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Estuarine crocodiles Crocodylus porosus inhabit freshwater, estuarine and marine environments. Despite being known to undertake extensive movements throughout and between hypo- and hyperosmotic environments, little is known on the role of the cloaca in coping with changes in salinity. In addition to the well-documented functional plasticity of the lingual salt glands, we report here that the middle of the three cloacal segments (i.e. the urodaeum), responds to increased ambient salinity to enhance solute-coupled water absorption. This post-renal modification of urine serves to conserve water when exposed to hyperosmotic environments and, in conjunction with lingual salt gland secretions, enables C. porosus to maintain salt and water balance and thereby thrive in hyperosmotic environments. Isolated epithelia from the urodaeum of 70% seawater-acclimated C. porosus had a strongly enhanced short circuit current (indicator of active ion transport) compared to freshwater-acclimated crocodiles. This enhanced active ion absorption was driven by increased Na+/K+-ATPase activity, and possibly enhanced proton pump activity, and was facilitated by the apical epithelial Na+ channel (ENaC) and/or the apical Na+/H+ exchanger (NHE2), both of which are expressed in the urodaeum. NHE3 was expressed at very low levels in the urodaeum and likely does not contribute to solute-coupled water absorption in this cloacal segment. Since C. porosus does not appear to drink water of salinities above 18 ppt, observations of elevated short circuit current in the rectum as well as a trend for increased NHE2 expression in the esophagus, the anterior intestine, and the rectum, suggests that dietary salt intake may stimulate salt, and possibly water absorption by the gastro-intestinal tract of C. porosus living in hyperosmotic environments.

The diamondback terrapin is the only temperate turtle species that exclusively inhabits estuarine habitats. Morphological, behavioral and physiological features contribute to the terrapin's ability to regulate body fluid osmotic pressure in a euryhaline environment. Low integument permeability combined with aquatic-terrestrial shuttling behavior limits passive exchange of water and salts with the environment, and terrapins regulate active uptake of salts via alterations in drinking and feeding behavior. The lachrymal salt gland facilitates excretion of excess sodium (Na+) and chloride (Cl−) ions through active transport mechanisms. We investigated body fluid dynamics, oxygen consumption (V̇O2), and osmotic status of terrapins exposed to an acute increase in salinity (12 to 35 psu) at 10°C and 25°C to gain insight into the relative importance of behavioral vs. physiological osmoregulatory adjustments over a range of seasonally-relevant temperatures. Linear mixed models were used to evaluate the effects of experimental temperature, salinity, and mass. Overall, temperature effects were stronger than salinity effects. Terrapins acclimated to 25°C had significantly lower blood osmolality and Na+, and higher water turnover rates, daily water flux (DWF), and V̇O2 compared with terrapins acclimated to 10°C. Salinity effects were restricted to DWF, which significantly decreased in response to acute exposure to 35 psu. Our results support the notion that behavioral adjustments predominate in the osmoregulatory strategy of terrapins.

SUMMARY To understand renal responses to salinity change in aquatic reptiles, we examined the structure and function of the kidney in three species of snake: a marine species with a salt gland (Laticauda semifasciata), a marine species without a salt gland (Nerodia clarkii clarkii) and a freshwater species without a salt gland (Nerodia fasciata). Both marine species maintained relatively constant plasma ions, even after acclimation to saltwater. By contrast, both plasma Cl– and mortality increased with salinity in the freshwater species. To investigate putative renal ion regulatory mechanisms, we examined the distribution and abundance of Na+/K+-ATPase (NKA) and the Na+/K+/2Cl– cotransporter (NKCC2). In all species, NKA localized to the basolateral membranes of the distal tubule and the connecting segments and collecting ducts only; there was no effect of salinity on the distribution of NKA or on the abundance of NKA mRNA in any species. NKCC2 protein was undetectable in the kidney of any of the species and there was no effect of salinity on NKCC2 mRNA abundance. We also examined the distribution and abundance of aquaporin 3 (AQP3) in the kidney of these species; although putative AQP3 localized to the basolateral membranes of the connecting segments and collecting ducts of all three species, there was no effect of salinity on the localization of the protein or the abundance of the transcript. Interestingly, we found very few differences across species, suggesting that the snake kidney may play a trivial role in limiting habitat use.

SUMMARYSaltwater crocodiles, Crocodylus porosus, possess lingual salt glands which function to remove excess Na+ and Cl− accumulated as a consequence of living in salt water. Little is known about the nature of ion transport systems in C. porosus salt glands and how these systems respond to an osmotic challenge. In the present study, we examined the distribution and regulation of the Na+/K+-ATPase (NKA) pump, specifically the α-(catalytic) subunit in the salt glands of C. porosus chronically acclimated (6 months) to freshwater (FW) or 70% seawater (SW). We hypothesised that in the SW-acclimated C. porosus there would be an up-regulation of the abundance, activity and gene expression of the NKA transporter. NKA was immunolocalised to the lateral and basal membrane of secretory cells. As predicted, the NKA α-subunit was 2-fold more abundant in SW-acclimated C. porosus salt glands. NKA gene expression was also elevated in the salt glands of SW- vs FW-acclimated crocodiles. There was no increase in the specific activity of NKA in SW-acclimated animals and the in vitro rate of oxygen consumption by salt gland slices from SW-acclimated animals was not significantly different from that of FW-acclimated animals. The proportion of tissue oxygen consumption rate attributable to NKA activity was not different between SW- and FW-acclimated animals (approximately 50%). These data suggest that either chronic SW acclimation does not affect NKA in crocodile salt glands in the same manner as seen in other models or crocodiles possess the capacity to moderate NKA activity following prolonged exposure to SW.

SUMMARY Adult leatherback turtles are gigantothermic/endothermic when foraging in cool temperate waters, maintaining a core body temperature within the main body cavity of ca. 25°C despite encountering surface temperatures of ca. 15°C and temperatures as low as 0.4°C during dives. Leatherbacks also eat very large quantities of cold, gelatinous prey (medusae and pyrosomas). We hypothesised that the head and neck of the leatherback would have structural features to minimise cephalic heat loss and limit cooling of the head and neck during food ingestion. By gross dissection and analytical computed tomography (validated by ground truthing dissection) of an embalmed specimen we confirmed this prediction. 21% of the head and neck was occupied by adipose tissue. This occurred as intracranial blubber,encapsulating the salt glands, medial portions of the eyeballs, plus the neurocranium and brain. The dorsal and lateral surfaces of the neck featured thick blubber pads whereas the carotid arteries and jugular veins were deeply buried in the neck and protected laterally by blubber. The oesophagus was surrounded by a thick sheath of adipose tissue whereas the oropharyngeal cavity had an adipose layer between it and the bony proportion of the palate,providing further ventral insulation for salt glands and neurocranium.

SUMMARY We compared in vivo responses of rectal gland secretion to carbonic anhydrase (CA) inhibition (10-4 mol l-1acetazolamide) in volume-loaded dogfish with in vitro responses in an isolated-perfused gland stimulated with 5×10-6 mol l-1 forskolin and removed from systemic influences. We also measured respiratory gas exchange in the perfused gland, described the acid-base status of the secreted fluid, and determined the relative importance of various extracellular and intracellular acid-base parameters in controlling rectal gland secretion in vitro. In vivo, acetazolamide inhibited Cl- secretion and decreased pHi in the rectal gland, but interpretation was confounded by an accompanying systemic respiratory acidosis, which would also have contributed to the inhibition. In the perfused gland, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{CO}_{2}}\) \end{document} and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{O}_{2}}\) \end{document}increased in linear relation to increases in Cl- secretion rate. CA inhibition (10-4 mol l-1 acetazolamide) had no effect on Cl- secretion rate or pHi in the perfused gland, in contrast to in vivo, but caused a transitory 30% inhibition of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{CO}_{2}}\) \end{document}(relative to stable \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{O}_{2}}\) \end{document}) and elevation in secretion PCO2 effects, which peaked at 2 h and attenuated by 3.5-4 h. Secretion was inhibited by acidosis and stimulated by alkalosis; the relationship between relative Cl-secretion rate and pHe was almost identical to that seen in vivo. Experimental manipulations of perfusate pH, PCO2 and HCO3-concentration, together with measurements of pHi, demonstrated that these responses were most strongly correlated with changes in pHe, and were not related to changes in PCO2, extracellular HCO3-, or intracellular HCO3-levels, though changes in pHi may also have played a role. The acid-base status of the secreted fluid varied with that of the perfusate, secretion pH remaining about 0.3-0.5 units lower, and changing in concert with pHe rather than pHi; secretion HCO3- concentrations remained low,even in the face of greatly elevated perfusate HCO3-concentrations. We conclude that pH effects on rectal gland secretion rate are adaptive, that CA functions to catalyze the hydration of CO2,thereby maintaining a gradient for diffusive efflux of CO2 from the working cells, and that differences in response to CA inhibition likely reflect the higher perfusion-to-secretion ratio in vitro than in vivo.

Secretion of chloride from blood to lumen is accomplished in the rectal gland of elasmobranchs by a process of secondary active transport involving the co-transport of Cl− with Na+ across the basolateral membranes of rectal gland cells. Energy is provided by ATP via membrane Na-K-ATPase, which establishes an electrochemical gradient favouring Na+ influx into the cell. The involvement of K+ in the co-transport mechanism, so as to provide a ratio of 1 Na+: 1 K+: 2 Cl− entering the cell, would increase the energetic efficiency of the process, and is consistent with the CI/O2 ratio of 27–30 observed in secreting rectal glands. Secretion is stimulated by cyclic AMP (cAMP) and by vasoactive intestinal peptide (VIP) and adenosine, which activate adenylate cyclase. Activation of the gland in vivo probably occurs via VIP-secreting nerves as well as circulating agents; it is inhibited by somatostatin. Cyclic AMP probably stimulates chloride secretion by at least three mechanisms: (1) increasing chloride conductance across the luminal cell membrane, (2) enhancing the co-transport pathway for transmembrane movements of Na+, K+ and Cl− and (3) activating Na-K-ATPase.

We measured the venous and arterial pressure, as well as the rate of secretion and content of cyclic AMP and high energy phosphate compounds, of the rectal gland of the anaesthetized dogfish, Squalus acantinas (L.). Intravenous infusion of isotonic solutions produced a very large increase in the rate of secretion by the rectal gland. The increase in secretion was preceded by an increase in venous blood pressure, but arterial blood pressure was not modified. Injections of small doses of veratridine stimulated gland secretion when given in the vicinity of the heart but not when given in the dorsal aorta. During volume expansion the creatine phosphate and ATP content of the gland were markedly reduced, while ADP and AMP as well as cyclic AMP content were increased. We conclude that: (a) volume expansion leads to the release of a message that activates adenyl cyclase in the gland; (b) the increased venous pressure may be the initial signal in the sequence that leads to the release of the activating messenger; (c) there is a receptor mechanism in the atrial and cardiac region that triggers the sequence that activates glandular secretion; (d) the reduction in the content of high energy phosphate compounds during volume expansion is caused by an increase in energy expenditure, probably due to gland secretion.

When salt-gland-cannulated green sea turtles were stimulated by the intravenous injection of various salt loads the secretion osmolality rose quickly from a basal value of 300–400 mosmol kg-1 to a plateau of 1680–2000. The height of the plateau and the duration of the response were dependent on the amount of salt given. An increase in flow rate accompanied the initial rise in concentration, but the flow rate thereafter declined while the plateau concentration was held. The fluid was protein-free, and was mainly composed of Na+ and Cl-, in similar relative concentrations to those in sea water, but had substantial amounts of K+, Mg2+ and HCO3- and negligible amounts of glucose. Urea concentrations did not change with osmolarity, and were similar to that of plasma, suggesting that the walls of the salt gland ducts are permeable to urea. The composition of the tears was uninfluenced by the concentration of the injected solutions, and its ionic content did not directly depend upon the identity of the added salts. The two eyes of individual turtles frequently did not function in synchrony. The average amount of the salt load remaining in a turtle at the end of salt gland activity, 4·5 mmol ion kg body mass-1, was very similar to the median load required to stimulate the salt gland (4·0mmolion kg body mass-1). This suggests that there is a threshold salt load level of around 4·5 mmol ion kg body mass-1 that determines salt gland on/off activity. The flow/concentration relationships of the salt gland fluid are consistent with water reabsorption from a primary isosmotic secretion being the major mechanism for concentrating the tears. A high maximal secretory rate is indicated for the primary fluid, 0·32 ml g salt gland-1 min-1, equivalent to that of the mammalian kidney. Calculations suggest that 83% of the initial fluid is recovered, Na+ is passively concentrated, Cl- and Mg2+ concentrations are enhanced, and that the duct walls are either impermeable to glucose or that glucose is actively taken up.

Aedes dorsalis larvae are one of the few organisms capable of inhabiting hypersaline NaHCO3-CO3 lakes. Under laboratory conditions larvae can survive and develop normally in saline media with pH values of 10·5, HCO3− concentrations of 250 mm, or CO32− concentrations of 100 mm. Despite ingestion of alkaline media at a rate equal to 130% of larval body wt/day, these insects regulated haemolymph pH (7·55–7·70) and HCO3− concentration (8·0–18·5 mm) within narrow limits. Analysis of fluid obtained by micropuncture or microcannulation indicated that rectal secretion is an important mechanism of pH and HCO3− regulation. The lumen-to-haemocoel HCO3− and CO32− gradients generated by isolated, microcannulated recta were 21 :1 and 241 :1, respectively. Transepithelial potential in cannulated recta was – 14·2 to –23·3 mV (lumen negative), demonstrating that net HCO3− secretion occurs against a large electrochemical gradient.

Perfusion and sodium secretion parameters were measured in the isolated rectal gland of Scyliorhinus canicula L. perfused at in vivo pressures, and the effect of stimulation of secretory activity by cyclic AMP and theophylline on these parameters was determined. Stimulation resulted in large increases in secretion flow rate, percentage extraction of sodium from the perfusing fluid, and arteriovenous sodium concentration difference, but did not affect perfusion flow rate or the sodium concentration of the secreted fluid. Reduction of perfusion flow rate to values below 65% of the control level, achieved by reducing perfusion pressure, produced a marked decline in sodium secretion -a process accompanied by increases in the percentage extraction of sodium and arteriovenous concentration difference of sodium, but again without any change in the sodium concentration of the secreted fluid. The in vivo consequences of these findings are discussed with reference to related findings for the avian nasal salt gland. The normal rate of secretion, its sodium concentration, and the nature of the dependence of secretion rate on perfusion flow below certain levels, were essentially unaffected by a reduction in the availability of oxygen to the gland by approximately 80%. It is concluded that the observed relationship between perfusion flow and sodium secretion rate in the stimulated gland is not related to oxygen availability, and hence that the primary underlying function of the synchronized secretion-related vasodilation seen in the gland is not to increase the supply of oxygen to the stimulated secretory tissue. We discuss possible reasons why this erroneous conclusion has been reached by other workers.

1. Regardless of the composition of the incubation medium, the nasal gland tissue from ducks adapted to hypertonic saline always released inorganic phosphate from the ATP in the medium at a higher rate than tissue derived from freshwater-adapted ducks. 2. Changes in the nasal gland ATP-ase activities during the period of adaptation to saline followed the same time course as the changes which occurred in the sodium-excretory capacity of the tissue. 3. Adaptation to saline resulted in a three- to fourfold increase in the weight-specific ATP-ase activity which was accompanied by a three- to fourfold increase in the weight-specific sodium-excretory capacity. 4. In the saline-adapted birds a positive correlation was found between the sodium-excretory capacity of the nasal gland tissue and the corresponding ATP-ase activity. 5. The moles of sodium excreted per mole of ATP hydrolysed was constant irrespective of whether the birds were adapted to either fresh water or salt water. 6. Concommitant with the changes in weight-specific sodium excretion and ATP-ase activity, an increase in the sodium concentration of the nasal gland fluid was observed. An approximately twofold increase in nasal gland weight also occurred during this period. 7. All changes which were observed to occur during the period of adaptation to saline were reversed when the birds were returned to fresh water. 8. The observed changes in enzyme activity and sodium excretory rates of the nasal glands are discussed in relation to possible cellular changes which might occur and their importance relative to the birds's ability to adapt to a marine environment.