Electron microprobe analysis of intracellular electrolytes in resting and isoproterenol-stimulated exocrine glands of frog skin

Role of cutaneous surface fluid in frog osmoregulation

The study investigated whether evaporative water loss (EWL) in frogs stems from water diffusing through the skin or fluid secreted by mucous glands. Osmolality of cutaneous surface fluid (CSF) of Rana esculenta (Pelophylax kl. esculentus) subjected to isoproterenol or 30°C-34°C was 191±9.3 and 181±7.5 mosm/kg, respectively, as compared to lymph osmolality of, 249±10 mosm/kg. Cation concentrations of CSF were likewise independent of pre-treatment with averages of, [Na(+)]=65.5±5.1 and [K(+)]=14.9±1.6 mmol/L, and lymph concentrations of 116 mmol Na(+)/L and 5.1 mmol K(+)/L. The relatively high [K(+)] confirms that CSF is produced by submucosal glands. Since the chemical energy of water of CSF was always higher than that of body fluids, diffusion of water would be from CSF to the interstitial fluid and not in the opposite direction. It is concluded that volume and composition of CSF are regulated by subepidermal exocrine gland secretion balanced by EWL into the atmosphere and ion reuptake by the epidermal epithelium. Previously discovered regulatory mechanisms of epithelial ion absorption, hitherto not ascribed a body function, fit well with a role in regulating turnover of CSF. As a regulated external physiological compartment, CSF would be of importance for the immune defenses that amphibians employ in protecting their skin.

Reconciling the Krogh and Ussing interpretations of epithelial chloride transport - presenting a novel hypothesis for the physiological significance of the passive cellular chloride uptake

In 1937, August Krogh discovered a powerful active Cl(-) uptake mechanism in frog skin. After WWII, Hans Ussing continued the studies on the isolated skin and discovered the passive nature of the chloride uptake. The review concludes that the two modes of transport are associated with a minority cell type denoted as the γ-type mitochondria-rich (MR) cell, which is highly specialized for epithelial Cl(-) uptake whether the frog is in the pond of low [NaCl] or the skin is isolated and studied by Ussing chamber technique. One type of apical Cl(-) channels of the γ-MR cell is activated by binding of Cl(-) to an external binding site and by membrane depolarization. This results in a tight coupling of the uptake of Na(+) by principal cells and Cl(-) by MR cells. Another type of Cl(-) channels (probably CFTR) is involved in isotonic fluid uptake. It is suggested that the Cl(-) channels serve passive uptake of Cl(-) from the thin epidermal film of fluid produced by mucosal glands. The hypothesis is evaluated by discussing the turnover of water and ions of the epidermal surface fluid under terrestrial conditions. The apical Cl(-) channels close when the electrodiffusion force is outwardly directed as it is when the animal is in the pond. With the passive fluxes eliminated, the Cl(-) flux is governed by active transport and evidence is discussed that this is brought about by an exchange of cellular HCO(3) (-) with Cl(-) of the outside bath driven by an apical H(+) V-ATPase.

Patch clamp on the luminal membrane of exocrine gland acini from frog skin (Rana esculenta) reveals the presence of cystic fibrosis transmembrane conductance regulator-like Cl- channels activated by cyclic AMP

Chloride channels in the luminal membrane of exocrine gland acini from frog skin (Rana esculenta) constituted a single homogeneous population. In cell-attached patches, channels activated upon exposure to isoproterenol, forskolin, or dibutyryl-cAMP and isobutyl-1-methyl-xanthine rectified in the outward direction with a conductance of 10.0 +/- 0.4 pS for outgoing currents. Channels in stimulated cells reversed at 0 mV applied potential, whereas channels in unstimulated cells reversed at depolarized potentials (28.1 +/- 6.7 mV), indicating that Cl- was above electrochemical equilibrium in unstimulated, but not in stimulated, cells. In excised inside-out patches with 25 mM Cl- on the inside, activity of small (8-pS) linear Cl--selective channels was dependent upon bath ATP (1.5 mM) and increased upon exposure to cAMP-dependent protein kinase. The channels displayed a single substate, located just below 2/3 of the full channel amplitude. Halide selectivity was identified as PBr > PI > PCl from the Goldman equation; however, the conductance sequence when either halide was permeating the channel was GCl > GBr >> GI. In inside-out patches, the channels were blocked reversibly by 5-nitro-2-(3-phenylpropylamino)benzoic acid, glibenclamide, and diphenylamine-2-carboxylic acid, whereas 4, 4-diisothiocyanatostilbene-2,2-disulfonic acid blocked channel activity completely and irreversibly. Single-channel kinetics revealed one open state (mean lifetime = 158 +/- 72 ms) and two closed states (lifetimes: 12 +/- 4 and 224 +/- 31 ms, respectively). Power density spectra had a double-Lorentzian form with corner frequencies 0.85 +/- 0.11 and 27.9 +/- 2.9 Hz, respectively. These channels are considered homologous to the cystic fibrosis transmembrane conductance regulator Cl- channel, which has been localized to the submucosal skin glands in Xenopus by immunohistochemistry (Engelhardt, J.F., S.S. Smith, E. Allen, J.R. Yankaskas, D.C. Dawson, and J.M. Wilson. 1994. Am. J. Physiol. 267: C491-C500) and, when stimulated by cAMP-dependent phosphorylation, are suggested to function in chloride secretion.

Role of pars nervosa of the hypophysis in amphibian water economy: a re-assessment

1. Responses in renal function and in water permeability of skin and bladder to wet and dry environments are accomplished within the range of normal hydration of the amphibian organism. 2. Urine production is discontinued at moderate dehydration. 3. Strong dehydration is needed to raise plasma arginine vasotocin (AVT). 4. Surgical interference with hypophysial function may repress water balance responses because of pars distalis dysfunction, with no clear effect of elimination of pars nervosa function. 5. Antidiuretic hormones, along with adrenergic agonists, may be potent stimulators of the water permeability of membranes of variable permeability, such as skin of terrestrial anurans. 6. AVT does not play a key role in amphibian water economy, but may exert a modulatory role in the control of renal function, secondary to nervous control.

Isotonic secretion via frog skin glands in vitro. Water secretion is coupled to the secretion of sodium ions

In isolated frog skin at least three different types of cells are engaged in the transepithelial ion and water transport; these are the granular cells, the mitochondria-rich cells and the glandular cells. The experiments presented were carried out on isolated frog skin bathed in Cl- or NO3- Ringer's solution, where the active transepithelial Na+ uptake via the granular cells was blocked by amiloride. Transepithelial current and water flow were measured. When a negative current was passed across the skins (the skins were clamped at -100 mV), the current was mainly carried by a net influx of Cl- via the mitochondria-rich cells. The current had no effect on the transepithelial water movement. This finding indicates that there is nearly no coupling between the Cl- flux and the movement of water via the mitochondria-rich cells. Prostaglandin E2 activates the glandular cells of the exocrine glands in the skin. When prostaglandin E2 was added under these experimental conditions (the skins were clamped at -100 mV, with amiloride in the apical bathing solution, and the glandular secretion of ions was blocked by the use of NO3- Ringer's solution), then the transepithelial current became more negative. This change in current was mainly due to an increase in the Na+ efflux via the glands. Thus PGE2 increase the Na+ conductance of the skin glands. Together with this increase in the Na+ efflux a highly significant increase in the water secretion was observed. The water movement (secretion) across the skin was under these conditions coupled to the PGE2-induced efflux of Na+, and when one Na+ was pulled from the basolateral to the apical solution via this pathway 215 molecules of water followed. This must be due to electro-osmosis (friction between ions and water) or current-induced local osmosis.

The effect of amiloride and benzimidazoleguanidine added to the inside medium on electrolyte pathways in the frog skin glands

Serosal amiloride inhibits Na+, K+ and Cl- efflux and reduces short-circuit current and transepithelial conductance in noradrenaline-stimulated frog skin in which the sodium channels in the apical membrane are blocked by amiloride. BIG (benzimidazoleguanidine) inhibits Na+ and K+ efflux and reduces the short-circuit current. In some skins BIG decreased and in others it increased Cl- efflux and conductance. The variable response appears to be due not only to inhibition of salt extrusion from the gland cells but also to activation of Cl- transport through the mitochondria-rich cells, since it was shown that BIG could increase Cl- efflux in gland-free preparations. Different target mechanisms in the gland cells where the two substances may exert their effect are discussed.

Forskolin activates gated Cl- channels in frog skin

We investigated the effect of forskolin on Cl- movements across the isolated epithelium of frog skin. With Cl- on both sides, forskolin (50 mumol/l) increased the transepithelial conductance considerably and elicited significant Cl- secretion. Establishing transepithelial Cl- gradients markedly increased the Cl- currents (ICl). During forskolin treatment, the power density spectra (PDS) of the fluctuation in transepithelial current contained a Lorentzian component that depended on the presence of Cl- in the bathing solutions. Mucosal as well as serosal diphenylamine-2-carboxylic acid (DPC; 1 mmol/l) partially depressed ICl as well as the Lorentzian noise component. Microelectrode recordings from cells involved in transepithelial Na+ absorption showed that forskolin activates gated Cl- channels in a cellular pathway in parallel with the Na+-transporting granulosum cells of the frog skin. The activation of the Cl- -dependent currents and Lorentzian noise was rather variable, and adaptation of the animals to solutions that contained 40 or 60 mmol/l NaCl increased the sensitivity to forskolin. In skins of salt-adapted animals, oxytocin (0.1 U/ml) also slightly activated the Cl- pathway. On the other hand, oxytocin and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP; 1 mmol/l) were without effect in control skins.

Mechanism of calcium ionophore stimulated Cl secretion from frog skin glands

The aim of the present study was to investigate the mechanism by which the calcium ionophore A23187 stimulates Cl and water secretion from exocrine glands in the frog skin. The Cl secretion was visualized as changes in short-circuit current (SCC) in skins where the Na absorption was blocked by amiloride applied to the apical membrane. Measurements of A23187 stimulated ion fluxes showed that the ionophore induced a net secretion of Cl, Na and K. The active Cl secretion was enhanced more than the Na and K secretion, resulting in a net secretion of negative ions which closely resembled the A23187-stimulated SCC. The effect of A23187 was abolished in skins pretreated with indomethacin, implying the involvement of prostaglandins in the response. Furthermore, the effect of A23187 was inhibited in the presence of quinacrine, indicating that the activation of the cyclooxygenase pathway is dependent on phospholipase A2 activity. In addition, the A23187, but not the arachidonic acid stimulated Cl secretion was abolished in the presence of trifluoperazine, suggesting that the effect of the ionophore may be mediated via a Ca2+ -calmodulin-dependent step located before the activation of the cyclooxygenase. The net water flow and the Cl secretion were measured simultaneously under the conditions outlined above. The stimulation, inhibition, and time-course of the water secretion were similar to the changes observed for the Cl secretion. The A23187 stimulated Cl secretion was enhanced by the phosphodiesterase inhibitor, theophyllin, indicating that the effect of A23187 was caused by an increase in the intracellular cAMP level in the gland cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Frontiers in electron probe microanalysis: application to cell physiology

The application of electron probe microanalysis techniques, using X-ray and electron energy loss instruments, to problems in cell physiology is reviewed. The details of the special methodological requirements for the analysis of cryosections at high spatial resolution in an analytical electron microscope are discussed together with a comprehensive review of data obtained on major organ systems and cell types.

Prostaglandin E2-stimulated glandular ion and water secretion in isolated frog skin (Rana esculenta)

Prostaglandins are known to stimulate the active sodium absorption in frog skin. In this paper it is shown that prostaglandin E2 (PGE2) stimulates an active secretion of Cl-, Na+, and K+ from the skin glands in Rana esculenta. The active Cl secretion is enhanced more than the Na and K secretion. Therefore, in skins where the Na absorption is inhibited by amiloride, the addition of PGE2 results in an increase in the short-circuit current (SCC). The PGE2-stimulated Cl secretion could be inhibited by the presence of ouabain or furosemide in the basolateral solution or diphenylamine-2-carboxylate in the apical solution. The PGE2-stimulated Cl secretion was enhanced by the phosphodiesterase inhibitor, theophylline, indicating that the effect of PGE2 was caused by an increase in the intracellular cAMP level in the gland cells. The calcium ionophore A23187, which increases the PGE2 synthesis in frog skin, stimulated the glandular Cl secretion. This secretion could be blocked by the prostaglandin synthesis inhibitor indomethacin, indicating that A23187 acts by increasing the prostaglandin synthesis and not by a direct action of Ca2+ ions per se. The net water flow (Jw) and the Cl secretion were measured simultaneously under the conditions outlined above. The stimulation, inhibition, and the time-course of the outward-directed Jw were similar to the change observed for the Cl secretion. These results show that PGE2 stimulates a glandular secretion of Cl and water in frog skin, probably by increasing the cAMP level in the gland cells.