Renal handling of magnesium in fish: from whole animal to brush border membrane vesicles

Abstract

Of all known vertebrate tissues, the kidneys of fish are the champions of Mg transport. They can switch from Mg conservation in fresh water to Mg wasting in seawater. High rates of tubular transport and the ability to alternate between Mg reabsorption and secretion make fish kidneys the model of choice investigating the mechanisms of transepithelial and membrane Mg transport and its regulation by extracellular hormones and intracellular messengers. Studies in isolated proximal tubules indicate active transepithelial Mg transport that requires metabolic energy for both tubular reabsorption and secretion. Whether active transport is primary and mediated by a Mgpump, or secondary and mediated via cotransport or antiport, is unknown. In fresh water fish, the active transport pathway appears to include a Mg-channel located in brush-border membranes of proximal tubules. Although plasma Mg concentrations are well protected, a primary hormone controlling Mg balance has yet to be identified in any animal. Moreover, the mechanisms of intracellular Mg homeostasis, especially in epithelial cells with high Mg throughput, are unknown. New methods, including Mg imaging and genetic/molecular approaches promise to unravel Mg transport mechanisms in teleost renal tubules.

Przewalski’s Naked Carp (Gymnocypris przewalskii): An Endangered Species Taking a Metabolic Holiday in Lake Qinghai, China

“…In river water, the physiology of Przewalski's naked carp appears very typical of most freshwater teleosts with respect to plasma ions (e.g., Holmes and Donaldson 1969), renal function (e.g., Hickman and Trump 1969;Beyenbach 2000), N-waste excretion (e.g., Wood 1993;Wilkie 2002), and acid-base status (e.g., Albers 1970). Despite the low ambient at high altitude, Po 2 blood gases and resting metabolic rates are not unusual for teleosts at comparable size and temperature (e.g., Perry and McDonald 1993;Clarke and Johnston 1999).…”

“…On this basis, it is apparent that the "final" values for osmolality, [Na ϩ ], [Cl Ϫ ], and [K ϩ ] are all approximately equilibrated with the external lake water; the overall response appears to be one of osmoconformity rather than osmoregulation. Thus, the naked carp behaves similarly to stenohaline freshwater cyprinids that tolerate moderate salinities (Lahlou et al 1969;Maceina et al 1980;Hegab and Hanke 1982;De Boeck et al 1997, 2000. This pattern is very different from euryhaline freshwater teleosts such as the rainbow trout (Oncorhynchus mykiss; Shehadeh and Gordon 1969;Bath and Eddy 1979;Richards et al 2003), where Figure 11.…”

“…Equilibration with lake water is clearly not the explanation for the observed response in plasma [Mg 2ϩ ]; indeed, had this occurred, plasma [Mg 2ϩ ] would have increased to 36 mmol L Ϫ1 (Table 1), and the fish would have undoubtedly died. Even in full-strength seawater containing 54 mmol L Ϫ1 [Mg 2ϩ ], euryhaline and stenohaline marine teleosts normally keep this ion below 3 mmol L Ϫ1 in the blood plasma to avoid its toxic effects on neuromuscular function (Holmes and Donaldson 1969;Beyenbach 2000;Marshall and Grosell 2005). In the naked carp in lake water, plasma [Mg 2ϩ ] is clearly regulated but stabilizes at a value as high as 12 mmol L Ϫ1 (Fig.…”

“…However, as with K ϩ , there is apparently a Mg 2ϩ uptake mechanism on the gills to cope with times of dietary Mg 2ϩ deficiency (Flik et al 1993;Bijvelds et al 1998). In the naked carp in lake water, it is probable that Mg 2ϩ loading occurs by diffusion across the gills and also possibly across the intestine (if drinking occurs; see below) and that the high plasma set point is determined by the kidney's ability to excrete Mg 2ϩ (Oikari and Rankin 1985;Beyenbach 2000).…”

The osmoregulatory ability of the invasive species sunbleak Leucaspius delineatus and topmouth gudgeon Pseudorasbora parva at elevated salinities, and their likely dispersal via brackish waters

“…The present data indicate that both sunbleak and topmouth gudgeon are capable of limiting the changes in plasma ion concentrations to a certain extent, but unidirectional ion flux measurements are needed to elucidate these processes further. The rising plasma Mg 2+ levels of both species during brackish‐water exposures, particularly in topmouth gudgeon held in water of salinity 13·7, suggests that divalent ion excretion by the kidneys, which is well developed in seawater‐acclimated euryhaline fishes (Beyenbach, 2000), may be limited in sunbleak and topmouth gudgeon. The plasma Mg 2+ concentration of fishes held in brackish water, however, was always well below the environment suggesting that some renal excretory processes may have been involved.…”