Glomerular ultrastructure of the trout, Salmo gairdneri. Glomerular capillary epithelium and the effects of environmental salinity

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Comparative anatomy of the podocyte: A scanning electron microscopic study

The three-dimensional structure of the renal glomerular podocyte was comparatively analyzed with special reference to its cellular interdigitation. Kidneys of lampreys, carps, eels, xenopuses, bullfrogs, iguanas, rats, and rabbits were used as materials. Urinary and basal surfaces of podocytes were exposed by a conventional freeze-fracture method and by NaOH maceration, respectively, and subsequently examined by scanning electron microscopy. In accordance with previous reports, each podocyte consisted of a round cell body protruding into Bowman's space, four to six major processes embracing glomerular capillary, and numerous pedicels on both sides of the major processes. The podocyte pedicels exhibited uniform needle-like shapes, about 0.2 microm thick, interdigitated with those of adjoining cells along the entire length of the cell margins in all the animal species examined. This finding suggests that the fine pedicel interdigitation is a primary event in morphogenesis of the podocyte. The basal aspect of the glomerular epithelium was mosaicked with pedicels which were laid at various angles to the capillary axis, in favor of its possible role as a mechanical support of the capillary wall.

Angiotensin II-induced calcium signalling in isolated glomeruli from fish kidney (Oncorhynchus mykiss) and effects of losartan

Glomeruli were isolated from the kidney of freshwater-adapted rainbow trout, Oncorhynchus mykiss, to qualitatively evaluate changes in cellular calcium associated with angiotensin II ([Asn1Val5]-Ang II) receptor stimulation and antagonism by the Ang II receptor antagonist losartan. Microspectrofluorometry using the fluorescent calcium indicator dye Calcium Green recorded fluorescence changes in isolated single glomeruli. Isolated glomeruli containing ester-loaded Calcium Green showed an Ang-II-induced transient rise in fluorescence. This transient rise showed an increased peak amplitude with increased Ang II concentration (10(-9) to 10(-6) M), but only a very small response was detectable in glomeruli exposed to 10(-9) M Ang II. The biphenylimidazole compound losartan (=DuP 753), an antagonist of the mammalian AT1 subtype Ang II receptor, initiated a transient agonistic rise in glomerular fluorescence at high concentration (10(-5), 10(-4), and 10(-3) M). However, the responses to 10(-6) 10(-7) M losartan were small or very low in each case. Losartan (10(-4) or 10(-7) M) antagonised the Ang-II-induced signalling in isolated glomeruli exposed to 10(-7) or 10(-6) M Ang II, respectively. This is the first evidence for functional AT1-like Ang II receptors coupled to cellular calcium signalling in the glomeruli of rainbow trout.

Prostaglandins in the kidney, urinary bladder and gills of the rainbow trout and European eel adapted to fresh water and seawater

Prostaglandins in kidney, gills, and urinary bladder of freshwater-adapted and seawater-adapted rainbow trout, Oncorhynchus mykiss (= Salmo gairdneri), and European eel, Anguilla anguilla, were determined by solid-phase extraction of tissue homogenates and high-pressure liquid chromatography. Prostaglandins E2, E1, F1 alpha, F2 alpha, and D2 and the more stable metabolite of prostacyclin, 6-keto F1 alpha, occurred in these osmoregulatory tissues. In gill filaments and kidneys of both eel and trout, prostaglandins D2 and 6-keto F1 alpha were major prostaglandins. Concentrations of these prostaglandins were significantly lower in the eel after seawater adaptation, but not in the trout. The urinary bladder of the trout contained the highest levels of prostaglandins; bladders of seawater-adapted trout contained prostaglandin D2 at 6.7 ng/mg wet tissue, the highest level of any prostaglandin determined in the present studies. Prostaglandin D2 was not detected in bladders of freshwater-adapted trout.

The elasmobranch renal corpuscle: fine structure of Bowman's capsule and the glomerular capillary wall

The fine structure of the renal corpuscle of the marine elasmobranch fish, the little skate (Raja erinacea), and two species of dogfish sharks, the spiny dogfish (Squalus acanthias) and the smooth dogfish (Mustelus canis), was studied by light microscopy and by transmission (thin sections, freeze-fracture replicas) and scanning electron microscopy. Bowman's capsule was lined by ciliated cells, similar to those of the first part of the tubule, at the urinary pole and squamous cells in the zone between urinary and vascular poles. At the vascular pole the visceral epithelial cells had some closely apposed cuboidal cell bodies with a few processes inserted along the basement membrane, but foot processes were absent. These cuboidal cells were continuous with podocytes, which had primary, secondary and tertiary processes from which the pedicels arose. An inconsistently present slit membrane bridged the pedicels at varying distance from the urinary space. Small maculae and large fasciae occludentes joined the podocytes and/or their processes among which gap junctions were observed. In the skate kidney, the podocyte plasma membrane facing the basement membrane contained orthogonal arrays of particles. The epithelial basement membrane of the glomeruli in all elasmobranchs was consistently thick. The mesangial cells were numerous and partially enveloped in a basement membrane; their long processes almost completely circumscribed the capillary walls. The mesangial matrix was abundant and consisted of loosely arranged collagen fibrils, microfibrils and occasional anchoring fibrils. The endothelial cells had irregularly distributed fenestrations of various sizes and lay on their discontinuous basement membrane which was separate from that under the epithelial cells. The filtration apparatus resembles that of immature mammals and lower vertebrates.

Antidiuretic and cardiovascular actions of prostaglandin E2 in the rainbow trout Salmo gairdneri

The renal and cardiovascular actions of intravenous injection or infusion of prostaglandin E2 (PGE2) have been investigated in the anaesthetised rainbow trout, Salmo gairdneri. Amounts of 1-2500 ng PGE2/kg body wt had a potent vasodilatory action and caused rapid dose-related decreases in dorsal aortic blood pressure. Intravenous infusion of PGE2 was associated with a secondary tachycardia and vasoconstriction. Injection of PGE2 resulted in transient dose-related decreases of urine production. Infusion of PGE2 at 200 ng/min/kg rapidly decreased glomerular filtration rates by reducing the populations of filtering nephrons and later by increasing the renal tubular water reabsorption. The possible endocrine systems which may be involved in the renal responses to PGE2 are discussed.

The characteristics and distribution of monoamine oxidase (MAO) activity in different tissues of the rainbow trout, Salmo gairdneri

Monoamine oxidase (MAO) activity was determined fluorometrically in brain, intestine, kidney and liver tissues of the rainbow trout, Salmo gairdneri. MAO activity was inhibited by various drugs in a concentration-related manner, with single sigmoid inhibition curves, the inhibitors of type A MAO, harmaline and clorgyline being more effective than deprenyl, an inhibitor of type B MAO. Intestine exhibited greatest MAO activity followed by liver and brain with kidney showing least activity. The Michaelis constants (Km) also showed variability between tissues. Inhibition of MAO by harmaline was non-competitive and dependent on the concentration of substrate present.

Renal microvasculature of the rainbow trout, Salmo gairdneri: scanning electron microscopy of corrosion casts of glomeruli

Scanning electron microscopy of corrosion casts of blood vessels permits detailed and accurate study of the microcirculation. The present study examined the renal microvasculature of the rainbow trout, Salmo gairdneri. The conventional picture of a glomerulus with one afferent arteriole was common, but glomeruli were often supplied by two afferent arterioles. In the majority of these, the intrarenal artery gave rise to a single afferent arteriole that branched to form two smaller vessels before reaching the glomerulus. Glomeruli with two afferent arterioles that arose independently from the intrarenal artery also occurred. The majority of glomeruli had a single efferent arteriole, but a proportion of glomeruli had two efferent arterioles. Efferent arterioles were smaller in diameter than the afferent arterioles. The glomerular capillaries were arranged in lobules, with few anastomoses between lobules, so that, for glomeruli with two afferent or two efferent arterioles, vascular perfusion and thus filtration within discrete lobules is probable.

Renal and cardiovascular effects of angiotensin II in the rainbow trout, Salmo gairdneri

The renal and cardiovascular effects of an intravenous infusion of angiotensin II in the freshwater anaesthetised rainbow trout, Salmo gairdneri, have been reappraised, without the infusion of noradrenaline. In these fish the previously reported antidiuretic response to angiotensin II was not maintained although a systemic pressor response persisted and the tubular transport maximum for glucose remained depressed. The data suggest that angiotensin has an intrarenal action which may play a role in renal adaptation to increased environmental salinities.

The anionic barrier system on the mesonephric renal glomerulus of the brown hagfish, Paramyxine atami Dean (Cylostomi)

The distribution of anionic groups of acid mucopolysaccharides on the surface of glomerular constituents of a brown hagfish, Paramyxine atami Dean, has been studied morphologically. The ionized anionic groups of acid mucopolysaccharides were labeled on fixed tissues by staining with cationic cacodylate iron colloid (Fe-Cac) at pH 4.0. The glomerular permeability to cationic and anionic macromolecules was observed morphologically in the kidney of the animal injected with native anionic ferritin (NF) or cationized ferritin (CF) into the dorsal aorta. Histochemical staining of tissues with Fe-Cac (pH 4.0) revealed the ionized anionic groups of acid mucopolysaccharides on both luminal and abluminal surfaces of endothelial cells, within the glomerular basement membrane (GBM), and on the visceral epithelial cell surface facing the urinary space. The CF molecules introduced into the dorsal aorta easily passed through the fenestrae of the capillary endothelial cell layer and the thick fibrillar GBM, reaching the urinary space to be adsorbed to the visceral epithelial cell surface or taken up by these visceral epithelial cells. On the other hand, NF hardly passed through the capillary wall. These results show that the nonosmoregulating mesonephric glomerulus of the brown hagfish has a working anionic barrier system. The function of its glomerulus is compared to that of the mammalian metanephric glomerulus.