Peptidergic and adrenergic regulation of electrogenic ion transport in isolated gills of the flounder (Platichthys flesus L.)

Gill Na+-K+-2Cl−cotransporter abundance and location in Atlantic salmon: effects of seawater and smolting

Na+-K+-2Cl−cotransporter abundance and location was examined in the gills of Atlantic salmon ( Salmo salar) during seawater acclimation and smolting. Western blots revealed three bands centered at 285, 160, and 120 kDa. The Na+-K+-2Cl−cotransporter was colocalized with Na+-K+-ATPase to chloride cells on both the primary filament and secondary lamellae. Parr acclimated to 30 parts per thousand seawater had increased gill Na+-K+-2Cl− cotransporter abundance, large and numerous Na+-K+-2Cl− cotransporter immunoreactive chloride cells on the primary filament, and reduced numbers on the secondary lamellae. Gill Na+-K+-2Cl− cotransporter levels were low in presmolts (February) and increased 3.3-fold in smolts (May), coincident with elevated seawater tolerance. Cotransporter levels decreased below presmolt values in postsmolts in freshwater (June). The size and number of immunoreactive chloride cells on the primary filament increased threefold during smolting and decreased in postsmolts. Gill Na+-K+-ATPase activity and Na+-K+-2Cl− cotransporter abundance increased in parallel during both seawater acclimation and smolting. These data indicate a direct role of the Na+-K+-2Cl− cotransporter in salt secretion by gill chloride cells of teleost fish.

Transport mechanisms of seawater teleost chloride cells: an inclusive model of a multifunctional cell

This review assembles recent information on seawater-type chloride cells of marine teleost fish and evaluates the secretion of Na+, Cl-, K+, H+ and NH4+ and the absorption of Ca2+. The evidence for the distribution (apical vs basolateral) and the abundance of the various ion pumps, cotransporters, channels and exchangers is assessed and an inclusive model is constructed. Relationships among the transport systems are presented to suggest that many, if not all, of these systems may be operating simultaneously in individual, multifunctional chloride cells.

Maturation of the endocrine pancreas in the sea bass, Dicentrarchus labrax L. (Teleostei): an immunocytochemical and ultrastructural study. I. Glucagon-producing cells

The structure of the endocrine pancreas in the sea bass (Dicentrarchus labrax L.) was studied with special reference to glucagon-immunoreactive cells. As described in most of the teleosts, the sea bass was found to have a diffuse pancreas. In the adult, endocrine cells were clustered in a principal islet and numerous accessory islets where the glucagon A cells were localized peripherally. Under electron microscopy, the A cells displayed a clear hyaloplasm with granules having typical spherical or polyhedral cores, as in other vertebrates. The maturation of the endocrine pancreas was monitored under rearing conditions. The endocrine pancreas appeared during the prelarval stage, 3 days after hatching, and consisted of a single cluster of morphologically similar cells, containing very small cytoplasmic granules. During the larval stage, cytodifferentiation resulted in modifications of cell shape and increased granule size. Typical granules appeared in 8-mm-long larvae. Cells immunoreactive with mammalian glucagon antibodies appeared only at the beginning of the juvenile stage (3 months/20 mm). Electron microscope observations revealed that the storage of hormone in numerous cytoplasmic granules began at this stage.

Pancreatic and thyroid hormones in rainbow trout (Salmo gairdneri): what concentration does the liver see?

Blood was sampled within 1 min from the hepatic portal vein, caudal vein, and heart (bulbus arteriosus) of individual rainbow trout (N = 26). Blood plasma levels of the pancreatic hormones, insulin, glucagon, and glucagon-like peptide (GLP) and the thyroid hormones, thyroxin and 3,5,3'-triiodo-L-thyronine, were measured by specific radioimmunoassays. Blood levels of insulin, glucagon, and GLP were significantly higher in the portal vein than in either the caudal vein or heart. Thyroid hormone concentrations did not vary between blood vessels. Based on the higher hormone titers in portal blood as compared to blood sampled from the other sites, the liver appears to be exposed to concentrations of pancreatic hormones that are three- to eightfold higher than those experienced by other tissues. There was no statistically significant difference between the caudal vein or the heart in blood concentrations of the different pancreatic hormones, although the average concentration of GLP was higher in the heart (0.22 +/- 0.03 ng/ml) than in the caudal vein (0.15 +/- 0.03 ng/ml). The molar ratio of average blood insulin/GLP concentrations was lower in the heart (3.9) than in either the hepatic portal vein (6.9) or the caudal vein (7.2), which may be favorable for potential physiological effects of GLP on the heart and gills. The results of this study imply some role for GLP in the regulation of cardiac and gill metabolism or function. The higher hormonal titers to which fish liver, as compared to other target tissues, is exposed should be taken into account when a dosage of pancreatic hormones is calculated for in vivo or in vitro experiments.

Circulatory catecholamines in the eel: origins and functions

While the three catecholamines (CAs) dopamine (DA), norepinephrine (NE) and epinephrine (E), are wide-spread in tissues of the American eel (Anguilla rostrata), the bulk of these CAs in the systemic blood originates from chromaffin cells in the wall of the posterior cardinal veins. In addition, the brain and unidentified structures in the opisthonephric kidney also release appreciable quantities of CAs. The functional realms attributed to systematically circulating CAs in teleosts comprise cardiovascular, respiratory, osmoregulatory, metabolic and endocrinotropic actions. In the eel, cardiovascular and respiratory effects are well established. However, we were unable to prove a physiological role of the CAs in osmoregulation. On the other hand, the eel is the only species among five vertebrates of greatly varying phylogenetic position (the others: hagfish, lamprey, rat, human) in which physiological doses of E were hyperglycemic. As in lamprey and rat, DA and NE are released in the eel by physiological doses of E. In addition, DA and NE also release the respective other two CAs. The physiological significance of the catecholaminotropic (CA-tropic) interactions remains to be established; however, the CA-tropic effect of E does not require the presence of the brain or 'preganglionic' nerve cells. In the eel, mild stress causes an immediate 'unorthodox' drop of plasma CAs, while stronger stress is followed by the expected increase of plasma CAs.

Sites of interactions of surfactants with beta-adrenergic responses in trout (Salmo gairdneri) gills

The aims of this study were to investigate the effects of surfactants on the beta-adrenergic responses of the arterio-arterial (lamellar) vascular pathway of isolated gills from the trout Salmo gairdneri. The resistance of the arterio-arterial vasculature was monitored by measuring the input pressure of isolated gills perfused at a constant flow and at constant efferent pressure. Specific beta-adrenergic stimulation with isoprenaline produced a dose-dependent reduction in vascular resistance which was mimicked by the presence of cyclic 8-(4-chlorophenylthio) adenosine-3':5'-monophosphate (8-cpt cyclic AMP). These results suggest that cyclic AMP is involved as an intracellular second messenger of beta-adrenergic regulatory responses of branchial haemodynamics. Surfactants in the perfusate at nominal concentrations of 1.0 mumol 1(-1) (0.3-0.5 mg 1(-1)) inhibited the response of the branchial tissue to isoprenaline but did not affect the response to 8-cpt cyclic AMP. This suggests that the surfactant effects are the result of events occurring within the cell membrane. Furthermore because the surfactants used affected the efficacy, rather than the affinity, of the isoprenaline response then the mechanism of action is unlikely to be an effect on agonist/receptor interaction but may be further along the stimulus response chain, perhaps affecting receptor/regulatory nucleotide/adenylate cyclase interaction or via direct inhibition of the cyclase itself.

Pancreatic peptides regulate Cl- secretion in the marine teleost gill

Hormonal regulation of the ionoregulatory function of the branchial epithelium in marine teleosts has been investigated using an isolated gill preparation of the seawater-adapted flounder Platichthys flesus. Agents which are assumed to elevate the effective intracellular concentration of cyclic AMP caused a stimulation of Cl- secretion as determined by measurement of the transepithelial electrical potential difference in the isolated gill. The pancreatic endocrine glucagon may act as an endogenous regulator of ion excretion in the gill as this peptide stimulated the transepithelial potential. Another islet peptide, somatostatin, inhibited branchial electrogenic ion transport. Attempts to investigate the mode of action of these peptides by measuring cyclic AMP levels in isolated gills were unsuccessful, since it appears that the pool of cyclic nucleotide which specifies the epithelial Cl- transport rate is small compared to total cyclic AMP content. A second approach investigating the ability of somatostatin to inhibit the transepithelial potential in gills stimulated by different secretagogues suggested dual modes of action for this peptide.

Converging adrenergic and cholinergic mechanisms in the inhibition of Cl secretion in fish opercular epithelium

The rate of Cl secretion (Isc) by the opercular epithelium of Fundulus heteroclitus is stimulated by elevations in cyclic AMP (cAMP) levels elicited via beta 1-adrenergic receptor activation, and inhibited by both alpha 2-adrenergic and muscarinic cholinergic receptor activation via mechanisms presently unknown. A comparison of these two inhibitory responses was made using clonidine, an alpha 2-adrenergic agonist, and acetylcholine (ACh), a cholinergic agonist. The dose required for maximum inhibition was 100 times greater for ACh, but in all other respects the responses elicited by both agonists were statistically indistinguishable. Adrenergic antagonists did not diminish the ACh inhibition, and cholinergic antagonists did not diminish the clonidine inhibition, indicating that the two receptor types were distinct from each other. In control tissues and tissues pretreated with agents that increase cAMP levels (isoproterenol, IBMX, forskolin), both ACh and clonidine had no effects on cyclic AMP levels, indicating an inhibitory mechanism independent of adenylate cyclase. Neither Ca-free media nor a variety of calcium antagonists diminished the ACh or clonidine inhibitions. These results suggest that the alpha 2-adrenergic and muscarinic cholinergic pathways converge into a common pathway to inhibit Cl secretion by a mechanism not involving adenylate cyclase or the mobilization of either extracellular or intracellular calcium stores.