased by 34.9+/-4.3 and 8.6+/-3.2 bpm in hypoxic and hypercarbic fish, respectively. Pre-treating fish with atropine (1micromol kg(-1)) attenuated or abolished the bradycardia during hypoxia and hypercarbia, respectively. However, there were no significant differences in the arterial blood gases between the control and atropinized fish. Dorsal aortic blood pressure was increased maximally by 11.3+/-2.8 and 17.7+/-2.0mm Hg in the hypoxic and hypercarbic fish. Pre-treatment of fish with prazosin (2.4micromol kg(-1)) prevented these increases in blood pressure. Blood gases were unaltered by prazosin treatment in the hypercarbic fish. However, in the hypoxic fish, gas transfer appeared to be impaired by prazosin on the basis of lowered PaO2 (by approximately 35 mm Hg compared to control fish) and increased PaCO2 (by approximately 0.3mm Hg). Because the normal hyperventilatory response to hypoxia was prevented by prazosin, it is possible that the impairment of gas transfer was related to inadequate ventilation rather than to any differences in the pressor response. The present results provide no evidence that gas transfer in rainbow trout is enhanced by bradycardia nor do they reveal any obvious benefit associated with the increases in blood pressure that accompany hypoxia and hypercarbia.

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

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.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Branchial membrane-associated carbonic anhydrase activity maintains CO2excretion in severely anemic dogfish

Plasma CO2reactions in Pacific spiny dogfish ( Squalus acanthias) have access to plasma and gill membrane-associated carbonic anhydrase (CA). Acute severe experimental anemia and selective CA inhibitors were used to investigate the role of extracellular CA in CO2excretion. Anemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering hematocrit from 14.2 ± 0.4% (mean ± SE; N = 31) to 5.2 ± 0.1% ( N = 31) had no significant impact on arterial or venous CO2tensions (PaCO2and PvCO2, respectively) over the subsequent 2 h. Pco2was maintained despite the reduction in red cell number and a significant 32% increase in cardiac output (V̇b), both of which have been found to cause PaCO2increases in teleost fish. By contrast, treatment of anemic dogfish with the CA inhibitors benzolamide (1.3 mg/kg) or F3500 (50 mg/kg), to selectively inhibit extracellular CA, elicited rapid and significant increases in PaCO2of 0.68 ± 0.17 Torr ( N = 6) and 0.53 ± 0.11 Torr ( N = 7), respectively, by 30 min after treatment. These findings provide a functional context in which extracellular CA in dogfish contributes substantially to CO2excretion. Additionally, the apparent lack of effect of V̇bchanges on Pco2suggests that, in contrast to teleost fish, CO2excretion in dogfish does not behave as a diffusion-limited system.

Apparent diffusion limitations on branchial CO2 transfer are revealed by severe experimental anaemia in brown bullhead (Ameiurus nebulosus)

In this study, the rapid (within 2 h) effects of acute anaemia on blood gas and acid-base status, as well as cardiorespiratory variables, were examined in brown bullhead (Ameiurus nebulosus). Anaemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering haematocrit from the control value of 23.5+/-1.0% (mean+/-S.E.M.; N=37) to 5.9+/-0.3% (N=37) resulted in a significant increase (by 2.63+/-0.51 torr; N=7) in arterial CO(2) tension (PaCO(2)) over the subsequent 2-h period in the absence of a change in arterial O(2) tension (PaO(2)). Treatment with bovine carbonic anhydrase (CA) reduced the extent of the PaCO(2) increase to the point where it was not statistically significant. In both control and CA-treated fish, arterial pH decreased during acute anaemia; the acidosis was of mixed respiratory and metabolic origin in control fish and primarily metabolic in CA-treated fish. Inducing anaemia caused increases in both cardiac output (V*b) and heart rate that were similar in control and CA-treated fish. Experimental elevation of V*b equivalent to that observed during anaemia, but in the absence of lowered haematocrit, increased PaCO(2) significantly by 1.49+/-0.74 to 1.64+/-0.78 torr (N=5) without affecting PaO(2). These findings suggest that CO(2) excretion in bullhead, as in rainbow trout, is effectively diffusion-limited, and that approximately half of the increase in PaCO(2) measured during the initial 2 h of anaemia results from the impact of increased blood flow (hence decreased gill transit time) in a diffusion-limited system.

Peripheral O2 chemoreceptors mediate humoral catecholamine secretion from fish chromaffin cells

This study addressed the hypothesis that the secretion of catecholamines from trout (Oncorhynchus mykiss) chromaffin cells, during hypoxia, is triggered by stimulation of O(2) chemoreceptors located within the gills. Sodium cyanide was administered into the inspired water (external cyanide) or injected into the gill circulation (internal cyanide) to pharmacologically stimulate external (water sensing) or internal (blood sensing) O(2) chemoreceptors, respectively. Both of these treatments caused an elevation of circulating catecholamine levels. The response to external, but not internal, cyanide was abolished by removal of the first gill arch. Hypoxia produced an increase in circulating catecholamine levels that was unaffected by removal of the first gill arch or by denervation of the pseudobranch. Cyanide and hypoxia both caused the well-documented cardiorespiratory reflexes normally observed in this species. This study demonstrates, for the first time, that gill O(2) chemoreceptors can initiate the reflex that leads to catecholamine release from the chromaffin cells and that stimulation of internally oriented O(2) receptors on all gill arches appears to be the physiologically important mechanism for initiating release.

Cardiorespiratory adjustments during hypercarbia in rainbow troutOncorhynchus mykissare initiated by external CO2receptors on the first gill arch

Experiments were performed to test the hypothesis that the marked ventilatory and cardiovascular responses to hypercarbia in rainbow trout Oncorhynchus mykiss arise from specific stimulation of chemoreceptors localised to the first gill arch. This was accomplished by measuring cardiorespiratory variables during acute hypercarbia (20 min at PCO2=8 mmHg; 1 mmHg=0.133 kPa) in fish subjected to selective bilateral extirpation of the first gill arch. The cardiovascular responses to hypercarbia in the intact fish included a significant bradycardia (from 75.0±1.6 to 69.0±2.0 beats min-1; means ± S.E.M.; N=16), an increase in dorsal aortic blood pressure (from 30.8±1.5 to 41.9±2.5 mmHg; N=16) and a rise in systemic vascular resistance (from 1.1±0.1 to 1.4±0.1 mmHg ml-1 kg-1 min-1; N=16). Removal of the first gill arch or pre-treatment with the muscarinic receptor antagonist atropine prevented the hypercarbic bradycardia without affecting the pressure or resistance responses. Correlation analysis,however, revealed shallow but significant inverse relationships between water PCO2 and cardiac frequency in both atropinised(r2=0.75) and gill-extirpated(r2=0.90) fish, suggesting a direct mild effect of CO2 on cardiac function. The ventilatory response to hypercarbia in the intact fish consisted of an increase in ventilation amplitude (from 0.62±0.06 to 1.0±0.13 cm; N=16) with no change in breathing frequency. Removal of the first gill arch lowered resting breathing frequency and prevented the statistically significant elevation of breathing amplitude. Gill extirpation, however, did not totally abolish the positive correlation between water PCO2 and ventilation amplitude (r2=0.84), suggesting the presence of additional(although less important) chemoreceptive sites that are not confined to the first gill arch. Plasma catecholamine levels were elevated during hypercarbia,and this response was unaffected by prior gill extirpation.

To assess whether the CO2 chemoreceptors of the first gill arch were sensing water and/or blood PCO2, bolus injections of CO2-enriched water or saline were made into the buccal cavity or caudal vein, respectively. Injections of CO2-enriched water to preferentially stimulate external receptors evoked catecholamine release and cardiorespiratory responses that closely resembled the responses to hypercarbia. As in hypercarbia, extirpation of the first gill arch prevented the bradycardia and the increase in ventilation amplitude associated with externally injected CO2-enriched water. Except for a slight decrease in cardiac frequency (from 73.0±2.8 to 70.3±3.5 beats min-1; N=11), injection of CO2-enriched saline to preferentially stimulate internal chemoreceptors did not affect any measured variable. Taken together, these data indicate that, in rainbow trout, the bradycardia and hyperventilation associated with hypercarbia are triggered largely by external CO2chemoreceptors confined to the first gill arch.

Sensing and transfer of respiratory gases at the fish gill

The gill is both a site of gas transfer and an important location of chemoreception or gas sensing in fish. While often considered separately, these two processes are clearly intricately related because the gases that are transferred between the ventilatory water and blood at the gill are simultaneously sensed by chemoreceptors on, and within, the gill. Modulation of chemoreceptor discharge in response to changes in O(2) and CO(2) levels, in turn, is believed to initiate a series of coordinated cardiorespiratory reflexes aimed at optimising branchial gas transfer. The past decade has yielded numerous advances in terms of our understanding of gas transfer and gas sensing at the fish gill, particularly concerning the transfer and sensing of carbon dioxide. In addition, recent research has moved from striving to construct a single model that covers all fish species, to recognition of the considerable inter-specific variation that exists with respect to the mechanics of gas transfer and the cardiorespiratory responses of fish to changes in O(2) and CO(2) levels. The following review attempts to integrate gas transfer and gas sensing at the fish gill by exploring recent advances in these areas.

Sensitivity of CO2 excretion to blood flow changes in trout is determined by carbonic anhydrase availability

The blood transit time through the gills of rainbow trout (Oncorhynchus mykiss) was modified by manipulation of cardiac output (Vb). The experiments tested the hypothesis that efficiency of CO2 excretion is sensitive to changes in blood flow owing to chemical equilibrium limitations. An extracorporeal blood shunt was used to continuously monitor blood gases in fish in which Vb was elevated (by 13.3 +/- 2.4 ml x min(-1) x kg(-1)) by intravascular saline injection or reduced (by 10.8 +/- 1.8 ml x min(-1) x kg(-1)) by removal of plasma. The arterial partial pressure of CO2 (Pa(CO2); an index of CO2 excretion efficiency) was increased with elevated Vb and was decreased with reduced Vb such that the changes in Pa(CO2) exhibited a significant positive sigmoidal relationship with the changes in Vb (r2 =0.75; P < 0.05). In contrast, there was no significant relationship between changes in the arterial partial pressure of O2 (Pa(O2); an index of O2 uptake efficiency) and changes in Vb (r2 = 0.07; P > 0.05). The intravenous administration of carbonic anhydrase (CA; 10 mg/kg) before vascular volume loading eliminated the increase in Pa(CO2) with increased Vb that was observed in control fish.

The CO2/pH ventilatory drive in fish

That ventilation in fish is driven by O2 has long been accepted. The O2 ventilatory drive reflects the much lower capacitance of water for O2 than for CO2, and is mediated by O2 receptors that are distributed throughout the gill arches and that monitor both internal and external O2 levels. In recent years, however, evidence has amassed in support of the existence of a ventilatory drive in fish that is keyed to CO2 and/or pH. While ventilatory responses to CO2/pH may be mediated in part by the O2 drive through CO2/pH-induced changes in blood O2 status, CO2/pH also appear to stimulate ventilation directly. The receptors involved in this pathway are as yet unknown, but the experimental evidence available to date supports the involvement of branchial CO2-sensitive chemoreceptors with an external orientation. Internally-oriented CO2-sensitive chemoreceptors may also be involved, although evidence on this point remains equivocal. In the present paper, the evidence for a CO2/pH-keyed ventilatory drive in fish will be reviewed.