Salt and water permeability of the epithelium of the coprodeum and large intestine in the normal and dehydrated fowl (Gallus domesticus). In vivo perfusion studies

Effects of the environment on the evolution of the vertebrate urinary tract

Evolution of the vertebrate urinary system occurs in response to numerous selective pressures, which have been incompletely characterized. Developing research into urinary evolution led to the occurrence of clinical applications and insights in paediatric urology, reproductive medicine, urolithiasis and other domains. Each nephron segment and urinary organ has functions that can be contextualized within an evolutionary framework. For example, the structure and function of the glomerulus and proximal tubule are highly conserved, enabling blood cells and proteins to be retained, and facilitating the elimination of oceanic Ca+ and Mg+. Urea emerged as an osmotic mediator during evolution, as cells of large organisms required increased precision in the internal regulation of salinity and solutes. As the first vertebrates moved from water to land, acid-base regulation was shifted from gills to skin and kidneys in amphibians. In reptiles and birds, solute regulation no longer occurred through the skin but through nasal salt glands and post-renally, within the cloaca and the rectum. In placental mammals, nasal salt glands are absent and the rectum and urinary tracts became separate, which limited post-renal urine concentration and led to the necessity of a kidney capable of high urine concentration. Considering the evolutionary and environmental selective pressures that have contributed to renal evolution can help to gain an increased understanding of renal physiology.

Does the ostrich (Struthio camelus) coprodeum have the electrophysiological properties and microstructure of other birds?

The ostrich is unique among birds in having complete separation of urine and faeces. The coprodeal epithelium is thus during dehydration exposed to a fluid 500 mOsm hyperosmotic to plasma. We have investigated whether the coprodeum is adapted like a mammalian bladder. The coprodeal epithelium was studied by electrophysiology in the Ussing chamber, and the anatomy by light microscopy and scanning electron microscopy.

ELECTROPHYSIOLOGY

The short-circuit current (SCC) and open circuit electrical potential difference were recorded. The change induced by 0.1 mmol mucosal amiloride was recorded. An average basal SCC of 162+/-29 microA/cm(2) was observed, and a resistance of 297+/-34 Omega cm(2) calculated. These values are as observed in other avian coprodea. The resistance is much lower than in mammalian bladders (10000 Omega cm(2)). The amiloride-sensitive SCC, equal to net sodium absorption, was approximately 5 micromol/cm(2)h as observed in other avian species.

ANATOMY

The mucosal membrane is composed of broad irregular folds with very short intestinal glands containing an unusually high proportion of goblet cells.

CONCLUSION

The ostrich coprodeum is not adapted like a mammalian bladder. The abundance of goblet cells results in a copious secretion of mucus that establishes a thick unstirred layer giving effective osmotic protection.

Ontogenetic development of proline intestinal transport in the domestic fowl

1. The proline disappearance from the jejunal and ileal lumen of chickens aged from 1 day to 15 weeks were studied using a perfusion method "in vivo". 2. A decrease in proline transport was observed from younger to older animals. The 2 intestinal segments showed different behaviours, the 1-day and 1-week-old animals showed the same value in the jejunum, whereas in the ileum a progressive decrease in proline transport was observed from the first week of life. 3. The differences observed in the 2 segments could be attributed to the different rates of growth of the jejunum and ileum with age; the jejunum showed a peak of growth in the second week of life whereas the ileum showed a peak of growth in the first week. Proline transport in the jejunum decreased until the fifth week and remained constant thereafter.

Experimental studies of intestinal ion and water transport

A major advance in transport physiology was H. H. Ussing's development of the voltage-clamp method, and later the Koefoed-Johnsen-Ussing model for Na+ transport. In the same decade, J. C. Skou identified the Na(+)-K(+)-ATPase, which maintains the Na+ and K+ gradients that drive most epithelial transport processes. With this foundation, Danish scientists have pursued the mechanism of ion transport and the resulting solute-linked water flow. Recent contributions have been on isosmotic transport, suggesting solute recycling, and KCl-water cotransport in the basolateral epithelial cell membrane. Efficient small intestinal nutrient absorption is dependent on coupling to the Na+ gradient. Cotransport of Na+ and glucose is quantitatively the most important absorptive mechanism in the small intestine, as illustrated by the success of oral rehydration solutions in diarrhoea. The majority of amino acids are likewise transported by Na+ dependent carriers, but recent experiments have identified a concomitant Cl- dependency for some. Regulation of intestinal secretion, both under normal digestive processes, and in response to enterotoxins, has turned out to be very complex. It involves local and central neuronal regulation through an array of neurotransmitters and local actions of gastrointestinal hormones. Major effectors are the submucosal neurons and the main transmitters serotonin, vasoactive intestinal peptide, acetylcholine, substance P, and neurotensin. Development of antisecretagogues is impeded by the existence of several receptor subtypes and significant species differences. The Na+ and water-conserving properties of the large intestine have been shown to be regulated by adrenocortical hormones, with aldosterone as a potent stimulator of colonic Na+ absorption. A major colonic function is the symbiosis with the anaerobic bacterial population. The fermentation of carbohydrate to short-chain fatty acids, which can be absorbed, supplements small intestinal digestive function.

The integrative segment of the quail Coturnix coturnix japonica. Occurrence and distribution of carbonic anhydrase and complex carbohydrates

As part of a more extensive study into the involvement of carbonic anhydrase in avian excretory function, the occurrence and distribution of this enzyme was investigated in the quail integrative segment. The integrative segment represents, in birds, that part of the intestinal tract where ureteral urine undergoes postrenal modification to form definitive urine. To define the structural peculiarities within the intestinal epithelium, the constituent parts, namely cloaca, rectum and caecum, as well as the posterior ileum, were examined histochemically to visualise complex carbohydrates. The histochemical findings for carbonic anhydrase activity were compared with the results from a correlative immunohistochemical approach performed with a specific antiserum to avian CA II. Most of the enzyme activity unhomogeneously distributed in the intestinal enterocytes within the mucosal epithelium, was shown to be due to the cytosolic isoenzyme CA II. Additional carbonic anhydrase isoenzymes, distinct from CA II, seem to occur both at the enterocyte brush border and at the smooth muscle layer of the muscularis externa.

Double luminal and vascular perfusion of chicken jejunum: studies on 3-O-methyl-D-glucose absorption

The aims of the present study were: (1) to set up a procedure for simultaneous vascular and luminal perfusion of the chicken jejunum; (2) to assess the transport capacity of the tissue under such conditions, and (3) to study the effects of phloretin and theophylline, given through the vascular perfusate, on 3-O-methyl-D-glucose intestinal transport. The perfusion procedure described allowed the control of intestinal and vascular inflow rates and perfusion pressures so that these parameters could be adjusted to physiological values. A perfluorochemical emulsion was used as oxygen carrier for the vascular perfusate. The absorptive function of the perfused tissue was assessed by means of its ability to transport 3-O-methyl-D-glucose. Furthermore, ultrastructure preservation was evaluated by scanning and transmission electron microscopy. Results indicate that the perfused tissue kept its transport capacity and morphology intact throughout the 120-min experimental period. Moreover, no hypersecretion was observed as indicated by the constancy of perfusate volumes and perfusion pressures. Phloretin (1 mM) or theophylline (10 mM) added to the vascular perfusate markedly reduced the transfer of 3-O-methyl-D-glucose from the enterocyte to the vascular fluid without affecting the uptake from the lumen. Our results suggest that this preparation may be used as an alternative tool for the study of intestinal absorption processes in avian species, particularly when complete examination is required of the efflux of substrates from the intestinal lumen to the vascular fluid.

Water and electrolyte transport by the avian ceca

In galliform birds with well developed ceca, these are functionally important components of the lower intestinal complex, which is capable as a whole of substantial, hormone-regulated homeostatic modification of mixed urine and intestinal luminal fluid before final excretion. Active Na+ transport drives Na-linked absorption of water and Cl- and secretion of K+; there are also lesser Na-independent components of the latter fluxes. These transport processes are appropriately enhanced by dehydration, Na depletion, or exogenous aldosterone.

Modulation of Na and Cl transport by mineralocorticoids

1. The epithelia of the hen lower intestine show a Na-channel, Na-cotransport, chloride cells, and chloride absorption and secretion. 2. The short circuit current is affected by low salt levels, amiloride, glucose, lysine, leucine, galactose, ouabain, bumetanide, aldosterone, dexamethasone and spironolactone. 3. The properties of the different sodium and chloride channels are described.

Jejunal and cecal 3-oxy-methyl-D-glucose absorption in chicken using a perfusion system in vivo

3-oxy-methyl-D-glucose (3-OMG) absorption by jejunum and caecum has been studied in the domestic fowl in vivo, with luminal perfusion, during 5 min periods. The diffusion component was evaluated in the presence of phloridzin (10(-3) M) that inhibits the active transport mechanism. Kd of jejunal and cecal diffusion of the monosaccharide have been calculated, showing a similar value. The Kt and Vmax of 3-OMG absorption were calculated using a graphical method for the two intestinal segments. The caecum showed a lower Kt and Vmax than the jejunum did.

Absorption of 3-oxy-methyl-D-glucose by chicken cecum and jejunum in vivo

Jejunal and cecal 3-oxy-methyl-D-glucose (3-OMG) absorption was studied in 4- to 8-week-old chickens by an in vivo perfusion technique (perfusion rate 1.5 ml/min). Total and phloridzin-insensitive 3-OMG absorption was tested for lumenal substrate concentrations ranging from 1.25 to 20 mmol/l. The estimated apparent Michaelis constants in jejunum and cecum were 5.1 and 4.0 mmol/l (Lineweaver-Burk method) and 3.2 and 3.1 mmol/l (visual inspection method), respectively. Vmax were similar in both segments with either method, about 0.3 mumol/cm2 X 5 min. Passive permeability coefficients were the same in both regions (about 45 l/cm2 X 5 min X 10(3)). The transport properties of the cecal epithelium in vivo suggest a role of these intestinal segments in the absorption of nutrients originated from digestive processes.

Serum electrolyte and enzyme responses to heat stress and dehydration in the fowl (Gallus domesticus)

1. Serum electrolytes, enzymes and various metabolites were determined in the hyperthermic and dehydrated fowl. 2. In normally-hydrated fowls, heat stress did not significantly affect blood constituents. 3. Water deprivation for 48 hr (dehydration) significantly (p less than 0.05) increased Na+, osmolality, SGPT and T3-retention. 4. During hyperthermic dehydration, Na+, Cl-, osmolality (p less than 0.01), BUN, glucose, T3-retention (p less than 0.02) and uric acid (p less than 0.001) significantly increased. 5. The present findings are consistent with the suggestion that changes in Na+/Ca2+ ratio might raise the hypothalamic thermoregulatory set-point and support our previous findings that acclimated fowls could efficiently regulate body temperature and acid-base status while avoiding extreme metabolic and enzymatic changes during heat exposure and dehydration.

Transport of electrolytes and water in the upper jejunum of the fowl in vivo perfusion

The mechanism(s) of electrolytes (JNa, JC1, JCa and JK) and water transport (JV) were studied in the upper jejunum of the laying hen by an in vivo perfusion procedure. Water secretion is modified by the difference of osmotic pressure between the lumen and plasma and we have estimated the osmotic permeability coefficient (P osm = 17.4 microliter x h-1 x mosm-1 x g-1 DW) and the reflexion coefficient (0.72). At sodium concentrations of 0 and 80 mM x 1-1 in the lumen, there is a large secretion of Na. When the water flow is modified by osmotic pressure, this secretion is markedly influenced by JV. Similarly net chloride flux occurred mainly by solvent drag. Conversely water flow does not modify the large potassium absorption along the concentration gradient and only slightly affects the flux of calcium. Transport of calcium occurred along the gradient of concentration between lumen and plasma without saturation until a lumen concentration of 20 mM x 1-1. This driving force seemed to be the main one since JCa is close to zero when there is no gradient. This observation supports the hypothesis that the variations of net Ca absorption during the laying cycle is due to a modification of concentration of soluble calcium in the contents of the intestine.

Salt and water excretion by birds: the lower intestine as an integrator of renal and intestinal excretion

1. In the fowl, the small intestine is important for net absorption of Ca2+ and K+, but not for Na+ nor water (in this and several other species). 2. Net water absorption in birds with large saccate caeca occurs in caeca greater than rectum greater than coprodeum, but net Na+ absorption (an active process motivating other absorptive functions) occurs in rectum less than caeca and coprodeum. 3. Interspecific variability and the scarcity of comparative studies militate against broad, well-founded generalisations in this subject.

Dietary Na+ effects on transepithelial transport of NaCl by hen (Gallus domesticus) lower intestine (colon and coprodeum) perfused luminally in vivo

The colon and coprodeum of anaesthetised laying hens was perfused luminally with solutions of varying compositions, to elucidate the influence of three levels of dietary NaCl and of luminal NaCl concentrations on coprodeal and colonic transport of NaCl. Net Na+ and Cl- absorption rates were increased in response to low dietary Na+ levels and higher luminal NaCl concentrations. Net Na+ absorption was always against the prevailing electrochemical gradient, saturably dependent on luminal Na+ concentrations in low Na+ diet birds, and linearly dependent on luminal concentrations in medium and high Na+ diet birds. Net Cl- absorption was usually down its electrochemical gradient. The Cl- flux rates were strongly linearly dependent on Na+ flux rates, and statistically this relationship could account for all significant variation of Cl- fluxes associated with dietary and perfusate treatments. Transmural electrical potential differences were invariably serosa positive, and partly correlated with Na+ fluxes.

Transepithelial transport of K+, NH4+, inorganic phosphate and water by hen (Gallus domesticus) lower intestine (colon and coprodeum) perfused luminally in vivo

The colon and coprodeum of anaesthetised laying hens was perfused luminally with solutions of varying compositions to elucidate the influence of three levels of dietary Na+ and of luminal fluid composition on transport of electrolytes and water. Net rates of secretion of K+ and absorption of NH4+ were increased in response to dietary Na+ depletion and increased luminal Na+ concentrations, but these fluxes were linked to Na+ transport and no dietary or perfusion treatment effects could be detected on them independently of effects on Na+ flux. Similar results were found for Na+-linked water absorption from isosmotic perfusates. Serosal-mucosal net osmotic flow with Na+-free perfusates was significantly reduced by Na+-depletion, but no significant dietary effect could be detected on the serosal-mucosal osmotic permeability coefficient. The reflexion coefficient was effectively unity. Mucosal-serosal inorganic phosphate flux was small. The flux was independent of Na+ flux and dietary Na+ levels, and apparently facilitated by serosalmucosal osmotic flow. The data allowed determination of the absorption/secretion in coprodeum and colon of the ions investigated, as compared to the renal excretion rate.

Structural-functional correlations in the kidneys and observations of colon and cloacal morphology in certain Australian birds

1. Variations in renal microstructure between the zebra finch and Senegal dove were consistent with their relative renal concentrating abilities (urine/plasma ratios of 2-8 and 1-7, respectively). Compared with dove kidneys, those of the finch contained a higher fraction of mammalian-type nephrons (with Henle's loops), and a lower fraction of reptilian-type nephrons (without loops). 2. Singing honeyeaters concentrated their urine almost as well as zebra finches, although honeyeater kidneys were less specialized (fewer mammalian-type nephrons). Such findings emphasize the need to clarify other osmoregulatory parameters. 3. No significant microstructural differences were found in the kidneys of domesticated as compared with those of wild zebra finches. Hence, osmoregulatory differences between tame and wild birds must be related to physiological factors rather than morphological. 4. Thickness of the renal medulla seemed to be directly correlated with urine concentrating ability. However, certain inconsistencies obscure this relationship such that its resolution will require further research. 5. Histological features of the mucosae of the colon and cloaca are described. The galah and kookaburra displayed a mammalian (non-villous) pattern of mucosal organization. Zebra finches, singing honeyeaters, and particularly emus, possessed colonic and cloacal villi and hence an increased surface area per volume in this region of the gut. This raises the possibility that the colon and cloaca are involved in uring concentration and osmoregulatory activities in these species.

Cloacal resorption of salt and water in the Galah (Cacatua roseicapilla)

1. The transmural net flow of salt and water in the coprodeum and large intestine of the Galah (Cacatua roseicapilla), an Australian xerophilic parrot, was measured by an in vivo perfusion technique. The main goal of the study was to understand what happens when the hyperosmotic ureteral urine formed in the dehydrated state is regurgitated into the cloaca. Buffered perfusion fluids, hyper- and isosmotic to plasma, with varying NaCl and KCl concentrations, were used. [(14)C]polyethylene glycol served as an unabsorbable water marker.2. The cloacal Na(+) and Cl(-) absorption rates were nearly parallel and at low luminal concentrations roughly proportional to the intraluminal concentrations. At higher concentrations the Na(+) absorption rate showed saturation. The maximal Na(+) flow was 217 muequiv/kg.hr, the concentration at half maximal flow 181 m-equiv/l. The Na(+) absorption rate was not impaired by a high K(+) concentration. K(+) was secreted into the intestine, except at high intraluminal K(+) concentrations where resorption was observed.4. The apparent osmotic permeability coefficient was 0.85 mul./kg.hr.m-osmole at an average osmotic difference of 446 m-osmole between lumen and plasma; it was higher at lower differences. In the (near) absence of an osmotic difference across the cloacal epithelium the solute-linked water flow was 5.0 mul. H(2)O/muequiv Na(+).5. It is concluded that the hyperosmotic ureteral urine formed in the dehydrated state can pass into the cloaca without a water loss. A Na(+) absorption of around 70% of the ureteral output is likely.

Effects of exteriorization of the ureters on the water metabolism of the domestic fowl

1. Six domestic fowls were operated for exteriorization of the ureters.2. Three weeks after the operation their food and water intake was compared with that of six unoperated control fowls of similar weight.3. Water intake was calculated from the amount of water drunk, the metabolic water and the water content of the food eaten; while water loss was estimated from the water content of urine and faeces excreted and from evaporation.4. Fowls with exteriorized ureters drank more than the control birds. The excess of water drunk by these birds approximated the amount of water lost in the urine.

Sodium chloride absorption and solute-linked water flow across the epithelium of the coprodeum and large intestine in the normal and dehydrated fowl (Gallus domesticus). In vivo perfusion studies

1. The transmural net flow of salt and water in the coprodeum and large intestine of normal and dehydrated hens was investigated by means of an intraluminal in vivo perfusion technique. The lumen was perfused with hypo-, iso-, and hyperosmotic salt solutions. Polyethylene glycol (PEG 4000) and [(14)C]inulin served as water markers.2. The maximal net Na(+) flow (J(Na)) from the mucosal to the serosal side was nearly the same in the two states of hydration: normal birds 308 mu-equiv/kg.hr, dehydrated birds 281 mu-equiv/kg.hr, while the J(Na) was half maximal at luminal Na(+) concentrations of 99 and 43 mu-equiv/l. respectively. The ;affinity' for Na(+) in the dehydrated bird was thus twice that in the normal bird. K(+) was secreted into the gut lumen at a constant rate against the electrochemical gradient, J(K) = -97 mu-equiv/kg.hr (S.E. = 5). Cl(-) was absorbed from lumen to plasma down the electrochemical gradient with J(Cl) ranging from 0 to 94 mu-equiv/kg.hr. The low J(Cl) was observed at low luminal NaCl concentrations when the J(Na) was also small.3. The solute-linked water flow, J(vs), occurring in the absence of an osmolality difference across the epithelium, was 1.1 mul. H(2)O/mu-equiv Na(+) in normal birds and 1.5 in dehydrated birds. The J(vs) was calculated as an operational parameter in experiments with luminal osmolalities different from plasma osmolality by subtracting the water flow observed in an experiment without Na(+) in the perfusion fluid from the water flow in an experiment with Na(+) containing perfusion fluids, both fluids being of the same osmolality. J(vs) was maximal at luminal osmolalities close to plasma osmolality. These observations are compatible with the hypothesis that the J(vs) is due to an osmotic flow into a confined region between the cells.4. When the perfusion rate was lowered from 5-9 ml./kg.hr to 0.8-1.0 the incoming perfusion fluid osmolality at which net water flow across the epithelium was zero went up from 100 to 180 m-osmolal higher than plasma osmolality. This observation suggests that a significant fraction of ureteral urine in the dehydrated bird may be absorbed in the coprodeum and large intestine.