Gill Permeability Research Articles

Active urea transport and an unusual basolateral membrane composition in the gills of a marine elasmobranch.

In elasmobranch fishes, urea occurs at high concentrations (350-600 mM) in the body fluids and tissues, where it plays an important role in osmoregulation. Retention of urea by the gill against this huge blood-to-water diffusion gradient requires specialized adaptations to the epithelial cell membranes. Experiments were performed to determine the mechanisms and structural features that facilitate urea retention by the gill of the spiny dogfish Squalus acanthias. Analysis of urea uptake by gill basolateral membrane vesicles revealed the presence of a phloretin-sensitive (half inhibition 0.09 mM), sodium-coupled, secondary active urea transporter (Michaelis constant = 10.1 mM, maximal velocity = 0.34 micromol. h(-1). mg protein(-1)). We propose that this system actively transports urea out of the gill epithelial cells back into the blood against the urea concentration gradient. Lipid analyses of the basolateral membrane revealed high levels of cholesterol contributing to the highest reported cholesterol-to-phospholipid molar ratio (3.68). This unique combination of active urea transport and modification of the phospholipid bilayer membrane is responsible for decreasing the gill permeability to urea and facilitating urea retention by the gill of Squalus acanthias.

Influence of temperature and membrane lipid composition on the osmotic water permeability of teleost gills.

Osmotic water uptake was measured gravimetrically in isolated, ligated gill arches from trout (acclimated to and incubated at 5 degrees and 20 degrees C) and tilapia (21.5 degrees and 33 degrees C). For both species, incubation of arches at the higher temperature led to 1.5- to 3-fold greater measures of water weight gain. However, gills from warmer-acclimated trout and tilapia had 1- to >3-fold lower the initial rate and 1.5- to >2.5-fold lower the extent of water uptake seen in colder-acclimated conspecifics. Both the incubation temperature sensitivity and the acclimation effects are consistent with transmembrane water permeation. Calcium-free incubations (permitting paracellular water movement) also indicated that interfacial cell membranes contribute to gill permeability characteristics; without calcium, trout gill osmotic water uptake values increased 1.5- to 2-fold, and the temperature dependence of water uptake decreased (initial rate) or was eliminated (extent). The specific contribution of cholesterol to restricting barrier membrane water permeability was indicated by concentration-dependent increases in water uptake in the presence of either nystatin (a cholesterol-complexing, pore-forming agent) or methyl-beta-cyclodextrin (which selectively depletes membrane cholesterol). In addition, a cholesterol-specific cytochemical probe (filipin) intensely labeled the apical surface membranes of trout and tilapia gill epithelium. In summary, these studies implicate membrane cholesterol in determining water permeability in fish gills.

Copper uptake kinetics across the gills of rainbow trout ( Oncorhynchus mykiss) measured using an improved isolated perfused head technique

Measurements of branchial Cu uptake in vivo are complicated by rapid changes in the Cu stores of other body compartments such as the blood and liver. This study eliminates these problems by adopting an in vitro approach. The perfused head technique was improved and viability extended to 2 h or more, so that kinetic studies on Cu uptake could be performed. The improvements included the use of Leibovitz L-15 culture medium instead of saline perfusates, ventilation of the gills with single pass flowing water to allow control of metal speciation, and the use of surgical glue rather than sutures to greatly reduce surgery time. Viability was assessed by perfusion pressure of <50 cm H 2O, demonstration of net Na influx, <7.1 IU ml −1 of lactate dehydrogenase (LDH) activity in the perfusate, and a steady-state ethanol permeability of the gills to indicate the absence of changes in functional gill area. Gill morphology was also checked histologically. Concentrations of Cu >10 μmol l −1 caused only slight effects on some secondary lamellae, but this did not effect gill haemodynamics, gill permeability (ethanol uptake), or perfusate LDH activity when compared to similar measurements in non-viable preparations. Cumulative Cu uptake into the perfusate did not follow changes in perfusate flow and reached a steady state in about 1 h. Copper uptake kinetics based on the total Cu concentration in the water fitted a rectangular hyperbole (the Michaelis equation) with a regression coefficient ( r 2) of 0.98. The K m and V max values were 11.85±2.8 μmol l −1 and 52.2±4.9 nmol kg −1 h −1, respectively (mean±SE, n=5). Copper uptake rate plotted against free divalent Cu (Cu 2+) in the water did not fit the Michaelis equation, but showed second order reaction kinetics ( r 2=0.87) with a rate constant ( k) of 4.43. Copper uptake was abolished by the serosal application of 50 μmol l −1 vanadate. These observations suggest that Cu uptake by the gills is mediated through a vanadate sensitive P-type ATPase which has a similar K m (total Cu) to the mammalian Cu-ATPase. The second order reaction kinetics for Cu 2+ uptake from the water is consistent with the reduction of Cu 2+ to Cu + prior to membrane transport, as observed in mammalian cells.

Toxicological aspects associated with the ecology of Donax trunculus (Bivalvia, Mollusca) in a polluted environment

This study provides data on the ecology and toxicology observed in the population of Donax trunculus, a sand dwelling mussel, in the shallow subtidal of Haifa Bay (Mediterranean Sea, Israel). The studied population of the mussel forms a dominant fraction in a community of sand-dwelling molluscs in a zone located 5–25 m from the shoreline, and at depths of 20–120 cm, numbering up to 2000 per m2. Samples of the mussel were collected from three sites, located within 9 km of shore in Haifa Bay. These included a clean site (Akko), a site polluted by a chemical PVC-polymer industry (Frutarom), and a site polluted with oil and waste from the petrochemical industry (Qiryat Yam). Metal analysis indicated site-dependent variations in levels of cadmium, lead, copper and mercury in the mussel soft tissues. Copper levels were similar in the bivalves collected from all the sites. Levels of mercury were significantly higher in specimens from the PVC-polluted site (Frutarom) while levels of cadmium were higher in Akko and Qiryat Yam compared to Frutarom. Lead residues were found only in Donax from Akko. The residual contents of mercury, copper and cadmium were relatively high in the young and noticeably low in Donax of medium body size. The main site of deposition of metals was in the soft tissues of the bivalve, but bioaccumulation of metals was also found in the shells. A marked increase in permeability of gills and mantle to the fluorescent anionic dye — fluorescein (FLU) was detected in Donax from Qiryat Yam and especially Frutarom, compared to the bivalves sampled from Akko. Multiple foci of enhanced permeability (multiple fluorescent spots) were detected in all the individuals sampled from Frutarom but none in the bivalve samples from Akko. Lysosomal accumulation of the metachromatic fluorescent cationic probe, acridine orange — (AO), was significantly decreased in the tissues of D. trunculus from polluted sites, especially polluted by the PVC factory. This decrease correlated with lysosomal enlargement and the formation of secondary lysosomes. D. trunculus appears to possess the most effective biochemical and physiological defense mechanisms enabling it to survive in habitats of polluted shallow waters, where other sand dwelling mollusc species were absent or found only in waters deeper then 2.5 m.

Urea and water permeability in the ureotelic gulf toadfish (Opsanus beta).

The permeability of toadfish gills and skin to urea and water has been measured in order to investigate the mechanisms behind the pulsatile excretion of urea previously described in this species. A perfused gill preparation was used in the gill studies and isolated pieces of skin mounted in an Ussing chamber in the skin studies. Simultaneously, urea and water permeability was measured in vivo in free swimming fish. In vivo the nonpulsing urea permeability was exceptionally low compared to other teleosts, while the tritiated water permeability was similar to that of other teleosts. The urea permeability increased 30-fold during a pulse while water permeability stayed unaffected. Compared to in vivo, tritiated water permeability was approximately 50% lower in the gills and the skin when measured directly in the isolated preparations. The urea permeability was almost identical between the three preparations. Four out of 20 perfused gill preparation showed a spontaneous urea pulse during perfusion. Several treatments were tested to elicit the pulse artificially but without success. Hormones and drugs tested were: arginine-vasotocin (AVT), 10(-10) M; adrenaline, 10(-7) M; isoprenaline, 10(-5) M; acetylcholine, 10(-7) and 10(-6) M; serotonin, 10(-7) and 10(-6) M; adenosine, 10(-6) M; cortisol, 10(-7) M; and combinations of AVT, adrenaline, and cortisol. Adrenaline and isoprenaline increased tritiated water permeability without affecting urea permeability. Gradually increasing the ammonia levels in the perfusate from 0.1 mM to 1.6 mM caused a slight increase in water permeability but a marked and progressive increase in urea permeability. No indications of an ammonia trapping mechanism in the gills were found. There was no effect of AVT (10(-10) mol l-1) in the urea permeability of the skin preparation while cortisol (10(-7) M) led to a modest increase in urea permeability. Based on a comparison between the in vivo and in vitro preparations used here, we conclude that the urea pulse in a urea-pulsing toadfish occurs through the gills and not the skin. We still do not know which internal mechanism or signal triggers the urea pulse in the toadfish.

Pulsatile urea excretion in gulf toadfish (Opsanus beta): evidence for activation of a specific facilitated diffusion transport system.

When toadfish are made ureotelic by a crowding/confinement protocol, they excrete approximately 90 % of their urea nitrogen (urea-N) production in large, irregular pulses (1-2 pulses per day) from the gill region. We investigated three hypotheses as to the mechanism of pulsatile excretion: (i) the presence of an active reabsorptive 'back-transport' mechanism that is periodically inhibited to allow urea-N excretion to occur; (ii) the periodic occurrence of a generalized, non-specific increase in gill permeability; and (iii) the presence of a specific facilitated diffusion transport system that is periodically activated. Exposure of toadfish during non-pulse periods to treatments designed to block a 'back-transport' mechanism (Na+-free sea water or the urea analogues 30 mmol l-1 thiourea or 30 mmol l-1 acetamide in the external water) did not stimulate a leakage of urea-N, thereby opposing the first hypothesis. The second hypothesis was opposed by several results. Neither injection of the potent branchial vasodilator L-isoprenaline (10(-5) mol l-1) nor infusion of NH4Cl, the latter at levels known to stimulate urea-N efflux in perfused gills, had any effect on urea-N excretion. Furthermore, during natural pulse events, when the normally very low gill permeability to urea (3x10(-7) cm s-1) increased at least 35-fold, there was no accompanying increase in permeability to either 3H2O (1.5x10(-5) cm s-1) or the paracellular marker [14C]PEG-4000 (10(-8) cm s-1). However [14C]thiourea permeability (1.5x10(-7) cm s-1) increased approximately fivefold, in support of the third hypothesis. Furthermore, when 30 mmol l-1 urea was placed in the external water, a concentration (60 000 micromol-N l-1) approximately three times that of blood (20 000 micromol-N l-1), each efflux pulse event (measured with [14C]urea) was accompanied by a net uptake, such that blood urea-N levels rose rather than fell. A proportional 1:1 relationship between influx per unit external concentration and efflux per unit internal (i.e. plasma) concentration indicated a fully bidirectional transport system. The simultaneous presence of 60 mmol l-1 thiourea in the external water inhibited the influx component by 73 %, further supporting this conclusion. These data, together with recent molecular, morphological and endocrinological evidence, strongly suggest that pulsatile urea-N excretion is caused by the periodic activation of a facilitated urea transporter in the gills, similar to the vasopressin-regulated urea transporter in the mammalian kidney.

Acclimation to hard or soft water at weakly alkaline pH influences gill permeability and gill surface calcium binding in rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) were acclimated for 10 d to soft (0.1 mM as CaCO3) or hard (1.0 mM as CaCO3) water at weakly alkaline pH (8.06–8.34). Following acclimation, individual gill arches were removed for examining the effects of low hardness or high hardness acclimation on gill water permeability, gill Ca2+interactions, and gill aluminum interactions. Isolated gill arches were exposed to water of varying Ca2+(0.0–1.0 mM) and aluminum (3.7–37 μM) concentration for osmotic permeability experiments. High hardness acclimated gills had significantly greater percent weight gain over time caused by osmotic water entry than low hardness acclimated gills, when exposed to distilled water (32.34 ± 1.15 and 24.86 ± 0.62%, respectively, after 60-min incubations); these differences were absent when Ca2+(0.1–1.0 mM) was added to the incubation medium. Gill arch Ca2+binding experiments resolved two gill surface binding site populations, which differed in their Ca2+binding affinity. The higher affinity sites were probably associated with gill membrane permeability, because low hardness acclimated gills had more such sites (binding capacity, 0.322 ± 0.027 μmol Ca2+∙g−1) and less permeable gills than high hardness acclimated gills (binding capacity, 0.198 ± 0.004 μmol Ca2+∙g−1). Aluminum had little influence on gill permeability and gill Ca2+binding.

Electrogenic action of calcium on crayfish gill

When intact crayfish are in an ion-poor medium (KCl, 0.1 mmol·l-1+KHCO3, 0.1 mmol·l-1) there is a large potential difference (transepithelial potential difference),-20 to-40 mV (hemolymph negative), across the gills. Addition of Ca2+ to the medium is followed by a rapid change in transepithelial potential difference to near 0 mV. The transepithelial potential difference showed a non-linear dependence on [Ca2+]out with a limiting value of+2 to+10 mV at>1 mmol·l-1. The concentration generating a half-maximum transepithelial potential difference change (15–20 mV) was 0.1 to 0.2 mmol·l-1. Three other alkaline earth ions were also electrogenic; Ba2+ caused slightly larger transepithelial potential difference changes, Sr2+ and Mg2+ were a little less effective. It has been suggested that the transepithelial potential difference in ion-poor medium (in fish) is due to the diffusive efflux of NaCl across the gills, with a Cl-/Na+ permeability ratio of <1. Evidence is presented that this might be the case in crayfish. The electrogenic effect of Ca2+ might then be due to its effect on gill permeability to Na+ and Cl- such that the permeability ratio increased and approached unity as the transepithelial potential difference approached 0. However, this was shown to be unlikely. An alternative explanation for Ca2+ dependence of the transepithelial potential difference is that active inward Ca2+ transport is electrogenic.

Differential responses to copper in rainbow trout (Oncorhynchus mykiss) acclimated to sea water and brackish water

Rainbow trout, Oncorhynchus mykiss, acclimated to 33% sea water (12 mg·ml-1 salinity) experienced significant (10 meq·1-1) increases in plasma [Na+] and [Cl-] within 5 h of exposure to 6.3 μmol copper·1-1 indicating severe impairment of branchial ionoregulatory capacity. All plasma ion levels subsequently stabilised once the transbranchial [Na+] gradient was reduced to zero. The similar ionic strength of the external medium and their body fluids appeared to protect trout maintained in 33% sea water from further ionoregulatory stress and any secondary physiological disturbances during exposure to copper. Despite three- and fourfold greater transbranchial [Na+] and [Cl-] gradients, trout acclimated to full-strength sea water (35 mg·ml-1 salinity) suffered no major changes in plasma Na+, Cl-, K+, or Ca2+, blood gases or haematology during 24 h exposure to 6.3 μmol copper·1-1. This reduction in toxicity in full strength sea water cannot be explained by differences in copper speciation. We suggest that during acute exposure to waterborne copper, active NaCl extrusion is unaffected due to the basolateral location of the gill Na+/K+-ATPase, but that ionoregulatory disturbances can occur due to gill permeability changes secondary to the displacement of surface-bound Ca2+. However, in full strength sea water the three-fold higher ambient [Ca2+] and [Mg2+] appear to be sufficient to prevent any detrimental permeability changes in the presence of 6.3 μmol copper·1-1. Plasma [NH + 4 ] and [HCO - 3 ] were both significantly elevated during exposure to copper, indicating that some aspects of gill ion transport (specifically the apical Na+/NH + 4 and Cl-/HCO - 3 exchanges involved in acid/base regulation and nitrogenous waste excretion) are vulnerable to inhibition in the presence of waterborne copper.

Rubidium fluxes and ion permeability studies in the isolated perfused Carcinus gills

1. 1. Rubidium transport and inorganic osmolyte permeabilities were studied in isolated, perfused Carcinus gills, 2. 2. The gill permeability coefficient calculated from the unidirectional ion fluxes in the perfused gills in diluted sea water identical to the bath showed more cation than anion selectivity, showing the following permeability ratios P Rb: P Na: P Cl = 1.7:1.0:0.2. 3. 3. Net Rb secretive fluxes ( J b→a from the haemolymph across the basolateral and apical membrane surfaces into the bathing solution were inhibited (51%) by basolaterally applied ouabain (5 × 10 −3M). 4. 4. The results suggest the presence of ouabain-sensitive active Rb secretion where the mechanisms are basolaterally located. Net rubidium fluxes were not changed by externally applied 10 −4M amiloride.

Transepithelial ion exchange in smolting atlantic salmon (Salmo Salar L.)

Whole-body (but predominantly gill) Na(+) exchange, gill Na(+)/K(+)/ATPase activity and seawater tolerance were examined in juvenile Atlantic salmon during the smolting period. Transepithelial net Na(+) gain decreased steadily from late February showing a net loss in April and early May, returning to approximate equilibrium in mid-May. This seasonal net loss of Na(+) to the environment occurred slightly after maximal gill epithelial Na(+)/K(+)/ATPase activity and preceded maximal seawater tolerance. The results are discussed in relation to changes in gill permeability and salt intake via the diet.

Physiological Correlates of Interspecific Variation in Acid Tolerance in Fish

ABSTRACT This study investigated ion regulation in relation to water pH in three species of fish of differing tolerance to low pH (common shiners, Notropis cornutus, most sensitive; rainbow trout, Salmo gairdneri, intermediate; yellow perch, Perea flavescens, least sensitive). Increasing sensitivity to exposure to low pH was characterized by shorter survival times, greater losses of whole-body ions, more complete inhibition of Na+ uptake, and greater stimulation of Na+ efflux, the latter being the most important factor in determining survival. Interspecific variations in acid tolerance were also correlated with Na+ transport characteristics at circumneutral pH; Km was directly correlated Vmax and inversely correlated with acid tolerance. In addition, there were large qualitative differences among the species in the Ca2+-dependence of Na+ efflux. Sodium efflux induced by low pH was markedly Ca2+-dependent in both trout and shiners in a manner consistent with a simple competition between Ca2+ and H+ for gill binding sites. The increased sensitivity of shiners relative to trout was related to lowered Cam binding activity. In contrast, Na+ efflux in perch was virtually unaffected by water [Ca2+], Similarly, La3+ (a Ca2+ antagonist) stimulated higher Na+ losses from shiners than from trout, but had little effect upon perch. Ionic losses produced by saturating La3+ concentrations were generally lower than those produced by H+, suggesting that Ca2+ displacement is not the only mechanism for increased gill permeability at low pH. Nonetheless, the results obtained are consistent with the notion that acid tolerance may be related to Ca2+-binding activity in some species (e.g. trout and shiners) although not in others (e.g. perch).

The Effect of External Sodium and Calcium Concentrations on Sodium Fluxes by Salt-Depleted and Non-Depleted Minnows, Phoxinus Phoxinus (L.)

ABSTRACT The relationship between sodium influx and external sodium concentration in Phoxinus is complex and unusual. In non-depleted fish the relationship is approximately that given by the Michaelis–Menten equation of enzyme kinetics. However, the Km value (a measure of the affinity of the sodium uptake mechanism for sodium) is very high (3 mmol 1−1), indicating a low affinity of the uptake mechanism for sodium. On sodium depletion, the relationship between sodium influx and external sodium concentration changes to produce a curve which has a stepped appearance, and is unusual in that the maximum influx is not increased above that in non-depleted fish. The overall Km alters very little; however, the Km for the lower part of the curve is very low (0·05 mmol 1−1). A model is proposed to explain these results in the form of two sodium uptake mechanisms working in parallel across the gill. The second carrier is only active when the fish is sodium-depleted and kept in low external sodium concentrations. Neither the external sodium concentration nor the external calcium concentration has any direct effect on sodium efflux. However, fish depleted in 1 mmol 1−1 calcium have a lower sodium efflux than fish depleted in distilled water. Calcium appears to reduce the permeability of the gill to ions such as sodium. Since calcium has no effect on sodium influx, changes in gill permeability do not involve the sodium influx mechanism.

Permeability of the integument of the horseshoe crab, Limulus polyphemus, to water, sodium, and bromide

AbstractMean water movements across the double chitin membrane of the Limulus gill are 5,000–7,000 μmoles/cm2 · h. The carapace is much less permeable to water than the gill; this difference is probably due primarily to the greater thickness of the carapace. Variations in serum sodium concentration due to salinity changes do not affect the rates of water diffusion across the gills or carapace. The cause of a substantial variation between individuals in integumental water permeability is unknown. The sodium influx across the carapace of Limulus (0.7–1.6 μmoles/cm2 · h) is only a fraction that of the unidirectional water flux under the same conditions (0.5 M NaCl on both sides). This difference may be attributed to the greater size of the sodium ion in solution. There is no apparent effect of variations in carapace thickness on the sodium influx. The double chitin gill membrane is similarly low in sodium and bromide permeability (about 0.5–0.6 μmole/cm2 · h). Thus it appears that the gills may have a relatively passive role in ionic regulation. Their low rates of ionic diffusion help to restrict the loss or intake of salts as the salinity changes. However, the high gill permeability to water must pose a severe problem for an animal coping with changes in external salinity.

Changes in osmotic water permeability of the eel gills during seawater and freshwater adaptation

Changes in osmotic water permeability of the isolated gills of the Japanese eel,Anguilla japonica were studied during transfer to seawater or to fresh water. The water permeability increased gradually during the course of seawater transfer and attained a maximal level after 2 weeks. The water permeability of the freshwater eel gills was reduced when calcium ions were added to the incubation medium at a concentration of 1 mM, where-as no effect of the ion was observed on the gills of the seawater-adapted eel even at a higher concentration (10 mM). In contrast to seawater transfer, the water permeability decreased to a low level almost immediately (3 h) after transfer from seawater to fresh water. The acute reduction of the water permeability was also seen in the gills of the hypophysectomized eel after transfer to fresh water.

Pardaxin increases solute permeability of gills and rectal gland in the dogfish shark ( Squalus acanthias)

1. The action of the ichthyotoxic secretion of the Red Sea flatfish Pardachirus marmoratus and its derived toxin, pardaxin, was examined in the dogfish shark ( Squalus acanthias). 2. Pardaxin was more toxic when administered to the bathing medium than when injected into a dorsal artery and it transiently diminished the spiracular rate and caused a severe struggling response in the adult shark only when administered to the head region of the shark. 3. Pardaxin caused a transient leakage to urea and sodium between the shark and the seawater. 4. In the isolated perfused rectal gland pardaxin irreversibly reduced the rate of chloride secretion and concentration gradient of urea between perfusate and rectal gland fluid. In addition, ultrastructural studies on the rectal gland showed that ionic lanthanum penetrated the tight junctions and foci of cell necrosis were observed. 5. These studies indicate that in shark the gills are the most probable target of the toxicity of pardaxin.

Effects of external Mg 2+ and Ca 2+ on branchial osmotic water permeabiligy and prolactin secretion in the teleost fish Sarotherodon mossambicus

In Sarotherodon mossambicus prolactin cell activity is related to ambient Ca 2+ levels, and prolactin has hypercalcemic activity in this species. To study whether prolactin has a direct action on calcium metabolism, or whether prolactin's relationship with calcium is indirect and connected with control of gill permeability, the effects of external Ca 2+ and Mg 2+ on prolactin secretion and gill permeability were compared. It appeared that high external Mg 2+ was associated with reduced prolactin secretion, even though high Mg 2+ resulted in a marked hypocalcemia. Exposure of fish to high Ca 2+ levels led to hypercalcemia. Both high Mg 2+ and high Ca 2+ concentrations in the ambient water reduced the osmotic water permeability of the gills. These results represent further evidence that prolactin secretion in S. mossambicus may be affected by any external factor that interferes with branchial permeability. It is concluded that prolactin's main function in this species is connected with control of branchial permeability rather than calcium metabolism, although internal calcium may be implicated in permeability control.

Studies of Ammonia in the Rainbow Trout Physico-Chemical Parameters, Acid-Base Behaviour and Respiratory Clearance

ABSTRACT Ammonia (NH3) is only slightly more soluble (+3·4%) in the plasma of rainbow trout than in water, and its pK′ is only 0·14 units higher than in fresh water at 15 °C. Determination of these physico-chemical parameters together with measurements of blood and water pH and total ammonia concentrations allowed calculation of the mean resting partial pressure gradient across the gills (54 × 10−s6Torr) and estimation of the gill permeability coefficient (D = 1·3 × 10−5cm−1). Under normal resting conditions of low external NH3 and pH, diffusive movement of NH3 appears to account adequately for ammonia excretion in the rainbow trout; 90 ± 10% of the excreted ammonia appears to originate from the blood, rather than from de novo synthesis of ammonia in the gills. During the high external ammonia treatments, the fish reached a steady state with a net inward ammonia gradient, which could be accounted for by a counter-balancing Na+/NH4+ exchange. Ammonium salt infusions or injections which stimulate Na+ influx are not sufficient demonstrations of Na+/NH4+ exchange, since acidosis is produced, and alternate interpretations of the Na+ flux stimulation are possible, such as enhanced Na+/H+ exchange.

Water Quality Modifies Uptake of Waterborne Methylmercury by Rainbow Trout, Salmo gairdneri

We measured the efficiency of uptake of waterborne methylmercury relative to oxygen consumption for rainbow trout, Salmo gairdneri, in hard or soft water and during exposure to sublethal concentrations of mercuric chloride or zinc sulphate. The relative efficiency of methylmercury uptake in soft water was more than double that measured in hard water. When mercuric chloride was added with waterborne methylmercury, uptake efficiency was further increased, with similar values obtained in hard and soft water. In contrast, addition of zinc sulphate decreased the relative efficiency of methylmercury uptake. Water quality thus significantly affects the accumulation of waterborne methylmercury by fish. In particular, calcium-dependent changes in gill permeability may explain elevated methylmercury residues observed in fish from lakes of low alkalinity and pH.

Effects of environmental salinity on branchial permeability of rainbow trout, Salmo gairdneri.

1. The gill fluxes of various non-electrolytes were measured in fresh-water- and sea-water-adapted trout (Salmo gairdneri). The studies were performed in vitro with a 'perfused-head' preparation. 2. The results allow one to specify different transepithelial pathways according to the physico-chemical characteristics of the permeant molecules: (1) for hydrophilic the physico-chemical characteristics of the permeant molecules: (1) for hydrophilic and lipophilic molecules of small molecular radius, a transcellular pathway in the respiratory cells of the secondary lamellae, (2) for hexose, all paracellular pathways in the gill epithelium, and (3) for hexose polymers (inulin, dextran), a transcellular pathway in the chloride cells of the primary lamellae. 3. The selectivity of the respiratory cells as a function of the liposolubility of the molecules tested is low. The decrease of this selectivity in the course of salt-water adaptation taken together with the modification of lipid composition of membranes and the effect of adrenaline on the branchial permeability suggests that non-electrolytes diffuse through a lipid phase able to form hydrogen bonds. 4. The high permeability of gills to hexose polymers of high molecular weight suggests a vesicular transport, especially in fish adapted to fresh water.