Kidney and Cloaca Function in the Estuarine Crocodile (Crocodylus porosus) at Different Salinities: Evidence for Solute-linked Water Uptake

Salt-water acclimation of the estuarine crocodile Crocodylus porosus involves enhanced ion transport properties of the urodaeum and rectum

Estuarine crocodiles, Crocodylus porosus, inhabit freshwater, estuarine and marine environments. Despite being known to undertake extensive movements throughout and between hypo-osmotic and hyperosmotic environments, little is known about the role of the cloaca in coping with changes in salinity. We report here that, in addition to the well-documented functional plasticity of the lingual salt glands, 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 (an indicator of active ion transport) compared with 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 probably does not contribute to solute-coupled water absorption in this cloacal segment. As 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 oesophagus, the anterior intestine and the rectum suggest that dietary salt intake may stimulate salt and possibly water absorption by the gastrointestinal tract of C. porosus living in hyperosmotic environments.

Comparison of the osmoregulatory capabilities among three amphibious sea snakes (Laticauda spp.) in Taiwan

Background

The three species of amphibious sea snakes (Laticauda semifasciata, L. laticaudata, and L. colubrina) in Taiwan were described as having different habitat affinities from terrestrial to marine. In this study, the osmoregulatory capabilities of three species were compared to test if their capabilities were associated with different habitat affinities.

Results

The sea snakes were transferred from a terrestrial environment to freshwater (FW) or seawater (SW) for 1 week, and then, Na+/K+-ATPase (NKA) activities of the salt gland (sublingual glands) and kidneys, the water content of the muscles, the body fluid osmolality, and Na+, Cl−, and K+ concentrations were measured. Results showed that the body fluid osmolality, Na+ and Cl− levels, and muscle water content of most marine species, L. semifasciata, remained relatively constant, and the NKA activity of its salt gland was approximately threefold higher than those of L. laticaudata and L. colubrina. In both L. semifasciata and L. laticaudata, NKA activities of the salt glands were higher in SW than in FW; however, no significant change was found in L. colubrina (the most terrestrial species).

Conclusions

This study suggests that the NKA activity of the sublingual gland is associated with salt excretion, and the three species possess different osmoregulatory strategies which are associated with their habitat affinities.

Arginine vasotocin: site and mode of action in the reptilian kidney

Recent morphological, physiological, and molecular studies that shed light on the functional significance of the neurohypophysial peptide arginine vasotocin (AVT) in the reptilian kidney are reviewed. Several structural features of the reptilian nephron are highlighted, including the presence of an incipient juxta-glomerular apparatus, aglomerular nephrons, and the thin, very short intermediate segment (IS) joining proximal and distal convoluted tubules. Although the V(2)-like AVT receptor and its gene have yet to be sequenced in any reptile, AVT receptors have been located in both the IS and the branched collecting duct system in the agamid lizard Ctenophorus ornatus. These findings suggest that AVT may have a dual effect in the reptilian kidney, first diluting the urinary fluid in the thin-intermediate segment, prior to its entering the collecting duct system, and then facilitating water reabsorption along an osmotic gradient as the urine passes through the final segments of the nephron. The IS may thus prove to be the evolutionary homologue of the thin-ascending limb of the loop of Henle in the mammalian kidney.

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.