Ion uptake in naturally acidic water

The first studies on ion regulation in fish exposed to low pH, which were inspired by the Acid Rain environmental crisis, seemed to indicate that ion transport at the gills was completely and irreversibly inhibited at pH 4.0-4.5 and below. However, work on characid fish native to the Rio Negro, a naturally acidic, blackwater tributary of the Amazon River, found that they possess ion transport mechanisms that are completely insensitive to pHs as low as 3.25. As more species were examined it appeared that pH-insensitive transport was a trait shared by many, if not most, species in the Order Characiformes. Subsequently, a few other species of fish have been shown to be able to transport ions at low pH, in particular zebrafish (Danio rerio), which show rapid recovery of Na+ uptake at pH 4.0 after initial inhibition. Measurements of rates of Na+ transport during exposure to pharmacological agents that inhibit various transport proteins suggested that characiform fish do not utilize the generally accepted mechanisms for Na+ transport that rely on some form of H+ extrusion. Examination of zebrafish transport at low pH suggest the rapid recovery may be due to a novel Na+/K+ exchanger, but after longer term exposure they may rely on a coupling of Na+/H+ exchangers and NH3 excretion. Further work is needed to clarify these mechanisms of transport and to find other acid-tolerant species to fully gain an appreciation of the diversity of physiological mechansisms involved.

Effects of rearing salinity on expression and function of ion motive ATPases and ion transport across the gastric caecum of Aedes aegypti larvae

Larvae of Aedes aegypti, the yellow fever vector, inhabit a variety of aquatic habitats ranging from fresh water to brackish water. This study focuses on the gastric caecum of the larvae, an organ that has not been widely studied. We provide the first measurements of H+, K+, and Na+ fluxes at the distal and proximal gastric caecum, and have shown that they differ in the two regions, consistent with previously reported regionalization of ion transporters. Moreover we have shown that the regionalization of vacuolar H+-ATPase and Na+/K+ -ATPase is altered when larvae are reared in brackish water (30% seawater) relative to fresh water. Measurements of luminal Na+ and K+ concentrations also show a 5-fold increase in Na+/K+ ratio in the caecal lumen in larvae reared in brackish water relative to fresh water, whereas transepithelial potential and luminal pH were unchanged. Calculated electrochemical potentials reveal changes in the active accumulation of Na+ and K+ in the lumen of the gastric caecum of fresh water versus brackish water larvae. Together with the results of previous studies of the larval midgut, our results show that the caecum is functionally distinct from the adjacent anterior midgut, and may play an important role in osmoregulation as well as uptake of nutrients.

Impact of ontogenetic changes in branchial morphology on gill function in Arapaima gigas

Soon after hatching, the osteoglossid fish Arapaima gigas undergoes a rapid transition from a water breather to an obligate air breather. This is followed by a gradual disappearance of gill lamellae, which leaves smooth filaments with a reduced branchial diffusion capacity due to loss of surface area, and a fourfold increase in diffusion distance. This study evaluated the effects these changes have on gill function by examining two size classes of fish that differ in gill morphology. In comparison to smaller fish (approximately 67.5 g), which still have lamellae, larger fish (approximately 724.2 g) without lamellae took up a slightly greater percentage of O2 across the gills (30.1% vs. 23.9%), which indicates that the morphological changes do not place limitations on O2 uptake in larger fish. Both size groups excreted similar percentages of CO2 across the gills (85%-90%). However, larger fish had higher blood PCO2 (26.51.9 vs. 16.51.5 mmHg) and HCO3(-) (40.2 +/- 2.9 vs. 33.6 +/- 4.5 mmol L(-1)) concentrations and lower blood pH (7.58 +/- 0.01 vs. 7.70 +/- 0.04) than did smaller fish, despite having lower mass-specific metabolisms, suggesting a possible diffusion limitation for CO2 excretion in larger fish. With regard to ion regulation, rates of diffusive Na+ loss were about 3.5 times higher in larger fish than they were in smaller fish, despite the lowered branchial diffusion capacity, and rates of Na+ uptake were higher by about the same amount despite 40% lower activity of branchial Na+/K+-ATPase. Kinetic analysis of Na uptake revealed an extremely low-affinity (K(m) = 587.9 +/- 169.5 micromol L(-1)), low-capacity (J(max) = 265.7 +/- 56.8 nmol g(-1) h(-1)) transport system. These data may reflect a general reduction in the role of the gills in ion balance. Renal Na+/K+-ATPase activity was 5-10 times higher than Na+/K+-ATPase activity in the gills, and urine: plasma ratios for Na+ and Cl(-) were very low (0.001-0.005) relative to that of other fish, which suggested an increased role for dietary salt intake and renal salt retention and which was representative of a more "terrestrial" mode of ion regulation. Such de-emphasis of branchial ion regulation confers greatly reduced sensitivity of diffusive ion loss to low water pH. Ammonia excretion also appeared to be impacted by gill changes. Rates of ammonia excretion in larger fish were one third less than that in smaller fish, despite larger fish having blood ammonia concentrations that were twice as high.

Metabolic and ionoregulatory responses of the Amazonian cichlid, Astronotus ocellatus, to severe hypoxia

We examined the metabolic and ionoregulatory responses of the Amazonian cichlid, Astronotus ocellatus, to 20 h exposure to severe hypoxia (0.37 +/- 0.19 mg O(2)/l; 4.6% air saturation) or 8 h severe hypoxia followed by 12 h recovery in normoxic water. During 20 h exposure to hypoxia, white muscle [ATP] was maintained at normoxic levels primarily through a 20% decrease in [creatine phosphate] (CrP) and an activation of glycolysis yielding lactate accumulation. Muscle lactate accumulation maintained cytoplasmic redox state ([NAD(+)]/[NADH]) and was associated with an inactivation of the mitochondrial enzyme pyruvate dehydrogenase (PDH). The inactivation of PDH was not associated with significant changes in cytoplasmic allosteric modulators ([ADP(free)], redox state, or [pyruvate]). Hypoxia exposure caused an approximately 65% decrease in gill Na(+)/K(+) ATPase activity, which was not matched by changes in Na(+)/K(+) ATPase alpha-subunit protein abundance indicating post-translational modification of Na(+)/K(+) ATPase was responsible for the decrease in activity. Despite decreases in gill Na(+)/K(+) ATPase activity, plasma [Na(+)] increased, but this increase was possibly due to a significant hemoconcentration and fluid shift out of the extracellular space. Hypoxia caused an increase in Na(+)/K(+) ATPase alpha-subunit mRNA abundance pointing to either reduced mRNA degradation during exposure to hypoxia or enhanced expression of Na(+)/K(+) ATPase alpha-subunit relative to other genes.

P-type Na+/K+-ATPase and V-type H+-ATPase expression patterns in the osmoregulatory organs of larval and adult mosquitoAedes aegypti

This study describes the expression patterns of P-type Na+/K+-ATPase and V-type H+-ATPase in the larval and adult forms of the mosquito Aedes aegypti and provides insight into their relative importance in ion transport function of key osmoregulatory organs. RT-PCR assays indicate that, at the level of the gene,both ATPases are expressed in all of the osmoregulatory tissues of larvae(midgut, Malpighian tubules, rectum and anal papillae) and adults (stomach,Malpighian tubules, anterior hindgut and rectum). Immunohistochemical studies determined that both ATPases are present in high levels in all the relevant organs, with the exception of the larval rectum (P-type Na+/K+-ATPase only). In larval gastric caeca, ATPase location corresponds to the secretory (basal P-type Na+/K+-ATPase, apical V-type H+-ATPase) and ion-transporting (V-type H+-ATPase on both membranes) regions as previously described. The two ATPases switch membrane location along the length of the larval midgut, indicating three possible regionalizations,whereas the adult stomach has uniform expression of basolateral P-type Na+/K+-ATPase and apical V-type H+-ATPase in each cell. In both larval and adult Malpighian tubules, the distal principal cells exhibit high expression levels of V-type H+-ATPase (apically and cytoplasmically) whereas P-type Na+/K+-ATPase is highly expressed in stellate cells found only in the distal two-thirds of each tubule. By contrast, the proximal principal cells express both P-type Na+/K+-ATPase (basal) and V-type H+-ATPase(apical). These results suggest a functional segregation along the length of the Malpighian tubules based on cell type and region. P-type Na+/K+-ATPase is the only pump apparent in the larval rectum whereas in the larval anal papillae and the adult hindgut (including the anterior hindgut and rectum with rectal pads), P-type Na+/K+-ATPase and V-type H+-ATPase localize to the basal and apical membranes, respectively. We discuss our findings in light of previous physiological and morphological studies and re-examine our current models of ion transport in these two developmental stages of mosquitoes that cope with disparate osmoregulatory challenges.

Secretion of water and ions by malpighian tubules of larval mosquitoes: effects of diuretic factors, second messengers, and salinity

The effects of changes in the salinity of the rearing medium on Malpighian tubule fluid secretion and ion transport were examined in larvae of the freshwater mosquito Aedes aegypti and the saltwater species Ochlerotatus taeniorhynchus. For unstimulated tubules of both species, the K(+) concentration of secreted fluid was significantly lower when larvae were reared in 30% or 100% seawater (O. taeniorhynchus only), relative to tubules from freshwater-reared larvae. The Na(+) concentration of secreted fluid from unstimulated tubules of O. taeniorhynchus reared in 30% or 100% seawater was higher relative to tubules from freshwater-reared larvae. The results suggest that changes in salinity of the larval rearing medium lead to sustained changes in ion transport mechanisms in unstimulated tubules. Furthermore, alterations of K(+) transport may be utilized to either conserve Na(+) under freshwater (Na(+)-deprived) conditions or eliminate more Na(+) in saline (Na(+)-rich) conditions. The secretagogues cyclic AMP [cAMP], cyclic GMP [cGMP], leucokinin-VIII, and thapsigargin stimulated fluid secretion by tubules of both species. Cyclic AMP increased K(+) concentration and decreased Na(+) concentration in the fluid secreted by tubules isolated from O. taeniorhynchus larvae reared in 100% seawater. Interactions between rearing salinity and cGMP actions were similar to those for cAMP. Leucokinin-VIII and thapsigargin had no effect on secreted fluid Na(+) or K(+) concentrations. Results indicate that changes in rearing medium salinity affect the nature and extent of stimulation of fluid and ion secretion by secretagogues.

Unusual physiology of scale-less carp, Gymnocypris przewalskii, in Lake Qinghai: a high altitude alkaline saline lake

The scale-less carp (Gymnocypris przewalskii) inhabits Lake Qinghai located on the Qinghai-Tibet plateau (elevation, 3200 m) in western China. The lake waters are alkaline (pH 9.4, titratable alkalinity=30 mmol l(-1)), Mg(2+)-rich (18.7 mmol l(-1)), Ca(2+)-poor (0.30 mmol l(-1)) and saline (9 per thousand ). These fish make annual spawning migrations into freshwater rivers. We investigated the physiology of nitrogen excretion and ionoregulation of fish from the lake and river. Fish from both waters were ammonotelic, although ammonia-N excretion rates were lower in lake fish (175 vs. 344 micromol kg(-1) h(-1), P<0.05) resulting in unusually high levels of ammonia in blood plasma (2.23 vs. 0.32 mmol l(-1)), bile, liver, muscle and brain. Exposure to 0.4 mmol l(-1) total ammonia in lake water ([NH(3)]=0.16 mmol l(-1)) killed fish within 8 h. River fish survived exposure to 1.0 mmol l(-1) total ammonia in river water at pH 8.0 ([NH(3)]=0.023 mmol l(-1)) for 24 h suggesting high ammonia tolerance in lake fish. High glutamate dehydrogenase and glutamine synthetase activities in tissues probably allow the fish to alleviate ammonia toxicity by amino acid accumulation. Neither lake nor river fish relied on urea excretion to remove excess N. Urea-N excretion rates were below 20 micromol kg(-1) h(-1) for both groups, and levels of urea in plasma and tissues were moderate. When exposed to elevated ammonia, urea-N excretion increased slightly (approximately 50 micromol kg(-1) h(-1)) and liver and muscle urea levels increased in the river fish. Plasma ion levels were within the range typical of cyprinids, but river fish had significantly higher plasma [Na(+)] and [Cl(-)] and lower [K(+)] than fish from the lake. During 48-h lake-to-river water transfer, plasma Na(+) and Cl(-) levels rose significantly. Significantly higher Na(+)/K(+)-ATPase activity in the gills of river fish may be related to the higher plasma ion levels. Plasma [Mg(2+)] and [Ca(2+)] were tightly regulated despite the great differences in the lake and river water levels.

Protection by natural blackwater against disturbances in ion fluxes caused by low pH exposure in freshwater stingrays endemic to the Rio Negro

Stenohaline freshwater stingrays (Potamotrygon spp.) are endemic to the very dilute (Na(+), Cl(-), Ca2(+) <or=30 micromol L(-1)), often acidic blackwaters of the Rio Negro despite gill Na(+) and Cl(-) transport characteristics that appear unfavorable (high K(m), low J(max)). We evaluated the possible protective role of blackwater itself, which is rich in dissolved organic carbon (DOC), as well as the importance of Ca(2+) in allowing this tolerance of dilute, acidic conditions. Responses of stingrays in natural blackwater (DOC=8.4 mg L(-1)) were compared with those in a natural reference water with similar ion levels but low DOC (0.6 mg L(-1)). Comparing these two water types, we found that differences in Na(+) and Cl(-) unidirectional fluxes (JXin, JXout; measured with radiotracers) and net fluxes (JXnet), influx and outflux kinetic relationships, and net ammonia excretion (J(Amm)) were generally small at pH 6.3, though the balance points where Jin=Jout shifted from >300 micromol L(-1) in reference water (low DOC) to about 100 micromol L(-1) in blackwater (high DOC). In reference water, both JNain and JClin were inhibited >90%, both JNaout and JClout more than doubled, and J(Amm) did not change at pH 4.0. In blackwater, the inhibition of influxes was attenuated, the increases in outflux did not occur, and J(Amm) increased by 60% at pH 4.0. Addition of 100 micromol L(-1) Ca(2+) to reference water prevented the increases in JNaout and JClout and allowed J(Amm) to increase at pH 4.0, which demonstrates that the gills are sensitive to Ca(2+). However, addition of Ca(2+) to blackwater had no effect on the responses to pH 4.0. Addition of commercial humic acid to reference water did not duplicate the effects of natural Rio Negro blackwater at the same DOC level; instead, it greatly exacerbated the increases in JNaout and JClout at low pH and prevented any protective influence of added Ca(2+). Thus, blackwater DOC appears to be very different from commercial humic acid. Biogeochemical modeling indicated that blackwater DOC prevents Ca(2+) binding, but not H(+) binding, to the gills and that the protective effects of blackwater cannot be attributed to its higher buffer capacity or its elevated Al or Fe levels. Natural DOC may act directly at the gills at low pH to exert a protective effect and, when doing so, may override any protective action of Ca(2+) that might otherwise occur.

Mechanisms of ion transport inPotamotrygon, a stenohaline freshwater elasmobranch native to the ion-poor blackwaters of the Rio Negro

Stingrays of the family Potamotrygonidae are the only stenohaline freshwater elasmobranchs. Potomotrygon sp. collected from the ion-poor blackwaters ([Na+], [Cl-] and[Ca2+]=10-30 μmol l-1, pH 6.1) of the Rio Negro,Amazonas, Brazil, were ammoniotelic (91% ammonia-N, 9% urea-N excretion) and exhibited blood chemistry (Na+, Cl-, urea, ammonia and glucose levels and osmolality) typical of freshwater teleosts. Unidirectional Na+ and Cl- influx rates, measured with radiotracers,displayed saturation kinetics. The relationships for Cl- and Na+ had similar Km values (300-500 μmol l-1), but Jmax values for Cl-(approximately 950 μmol kg-1 h-1) were almost twice those for Na+ (approximately 500 μmol kg-1h-1). Cl- efflux rates varied with external concentration, but Na+ efflux rates did not. There were no differences in the kinetic variables (Km, Jmax) for influx between animals acclimated to their native ion-poor blackwater or to ion-rich hard water, but efflux rates for both Na+ and Cl- were lower in the former, yielding much lower balance points (external Na+ or Cl- levels at which influx and efflux were equal). Na+, Cl- and Ca2+ uptake were all strongly inhibited by acute exposure to pH 4.0, but efflux rates and Ca2+ binding to the body surface did not change. Na+ influx was inhibited by amiloride (10-4 mol l-1) and by two of its analogs, phenamil (4×10-5mol l-1) and HMA (4×10-5 mol l-1), with the latter being slightly more potent, while Cl- fluxes were unaffected. Cl- fluxes were insensitive to DIDS(2×10-5 mol l-1 or 10-4 mol l-1) and SITS (10-4 mol l-1), but both influx and efflux rates were strongly inhibited by DPC (10-4 mol l-1) and thiocyanate (10-4 mol l-1). Ammonia excretion was unresponsive to large changes in water Na+concentration, but was elevated by 70% during acute exposure to pH 4.0 and transiently inhibited by approximately 50% by amiloride and its analogues. The strategy of adaptation to ion-poor blackwater appears similar to that of some Rio Negro teleosts (Cichlidae) in which low-affinity transport systems are relatively sensitive to inhibition by low pH but are complemented by low diffusive loss rates. Ionic transport systems in these freshwater elasmobranchs, although superficially similar to those in some freshwater teleosts, may bear more resemblance to their presumed evolutionary precursors in marine elasmobranchs.

The characterization of ion regulation in Amazonian mosquito larvae: evidence of phenotypic plasticity, population-based disparity, and novel mechanisms of ion uptake

This study is the first step in characterizing ion uptake mechanisms of mosquito larvae from the Amazon region of Brazil. Hemolymph NaCl levels and rates of unidirectional Na(+) and Cl(-) uptake were measured in larvae of Aedes aegypti and Culex quinquefasciatus in a series of environmental manipulations that are known to challenge ion regulation in other aquatic animals. Despite being reared for numerous generations in dilute media (20 micromol L(-1) NaCl), both species were able to maintain high hemolymph NaCl concentrations, a departure from previous studies. Exposure to distilled water or high-NaCl media did not affect hemolymph ion levels, but pH 3 caused significant decreases in hemolymph Na(+) and Cl(-) levels in both species. Exposure to water from Rio Negro (pH 5.5), an organically rich but ion-poor body of water, did not disturb hemolymph Na(+) and Cl(-) levels or the uptake of these ions. Acute exposure to control media or Rio Negro water titrated to pH 3.5 caused inhibition of Na(+) uptake and stimulation of Cl(-) uptake in C. quinquefasciatus, but A. aegypti larvae experienced only a significant reduction of Na(+) uptake in Rio Negro/pH 3.5 treatment. The stimulation of Cl(-) uptake at low pH has been documented only in aquatic insects and differs from all other invertebrate and vertebrate species. A similar pattern of Na(+) uptake inhibition and Cl(-) uptake stimulation was observed in A. aegypti larvae exposed to bafilomycin A(1), a blocker of V-type H(+) ATPase. Culex quinquefasciatus larvae were unaffected by this drug. Both Na(+) and Cl(-) uptake were reduced when C. quinquefasciatus larvae were exposed to acetazolamide, indicating that H(+) and HCO(3)(-), derived from hydration of CO(2), are involved with Na(+) and Cl(-) uptake. Kinetic analysis of Na(+) and Cl(-) uptake in C. quinquefasciatus, A. aegypti, and Anopheles nuneztovari larvae indicate that these Amazonian species share similar high-capacity and high-affinity mechanisms. Comparison of the Amazonian C. quinquefasciatus with a Californian population provided evidence of both phenotypic plasticity and population disparity in Na(+) and Cl(-) uptake, respectively. When the California population of C. quinquefasciatus was reared in a medium similar to that of the Amazonian group (60 micromol L(-1) NaCl) instead of 4,000 micromol L(-1) NaCl, larvae increased both Na(+) uptake capacity (J(max)) and affinity (i.e., reduced K(m)), yet Cl(-) uptake did not change from its nonsaturating, low-capacity pattern. In the reverse experiment, Amazonian C. quinquefasciatus demonstrated plasticity in both Na(+) and Cl(-) uptake by significantly reducing rates when held in 4,000 micromol L(-1) NaCl for 3 d.

Ion regulatory patterns of mosquito larvae collected from breeding sites in the Amazon rain forest

We examined the ion composition of mosquito breeding sites located in the Amazon rain forest and the ion regulatory patterns of larvae from these habitats. We found larvae of Toxorhynchites haemorroidalis, Limatus durhamii, Culex (Carrollia) bonnei, and Culex (Culex) sp. residing in fallen palm bracts, leaves, and tree holes that were filled with water. These breeding sites had micromolar levels of Na(+) (1.6-99 micromol L(-1)), but K(+) and Cl(-) concentrations were higher and varied over a large range (231-17,615 micromol L(-1) K(+); 355-2,700 micromol L(-1) Cl(-)). Despite the variability in environmental ion levels and ratios, all four species maintain high hemolymph NaCl levels (80-120 mmol L(-1) Na(+); 60-80 mmol L(-1) Cl(-)). However, the species differed in the means by which they maintain hemolymph ion balance, as indicated by the range of unidirectional Na(+) and Cl(-) uptake rates. Toxorhynchites haemorroidalis had extremely low rates of Na(+) uptake and undetectable Cl(-) uptake, whereas L. durhamii had high rates of uptake for both ions. This variability in rates of uptake may reflect species differences in rates of diffusive ion loss (i.e., permeability). We observed the same curious pattern of Na(+) inhibition and Cl(-) stimulation by low-pH exposure in all four species of mosquitoes, as has been documented in other mosquitoes and aquatic insects. Kinetic analyses of Na(+) and Cl(-) uptake in C. bonnei larvae revealed an unusual pattern of Na(+) uptake that increases linearly (nonsaturable) to extremely high rates, while Cl(-) uptake is a low-affinity, low-capacity system. This pattern contrasts with L. durhamii and Culex (Culex) sp. larvae, which had large increases in both Na(+) and Cl(-) uptake when external NaCl levels were increased. Our results suggest that although these rain forest mosquito larvae are residing in habitats with similar low Na(+), high Cl(-) composition and maintain similar hemolymph NaCl levels, the underlying mechanisms of ion regulation differ among the species.

Diverse strategies for ion regulation in fish collected from the ion-poor, acidic Rio Negro

We measured unidirectional ion fluxes of fish collected directly from the Rio Negro, an extremely dilute, acidic blackwater tributary of the Amazon. Kinetic analysis of Na(+) uptake revealed that most species had fairly similar J(max) values, ranging from 1,150 to 1,750 nmol g(-1) h(-1), while K(m) values varied to a greater extent. Three species had K(m) values <33 micromol L(-1), while the rest had K(m) values >or=110 micromol L(-1). Because of the extremely low Na(+) concentration of Rio Negro water, the differences in K(m) values yield very different rates of Na(+) uptake. However, regardless of the rate of Na(+) uptake, measurements of Na(+) efflux show that Na(+) balance was maintained at very low Na(+) levels (<50 micromol L(-1)) by most species. Unlike other species with high K(m) values, the catfish Corydoras julii maintained high rates of Na(+) uptake in dilute waters by having a J(max) value at least 100% higher than the other species. Corydoras julii also demonstrated the ability to modulate kinetic parameters in response to changes in water chemistry. After 2 wk in 2 mmol L(-1) NaCl, J(max) fell >50%, and K(m) dropped about 70%. The unusual acclimatory drop in K(m) may represent a mechanism to ensure high rates of Na(+) uptake on return to dilute water. As well as being tolerant of extremely dilute waters, Rio Negro fish generally were fairly tolerant of low pH. Still, there were significant differences in sensitivity to pH among the species on the basis of degree of stimulation of Na(+) efflux at low pH. There were also differences in sensitivity to low pH of Na(+) uptake, and two species maintained significant rates of uptake even at pH 3.5. When fish were exposed to low pH in Rio Negro water instead of deionized water (with the same concentrations of major ions), the effects of low pH were reduced. This suggests that high concentrations of dissolved organic molecules in the water, which give it its dark tea color, may interact with the branchial epithelium in some protective manner.

Sodium and chloride regulation in freshwater and osmoconforming larvae ofCulexmosquitoes

In this study, we examined aspects of Na+ and Cl– regulation in mosquito larvae of the genus Culex, a group that includes species that tolerate high salinity as well as other forms that are restricted to fresh water. When the euryhaline osmoconformer C. tarsalis was acutely transferred from 30 % to 50 % sea water, the patterns of hemolymph Na+ and Cl– regulation were similar. The underlying regulatory mechanisms for these two ions have very different characteristics. In C. tarsalis, Na+ efflux was significantly elevated compared with the rates measured in the freshwater-restricted C. quinquefasciatus, while Cl– influx was relatively lower. The modulation of Na+ efflux and Cl– influx allowed C. tarsalis to avoid a potential salt load and ionic disturbance in the hemolymph during an acute increase  in salinity. The observed adjustment of NaCl regulation departs from that determined for other euryhaline organisms and is integral to the osmoconforming response. At the other extreme of the salinity spectrum, we observed that C. tarsalis faces difficulties in ion regulation in habitats with low NaCl levels because of its inability to reduce ion efflux and adjust ion absorption rates to maintain hemolymph ion balance. In contrast, C. quinquefasciatus exhibited a reduced ion efflux and the ability to upregulate Na+ uptake, traits necessary to extend its lower salinity limit.

The physiology of salinity tolerance in larvae of two species of Culex mosquitoes: the role of compatible solutes

We investigated the physiological basis for differences in salinity tolerance ranges in mosquito larvae of the genus Culex. We examined the response of larvae of C. quinquefasciatus, a freshwater obligate, and C. tarsalis, a euryhaline osmoconformer, following transfer from fresh water to 34% sea water. Hemolymph Na(+) and Cl(−) levels increased similarly in both species, indicating that ion regulation does not differ under these conditions. C. quinquefasciatus responded to increased environmental salinity with increased hemolymph levels of serine, but suffered a significant reduction in levels of trehalose. C. tarsalis responded to increased environmental salinity with increased hemolymph levels of both proline and trehalose. When C. tarsalis larvae were held in 64% sea water, which C. quinquefasciatus larvae cannot tolerate, hemolymph proline and trehalose were accumulated approximately 50-fold and twofold, respectively, relative to freshwater values. We found that proline serves as both an intra- and extracellular compatible solute in C. tarsalis, the first such circumstance documented in an animal in response to increased environmental salinity. Analyses of the acute responses of the two species to an increase in salinity (from 30% to 50% sea water) indicate that larvae of C. tarsalis are able to volume-regulate via drinking and to attenuate increases in hemolymph NaCl concentration using unknown mechanisms during large, rapid increases in salinity.

Regulation of compatible solute accumulation in larvae of the mosquito Culex tarsalis: osmolarity versus salinity

In this study, we demonstrate that two of the osmolytes utilized in the osmoconforming strategy of larval Culex tarsalis are regulated by two fundamentally different signals. When the external osmolality was increased using salinity (sea salts), hemolymph NaCl, proline and trehalose concentrations increased significantly. When sorbitol was used to increase the external osmolality without an elevation in salt concentration, hemolymph NaCl and proline concentrations decreased, whereas hemolymph trehalose concentration increased. The results suggest that proline accumulation was cued by increases in salinity, whereas trehalose levels followed increases in osmolality. Interestingly, we found that C. tarsalis larvae accumulated the exogenous sorbitol in the hemolymph in an osmoconforming manner. We conducted further studies in which changes in hemolymph NaCl concentrations were manipulated using changes in environmental salinity. The results suggested that hemolymph proline accumulation was cued by the proximal signal of hemolymph NaCl levels. Regardless of which solute (sea salts, sorbitol or mixtures thereof) was used to raise the external osmolality, trehalose accumulation tracked the increase in total osmolality of the medium. These findings indicate that the synthesis and accumulation of these two osmolytes are regulated by two independent signals.

MRI phase mapping of temperature distributions induced in food by microwave heating

A range of temperature-sensitive MRI parameters of water (T2, T1, diffusion coefficient, and chemical shift) were evaluated to map in three dimensions the non-uniform temperature distributions induced by microwave heating in both model and real food systems. Phase mapping was found to be the most robust method, and evaluations of possible experimental errors were based on semi-quantitative studies of homogeneous and heterogeneous systems. The MRI protocol provides complementary phase and magnitude data, which are related to the sample temperature and structural heterogeneity, respectively. Used together, they relate the temperature changes to the differential thermal properties of the various components within a heterogeneous sample. The potential applications of this technique to microwave and other forms of heating is discussed.

Ion and acid-base balance in three species of Amazonian fish during gradual acidification of extremely soft water

Sensitivity to acid water was assessed in three species of Amazonian fish that encounter naturally acidic blackwaters to differing degrees in the wild: tambaqui (Colossoma macropomum), matrincha (Brycon erythropterum), and tamoatá (Hoplosternum littorale), in decreasing order of occurrence in blackwater. Fish were exposed to a graded reduction in water pH, from pH 6 to 5 to 4 to 3.5, followed by return to pH 6. Fish were exposed to each new pH for 24 h. During these exposures, net transfers of ions (Na+, K+, Cl-, and Ca2+) and acid-base equivalents to and from the external water were used as physiological indicators of acid tolerance. Exposure to pH 5 had a minimal effect on net ion fluxes. Significant net losses of all ions (except Ca2+) were recorded in all three species during the first few hours of exposure to pH 4. However, ion balance was usually restored within 18 h at pH 4. Exposure to pH 3.5 caused even greater ion losses in all three species and proved to be acutely lethal to tamoatá. Matrincha sustained irreversible physiological damage at pH 3.5, as ion fluxes did not recover following return to pH 6 and there was some mortality. Tambaqui suffered the least ionoregulatory disturbances at pH 3.5 and was the only species to make a full recovery on return to pH 6. In all species, there was a tendency for ammonia excretion to increase at low water pH, but even at pH 3.5, there was no significant net uptake of acid from the water. Overall, there was a strong relationship between the magnitude of ionic disturbances and the lethality of exposure to low pH. The relative insensitivity of the ionoregulatory system of tambaqui to low pH indicates that this is a feature of fish native to blackwater systems rather than one that is common to all Amazon fish.

Responses of an Amazonian teleost, the tambaqui (Colossoma macropomum), to low pH in extremely soft water

Our goal was to compare the internal physiological responses to acid challenge in an acidophilic tropical teleost endemic to dilute low-pH waters with those in nonacidophilic temperate species such as salmonids, which have been the subjects of most previous investigations. The Amazonian tambaqui (Colossoma macropomum), which migrates between circumneutral water and dilute acidic "blackwater" of the Rio Negro, was exposed to a graded low-pH and recovery regime in representative soft water (Na+ = 15, Cl- = 16, Ca2+ = 20 mumol L-1). Fish were fitted with arterial catheters for repetitive blood sampling. Water pH was altered from 6.5 (control) to 5.0, 4.0, 3.0, and back to 6.5 (recovery) on successive days. Some deaths occurred at pH 3.0. Throughout the regime, there were no disturbances of blood gases (O2 and CO2 tensions and contents) or lactate levels, and only very minor changes in acid-base status of plasma and red cells. However, erythrocytic guanylate and adenylate levels increased at pH's less than or equal to 5.0. Down to pH 4.0, plasma glucose, cortisol, and total ammonia levels remained constant, but all increased at pH 3.0, denoting a stress response. Plasma Na+ and Cl- levels declined and plasma protein concentration increased at pH 3.0, indicative of ionoregulatory and fluid volume disturbance, and neither recovered upon return to pH 6.5. Cortisol and ammonia elevations also persisted. Transepithelial potential changed progressively from highly negative values (inside) at pH 6.5 to highly positive values at pH 3.0; these alterations were fully reversible. Experimental elevations in water calcium levels drove the transepithelial potential positive at circumneutral pH, attenuated or prevented changes in transepithelial potential at low pH, and reduced Na+ and Cl- loss rates to the water during acute low-pH challenges. In general, tambaqui exhibited responses to low pH that were qualitatively similar but quantitatively more resistant than those previously documented in salmonids.

Effects of water pH and calcium concentration on ion balance in fish of the Rio Negro, Amazon

We examined the effects of acute low-pH exposure on ion balance (Na+, Cl-, K+) in several species of fish captured from the Rio Negro, a dilute, acidic tributary of the Amazon. At pH 5.5 (untreated Rio Negro water), the four Rio Negro species tested (piranha preta, Serrasalmus rhombeus; piranha branca, Serrasalmus cf. holandi; aracu, Leporinus fasciatus; and pacu, Myleus sp.) were at or near ion balance; upon exposure to pH 3.5, while Na+ and Cl- loss rates became significant, they were relatively mild. In comparison, tambaqui (Colossoma macropomum), which were obtained from aquaculture and held and tested under the same conditions as the other fish, had loss rates seven times higher than all the Rio Negro species. At pH 3.0, rates of Na+ and Cl- loss for the Rio Negro fish increased three- to fivefold but were again much less than those observed in tambaqui. Raising water Ca2+ concentration from 10 micromol L-1 to 100 micromol L-1 during exposure to the same low pH's had no effect on rates of ion loss in the three species tested (piranha preta, piranha branca, aracu), which suggests that either they have such a high branchial affinity for Ca2+ that all sites are saturated at 10 micromol L-1 and additional Ca2+ had no effect, or that Ca2+ may not be involved in regulation of branchial ion permeability. For a final Rio Negro species, the cardinal tetra (Paracheirodon axelrodi), we monitored body Na+ concentration during 5 d of exposure to pH 6.0, 4.0, or 3.5. These pH's had no effect on body Na+ concentration. These data together suggest that exceptional acid tolerance is a general characteristic of fish that inhabit the dilute acidic Rio Negro and raise questions about the role of Ca2+ in regulation of branchial ion permeability in these fish.

Ion regulation in ion-poor acidic water by the blackskirt tetra (Gymnocorymbus ternetzi), a fish native to the Amazon River

We examined the ionoregulatory capabilities of the blackskirt tetra (Gymnocorymbus ternetzi), which is native to ion-poor acidic waters of the Amazon River. Examination of Na+ uptake, which was only slightly sensitive to the uptake blocker amiloride, revealed several specializations for uptake in these waters. Kinetic analysis of Na+ uptake (at pH 6.5) revealed a high maximum rate of uptake and a low Michaelis-Menten constant, which allows the tetras to take up Na+ at high rates even at very low water levels. At pH 4.5, a pH where they experience sizable ion disturbances, they displayed several mechanisms to restore balance. Kinetic analysis at pH 4.5 revealed that the maximum uptake rate rose 67% while the Michaelis-Menten constant remained unchanged. Further tests showed that the upregulation of Na+ uptake occurred within 12 h in response to a doubling of Na+ efflux. Despite these specializations of the Na+ uptake mechanism, blackskirt tetras were not especially tolerant of low pH. Upon exposure to pH 4.0, they experienced a massive loss of Na+ due to a fourfold increase of Na+ efflux (relative to pH 5.0) and an 80% inhibition of uptake. Measurement of Na+ efflux in waters with different Ca2+ levels and in the presence of LaCl, a strong Ca2+ competitor, correlated the stimulation of Na+ efflux at low pH with a low branchial affinity for Ca2+. These tests indicate that blackskirt tetras possess abilities to resist the disruptive effects of moderately low pH but cannot survive in waters with a pH of 4.0 or less because of leaching of Ca2+ from branchial tight junctions, which stimulates ion losses.

Calcium regulation in the freshwater-adapted mummichog

In light of recent findings of an unusual pattern of ionoregulation (high Na+ uptake and negligible Cl- uptake) in the freshwater-adapted mummichog Fundulus heteroclitus, the pattern of Ca2+ regulation was examined. Under control conditions (water Ca2+=200 &mu;Eq l-1), unidirectional Ca2+ influx was 11&plusmn;4 nEq g-1 h-1. Acute variation of external Ca2+ levels revealed a saturable Ca2+ uptake system with a relatively high affinity (Km=125&plusmn;36 &mu;Eq l-1) and a transport capacity (Jmax=31&plusmn;4 nEq g-1 h-1) comparable to those of other teleosts. Lanthanum (equimolar to [Ca2+]) significantly blocked Ca2+ uptake by 67% whereas mag-nesium had no effect. Chronic low Ca2+ exposure (50 &mu;Eq l-1) stimulated Ca2+ uptake almost three-fold above control values, whereas chronic high Ca2+ exposure (20 000 &mu;Eq l-1) had no effect. Lanthanum and chronic low Ca2+ treatments disturbed the normally positive Ca2+ and Na+ balances of the animals whereas acid-base balance and ammonia excretion were undisturbed. The results indicate that Ca2+ regulation by the mummichog conforms to the model for freshwater Ca2+ transport whereby chloride cells on the gills take up Ca2+ actively from the water. However, the absence of extra-intestinal Cl- uptake and the recent demonstration of significant Ca2+ uptake by opercular epithelia raise questions about the relative roles of branchial and opercular epithelial chloride cells in freshwater F. heteroclitus. 1997 The Fisheries Society of the British Isles

Epidemiologic study of 4684 hospital-acquired infections in pediatric patients

During a 4-year period 4684 nosocomial infections occurred in a university pediatric hospital which admitted 78,120 patients (nosocomial infection rate (NIR) = 6.0). NIR varied from 0.17 to 14.0 on different wards or services; the highest rates (greater than or equal to 5.6) were found in the Neonatal Intensive Care Unit, infant neurosurgery, hematology/oncology, neonatal surgery, cardiology/cardiovascular surgery, Pediatric Intensive Care Unit and infant/toddler medicine areas. Infections were most common in patients less than or equal to 23 months (NIR = 11.5), were less common in the 2- to 4-year age group (NIR = 3.6) and occurred least frequently in patients greater than or equal to 5 years (NIR = 2.6). The median day of onset of infections was 15.3 days. The proportional frequencies of infections were: 35% gastrointestinal; 21% bacteremia; 16% respiratory (10% upper, 6% lower); 7% postoperative wound; 6% urinary tract; 5% skin (32% of these skin infections were related to intravascular lines); 5% eye; 3% cerebrospinal fluid; and 2% other. A similar proportional frequency of 379 infections in patients hospitalized for more than 100 days was observed. The etiologic agents were Gram-positive bacteria (50%), viruses (23%), Gram-negative bacteria (18%), fungi (4%) and mixed/other (5%).

Nosocomial Pseudomonas aeruginosa conjunctivitis in a pediatric hospital

Conjunctivitis accounted for 5% of nosocomial infections occurring in a university-affiliated pediatric hospital between January 1984 and April 1986. Pseudomonas aeruginosa was recovered from the conjunctiva of 30 patients. The primary diseases of these patients were chronic and debilitating. Eighty percent of patients were under 18 months of age although only 30% of admissions are represented in this age group. Seventy percent of cases occurred in pediatric intensive care unit/neonatal intensive care unit patients. Seventy percent of patients who had antecedent nasopharyngeal/endotracheal cultures obtained were colonized with P aeruginosa. All patients except one had one or more of the following interventions prior to the onset of conjunctivitis: tracheostomy, endotracheal tube, oxygen by hood, or suctioning. Two children (7.4%) have residual corneal scars. Improvements in eye care including protection of the eye during suctioning, other respiratory care, and nasogastric tube procedures are warranted.

Isolation usage in a pediatric hospital

In a prospective 12-month study at a university-affiliated pediatric hospital, isolation usage was quantitated by ward/service, season, isolation category and type of infection (community-acquired vs nosocomial). Such information may be helpful in designing hospitals, recognizing time utilization of the pediatric infection control nurse, and defining educational and isolation needs. Hospitals with multiple bed rooms and inadequate numbers of single rooms may be unable to meet current federal isolation guidelines. The mean number of isolation days was 153 per 1000 patient days or 15.3% of bed days used. This ranged from 18.5% on the infant/toddler/preschool medical ward to 2.8% on child/teenage orthopedic surgery. Isolation requirements vary seasonally and rose to 32% in winter on one ward. Proportional frequencies of isolation category included enteric--29%, protective--28%, strict--16%, barrier (contact)--10%, multiply resistant organism (MRO)--8%, wound--5%, pregnant women (careful handwashing)--3%, blood and body fluid precautions--1%. Isolation of patients with and contacts of nosocomial infections account for 32% of isolation usage. During one third of the 365-day year, the hospital is unable to provide adequate numbers of single rooms for one to 20 patients.