Intracellular voltage recordings in the opercular epithelium of Fundus heteroclitus

Cystic fibrosis transmembrane conductance regulator in the gills of the climbing perch, Anabas testudineus, is involved in both hypoosmotic regulation during seawater acclimation and active ammonia excretion during ammonia exposure

This study aimed to clone and sequence the cystic fibrosis transmembrane conductance regulator (cftr) from, and to determine the effects of seawater acclimation or exposure to 100 mmol l⁻¹ NH₄Cl in freshwater on its mRNA and protein expressions in, the gills of Anabas testudineus. There were 4,530 bp coding for 1,510 amino acids in the cftr cDNA sequence from A. testudineus. The branchial mRNA expression of cftr in fish kept in freshwater was low (<50 copies of transcript per ng cDNA), but significant increases were observed in fish acclimated to seawater for 1 day (92-fold) or 6 days (219-fold). Branchial Cftr expression was detected in fish acclimated to seawater but not in the freshwater control, indicating that Cl⁻ excretion through the apical Cftr of the branchial epithelium was essential to seawater acclimation. More importantly, fish exposed to ammonia also exhibited a significant increase (12-fold) in branchial mRNA expression of cftr, with Cftr being expressed in a type of Na⁺/K⁺-ATPase-immunoreactive cells that was apparently different from the type involved in seawater acclimation. It is probable that Cl⁻ excretion through Cftr generated a favorable electrical potential across the apical membrane to drive the excretion of NH₄⁺ against a concentration gradient through a yet to be determined transporter, but it led to a slight loss of endogenous Cl⁻. Since ammonia exposure also resulted in significant decreases in blood pH, [HCO₃⁻] and [total CO₂] in A. testudineus, it can be deduced that active NH₄⁺ excretion could also be driven by the exit of HCO₃⁻ through the apical Cftr. Furthermore, A. testudineus uniquely responded to ammonia exposure by increasing the ambient pH and decreasing the branchial bafilomycin-sensitive V-type H⁺-ATPase activity, which suggests that its gills might have low NH₃ permeability.

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.

Chloride cells and osmoregulation

The chloride cells of the gill secretory epithelium of fish that make the transition from fresh water to sea water adapt to the increased salinity by responding to a rapid signal that stimulates chloride secretion. In this paper, data are presented supporting the view that the transient increase in plasma osmolarity that can be measured during the transition is responsible for the stimulation of chloride secretion. A maximal increase of 65 mOsm in the plasma of Fundulus heteroclitus (the killifish) was found during acclimation to sea water. Similar or greater increases of osmolarity induced by mannitol on the basolateral side of isolated opercular epithelial membranes of the same species of fish containing great numbers of chloride cells produced stimulation of chloride secretion detected as the short circuit current. The shrinkage of the chloride cell activates the Na-K-2Cl cotransporter, and the Na/H exchanger and requires the integrity of apical chloride channels and normal levels of Ca. A Cl/HCO3 exchanger did not participate in this osmotic response to higher salinity. Chloride cell volume responses to osmolarity were studied with imagine and quantitative optics.

A diel rhythm of the short-circuit current expressed by the opercular epithelium of the killifish, Fundulus heteroclitus

The inner opercular epithelium of the killifish, Fundulus heteroclitus, may be separated as a single epithelial sheet and mounted in an Ussing-style chamber with the short-circuit current (Isc) approximating chloride secretion. Steady state opercular Isc values in seawater-adapted killifish were higher in tissues removed from fish in the afternoon than those observed in the morning. Opercular conductance was also higher in the afternoon than the morning. The opercular steady state Isc from killifish on a 13:11 light:dark cycle exhibited a diel variation with the peak in the middle of the light cycle and a nadir near the end of the dark phase. Temperature and salinity remained constant during the experimental period. Daily variation cued by the photoperiod should be considered when using this preparation as an experimental model for ion transport in the gill of a seawater-adapted fish.