In vivo perfusion studies of the cloacal water and electrolyte resorption in the fowl (Gallus domesticus)

Osmoregulatory role of the intestine in the sea lamprey (Petromyzon marinus)

Lampreys are the most basal vertebrates with an osmoregulatory strategy. Previous research has established that the salinity tolerance of sea lamprey increases dramatically during metamorphosis, but underlying changes in the gut have not been examined. In the present work, we examined changes in intestinal function during metamorphosis and seawater exposure of sea lamprey (Petromyzon marinus). Fully metamorphosed juvenile sea lamprey had 100% survival after direct exposure to 35 parts per thousand seawater (SW) and only slight elevations in plasma chloride (Cl^-) levels. Drinking rates of sea lamprey juveniles in seawater were 26-fold higher than juveniles in freshwater (FW). Na⁺-K⁺-ATPase (NKA) activity in the anterior and posterior intestine increased 12- and 3-fold, respectively, during metamorphosis, whereas esophageal NKA activity was lower than in the intestine and did not change with development. Acclimation to SW significantly enhanced NKA activity in the posterior intestine but did not significantly change NKA activity in the anterior intestine, which remained higher than that in the posterior intestine. Intestinal Cl^- and water uptake, which were observed in ex vivo preparations of anterior and posterior intestine under both symmetric and asymmetric conditions, were higher in juveniles than in larvae and were similar in magnitude of those of teleost fish. Inhibition of NKA by ouabain in ex vivo preparations inhibited intestinal water absorption by 64%. Our results indicate drinking and intestinal ion and water absorption are important to osmoregulation in SW and that preparatory increases in intestinal NKA activity are important to the development of salinity tolerance that occurs during sea lamprey metamorphosis.

Endocrine regulation of ion transport in the avian lower intestine

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.

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.

Altered spontaneous and osmotically induced drinking for fowls with permanent access to dilute quinine

Fowls were given a dilute quinine solution as their sole source of fluid, and effects on normal ingestion and on drinking responses to dipsogenic stimuli were examined. Compared to controls with water, daily fluid intakes were depressed by 25% with quinine. There was no significant effect of quinine on food intake, but growth was suppressed slightly. Drinking in response to hypertonic NaCl injections was attenuated with quinine, and whereas an initial peak in water intake was seen directly after hypertonic NaCl injection, this was absent with quinine. Moreover, increases in quinine intake after hypertonic NaCl injections were insufficient to restore normal osmolality. Plasma analyses indicated that birds drinking quinine were permanently dehydrated and, unlike birds with water, they appeared to reduce the hyperosmolality induced by hypertonic NaCl injections in the absence of drinking. In contrast, drinking responses to angiotensin were generally similar with quinine and water, although birds drinking quinine tended to stop sooner with the highest dose of angiotensin. These results support previous suggestions that osmotic thirst is of primary importance in control of normal drinking, but also demonstrate that birds tolerate a degree of dehydration if a suitable fluid source is unavailable.

Osmotic gradient dependence of osmotic water permeability in rabbit proximal convoluted tubule

To assess steady-state transepithelial osmotic water permeability (Pf), rabbit proximal convoluted tubules were perfused in vitro with the impermeant salt, sodium isethionate at 26 degrees C. Osmotic gradients (delta pi) were established by varying the bath concentration of the impermeant solute, raffinose. When lumen osmolality was 300 mOsm and bath osmolality was 320, 360 and 400 mOsm, apparent Pf decreased from 0.5 to 0.10 to 0.08 cm/sec, respectively. Similar data were obtained when lumen osmolality was 400 mOsm. Five possible causes of the delta pi dependence of apparent Pf were considered experimentally and/or theoretically: (1) external unstirred layer (USL); (2) cytoplasmic USL; (3) change in surface area; (4) saturation of water transport; (5) down-regulation of Pf. Apparent Pf was inhibited 83% by p-chloromercuribenzene sulfonate (pCMBS) at 20 mOsm, but not at 60 mOsm delta pi, suggesting presence of a serial barrier resistance to water transport. Increases in perfusate or bath solution flow rate and viscosity did not alter apparent Pf, ruling out an external USL. A simple cytoplasmic USL, described by a constant USL thickness and solute diffusion coefficient, could not account for the delta pi dependence of apparent Pf according to a mathematical model. The activation energy (Ea) for apparent Pf increased from 7.0 to 12.5 kcal/mol when delta pi was increased from 20 to 60 mOsm, not consistent with a simple USL or a change in membrane surface area with transepithelial water flow. These findings are most consistent with a complex cytoplasmic USL, where the average solute diffusion coefficient and/or the area available for osmosis decrease with increasing delta pi. These results (1) indicate that true Pf (at physiologically low delta pi) is very high (greater than 0.5 cm/sec) in the rabbit proximal tubule; (2) provide an explanation for the wide variation in Pf values reported in the literature using different delta pi, and (3) suggest the presence of a flow-dependent cytoplasmic barrier to water flow.

Transport function and control in bird caeca

1. The paired caeca at the junction of the ileum and rectum help determine the ionic composition of the voided excreta. 2. The caeca play a role in the transport of water, sodium, potassium, and chloride, the dominant effect being that of sodium transport.

Intravenous hypertonic saline injections and drinking in domestic fowls

Fowls were given intravenous (IV) injections of hypertonic solutions of NaCl, and subsequent water intakes were recorded. All concentrations of hypertonic NaCl increased drinking in the 90 min after injection, compared with control treatments. Increments in drinking in this time agreed closely with calculated amounts required to restore normal osmolality. In further experiments, delaying access to water by periods of 60-360 min after injection failed to reduce drinking elicited by hypertonic NaCl. Injections of 2.0 M NaCl caused increases in plasma osmolality and sodium concentration which were maintained throughout 360 min water deprivation, and caused prolonged reductions in hematocrit and plasma protein concentrations. These results demonstrate that cellular dehydration is a potent thirst stimulus in fowls, and imply that fowls do not reduce hyperosmolality by excretion of salt when water is unavailable.

Cecae of desert quail: importance in modifying the urine

The importance of the digestive ceca in the modification of ureteral urine was evaluated in conscious, unrestrained desert quail. Cecaectomized (Cx) birds took in slightly more food and water and excreted more solids and water than did sham operated controls. The percent utilization of food for the Cx birds was 76 and 79% for the sham operated controls. Urate output was 47 ml/kg day for Cx and 41 ml/kg day for the sham operated controls. Water content of the excrement was 90% for the Cx and 85% for the sham operated controls. There was a slight but insignificant increase in uric acid excretion in the droppings of cecaectomized birds.

NaCl transport across hen colon. Dependence on electro-chemical driving force

1. Sheets of isolated mucosa from the colon of chickens on either a high or a low NaCl diet were mounted in Ussing chambers. They were bathed in Krebs-phosphate medium and exposed to sodium concentrations ranging from 3.0 to 140 mmol/l (choline-chloride replacement). 2. In the short-circuited state the net fluxes of sodium in the mucosa (m)-serosa (s) direction, followed saturation kinetics with Km = 25 mmol/l in the low-NaCl chickens and 124 mmol/l in the high-NaCl chickens. 3. The unidirectional fluxes of sodium in the s-m direction were linearly related to the electro-chemical driving force. The apparent permeabilities were 8.6 +/- 0.3 x 10(-6) cm/s in the low-NaCl and 11.6 +/- 0.3 x 10(-6) cm/s in the high-NaCl birds. 4. The chloride fluxes were in the short-circuited state of equal magnitude in the m-s and the s-m direction, and in the two dietary states. Changes in the electrical driving force resulted in changes in chloride fluxes compatible with passive flow.

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.

Water consumption and urine volume in polydipsic and normal White Leghorn chickens

Experiments were conducted to study the route of passage of water by chickens exhibiting hereditary diabetes insipidus (di). Results showed that the excessive quantities of water consumed by the di chickens passes through the kidneys. Water reabsorption failure in the di chickens appears to occur in the kidneys.

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.

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

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.2. The lumen was perfused with a raffinose-electrolyte solution having a low sodium concentration (Na(+) = 1 m-equiv/l.). The osmolality of the solution was adjusted in the range 66-585 m-osmolal by adding raffinose. Polyethylene glycol 4000 (PEG) served as a water marker. The experiments permitted estimation of the passive transport parameters: the reflexion coefficient (sigma) of the penetrating solutes (predominantly NaCl), the Na(+) mobility (omega(s)), and the osmotic water permeability coefficient (P(osm)).3. When the luminal fluid had the same osmolality as plasma the net water flow (J(v)) was zero, indicating a sigma of unity. The net flow of Na(+) was zero, (at J(v) = 0) and a transmural electric potential difference close to zero was present both in normal and in dehydrated birds. This indicates an omega(s) of zero. When lumen osmolality was higher than that of plasma, no ;solvent drag' effect on Na(+) was demonstrated in the serosa to mucosa (s-m) direction.4. The P(osm) appeared to be independent of the luminal osmolality in the range of +/-200 m-osmolal from plasma osmolality. In normal birds the P(osms-m) was 3.2 mul./kg.hr.m-osmolal, the P(osmm-s) 5.8 mul./kg.hr.m-osmolal. In dehydrated birds these values were 3.6 and 10.0 respectively. Thus there seems to be rectification of water flow, and it varies with the state of hydration.5. A net K(+) flow of 15-50 mu-equiv/kg.hr in the m-s direction and a net Cl(-) flow of 10-50 mu-equiv/kg.hr in the s-m direction were observed. No relationship was observed between the flow of these solutes and the net water flow.

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.

Water transport in the cloaca of lizards: active or passive?

The withdrawal of water from the lizard cloaca can be a passive process resulting from the colloidal osmotic pressure of the plasma proteins. The forces necessary to withdraw water from lizard urine, the forces prevailing within the cloaca in vivo, and the counterbalancing of these forces by a protein solution placed in the cloaca all are in accord with this hypothesis.

The mechanism of salt and water absorption in the intestine of the eel (Anguilla anguilla) adapted to waters of various salinities

1. The absorption of NaCl and water was studied by intraluminal in vivo perfusion of the intestine of the yellow European eel (Anguilla anguilla) adapted to fresh water (FW), to sea water (SW), and to double strength SW (DSW).2. The net lumen to plasma NaCl transport from diluted SW perfusion fluids was independent of the NaCl concentration in the Na(+) concentration range tested. The NaCl absorption (expressed as mu-equiv/100 g.hr.) increased from FW (mean +/- S.E.): Na(+) 166 +/- 17, Cl(-) 205 +/- 24 to SW: Na(+) 363 +/- 33, Cl(-) 423 +/- 37, and again in DSW: Na(+) 640 +/- 110, Cl(-) 676 +/- 149.3. The osmolality of the perfusion fluid which resulted in zero net water transport was higher than plasma osmolality by 73 +/- 3 m-osmole in FW, 126 +/- 5 m-osmole in SW, and 244 +/- 32 m-osmole in DSW (mean +/- S.E.). A fairly constant ratio between net NaCl transport and this osmolality difference prevailed.4. The general osmotic permeability to water in the serosa-mucosa direction (expressed as mul./100 g. hr. m-osmole) measured from experiments with impermeant solute increased from FW: 3.7 +/- 0.5 to SW: 7.2 +/- 1.0 (mean +/- S.E.).5. These results are compatible with the interpretation that the water flow occurring in the absence of a general transmural osmotic gradient, the ;solute-linked water flow', is linearly related both to net NaCl transport and to the osmotic permeability to water. The findings support the view that the ;solute-linked water flow' is, indeed, secondary to the salt movement and is due to osmotic force.6. The amount of water absorbed from dilute SW perfusion fluids isosmotic with plasma was larger than in most other intestinal epithelia. FW: 650, SW: 1620 mul./100 g. hr. The NaCl concentration of the absorbate was hypertonic to plasma.7. The passive permeability of the intestine to NaCl was very low, and the reflexion coefficient was close to unity. Therefore metabolic energy will be used to absorb NaCl, even when the NaCl concentration in the gut is higher than that of plasma due to ingestion of SW. There appears to be a limited interaction in the intestinal wall between passive salt and water flow.8. In DSW the Na(+) ingestion with the oral intake of the surrounding fluid matched the gut absorption capacity. Since DSW is close to the tolerance limit, it is concluded that the gut NaCl transport capacity may be one of the factors limiting the tolerance to water or higher salinity.

Abnormal water balance in a mutant strain of chickens

Polydipsia and polyuria are pronounced in chickens of a selected strain and this diabetes insipidus is inherited. The kidneys of such birds are capable of an antidiuretic response when lysine vasopressin or arginine vasotocin is injected. Osmotic pressure and sodium concentration of the plasmas of normal and mutant chickens are identical. Chicks predicted to have diabetes insipidus on the basis of parental pedigree are polydipsic.