Differences in renal-cloacal function between Crocodylus porosus and Alligator mississippiensis have implications for crocodilian evolution

Major electrolytes and nitrogenous excretory products were analysed in the blood plasma, ureteral urine and cloacal urine of juvenile Alligator mississippiensis and Crocodylus porosus in fresh and hypoosmotic salt water (206 mosmol · l⁻¹). Both species coped well with saline water, showing little (Alligator) or no (Crocodylus) change in plasma composition. Comparisons of renal-cloacal function point to major differences in their osmoregulatory physiology. The cloaca of C. porosus is a very active osmoregulatory organ in salt and fresh water, contributing to water conservation and NaCl excretion through the lingual salt glands. In contrast, the cloaca of Alligator has little impact on the composition of excreted urine. It seems likely that A.␣mississippiensis is largely constrained to a renal response to osmotic and ionic stress while C. porosus is able to call on a more complex mix of renal response, post-renal modification of urine in the cloaca, and excretion of excess NaCl through the salt glands. The results support the idea that there are deep-seated differences in the osmoregulatory physiology of alligatorids and crocodylids (Eusuchia), an understanding of which should provide valuable insights into their evolution and zoogeography. Accepted: 7 September 1996     

Osmoregulatory mechanisms of the Australian freshwater crocodile, Crocodylus johnstoni, in freshwater and estuarine habitats

The estuary of the Limmen Bight River in Australia's Northern Territory is home to an unusual salt water-adapted population of the Australian `freshwater' crocodile, Crocodylus johnstoni. Crocodiles were captured from tidal reaches of the estuary ranging in salinity from 0.5–24‰ and from several permanent fresh water reaches more or less remote from saline waters. C. johnstoni is an effective osmoregulator in moderately saline waters and has osmoregulatory mechanisms very similar to its more marine-adapted relative, the estuarine crocodile Crocodylus porosus. Fasted C. johnstoni in brackish water appear to lose little sodium in cloacal urine, relying on their lingual salt glands for excretion of excess sodium chloride. The lingual glands show clear evidence of short-term and long-term acclimation to salt water. Like estuarine crocodiles, C. johnstoni drinks fresh water and will not drink sea water. Gross sodium and water fluxes in brackish water are very similar to those in other crocodilians, suggesting differences in integumental permeability are not a major influence on osmoregulatory differences between crocodilians. The data reinforce the hypothesis that crocodylids differ fundamentally from alligatorids in the structure and function of the renal-cloacal-salt gland complex and are of interest in current debate over the evolutionary and zoogeographical history of the eusuchian crocodilians. Accepted: 25 February 1999     

Osmoregulation in Crocodilians

Recent crocodilians live primarily in freshwater habitats. Howevertwo species (Crocodylus acutus and C. porosus) are estuarinespecialists; two others (C. niloticus and C. johnstoni) thatare primarily found in fresh water, have estuarine populations.Routes of uptake of water and sodium include drinking, feedingand associated incidental drinking, and integumental and buccaldiffusion. Routes of loss include faeces-cloacal fluid, lingualsalt glands, integumental and buccal diffusion, and respiratoryloss. The least understood route of salt and water exchangeis that of the oral and buccal epithelia, which are much morepermeable to water and sodium than the general integument. Thefreshwater alligator (Alligator mississippiensis) osmoregulatesin a manner typical for an amphibious reptile. Body sodium turnoveris low and the general integument is quite low in permeabilityto sodium. Water turnover is more rapid (in terms of molar exchange)but still relatively low for an aquatic reptile. Most waterexchange occurs across the integument and buccal epithelia.The presence of lingual salt glands in freshwater crocodiliansremains enigmatic, as does the failure of these exocrine glandsin estuarine species to respond to saline loading. Secretiondoes occur after injection of the parasympathetic stimulantmethacholine chloride. The "salt water crocodile" (C. porosus)possesses a suite of osmoregulatory adaptations similar to thosefound in other estuarine reptiles. Water and sodium balanceare maintained primarily by an extremely low general permeabilityto sodium, by economies in water loss, and by excretion of excesssodium by the lingual salt glands. Further work is needed toexamine newly hatched C. porosus, and the possibility of ontogeneticchange in lingual gland function in C. acutus. The importanceof incidental drinking of sea water during feeding (recentlydiscovered in turtles) needs to be evaluated in crocodilians.The use of osmoregulatory data in interpretation of the evolutionaryhistory of the genus Crocodylus needs to be viewed with caution.The hypothesis that all species of Crocodylus originated fromthe transoceanic migration of a saline-tolerant form may notbe the most parsimonious explanation.     

Sodium excretion rates and renal responses to acute salt loading in the European starling

Summary Renal clearance studies were performed in European starlings (Sturnus vulgaris) in order to determine the extent of ureteral sodium excretion under control conditions and during an acute, hyperosmotic salt stress. These experiments also estimated the contribution of the lower intestine (colon and cloaca) to postrenal solute reabsorption by making both cloacal and ureteral urine collections in the same birds. A comparison of ureteral vs cloacal excretion rates found significantly higher sodium (9.09±1.30 vs 1.03±0.38 Eq·kg^–1·min^–1) and chloride (4.15±0.56 vs 1.00±0.38 Eq·kg^–1·min^–1) excretion rates during the ureteral collections. Fractional excretion of sodium was also significantly higher during ureteral collections, but this value did not exceed 1% of the filtered sodium load during either collection series. Urine flow rate was significantly higher during cloacal collections, suggesting osmotic back-flux of water across the cloacal wall. Infusion of a 1M NaCl solution resulted in rapid increases in glomerular filtration rate (GFR), urine flow rate, and urine osmolality. Fractional sodium and water reabsorption decreased by 11% and 4%, respectively. Glomerular counts and size distribution profiles, measured by in vivo alcian blue labelling, provided no evidence for a reduction in the number of filtering glomeruli during hyperosmotic saline loading. We conclude that renal sodium excretion rates for the starling are similar to those seen in other avian species and in mammals. These studies also provide direct evidence for postrenal modification of urine in this species, even under conditions of continuous flow. Acute hyperosmotic salt stress can, under some conditions, cause increased rather than decreased GFR, indicating multiple regulatory pathways. Finally, there was no evidence in these studies for glomerular shutdown in response to salt loading.     

Osmoregulation of the Nile crocodile, Crocodylus niloticus, in Lake St. Lucia, Kwazulu/Natal, South Africa

Nile crocodiles of three age classes, hatched in captivity and reared in fresh water, when exposed acutely to water of 17 and 35 ppt NaCl, suffered marked dehydration, were lethargic, ceased to feed and lost mass. When exposed to gradually increasing salinities (3-35 ppt), with a short acclimation period at each salinity, crocodiles survived, continued to feed and increased in mass and size. All age classes had a relatively constant plasma osmolality across the salinity spectrum. Cloacal urine osmolality varied throughout the acclimation experiment, but did not increase with increasing salinity. No significant increase was found in plasma concentrations of any of the osmolytes. There was a trend of decreasing cloacal urine [Na(+)] and [Cl(-)] and increasing cloacal urine [K(+)] with increased salinity, indicating that urine was not an important route for Na(+) and Cl(-) excretion. Crocodiles exposed to saline conditions maintained relatively constant plasma uric acid concentrations, but urinary uric acid concentrations increased markedly with increasing salinities. This suggests that uric acid is the main constituent of nitrogenous waste excretion in saline exposed Nile crocodiles. As in Crocodylus porosus, C.niloticus has the physiological ability to survive and thrive in periodically hyper-osmotic environments. However, its euryhalinity is restricted, in that acute exposure to sea water leads to dehydration, but with an acclimation period at lower salinities, it survives and thrives in sea water.     

Effects of fresh and seawater ingestion on osmoregulation in Atlantic bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) are marine mammals with body water needs challenged by little access to fresh water and constant exposure to salt water. Osmoregulation has been studied in marine mammals for a century. Research assessing the effects of ingested fresh water or seawater in dolphins, however, has been limited to few animals and sampling times. Nine 16- to 25-h studies were conducted on eight adult dolphins to assess the hourly impact of fresh water, seawater, and seawater with protein ingestion on plasma and urine osmolality, urine flow rate (ufr), urinary and plasma solute concentrations, and solute clearance rates. Fresh water ingestion increased ufr. Fresh water ingestion also decreased plasma and urine osmolality, sodium and chloride urine concentrations, and solute excretion rates. Seawater ingestion resulted in increased ufr, sodium, chloride, and potassium urine concentrations, sodium excretion rates, and urine osmolality. Seawater with protein ingestion was associated with increased ufr, plasma osmolality, sodium excretion, and sodium, chloride, potassium, and urea urine concentrations. In conclusion, bottlenose dolphins appear to maintain water and plasma solute balance after ingesting fresh water or seawater by altering urine osmolality and solute clearance. Ingestion of protein with seawater appears to further push osmoregulation limits and urine solute concentrations in dolphins.     

Survival and growth of hatchling Crocodylus porosus in saltwater without access to fresh drinking water

Summary It has been suggested that C. porosus select nest sites which provide a source of freshwater for hatchlings during the dry season. From a mark-recapture study, we conclude that hatchling C. porosus can survive and grow in hyperosmotic saltwater without drinking freshwater. Hence, the siting of nests is unlikely to be the consequence of a requirement by hatchlings for freshwater. Considered along with other information, our observations imply that hatchling C. porosus have functional salt glands.     

Osmoregulation by the broad-snouted caiman, Caiman latirostris, in estuarine habitat in southern Brazil

The broad-snouted caiman Caiman latirostris, of South America mostly frequents freshwater but occurs also in estuaries. Nothing of substance is known of its osmoregulatory physiology but, in the light of accumulating evidence that alligatorids lack specialised adaptations for life in hyperosmotic waters, we anticipated its physiology would be more similar to that of Alligator mississippiensis than the euryhaline Crocodylus porosus, which has both lingual salt glands and a more complex renal:cloacal system. This proved to be the case. Caiman captured in estuaries of the Ilha do Cardoso in southern Brazil were effective hypo-osmotic osmoregulators in salinities of 0–24 ppt (seawater = 35 ppt). Plasma osmolarity, sodium and chloride were similar to those in other crocodilians and not influenced by salinity. Plasma urea was low and did not vary with salinity. We found no evidence of lingual or other salt glands. Urinary electrolyte concentrations varied considerably with salinity and in ways reminiscent of A. mississippiensis but very different from C. porosus. Ca. latirostris dehydrated in seawater more rapidly than C. porosus and had substantially higher integumental permeability to water. Caiman did not drink seawater but rehydrated rapidly when returned to freshwater (FW). We found small caiman (<500 g) only in very low salinities (<3 ppt) and larger caiman closer to the sea. We postulate that medium to large Ca. latirostris can take advantage of the feeding opportunities presented by the estuarine mangal despite lacking the physiological specialisations of crocodylids. Two individuals which we re-sighted by chance had travelled at least 600 m in 2–3 days, showing that every caiman we captured or saw was within easy reach of FW. Most likely their habitation of the estuary and its mangal is achieved through a combination of low surface area:volume ratio, relatively impermeable skin, and periodic access to FW. Accepted: 11 May 1998     

Mechanism of salt tolerance in wild rice (Oryza coarctata Roxb)

Summary Oryza coarctata, a highly salt-resistant wild rice species, is commonly found on the banks of coastal rivers in India. This species can also withstand saline water (20 to 40 dSm⁻¹ E.C) submergence for quite a long period. It was revealed thatO. coarctata has some special unicellular salt hairs (trichomes) on the adaxial surface of the leaves, by which they efficiently maintain a low concentration of toxic salts in the plant tissue. Sodium and chloride were the dominant ions in the excreted material but they also excrete potassium, magnesium and calcium. With the increase in soil salinity sodium, magnesium and chloride excretion increased.O. coarctata maintained the optimum mineral concentration in its tissues. Maximum accumulation of potassium was observed in the leaves. With the increase in salt stress total biomass production and osmotic potential increased over control but there was no change in the moisture percentage of leaves.     

Hormone-dependent dissociation of blood flow and secretion rate in the lingual salt glands of the estuarine crocodile, Crocodylus porosus

Salt and water balance in the estuarine crocodile, Crocodylus porosus, involves the coordinated action of both renal and extra-renal tissues. The highly vascularised, lingual salt glands of C. porosus excrete a concentrated sodium chloride solution. In the present study, we examined the in vivo actions of vasoactive intestinal peptide (VIP), B-type natriuretic peptide (BNP) and angiotensin II (ANG II) on the secretion rate and blood perfusion of the lingual salt glands. These peptides were selected for their vasoactive properties in addition to their reported actions on salt gland activity in birds and turtles and rectal gland activity in elasmobranchs. The femoral artery was cannulated in seven juvenile crocodiles for delivery of peptides and measurement of mean blood pressure and heart rate. In addition, secretion rate of, and blood flow to, the salt glands were recorded simultaneously using laser Doppler flowmetry. VIP stimulated salt secretion was coupled to an increase in blood flow and vascular conductance of the lingual salt glands. BNP was a potent stimulant of salt gland secretion, resulting in a maximal secretion rate of more than 15-fold higher than baseline; however, this was not coupled to an increase in perfusion rate, which remained unchanged. ANG II failed to stimulate salt gland secretion and there was a transient decrease in salt gland blood flow and vascular conductance. It is evident from this study that blood flow to, and secretion rate from, the lingual salt glands of C. porosus are regulated independently; indeed, it is apparent that maximal secretion from the salt glands may not require maximal blood flow.     

Circadian rhythm of water balance and aldosterone excretion in the whitebellied sunbird<Emphasis Type="Italic"> Nectarinia talatala</Emphasis>

Nectarivorous whitebellied sunbirds, Nectarinia talatala, demonstrate distinct circadian patterns in osmoregulatory parameters. We recorded intake of a 1 mol/l sucrose solution which enabled calculation of total water gain, and collected cloacal fluid for measurements of volume, osmolality and aldosterone concentration. These variables were assessed hourly over 12 h of photophase, and averaged over the 12-h scotophase period. Overnight, when sunbirds were in negative water balance, aldosterone concentrations and outputs were significantly higher than diurnal levels, reflecting a shut-down of cloacal fluid production. Early morning was marked by a high rate of osmotic excretion, disproportionate to water gain or cloacal fluid output, followed by steady intake and cloacal fluid output during the morning and early afternoon. Reduced water flux (decreased feeding and cloacal fluid output) during mid-afternoon was accompanied by a paradoxical decline in osmotic excretion, whilst a significant increase in the discrepancy between water intake and output was recorded as the birds effectively stored water before the scotophase. These patterns of intake and excretion may be informative in explaining drinking and foraging behaviour in the field.Abbreviations ALDO aldosterone - CF cloacal fluid - GFR glomerular filtration rate     

Effects of caecal ligation and saline acclimation on plasma concentration and organ mass in male and female Pekin ducks,Anas platyrhynchos

Summary The intestinal caeca reabsorb urinary sodium chloride (NaCl) and water (Rice and Skadhauge 1982). Free water may be generated if the reabsorbed NaCl is secreted via salt gland secretion (Schmidt-Nielsen et al. 1958). Therefore ceacal ligation should (a) reduce hingut NaCl and water reabsorption, (b) enhance the increase in plasma osmolality during saline acclimation, and (c) affect drakes more than ducks. Twelve Pekin drakes and 13 Pekin ducks, Anas platyrhynchos, were caecally ligated or sham operated before acclimation to 450 mmol · 1 NaCl. Body mass, hematocrit, plasma osmolality, and inonic concentrations of plasma, cloacal fluid, and salt gland secretion were measured after each increase in drinking water salinity. Osmoregulatory organ masses were determined. Caecal ligation did not effect plasma osmolality or ion concentrations of plasma, cloacal fluid, or salt gland secretion, but reduced salt gland size in ducks. Drakes and ducks drinking fresh water had the same hematocrit, plasma osmolality, and plasma concentrations of Na⁺ and Cl^–. In both sexes exposure to 75 mmol · 1^-1 NaCl significantly decreased plasma [Na⁺] and doubled cloacal fluid [Na⁺]. Exposure to 450 mmol · 1^-1 NaCl decreased body mass and increased hematocrit, plasma [Na⁺], [Cl^–], and plasma osmolality (more in drakes than in ducks); cloacal fluid osmolality nearly doubled compared to freshwater-adapted ducks, due mainly to osmolytes other than Na⁺ and Cl^–. The [Cl^–] in salt gland secretion only slightly exceeded drinking water [Cl^–].Abbreviations AVT antiduretic hormone - CF cloacal fluid - ECFV extraoellular fluid volume - FW freshwater acclimated - Hct hematocrit - MDWE mean daily water flux - [Na ⁺]_cf cloacal fluid sodium concentration - [Na ⁺]_pl plasma sodium concentration - Osm _cf cloacal fluid osmolality - Osm _pl plasma osmolality - SGS salt gland secretion - TBW total body water     

Nile crocodiles, Crocodylus niloticus, and estuarine crocodiles, Crocodylus porosus, show similar osmoregulatory responses on exposure to seawater

1. Nile crocodiles, reared in fresh water and exposed acutely to seawater, suffer marked dehydration and hypernatraemia. Cloacal urine osmolarity and potassium concentration increased markedly but urine sodium remains low. 2. Hypernatraemia is increased when secretion from the lingual salt glands is prevented. 3. C. niloticus appears not to drink seawater. 4. Similarities in osmoregulatory response between estuarine and Nile crocodiles suggest that the lingual salt glands of C. niloticus are functional in salt water, playing an important role in sodium balance. 5. Significant differences in the function of the renal/cloacal complex of Alligator and Crocodylus emphasize further the differences between these two groups of crocodilian and provide support for the postulated marine ancestry of many or all of the Crocodylidae.     

Effect of hydrochlorothiazide on urine saturation with brushite,in vitro collagen calcification by urine,and urinary inhibitors of collagen calcification

To clarify further the beneficial effect of thiazide diuretics on recurrent calcium nephrolithiasis, the effect of short-term hydrochlorothiazide therapy on urine saturation with brushite (CaHPO₄·2H₂O), in vitro collagen calcification by urine, and urinary inhibitors of calcification was studied.In 22 patients with idiopathic calcium oxalate/phosphate stones the urine calcium excretion decreased, the urine magnesium excretion increased and the urine magnesium/calcium ratio increased significantly (P < 0.001) during hydrochlorothiazide therapy. Supersaturation of the urine with brushite, which was present in 19 of the 22 patients, was reduced significantly (P < 0.001) in all during thiazide therapy, and to the undersaturated range in 16. The ability of urine to calcify collagen in vitro also decreased significantly (P < 0.001) during thiazide therapy, a change that correlated significantly (r = 0.4513, P < 0.05) with the decrease in brushite saturation. The concentration of urinary inhibitors of calcification, as determined with an in vitro collagen calcification system, was decreased significantly (P < 0.01) by thiazide therapy.It was concluded that, in addition to decreasing urine calcium excretion and increasing urine magnesium excretion, thiazide diuretics decrease the urinary brushite saturation and thus may prevent spontaneous nucleation or crystal growth, or both, of calcium phosphate. The ability of thiazides to decrease collagen calcification in vitro suggests that they may also prevent crystal growth on a nidus of organic matrix. Thiazides do not appear to act by increasing the excretion of urinary inhibitors of calcification.     

The temperature hysteresis of water-salt metabolism in the frog Rana temporaria]

Frogs acclimated to 4 degrees C were transported to a medium with temperature 20 degrees C, which caused polyuria; recovery of normal diuresis took about 24 h. During this period, hypernatremia was observed together with the increase in natriuresis, the rate of renal excretion of potassium ions with urea remaining constant. Water content of skeletal muscles decreased. Transportation of frogs acclimated to 20 degrees C into a medium with a temperature 4 degrees C decreased their diuresis. Renal excretion of sodium, calcium and magnesium ions remained unchanged, whereas that of potassium ions significantly decreased. The content of potassium and magnesium in the blood serum increased, that of sodium--decreased. Hydratation of muscled and kidneys was accompanied by the decrease of calcium, potassium and magnesium ions calculated per wet weight of the tissues, the level of sodium remaining unaffected. The data obtained indicate significant changes in the pattern of water and salt metabolism in frogs during temperature hysteresis.     

Osmoregulatory and metabolic costs of salt excretion in the Rufous-collared sparrow Zonotrichia capensis

Recent experiments on shorebirds have demonstrated that maintaining an active osmoregulatory machinery is energetically expensive. This may, in part, explain diet and habitat selection in birds with salt glands. However little is known about the osmoregulatory costs in birds lacking functional salt glands. In these birds, osmotic work is done almost exclusively by the kidneys. We investigated the osmoregulatory cost in a bird species lacking functional salt glands, the passerine Zonotrichia capensis. After 20 days of acclimation to fresh water (FW) and salt water (200 mM NaCl, SW), SW birds tended to be heavier than FW birds. However, this difference was not statistically significant. Total basal metabolic rate was higher in SW birds as compared with FW birds. Renal and heart masses were also higher in the SW group. We also found greater medullary development and an increase in urine osmolality in the SW group. In spite of Z. capensis' ability to tolerate a moderate salt load in the laboratory, we hypothesize that increased cost of maintenance produced by salt consumption may significantly affect energy budget, dietary, and habitat choices in the field.     

Effects of a prostacyclin analog iloprost on kidney function, renin-angiotensin and kallikrein-kinin systems, prostanoids and catecholamines in man

Iloprost (ZK 36 374), a stable analog of carbaprostacyclin, was infused for 72 h to nine patients with advanced obliterative arterial disease. Iloprost caused a marked vasodilation and a compensatory increase in cardiac output. The glomerular filtration rate increased by 45% and tubular reabsorption of sodium and water were reduced by 80% and 107%, respectively. The urine excretion rate increased by 122%. Tubular handling of potassium and calcium were not influenced by iloprost but magnesium reabsorption was stimulated. The renin-angiotensin system was not activated while serum angiotensin converting enzyme activity was decreased. Kallikrein excretion in urine was increased 4.4-fold but plasma kininogen, a substrate for kallikrein in producing vasoactive kinins, was unaffected by the drug. Plasma levels of 6-keto-PGF_1α and TxB₂ were decreased and their excretion in urine increased. Plasma catecholamines were not changed by iloprost. Several of the changes persisted for at least the first postinfusion day. The results indicate that iloprost increases urine excretion rate by increasing glomerular blood flow and by inhibiting sodium and water reabsorptions. The kinin-forming system, but not the renin-angiotensin system or plasma catecholamines, may be activated. The decrease in plasma level of prostanoids can be, at least partly, due to their increased excretions in urine.     

Idiopathic calcium nephrolithiasis. 1. Differences in urine crystalloids,urine saturation with brushite and urine inhibitors of calcification between persons with and persons without recurrent kidney stone formation.

The propensity of urine to promote calcium stone formation was compared in 64 patients with recurrent idiopathic calcium nephrolithiasis and 30 healthy individuals without such a history. The rates of excretion of urine crystalloids, the urine saturation with brushite (CaHPO4-2H2O), the ability of the urine to calcify collagen in vitro, and the concentration of urine inhibitors of collagen calcification were measured. The patients had a reduced urine citrate excretion rate in addition to an increased urine calcium excretion rate, while the rates for urine magnesium, phosphate, uric acid and oxalate were not significantly different in the two groups of subjects. The urine concentration of magnesium, phosphate and uric acid was decreased in the patients because of the higher urine volume. The urine creatinine excretion rate correlated with the rates of excretion of urine calcium, magnesium, phosphate, uric acid and oxalate in both groups, which suggested that increased lean body mass, possibly associated with greater food intake, may be an important determinant of crystalloid excretion. The urine of the patients was significantly more saturated with brushite than the urine of the control subjects and resulted in greater collagen calcification when incubated in vitro. The urine concentration of inhibitors of collagen calcification, however, was not significantly different in the two groups. Thus, the urine of patients with recurrent idiopathic calcium nephrolithiasis is more highly saturated with brushite, largely as a result of an increased urine calcium excretion rate, and contains a lower concentration of magnesium and citrate, substances that tend to prevent the precipitation and growth of crystals in urine.     

Arterial hypotension in ducks adapted to high salt intake

Summary A homogeneous group of 8-week-old Pekin ducks was divided into two groups: saltwater (SW) ducks received salt water of gradually increasing salinity (200–600 mOsm·kg^-1) from the 8th to 20th week of age; freshwater (FW) ducks were maintained on fresh water but otherwise treated identically. During the course of salt-adaptation SW ducks increased plasma osmolality, Na⁺ and Cl^- levels, and concentrations of the osmoregulatory peptide hormones arginine vasotocin and angiotensin II. The apparent volume of inulin distribution decreased in SW ducks, but blood volume was not reduced. SW ducks also developed arterial hypotension, bradycardia, and reduced cardiac output in the course of salt adaptation. This depressed cardiovascular performance was associated with enhanced vagal restraint of cardiac function and reduced plasma concentrations of norepinephrine. Salt water adaptation did not alter the degrees to which mean arterial pressure and heart rate changed in response to intravenous bolus injections of catecholamines. The same applied to the osmoregulatory peptides which were, however, effective only at supraphysiological concentrations. The Pekin duck, as a bird predisposed for adaptation to high salt loads, presumably adapts to chronic hypertonic saline intake by resetting the central autonomic control of blood pressure to a lower level.Abbreviations FW ducks fresh water ducks - SW ducks salt water ducks - ANGI angiotensin II - AVT arginine vasotocin - MAP mean arterial pressure - HR heart rate - IV intravenous - CO cardiac output - SV stroke volume - TPR total peripheral resistance - ISp virtual inulin space - ECFV extracellular fluid volume     

Morphology of the cloaca in the estuarine crocodile, Crocodylus porosus, and its plastic response to salinity

The cloacal complex of Crocodylus porosus is composed of three chambers (proctodaeum, urodaeum, and coprodaeum) separated by tight, muscular sphincters. The proctodaeum is proximal to the cloacal vent and houses the genitalia. The urodaeum is the largest chamber, is capable of storing large quantities of urine, and is lined with an epithelium with the capacity for transepithelial water and ion exchange. The coprodaeum, the most orad cloacal chamber, is a small, only marginally expandable chamber that has an epithelium composed almost entirely of mucus-secreting cells. The coprodaeum and lower intestine are reported to be the site(s) for urine modification in birds and bladderless lizards. A radiographic trace of urine storage in C. porosus kept for 2 months under hyperosmotic conditions showed no signs of retrograde movement of urine into the coprodaeum or rectum. Instead, urine was stored in the urodaeum of C. porosus. Examination of the mucosal surface of the urodaeum by SEM showed a plastic response to environmental salinity, with a possible increase in surface area in animals kept in hyperosmotic water compared with animals from fresh water. We propose the urodaeum as the primary site for postrenal modification of urine in C. porosus.