RESUMEN
A number of species of euryhaline teleosts have the remarkable ability to adapt and survive in environments of extreme salinity, up to two or even three times the osmolality of seawater. This review looks at some of the literature describing the adaptive changes that occur, primarily with intestinal water absorption and with the properties of the gill epithelium. While there is much that is still not completely understood, recent work has begun to look at these adaptations at the cellular and molecular level. As with seawater osmoregulation, fish adapting to hypersaline conditions generally increase drinking rates and water absorption across the intestine by solute-linked transport. This process requires increased activity, expression levels and possibly changes in subunit isoforms of Na(+)/K(+)-ATPase pumps, along with increases in other components of the NaCl absorptive pathway. Additionally, recent studies have demonstrated the importance of luminal anion exchange activity and of luminal alkalinization, which can support absorption against increasing osmotic gradients by promoting divalent ion precipitation (CaCO(3)) and by conversion of HCO(3)(-) ions to CO(2). The steepness of the lumen to blood osmotic gradient vis-a-vis the NaCl absorptive capacity, along with the accumulation of non-absorbed Mg(++) and SO(4)(--), likely become the limiting factors for survival in hypersaline conditions. Of interest is the observation of predicted hyperosmotic fluid absorption by the intestine in several species. Adaptive changes in the gill epithelium are also critical in this process, allowing for secretion of absorbed NaCl from the extracellular fluids. Most notably there are important changes in the numbers and size of mitochondrion-rich (MR) cells, the sites of active secretion of Cl(-), which ultimately drives the overall process of NaCl secretion. Balance studies of intake and output clearly indicate that a decrease in the osmotic permeability of the gill epithelium must also occur. The molecular correlates of this effect are not known, although decreased expression of one or more aquaporins seems to be a likely possibility. Finally, the regulatory changes seen with hypersaline adaptation may provide important new insights into epithelial function, including the role of organized transport assemblies ("metabolons") and changes in the expression of tight junction proteins such as claudins or occludins, which may modulate electrolyte permeabilities.
Asunto(s)
Adaptación Fisiológica , Peces/fisiología , Branquias/metabolismo , Salinidad , Animales , Branquias/citologíaRESUMEN
Experiments were designed to characterize an active, electrogenic transport of NH(4)(+) ions across the colonic epithelium of the domestic fowl (Gallus gallus). Colonic segments were isolated and stripped of underlying muscle. The mucosal epithelia were mounted in Ussing chambers and voltage-clamped to measure the short-circuit currents (I(SC)) associated with transport. Bilateral addition of NH(4)(+) caused a dose-dependent outward current (negative I(SC)), with a Km of 34+/-8 mM and a maximal current response of 311+/-47 microA cm(-2) (12+/-2 microEq cm(-2) h(-1)). A similar effect was seen with unilateral addition of NH(4)(+) to the serosal (s) side, but not with mucosal (m) addition. Pre-treatment with 10(-4) M amiloride exposed a net outward (negative) I(SC), and serosal NH(4)(+) addition further increased this outward current with a Km of 53+/-24 mM. Decreasing the bath pH from 7.3 to 6.0 did not affect the I(SC) response to NH(4)(+). Unidirectional NH(4)(+) flux measurements revealed a net secretory flux (8.8+/-3.1 micromol cm(-2) h(-1) s-m, versus 2.6+/-1.4 micromol cm(-2) h(-1) m-s). Furthermore, the secretory flux closely matched the resulting change in I(SC) with serosal NH(4)(+), showing that the transepithelial flux of NH(4)(+) could account for the outward current response. Addition of 50 nM bafilomycin A to the mucosal solution completely eliminated serosal to mucosal NH(4)(+) transport, implicating an apical V-type H(+)-ATPase in this transport process. The I(SC) response to NH(4)(+) was partially inhibited by ouabain, a blocker of the Na(+)/K(+)-ATPase, but only minimally affected by bumetanide, an inhibitor of the serosal Na(+)-K(+)-2Cl(-) cotransporter. Active NH(4)(+) extrusion across the mucosal membrane, combined with low permeability to NH(3) in this tissue, allow for maintenance of steep ammonia gradients across the colonic epithelium and protection from ammonia toxicity. Furthermore, these studies indicate that the hen colon may be a useful new model system for the study of NH(4)(+) transport.
Asunto(s)
Permeabilidad de la Membrana Celular/fisiología , Pollos/metabolismo , Colon/metabolismo , Mucosa Intestinal/metabolismo , Compuestos de Amonio Cuaternario/metabolismo , Animales , Transporte Biológico/fisiología , Colon/citologíaRESUMEN
Birds are a diverse vertebrate class in terms of diet and habitat, but they share several common physiological features, including the use of uric acid as the major nitrogenous waste product and the lack of a urinary bladder. Instead, ureteral urine refluxes from the urodeum into the more proximal coprodeum and portions of the hindgut (colon or rectum and ceca). This presents a potential problem in that hyperosmotic ureteral urine in contact with the permeable epithelia of these tissues would counteract renal osmotic work. This review describes and provides examples of different strategies used by avian species to balance renal and post-renal changes in urine composition. The strategies described include: 1. a "reptilian" mode, with moderate renal concentrating ability, but high rates of post-renal salt and water resorption; 2. the "mammalian" strategy, in which the coprodeum effectively functions like a mammalian urinary bladder, preserving the osmotic concentrating work of the kidney; 3. an interaction strategy, in which post-renal transport processes are hormonally regulated in order to optimize renal function under varying conditions of salt or water stress; 4. the salt gland strategy seen in marine or estuarine birds with functional salt glands, in which post-renal transport mechanisms are used to conserve urinary water and to recycle excess NaCl to the nasal salt glands. Finally, we also describe some features of an as-yet unstudied group of birds, the birds of prey. At least some species in this group are relatively good renal concentrators, and would be predicted to have post-renal mechanisms to preserve this work. This new synthesis illustrates the marked diversity of adaptive mechanisms used by avian species to maintain osmotic homeostasis.
Asunto(s)
Adaptación Biológica/fisiología , Aves/fisiología , Riñón/fisiología , Orina/fisiología , Animales , Riñón/anatomía & histología , Mamíferos/fisiología , Modelos Biológicos , Concentración Osmolar , Rapaces/fisiología , Reptiles/fisiología , Glándula de Sal/fisiologíaRESUMEN
The lower intestine (colon and coprodeum) of the domestic fowl maintains a very active, transporting epithelium, with a microvillus brush border, columnar epithelial cells, and a variety of transport systems. The colon of normal or high salt-acclimated hens expresses sodium-linked glucose and amino acid cotransporters, while the coprodeum is relatively inactive. Following acclimation to low salt diets, however, both colon and coprodeum shift to a pattern of high expression of electrogenic sodium channels, and the colonic cotransporter activity is simultaneously downregulated. These changes in the transport patterns seem to be regulated, at least in part, by aldosterone. Our recent work with this tissue has focused on whether aldosterone alone can account for the low salt pattern of transport. Other work has looked at the changes in morphology and in proportions of cell types that occur during chronic acclimation to high or low salt diets, and on a cAMP-activated chloride secretion pathway. Recent findings suggesting effects of other hormones on lower intestinal transport are also presented.
Asunto(s)
Aldosterona/metabolismo , Aves/fisiología , Transporte Iónico/fisiología , Tracto Gastrointestinal Inferior/metabolismo , Aclimatación , Aldosterona/farmacología , Animales , Dieta Hiposódica , Absorción Intestinal/efectos de los fármacos , Absorción Intestinal/fisiología , Tracto Gastrointestinal Inferior/anatomía & histología , Modelos Biológicos , Porfirinas/metabolismo , Aves de Corral/metabolismo , Sodio/metabolismo , Simportadores del Cloruro de Sodio/metabolismo , Gorriones/metabolismoRESUMEN
The effects of nitric oxide (NO) in the secretory response to the endogenous secretagogue 5-hydroxytryptamine (5-HT), the enterotoxins heat-labile enterotoxigenic Escherichia coli (ETEC) toxin (LT) and cholera toxin (CT), and various cultures of ETEC and Salmonella serotype Typhimurium in the porcine small intestine (Sus scrofa) were investigated. In anaesthetized pigs, jejunal tied-off loops were instilled with 5-HT, LT, CT, various cultures of ETEC or S. Typhimurium. Pigs were given intravenously isotonic saline or isotonic saline containing the NO synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME). L-NAME significantly induced an increased fluid accumulation in loops induced by 5-HT, ETEC and stn-mutated S. Typhimurium. Fluid accumulation in loops instilled with wild-type S. Typhimurium was increased by L-NAME, although not significantly, while there was no effect on fluid accumulation induced by an invH-mutated isogenic strain. No significant effect of L-NAME was observed on the fluid accumulation induced by the purified enterotoxins LT and CT. The results also demonstrated a relatively large difference in the ability to induce fluid accumulation between the bacteria strains. Diastolic, systolic and mean blood pressures were significantly increased and the body temperature was significantly decreased in groups of pigs treated with L-NAME. In conclusion, the results suggest that NO has a proabsorptive effect in the intact porcine jejunum and is involved in the systemic vascular tone.
Asunto(s)
Toxina del Cólera/farmacología , Escherichia coli/efectos de los fármacos , Escherichia coli/patogenicidad , Intestino Delgado/efectos de los fármacos , Intestino Delgado/microbiología , Óxido Nítrico/farmacología , Salmonella typhimurium/efectos de los fármacos , Salmonella typhimurium/patogenicidad , Serotonina/farmacología , Animales , Antiinfecciosos/metabolismo , Presión Sanguínea , Temperatura Corporal , Escherichia coli/metabolismo , Femenino , Mutación , NG-Nitroarginina Metil Éster/metabolismo , NG-Nitroarginina Metil Éster/farmacología , Neuronas/metabolismo , Óxido Nítrico Sintasa/metabolismo , Isoformas de Proteínas , Infecciones por Salmonella , Salmonella typhimurium/metabolismo , Porcinos , Factores de TiempoRESUMEN
The correlation between morphology of the mitochondria-rich cells (MR cells) in chicken lower intestine, coprodeum, and dietary sodium levels, has been investigated, using hens with differing dietary intake of NaCl and plasma aldosterone levels. Additionally, the function of the MR cells was evaluated in relation to proton secretion/exchange. Epithelium from the coprodeum was examined by optical, transmission and scanning electron microscopy, and Na(+)-transport across the coprodeal epithelium was measured electrophysiologically in Ussing-chambers. To investigate the function of MR cells, lectin-, enzyme- and immunohistochemistry methods were used. The MR cells were generally located in the epithelium on the upper parts of the sides of mucosal folds. Long microvilli, high but variable toluidine blue affinity/electrondensity and numerous mitochondria were the main features distinguishing them from the surrounding epithelial cells. Two main MR cell types were observed, differing in microvillous morphology, diameter and toluidine blue affinity/electrondensity. This probably reflected differences in maturity and activity. The MR cells expressed a positive carbonic anhydrase reaction and a proton exchange similar to the absorptive intestinal epithelial cells, but exhibited no specific demonstrable proton secretion. A close correlation between the ultrastructure of the MR-cells, dietary sodium levels, plasma aldosterone and transepithelial Na-transport was observed.
Asunto(s)
Pollos/fisiología , Mucosa Intestinal/citología , Mucosa Intestinal/metabolismo , Intestinos/citología , Mitocondrias/fisiología , Sodio/metabolismo , Animales , Anticuerpos/inmunología , Anhidrasas Carbónicas/análisis , Anhidrasas Carbónicas/metabolismo , Células Caliciformes/citología , Células Caliciformes/ultraestructura , Mucosa Intestinal/ultraestructura , Intestinos/ultraestructura , Transporte Iónico/fisiología , Lectinas/análisis , Lectinas/metabolismo , Microvellosidades/ultraestructura , ATPasas de Translocación de Protón/análisis , ATPasas de Translocación de Protón/metabolismo , Compuestos de Amonio Cuaternario/metabolismo , Intercambiador 3 de Sodio-Hidrógeno , Intercambiadores de Sodio-Hidrógeno/análisis , Intercambiadores de Sodio-Hidrógeno/inmunología , Intercambiadores de Sodio-Hidrógeno/metabolismoRESUMEN
Recent evidence indicates that soybean, which is widely used in animal nutrition, could directly alter intestinal ion and nutrient transport. However, the mechanisms involved are still unknown. The aim of the study was to investigate the effect of three differently treated soybean products on the glucose and Cl- transport capacity in porcine small intestine by the Ussing chamber technique. Jejunal and ileal piglet epithelial tissues were pre-incubated with extracts of raw soybean flour (RSF), heated soybean flour (HSF), or ethanol heat-treated soybean protein concentrate (SPC). The Na(+)-dependent glucose co-absorption capacity was then measured as an increase in the short-circuit current (ISC) after luminal addition of D-glucose. The effect of the soybean products on cAMP-dependent Cl- secretion was measured as the increase in ISC after the addition of the phosphodiesterase inhibitor, theophylline, while nervous regulation of Cl- secretion was investigated by the addition of the enteric neurotransmitters; 5-hydroxytryptamine (5-HT), substance P and vasoactive intestinal polypeptide (VIP). Incubation with RSF and HSF induced a 30% decrease of the Na(+)-dependent glucose absorption capacity in the jejunum. The effect was similar for RSF in the ileum. Theophylline-induced secretion was decreased by 30% after incubation with RSF, HSF and SPC but only in the jejunum. 5-HT-, substance P- and VIP-induced secretion were not altered by incubation with soybean extracts except in the HSF-incubated where the substance P-induced secretion was significantly reduced. In conclusion, soybean contains ethanol-sensitive heat-insensitive compounds impairing Na(+)-dependent glucose absorption in the jejunum and ileum, and ethanol- and heat-insensitive compounds causing an acute impairment of cAMP-dependent jejunal secretion.