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.

Buffering limits plasma HCO3- dehydration when red blood cell anion exchange is inhibited.

Theory suggests that HCO3- dehydration in the plasma of rainbow trout is limited by both the absence of carbonic anhydrase (CA) activity and the low non-bicarbonate buffer capacity of the plasma (betaplasma). The potential for betaplasma to limit plasma HCO3- dehydration was assessed in rainbow trout in which HCO3- dehydration via the red blood cell (RBC) was inhibited using the anion exchange blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). DIDS administration reduced the rate of RBC HCO3- dehydration by 68-80% for at least 6h, resulting in the elevation of arterial CO2 tension (PaCO2) by 3.07 +/- 0.45 Torr (N = 6). Addition of bovine CA to the circulation of DIDS-treated trout caused PaCO2 to decrease significantly. This effect was increased significantly in rainbow trout in which betaplasma was elevated experimentally by intravascular injection of N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES), supporting the hypothesis that CA-catalysed HCO3- dehydration in the plasma of rainbow trout is limited by proton availability.

The effects of exogenous extracellular carbonic anhydrase on CO2 excretion in rainbow trout (Oncorhynchus mykiss): role of plasma buffering capacity.

The buffering capacity (beta) of rainbow trout (Oncorhynchus mykiss) plasma was manipulated prior to intravascular injection of bovine carbonic anhydrase to test the idea that proton (H+) availability limits the catalysed dehydration of HCO3- within the extracellular compartment. An extracorporeal blood shunt was employed to continuously monitor blood gases in vivo in fish exhibiting normal plasma beta (-3.9+/-0.3 mmol 1(-1) pH unit(-1)), and in fish with experimentally (using N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid]) elevated plasma beta (-12.1+/-1.1 mmol 1(-1) pH unit(-1)). An injection of 5 mg kg(-1) carbonic anhydrase equally reduced (after 90 min) the arterial partial pressure of CO2 in trout with regular (-0.23+/-0.05 Torr) or high (-0.20+/-0.05 Torr) plasma beta; saline injection was without effect. Because ventilation and venous blood gases were unaffected by carbonic anhydrase, the effect of extracellular carbonic anhydrase in lowering arterial partial pressure of CO2 was likely caused solely by a specific enhancement of CO2 excretion owing to acceleration of HCO3- dehydration within the plasma. The lowering of arterial partial pressure of CO2 in trout after injection of exogenous carbonic anhydrase provides the first in vivo evidence that the accessibility of plasma HCO3- to red blood cell carbonic anhydrase constrains CO2 excretion under resting conditions. Because the velocity of red blood cell Cl-/HCO3- exchange governs HCO3- accessibility to red blood cell carbonic anhydrase, the present study also provides evidence that CO2 excretion at rest is limited by the relatively slow rate of Cl-/HCO3- exchange. The effect of carbonic anhydrase in lowering arterial partial pressure of CO2 was unrelated to plasma buffering capacity. While these data could suggest that H+ availability does not limit extracellular HCO3- dehydration in vivo at resting rates of CO2 excretion, it is more likely that the degree to which plasma beta was elevated in the present study was insufficient to drive a substantially increased component of HCO3- dehydration through the plasma.

Effect of oleoresin capsicum (OC) and ortho-chlorobenzylidene malononitrile (CS) on ciliary beat frequency.

Tear gases are largely used to control civil unrest. Their incapacitating effects involve the eyes, skin, and respiratory tract. We aimed to evaluate the effects of ortho-chlorobenzylidene malononitrile (CS) and oleoresin capsicum (OC) on ciliary beat frequency (CBF) of mouse tracheal rings. Addition of 0.05% OC or 0.01% CS induced a progressive decrease in CBF, from 11.5+/-0.5 to 4+/-0.1 Hz (P<0.05) and from 12.5+/-0.5 to 2.5+/-0.1 Hz (P<0.05), respectively, 30 min after exposure to the tear gas. Addition of exogenous ATP inhibited the effect of OC, suggesting that ATP could be used to counteract these adverse effects on CBF. However, ATP was inefficient against CS. Methylene blue and H7 inhibited the effects of OC, whereas indomethacin had no effect. None of these drugs affected the inhibitory action of CS. These results suggest that the inhibitory effect of OC is mediated through the guanylate cyclase-dependent pathway or protein kinase C-dependent phosphorylation. Another mechanism is probably involved in CS-induced inhibitory effect. Histological analysis of the trachea revealed an increase in mucus secretion after exposure to OC, and cytoplasmic vacuoles in epithelial cells after exposure to CS.

Evidence for a mineralocorticoid-like receptor linked to branchial chloride cell proliferation in freshwater rainbow trout.

Fish acclimated to ion-deficient water exhibit proliferation of branchial chloride cells. The objective of the present study was to investigate the role of cortisol in this response using the corticosteroid receptor antagonists RU486 and spironolactone. RU486 is a potent antagonist of the glucocorticoid actions of cortisol, whereas spironolactone exhibits high-affinity binding to mineralocorticoid receptors, with a resulting blockade of mineralocorticoid properties in mammals. Untreated rainbow trout, as well as rainbow trout given a single intraperitoneal implant of coconut oil alone, coconut oil containing RU486 (0.5 mg g(-1)) or coconut oil containing spironolactone (0.1 mg g(-1)), were exposed to either dechlorinated city-of-Ottawa tapwater or artificial softwater for 7 days. Neither corticosteroid antagonist nor acclimation condition affected circulating plasma cortisol levels, plasma ion concentrations or gill Na(+)-K(+)-ATPase activity. Kidney Na(+)-K(+)-ATPase activity was significantly higher in softwater-acclimated fish than in fish held in dechlorinated tapwater. In addition, whereas RU486 treatment was found to be without effect on gill morphometrics, treatment with spironolactone inhibited the proliferation of chloride cells normally associated with acclimation to ion-deficient water. The results of the present study provide further evidence for the mineralocorticoid actions of cortisol in freshwater fish, specifically in eliciting chloride cell proliferation. Furthermore, these results support the hypothesis that distinct glucocorticoid and mineralocorticoid receptor populations are present in teleost fish, despite the apparent absence of the classic mineralocorticoid hormone, aldosterone.

The effects of endothelin-1 on the cardiorespiratory physiology of the freshwater trout (Oncorhynchus mykiss) and the marine dogfish (Squalus acanthias).

The aim of the present study was to evaluate the effects of endothelin-l-elicited cardiovascular events on respiratory gas transfer in the freshwater rainbow trout (Oncorhynchus mykiss) and the marine dogfish (Squalus acanthias). In both species, endothelin-1 (666 pmol kg(-1)) caused a rapid (within 4 min) reduction (ca. 30-50 mmHg) in arterial blood partial pressure of O2. The effects of endothelin-1 on arterial blood partial pressure of CO2 were not synchronised with the changes in O2 partial pressure and the responses were markedly different in trout and dogfish. In trout, arterial CO2 partial pressure was increased transiently by approximately 1.0 mmHg but the onset of the response was delayed and occurred 12 min after endothelin-1 injection. In contrast, CO2 partial pressure remained more-or-less constant in dogfish after injection of endothelin-1 and was increased only slightly (approximately 0.1 mmHg) after 60 min. Pre-treatment of trout with bovine carbonic anhydrase (5 mg ml(-1)) eliminated the increase in CO2 partial pressure that was normally observed after endothelin-1 injection. In both species, endothelin-1 injection caused a decrease in arterial blood pH that mirrored the changes in CO2 partial pressure. Endothelin-1 injection was associated with transient (trout) or persistent (dogfish) hyperventilation as indicated by pronounced increases in breathing frequency and amplitude. In trout, arterial blood pressure remained constant or was decreased slightly and was accompanied by a transient increase in systemic resistance, and a temporary reduction in cardiac output. The decrease in cardiac output was caused solely by a reduction in cardiac frequency; cardiac stroke volume was unaffected. In dogfish, arterial blood pressure was lowered by approximately 10 mmHg at 6-10 min after endothelin-1 injection but then was rapidly restored to pre-injection levels. The decrease in arterial blood pressure reflected an increase in branchial vascular resistance (as determined using in situ perfused gill preparations) that was accompanied by simultaneous decreases in systemic resistance and cardiac output. Cardiac frequency and stroke volume were reduced by endothelin-1 injection and thus both variables contributed to the changes in cardiac output. We conclude that the net consequences of endothelin-1 on arterial blood gases result from the opposing effects of reduced gill functional surface area (caused by vasoconstriction) and an increase in blood residence time within the gill (caused by decreased cardiac output.

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

Experiments were performed in vivo to elucidate the underlying mechanism(s) of apparent diffusion limitations for CO(2) excretion in rainbow trout (Oncorhynchus mykiss). Ligation of two gill arches and the associated expected reduction in gill surface area of 30% caused pronounced respiratory acidosis as indicated by elevated arterial blood P(CO(2)) (Pa(CO(2))) and reduced arterial blood pH. Under conditions of normoxia, arterial blood P(O(2)) (Pa(O(2))) was not significantly (statistically) reduced. However, during hypoxia (water P(O(2))=70-80 mmHg), the apparent trend for reduced Pa(O(2)) values became statistically significant in fish with 15% surface area reduction. To determine whether the elevated Pa(CO(2)) in fish with reduced surface area (30%) reflected true diffusion limitations or chemical equilibrium limitations imposed by the relatively slow rate of red blood cell Cl(-)/HCO(3)(-) exchange, fish were injected with carbonic anhydrase (CA) to permit catalysis of HCO(3)(-) dehydration within the plasma. Injection of CA caused a lowering of Pa(CO(2)) by 0.87+/-0.32 mmHg within 120 min and thus essentially eliminated the increase in Pa(CO(2)) (1.04+/-0.33 mmHg) that was caused by the reduction in surface area. These results clearly demonstrate that the elevation in Pa(CO(2)) evoked by gill surface area reduction is a consequence of chemical equilibrium limitations rather than true diffusion limitations, per se.

Comparative acute toxicity of o-chlorobenzylidene malononitrile (CS) and oleoresin capsicum (OC) in awake rats.

Tear gases are largely used to control civil unrest. Their incapaciting effects involve eyes, skin and respiratory tract. This study was performed to compare acute respiratory effects of o-chlorobenzylidene malononitrile (CS), oleoresin capsicum (OC) and their respective solvents in awake rats, using an integrated system of nose-only exposure and multiple monitoring of breathing. Aerosols were generated by a Collison Nebulizer from the solutions held in tear gas sprays. The reduction of minute ventilation, observed during a 5 min exposure, was significantly more important with CS than with OC: minute ventilation represented 29+/-8 and 50+/-6% of pre-exposure minute ventilation respectively (P<0.05). The reduction of minute ventilation observed with CS and OC solvents alone was not significantly different from that observed with the tear gases themselves. The decrease in minute ventilation observed, between the second and the fifth minute of exposure, was of the same level for repeated exposure separated by 24 h. Time necessary to recover to 80% of pre-exposure minute ventilation was not significantly different between the two tear gases: 722+/-272 and 691+/-262 s for CS and OC respectively (NS). Histological analysis of the trachea, performed at the end of exposures, revealed an increase in mucus secretion after exposure to OC and cytoplasmic vacuoles in epithelial cells after exposure to CS. In the lungs, interstitial oedema was observed after exposure to OC and emphysema after exposure to CS.