Effects of epinephrine, glucagon and vasoactive intestinal polypeptide on chloride secretion by teleost opercular membrane

Paracellular pathway remodeling enhances sodium secretion by teleost fish in hypersaline environments

In vertebrate salt-secreting epithelia, Na(+) moves passively down an electrochemical gradient via a paracellular pathway. We assessed how this pathway is modified to allow Na(+) secretion in hypersaline environments. Mummichogs (Fundulus heteroclitus) acclimated to hypersaline [2× seawater (2SW), 64‰] for 30 days developed invasive projections of accessory cells with an increased area of tight junctions, detected by punctate distribution of CFTR (cystic fibrosis transmembrane conductance regulator) immunofluorescence and transmission electron miscroscopy of the opercular epithelia, which form a gill-like tissue rich in ionocytes. Distribution of CFTR was not explained by membrane raft organization, because chlorpromazine (50 μmol l(-1)) and filipin (1.5 μmol l(-1)) did not affect opercular epithelia electrophysiology. Isolated opercular epithelia bathed in SW on the mucosal side had a transepithelial potential (Vt) of +40.1±0.9 mV (N=24), sufficient for passive Na(+) secretion (Nernst equilibrium voltage≡ENa=+24.11 mV). Opercular epithelia from fish acclimated to 2SW and bathed in 2SW had higher Vt of +45.1±1.2 mV (N=24), sufficient for passive Na(+) secretion (ENa=+40.74 mV), but with diminished net driving force. Bumetanide block of Cl(-) secretion reduced Vt by 45% and 29% in SW and 2SW, respectively, a decrease in the driving force for Na(+) extrusion. Estimates of shunt conductance from epithelial conductance (Gt) versus short-circuit current (Isc) plots (extrapolation to zero Isc) suggested a reduction in total epithelial shunt conductance in 2SW-acclimated fish. In contrast, the morphological elaboration of tight junctions, leading to an increase in accessory-cell-ionocyte contact points, suggests an increase in local paracellular conductance, compensating for the diminished net driving force for Na(+) and allowing salt secretion, even in extreme salinities.

Exploring novel hormones essential for seawater adaptation in teleost fish

Marine fish are dehydrated in hyperosmotic seawater (SW), but maintain water balance by drinking surrounding SW if they are capable of excreting the excess ions, particularly Na(+) and Cl(-), absorbed with water by the intestine. An integrative approach is essential for understanding the mechanisms for SW adaptation, in which hormones play pivotal roles. Comparative genomic analyses have shown that hormones that have Na(+)-extruding and vasodepressor properties are greatly diversified in teleost fish. Physiological studies at molecular to organismal levels have revealed that these diversified hormones are much more potent and efficacious in teleost fish than in mammals and are important for survival in SW and for maintenance of low arterial pressure in a gravity-free aquatic environment. This is typified by the natriuretic peptide (NP) family, which is diversified into seven members (ANP, BNP, VNP and CNP1, 2, 3 and 4) and exerts potent hyponatremic and vasodepressor actions in marine fish. Another example is the guanylin family, which consists of three paralogs (guanylin, uroguanylin and renoguanylin), and stimulates Cl(-) secretion into the intestinal lumen and activates the absorptive-type Na-K-2Cl cotransporter by local luminocrine actions. The most recent addition is the adrenomedullin (AM) family, which has five members (AM1, 2, 3, 4 and 5), with AM2 and AM5 showing the most potent or efficacious vasodepressor and osmoregulatory effects among known hormones in teleost fish. Accumulating evidence strongly indicates that members of these diversified hormone families play essential roles in SW adaptation in teleost fish. In this short review, the author has attempted to propose a novel approach for identification of new hormones that are important for SW adaptation using comparative genomic and functional studies. The author has also suggested potential hormone families that are diversified in teleost fish and appear to be involved in SW adaptation through their ion-extruding actions.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Rapid regulation of NaCl secretion by estuarine teleost fish: coping strategies for short-duration freshwater exposures

This review summarizes the mechanism of Cl(-) active secretion and its regulation in estuarine teleost fish. Small estuarine fish such as the killifish, Fundulus heteroclitus, forage in shallow water following advancing tides and are exposed regularly to very dilute microenvironments. Using the killifish opercular epithelium and related teleost membranes containing mitochondria-rich cells, the regulation includes a reduction of active Cl(-) secretion and passive diffusive ion loss in a three-stage process spanning approximately 30 min. There is a combination of sympathetic neural reflex mediated by alpha(2)-adrenoceptors operating via intracellular inositol tris phosphate and intracellular Ca(2+) and a cellular hypotonic shock response, followed by covering over of ion-secreting cells by pavement cells. This effectively minimizes salt loss in dilute media. The upregulation of salt secretion on return to full strength seawater may be via hormones (arginine vasotocin and urotensin I) and neurotransmitter (vasoactive intestinal polypeptide) in combination with hypertonic shock. A hypothetical model includes involvement of protein kinase A and C and protein phosphatases 1 and 2A in regulation of the NKCC1 cotransporter on the basolateral side and protein kinase A regulation of the CFTR-like apical anion channel.

Cystic fibrosis transmembrane conductance regulator in teleost fish

The gills and intestinal epithelia of teleost fish express cystic fibrosis transmembrane conductance regulator (CFTR), and utilize this low conductance anion channel in the apical membrane for ion secretion in seawater gill and in the basolateral membrane for ion absorption in freshwater gill. Similarly, in the intestine CFTR is present in the basolateral membrane for intestinal absorption and also in the apical membrane of secreting intestine. The expression of CFTR and the directed trafficking of the protein to the apical or basolateral membrane is salinity-dependent. The CFTR gene has been cloned and sequenced from several teleost species and although all the major elements in the human gene are present, including two nucleotide binding domains that are common to all ATP binding cassette (ABC) transporters, the sequences are divergent compared to shark or human. In euryhaline fish adapting to seawater, CFTR, localized immunocytochemically, redistributes slowly from a basolateral location to the apical membrane while ion secretory capacity increases. The facility with which teleosts regulate CFTR expression and activation during salinity adaptation make this system an appealing model for the expression and trafficking operation of this labile gene product.

Regulation of Cl- secretion in seawater fish (Dicentrarchus labrax) gill respiratory cells in primary culture

1. Primary cultures of sea bass (Dicentrarchus labrax) gill cells grown on permeable membranes form a highly differentiated tight epithelium composed of respiratory-like cells. This preparation was also found to provide a functional model for investigating the hormonal regulation of Cl- secretion. 2. In control conditions, i.e. in the absence of hormones or other stimuli, the cultured epithelium showed a short-circuit current (Isc) of 8.8 +/- 0.4 microA cm-2, a transepithelial potential (Vt) of 28.6 +/- 0.6 mV (serosal side positive), and a transepithelial resistance (Rt) of 5026 +/- 127 Omega cm2. Addition of 50 nM PGE2 caused a stimulation of Isc, Vt and transepithelial conductance, Gt. The increase in Isc was probably due to the elevation in Cl- secretion, since it could be correlated with the stimulation of serosal to mucosal 36Cl- flux. Application of the neurohypophyseal peptide arginine vasotocin (AVT; 50 nM) or the beta-adrenergic agonist isoproterenol (isoprenaline; 0. 5 microM) evoked a stimulation in Cl- secretion, as was shown by the increases in Isc and Gt. The excitatory effect of isoproterenol followed by the inhibitory action of propranolol, a beta-adrenergic antagonist, suggested the presence of beta-adrenergic receptors. Noradrenaline (0.1 microM) elicited a reduction in Isc, Vt and Gt, which was counterbalanced by the addition of phentolamine, an alpha-adrenergic antagonist. This suggested an activation of alpha-adrenergic receptors. 3. This study provides evidence for hormonal control of the Cl- secretion in sea bass gill respiratory cells in culture, involving AVT, prostaglandin (PGE2), and beta- and alpha-adrenergic receptors.

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.

Angiotensin II binding sites in the rat pancreas and their modulation after sodium loading and depletion

Specific 125I angiotensin II binding sites were identified in the rat pancreas by radioreceptor assay, autoradiography and immunohistochemistry. Scatchard analysis of the binding in normal rats yielded a Kd of 0.51 +/- 0.23 nM with a Bmax of 15 +/- 3.5 fmol/mg protein (means +/- SD, n = 6). Changed plasma sodium concentration resulted in modifications in the binding affinity and capacity. Sodium loading depressed both Kd (0.36 +/- 0.1 nM) and Bmax (6.4 +/- 0.1 fmol/mg protein), while sodium depletion elevated both Kd (2.03 +/- 0.3 nM) and Bmax (45 +/- 3.5 fmol/mg protein) (means +/- SD, n = 6). Autoradiography using 125I Ang II and immunohistochemistry of the binding sites saturated with unlabeled Ang II and incubated with Ab-Ile5 Ang II, revealed localization of the binding sites on the islet cell membranes and in the exocrine pancreas.

Vasoactive intestinal peptide stimulates a cAMP-mediated Cl- current in avian salt gland cells

VIP plays an integral role in both protein and fluid secretion in many exocrine glands. By employing the perforated patch-clamp whole-cell recording technique we investigated the effects of VIP on membrane potential and transmembrane currents in avian exocrine salt gland cells. Prior to application of VIP, salt gland cells had a resting membrane potential close to -45 mV. When challenged with VIP (1-100 nM) a sustained depolarization to ECl- was induced which was mimicked by the application of cell-permeable cAMP analogues or forskolin (1 microM). By employing the voltage-clamp recording configuration a sustained increase in current was observed with a reversal potential which approximated ECl-. Ionic substitution experiments confirmed that the current was a Cl- conductance which was inhibited by the Cl- channel blockers flufenamic acid and niflumic acid and by the inhibitory cAMP isomer, adenosine-3',5'-cyclic monophosphothioate, Rp-isomer. Based on this, and the fact that the kinetic properties of the Cl- current activated by VIP are similar to those activated by cAMP, we propose that VIP-receptor interaction results in the activation of a cAMP-dependent Cl- current.

Dibutyryl cAMP activates bumetanide-sensitive electrolyte transport in Malpighian tubules

The effects of dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) and bumetanide (both 10(-4) M) on transepithelial Na+, K+, Cl-, and fluid secretion and on tubule electrophysiology were studied in isolated Malpighian tubules of the yellow fever mosquito Aedes aegypti. Peritubular DBcAMP significantly increased Na+, Cl-, and fluid secretion but decreased K+ secretion. In DBcAMP-stimulated tubules, bumetanide caused Na+, Cl-, and fluid secretion to return to pre-cAMP control rates and K+ secretion to decrease further. Peritubular bumetanide significantly increased Na+ secretion and decreased K+ secretion so that Cl- and fluid secretion did not change. In bumetanide-treated tubules, the secretagogue effects of DBcAMP are blocked. In isolated Malpighian tubules perfused with symmetrical Ringer solution, DBcAMP significantly hyperpolarized the transepithelial voltage (VT) and depolarized the basolateral membrane voltage (Vbl) with no effect on apical membrane voltage (Va). Total transepithelial resistance (RT) and the fractional resistance of the basolateral membrane (fRbl) significantly decreased. Bumetanide also hyperpolarized VT and depolarized Vbl, however without significantly affecting RT and fRbl. Together these results suggest that, in addition to stimulating electroconductive transport, DBcAMP also activates a nonconductive bumetanide-sensitive transport system in Aedes Malpighian tubules.

Adenylate cyclase activity in fish gills in relation to salt adaptation

The influence of salt adaptation on specific adenylate cyclase activity (measured by conversion of [alpha-32p]-ATP into [alpha-32p]-cAMP) was investigated in gill plasma membranes of rainbow trout (Salmo gairdneri) adapted to various salinities (deionized water, DW; fresh water, FW; 3/4 sea water, 3/4 SW; sea water, SW) and in sea water adapted-mullet (Mugil sp.). Basal activity declined by a factor of 2 in trout with increasing external salinity (pmoles cAMP/mg protein/10 min: 530 in DW, 440 in FW, 340 in 3/4 SW; 250 in SW) and was very low in SW adapted-mullet: 35. The Km for ATP was similar (0.5 mM) in both FW adapted- and SW adapted- trout in either the absence (basal activity) or in the presence of stimulating agents (isoproterenol; NaF) while the Vm varied. Analysis of stimulation ratios with respect to basal levels of the enzyme showed that hormones (glucagon, VIP) and pharmacological substances (isoproterenol, NaF) display a greater potency in high salt than in low salt adapted- fish gills. In contrast, salt adaptation did not have any effect on the regulation of adenylate cyclase by PGE1. These results are interpreted in relation to the general process of osmoregulation.

Production of eicosanoids by the killifish gills and opercular epithelia and their effect on active transport of ions

Gills and opercular epithelia of the killifish (Fundulus heteroclitus) homogenized and incubated with radiolabeled arachidonic acid were found to produce prostaglandins, leukotrienes, and hydroxyeicosatetraenoic acids. These metabolites were identified using thin-layer chromatography, autoradiography, reverse-phase high-performance liquid chromatography, and ultraviolet spectroscopy. Addition of glutathione and epinephrine to the incubation mixture caused a diminution in the production of most eicosanoids (cyclooxygenase and lipoxygenase products) whereas indomethacin decreased only the cyclooxygenase metabolites. The effects of eicosanoids on short-circuit and potential difference across opercular epithelia mounted in a Ussing-type chamber were examined. Prostaglandin E2 had an inhibitory effect on ion transport whereas the sulfidopeptide leukotrienes (LTC4, LTD4, and LTE4) had a stimulatory effect. These results indicate that gills and opercular epithelia have the capacity to synthesize eicosanoids and that some of these metabolites may play a role in the regulation of ion transport in the kill fish.

Atrial natriuretic factor inhibits Na-K-Cl cotransport in teleost intestine

Addition of atrial natriuretic factor (ANF) to the contraluminal side of the intestinal mucosa of a marine teleost, the winter flounder Pseudopleuronectes americanus, inhibits short-circuit current, net transepithelial fluxes of Na and Cl, and the unidirectional influx of Rb across the brush border membrane. This action of ANF is closely mimicked by addition of 8-bromo-guanosine 3',5'-cyclic monophosphate (8-BrcGMP). In contrast to the intestine, the opercular epithelium of the flounder did not respond to the in vitro addition of either ANF or 8-BrcGMP. Because intestinal salt and water absorption diminishes when marine fish enter water of lower salinity, ANF may be an important hormonal regulator through which euryhaline fish adapt to varying salinities.

The use of isolated fish opercular epithelium as a model tissue for studying intrinsic activities of loop diuretics

Isolated opercular epithelia of killifish (Fundulus heteroclitus) were mounted in an Ussing chamber. The epithelia displayed a transepithelial electrical potential difference (PD) of 10.6 +/- 0.3 mV (sea-water side negative) and a short-circuit current (SCC) of 72.1 +/- 2.1 microA cm-2. The electrical resistance was 160 +/- 3 omega cm2 (mean +/- SE, n = 269). The unidirectional flux of 36Cl from blood side to sea-water side compared well with the SCC. No net flux of 22Na or 24Na across the epithelium was observed. Raising of cyclic AMP levels by theophylline, 3-isobutyl-I-methyl-xanthine, isoprenaline and forskolin, increased SCC and PD. Prostaglandins PGE2 and to some extent PGF2 alpha inhibited SCC and PD. Inhibition of Na+-K+-ATPase by ouabain and orthovanadate reduced SCC and PD. Pretreatment of the epithelium with the stilbene disulphonic acid (DIDS) did not prevent the action of orthovanadate. Different types of diuretics were tested, but only the loop diuretics bumetanide, piretanide, and furosemide, rapidly and strongly inhibited PD and SCC and unidirectional 36Cl serosal to mucosal flux. Dose-response curves for these agents were parallel and EC50 values for effects on SCC were 40, 52, and 295 microM, respectively. The same relative activities of these diuretics have been seen in the renal thick ascending limb of Henle's loop (TALH). It is concluded that the killifish opercular skin responds to hormonal stimuli and various pharmacological agents in a manner similar to that of mammalian renal TALH. It should therefore be a useful model for studies of the modes of action and the structure-activity relationships of diuretics which act by inhibition of chloride transport or Na+-K+-ATPase activity.

Vibrating probe analysis of teleost opercular epithelium: correlation between active transport and leak pathways of individual chloride cells

We have utilized the vibrating probe technique to examine transport by individual chloride cells in the short-circuited fish opercular epithelium. Variability in the steady state and in response to rapid perturbations, including fast-acting hormones and ion replacement, was analyzed. Negative short-circuit currents, corresponding to chloride secretion, were associated with the apical crypts of all but five of 386 chloride cells sampled. Average chloride cell short-circuit current and conductance were 2.7 +/- 0.1 nA and 87.7 +/- 3.8 nS, respectively, or 19 mA cm-2 and 620 mS cm-2 (resistance = 1.6 omega cm2) when normalized to apical crypt surface area. Exposure to 1 microM epinephrine rapidly inhibited the tissue short-circuit current by inhibiting the current pumped by all chloride cells, i.e. all chloride cells have adrenergic receptors. The time course of inhibition for each cell mirrored that of the whole tissue. Reversal of epinephrine inhibition of the tissue short-circuit current by glucagon and phosphodiesterase inhibition was by reversal of epinephrine's inhibition of individual chloride cells, and not by turning on cells which were previously inactive or uninhibited, or by stimulating nonchloride cells. A great amount of variability existed among chloride cells in the ability of these agents to reverse epinephrine-inhibited current. Likewise, considerable variability in the response of chloride cell conductance to these perturbations was observed, and in many instances a clear dissociation between current and conductance was noted. In the steady state, variability among cells in a single tissue always defined a linear relationship between chloride cell current and conductance with zero-current conductance intercept at zero. Equivalent circuit modeling indicates that the leak conductance of chloride cells within a single tissue always contributes the same proportion to the total individual chloride cell conductance, such that the ratio between the conductances of the active and leak pathways of chloride cells is constant. The leak pathway is almost certainly dominated by a sodium-selective paracellular pathway. The results suggest that these cells control the permeability of their paracellular pathway. A possible mechanism for this control is discussed.

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.