Shark rectal gland

Sugar uptake, metabolism, and chloride secretion in the rectal gland of the spiny dogfish Squalus acanthias

The rectal gland of the spiny dogfish Squalus acanthias secretes a salt solution isosmotic with plasma that maintains the salt homeostasis of the fish. It secretes salt against an electrochemical gradient that requires the expenditure of energy. Isolated rectal glands perfused without glucose secrete salt, albeit at a rate about 30% of glands perfused with 5 mM glucose. Gradually reducing the glucose concentration is associated with a progressive decrease in the secretion of chloride. The apparent K_m for the exogenous glucose-dependent chloride secretion is around 2 mM. Phloretin and cytochalasin B, agents that inhibit facilitated glucose carriers of the solute carrier 2 (Slc2) family such as glucose transporter 2 (GLUT2), do not inhibit the secretion of chloride by the perfused rectal glands. Phloridzin, which inhibits Slc5 family of glucose symporters, or α-methyl-d-glucoside, which competitively inhibits the uptake of glucose through Slc5 symporters, inhibit the secretion of chloride. Thus the movement of glucose into the rectal gland cells appears to be mediated by a sodium-glucose symporter. Sodium-glucose cotransporter 1 (SGLT1), the first member of the Slc5 family of sodium-linked glucose symporters, was cloned from the rectal gland. No evidence of GLUT2 was found. The persistence of secretion of chloride in the absence of glucose in the perfusate suggests that there is an additional source of energy within the cells. The use of 2-mercapto-acetate did not result in any change in the secretion of chloride, suggesting that the oxidation of fatty acids is not the source of energy for the secretion of chloride. Perfusion of isolated glands with KCN in the absence of glucose further reduces the secretion of chloride but does not abolish it, again suggesting that there is another source of energy within the cells. Glucose was measured in the rectal gland cells and found to be at concentrations in the range of that in the perfusate. Glycogen measurements indicated that there are significant stores of glucose in the rectal gland. Moreover, glycogen synthase was partially cloned from rectal gland cells. The open reading frame of glycogen phosphorylase was also cloned from rectal gland cells. Measurements of glycogen phosphorylase showed that the enzyme is mostly in its active form in the cells. The cells of the rectal gland of the spiny dogfish require exogenous glucose to fully support the active secretion of salt. They have the means to transport glucose into the cells in the form of SGLT1. The cells also have an endogenous supply of glucose as glycogen and have the necessary elements to synthesize, store, and hydrolyze it.

A brief history of the study of fish osmoregulation: the central role of the Mt. Desert Island Biological Laboratory

The Mt. Desert Island Biological Laboratory (MDIBL) has played a central role in the study of fish osmoregulation for the past 80 years. In particular, scientists at the MDIBL have made significant discoveries in the basic pattern of fish osmoregulation, the function of aglomerular kidneys and proximal tubular secretion, the roles of NaCl cotransporters in intestinal uptake and gill and rectal gland secretion, the role of the shark rectal gland in osmoregulation, the mechanisms of salt secretion by the teleost fish gill epithelium, and the evolution of the ionic uptake mechanisms in fish gills. This short review presents the history of these discoveries and their relationships to the study of epithelial transport in general.

Natural feeding influences protein expression in the dogfish shark rectal gland: A proteomic analysis

The rectal gland is the principal salt-secreting organ in elasmobranchs, yet its functional response to normal physiological variation (e.g., due to feeding, stress) has only recently been examined. To complement studies on acid-base, digestive, and osmoregulatory physiology in response to natural feeding, we investigated protein-level responses in the rectal gland of spiny dogfish (Squalus acanthias) 6 h, 20 h, and 5 days (reference control) after a meal. Our objective was to identify proteins involved in regulation of osmoregulatory and metabolic processes in response to feeding. Proteins were separated by two-dimensional gel electrophoresis, and protein spots that were significantly up- or down-regulated >2 fold (i.e., abundance increased more than 100% or decreased more than 50%) were detected using gel image analysis software. Of 684 proteins analyzed on 2D gels, 16 proteins changed significantly 6 h after feeding vs. 5 day controls (5 decreased; 11 increased), and 12 proteins changed >2 fold 20 h after feeding vs. 5 day controls (2 decreased; 10 increased). Thirteen of these proteins were identified using mass spectrometry and classified into functional pathways using the PANTHER bioinformatics database. Rectal gland proteins that were regulated following feeding fell into three main categories: cytoskeletal/muscular (e.g., tropomyosin alpha chain, transgelin), energy metabolism (e.g., malate dehydrogenase, ATP synthase), and nucleotide metabolism (nucleoside diphosphate kinase). The data also revealed that previously documented increases in the activity of isocitrate dehydrogenase after feeding are at least partially due to increased abundance of a cytosolic, NADP-dependent isoform of this enzyme. One of the primary components of the rectal gland's response to feeding appears to be maintenance of the cellular supply of energy, which would be necessary to fuel increased activities of enzymes involved in salt secretion and oxidative metabolism in the rectal gland following a meal.

A critical analysis of carbonic anhydrase function, respiratory gas exchange, and the acid-base control of secretion in the rectal gland of Squalus acanthias

We compared in vivo responses of rectal gland secretion to carbonic anhydrase (CA) inhibition (10(-4) mol l(-1) acetazolamide) in volume-loaded dogfish with in vitro responses in an isolated-perfused gland stimulated with 5 x 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, M(CO(2)) and M(O(2)) 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 M(CO(2)) (relative to stable M(O(2))) and elevation in secretion P(CO(2)) 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, P(CO(2)) and HCO(3)(-) 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 P(CO(2)), extracellular HCO(3)(-), or intracellular HCO(3)(-) 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 HCO(3)(-) concentrations remained low, even in the face of greatly elevated perfusate HCO(3)(-) concentrations. We conclude that pH effects on rectal gland secretion rate are adaptive, that CA functions to catalyze the hydration of CO(2), thereby maintaining a gradient for diffusive efflux of CO(2) 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.

Metabolic organization and effects of feeding on enzyme activities of the dogfish shark (Squalus acanthias) rectal gland

In order to investigate the metabolic poise of the elasmobranch rectal gland, we conducted two lines of experimentation. First, we examined the effects of feeding on plasma metabolites and enzyme activities from several metabolic pathways in several tissues of the dogfish shark, Squalus acanthias, after starvation and at 6, 20, 30 and 48 h post-feeding. We found a rapid and sustained ten-fold decrease in plasma beta-hydroxybutyrate at 6 h and beyond compared with starved dogfish, suggesting an upregulation in the use of this substrate, a decrease in production, or both. Plasma acetoacetate levels remain unchanged, whereas there was a slight and transient decrease in plasma glucose levels at 6 h. Several enzymes showed a large increase in activity post-feeding, including beta-hydroxybutyrate dehydrogenase in rectal gland and liver, and in rectal gland, isocitrate dehydrogenase, citrate synthase, lactate dehydrogenase, aspartate amino transferase, alanine amino transferase, glutamine synthetase and Na(+)/K(+) ATPase. Also notable in these enzyme measurements was the overall high level of activity in the rectal gland in general. For example, activity of the Krebs' TCA cycle enzyme citrate synthase (over 30 U g(-1)) was similar to activities in muscle from other species of highly active fish. Surprisingly, lactate dehydrogenase activity in the gland was also high (over 150 U g(-1)), suggesting either an ability to produce lactate anaerobically or use lactate as an aerobic fuel. Given these interesting observations, in the second aspect of the study we examined the ability of several metabolic substrates (alone and in combination) to support chloride secretion by the rectal gland. Among the substrates tested at physiological concentrations (glucose, beta-hydroxybutyrate, lactate, alanine, acetoacetate, and glutamate), only glucose could consistently maintain a viable preparation. Whereas beta-hydroxybutyrate could enhance gland activity when presented in combination with glucose, surprisingly it could not sustain chloride secretion when used as a lone substrate. Our results are discussed in the context of the in vivo role of the gland and mechanisms of possible upregulation of enzyme activities.

Control of rectal gland secretion by blood acid-base status in the intact dogfish shark (Squalus acanthias)

In order to address the possible role of blood acid-base status in controlling the rectal gland, dogfish were fitted with indwelling arterial catheters for blood sampling and rectal gland catheters for secretion collection. In intact, unanaesthetized animals, isosmotic volume loading with 500 mmol L-1 NaCl at a rate of 15 mL kg-1 h-1 produced a brisk, stable rectal gland secretion flow of about 4 mL kg-1 h-1. Secretion composition (500 mmol L-1 Na+ and Cl-; 5 mmol L-1 K+; <1 mmol L-1 Ca2+, Mg2+, SO(4)2-, or phosphate) was almost identical to that of the infusate with a pH of about 7.2, HCO3- mmol L-1<1 mmol L-1 and a PCO2 (1 Torr) close to PaCO2. Experimental treatments superimposed on the infusion caused the expected disturbances in systemic acid-base status: respiratory acidosis by exposure to high environmental PCO2, metabolic acidosis by infusion of HCl, and metabolic alkalosis by infusion of NaHCO3. Secretion flow decreased markedly with acidosis and increased with alkalosis, in a linear relationship with extracellular pH. Secretion composition did not change, apart from alterations in its acid-base status, and made negligible contribution to overall acid-base balance. An adaptive control of rectal gland secretion by systemic acid-base status is postulated-stimulation by the "alkaline tide" accompanying the volume load of feeding and inhibition by the metabolic acidosis accompanying the volume contraction of exercise.

Salt and water regulation by the leatherback sea turtleDermochelys coriacea

We measured the salt and water balance of hatchling leatherback sea turtles, Dermochelys coriacea, during their first few days of life to investigate how they maintain homeostasis under the osmoregulatory challenge of a highly desiccating terrestrial environment and then a hyperosmotic marine environment. Hatchlings desiccated rapidly when denied access to sea water,with their hematocrit increasing significantly from 30.32±0.54 % to 38.51±1.35 % and plasma Na+ concentration increasing significantly from 138.2±3.3 to 166.2±11.2 mmoll-1 in 12 h. When hatchlings were subsequently put into sea water, hematocrit decreased and plasma Na+ concentration was unchanged but both were significantly elevated above pretreatment values. In other hatchlings kept in sea water for 48 h, body mass and plasma Na+ concentration increased significantly, but hematocrit did not increase. These data show that hatchlings were able to osmoregulate effectively and gain mass by drinking sea water. We stimulated hatchlings to secrete salt from the salt glands by injecting a salt load of 27 mmol kg-1. The time taken for secretion to begin in newly hatched turtles was longer than that in 4-day-old hatchlings, but the secretory response was identical at 4.15±0.40 and 4.13±0.59 mmol Na+ kg-1 h-1respectively. Adrenaline and methacholine were both potent inhibitors of salt gland secretion in a dose-dependent manner, although methacholine administered simultaneously with a subthreshold salt load elicited a transient secretory response. The results showed that hatchling leatherbacks are able to tolerate significant changes in internal composition and efficiently use their salt glands to establish internal ionic and water balance when in sea water.