Effects of cyclic AMP and theophylline on chloride conductance across toad skin

Integument structure and function in juvenile Xenopus laevis with disrupted thyroid balance

The skin is the largest organ in the body and is a barrier between the internal and external environment. The present study evaluates how PTU, a goitrogen, that is used to treat hyperthyroidism affects the structure and electrical properties of the frog (Xenopus laevis) skin. The results are considered in the context of the two-membrane model established in the seminal work of Ussing and collegues in the 1940s and 1950s. In vitro experiments with skin from Xenopus adults revealed that PTU can act directly on skin and causes a significant increase (p<0.05, One-way ANOVA) in short circuit current (Isc) via an amiloride-insensitive mechanism. Juvenile Xenopus exposed to waterborne PTU (5 mg/L) had a significantly bigger and more active thyroid gland (p<0.01, Student's t-test) than control Xenopus. The bioelectric properties of skin taken from Xenopus juveniles treated with PTU in vivo had a lower Isc, (3.05±0.4, n=13) and Rt (288.2±39.5) than skin from control Xenopus (Isc, 4.19±1.14, n=14; Rt, 343.3±43.3). A histological assessment of skin from PTU treated Xenopus juveniles revealed the epidermis was significantly thicker (p<0.01, Student's t-test) and had a greater number of modified exocrine glands (p<0.01, Student's t-test) in the dermis compared to control skin. Modifications in skin structure are presumably the basis for its changed bioelectric properties and the study highlights a site of action for environmental chemicals which has been largely neglected.

Chloride conductance in amphibian skin: regulatory control in the skin of Rana pipiens

Chloride conductance across the isolated skin of Rana pipiens shows a voltage-activated component (G(Cl)(V)) which requires the presence of mucosal Cl. G(Cl)(V) is normally low or dormant. It is stimulated by elevated intracellular cAMP, irrespective whether originating from application of ss-adrenergic agonists (isoproterenol), stimulators of the adenylyl-cyclase (forskolin), inhibitors of the phosphodiesterases (isobutyl-methyl-xanthine) or membrane-permeable cAMP analogues (CPT-cAMP). Baseline G(Cl) under inactivating conditions increases also with cAMP dose-dependently. The data indicate that cAMP is a central regulator of the passive, conductive chloride transport across amphibian skin.

Equine sweating and anhidrosis Part 1 ? equine sweating

Sweating has a variety of functions in mammals including pheromone action, excretion of waste products and maintenance of the skin surface ecosystem. In a small number of mammalian species, which includes humans and the Equidae, it also has an important role in thermoregulation. This review is focused specifically on the thermoregulatory role of sweat in Equidae and the causes of sweating failure (anhidrosis). The first part describes the glandular appearance, sweat composition, and output rates; and considers the latest theories on the glandular control and secretory mechanisms. It is concluded that the glands are not directly innervated but are controlled by the interplay of neural, humoral and paracrine factors. The secretory mechanism is not as simple as previously thought and is mediated by the dynamic interaction of activating pathways, including autocrine control not only of the secretory process but probably also of secretory cell reproduction, growth, and death.

Effect of mercuric chloride on electrical parameters and anion fluxes in the toad skin

The amphibian skin, widely used for studying the transepithelial passage of electrolytes, exhibits anion pathways relatively specific for Cl(-). We studied the effect of HgCl(2), 1.0 x 10(-4) M on its electrical parameters and unidirectional anion fluxes. In the presence of Cl(-), the transepithelial conductance (G) of the isolated skin of the Bufo arenarum toad increased considerably following exposure to HgCl(2), whereas short-circuit current (SCC)--reflecting transepithelial Na(+) transport-underwent only slight stimulation. Following the blockade of Na(+) intake by amiloride, 1.0 x 10(-4) M, the removal of Cl(-) from the solution bathing the epidermal border of the skin brought about a decrease in G, and gave rise to a gradient-induced SCC (SCCg) consistent with transepithelial passage of Cl(-) along its gradient. Addition of mercaptoethanol, 5.0 x 10(-3) M to the bath containing Hg(2+) fully reversed these effects. The increase in G was accompanied by an increase in the unidirectional (epidermal to dermal) fluxes of (36)Cl(-) and (131)I(-), and a decrease in the passage of (99m)TcO(4)(-). These results show the effects of HgCl(2) to be similar to those of theophylline, although exhibiting a different selectivity. Our data suggest that anion passage following exposure to HgCl(2) is, like that stimulated by theophylline, predominantly if not exclusively transcellular, and does not involve a significant opening of the tight junctions.

Xanthine derivatives without PDE effect stimulate voltage-activated chloride conductance of toad skin

The effect of xanthine derivatives on the voltage-activated Cl(-) conductance (G(Cl)) of amphibian skin was analyzed. 3-Isobutyl-1-methylxanthine (IBMX) and the recently synthesized xanthine derivatives 3,7-dimethyl-1-propyl xanthine (X-32) and 3,7-dimethyl-1-isobutyl xanthine (X-33), which lack inhibitory effects on phosphodiesterases in CHO and Calu-3 cells, increased voltage-activated G(Cl) without effect on baseline conductance at inactivating voltage. Half-maximal stimulation of G(Cl) occurred at 108 +/- 9 microM for X-32 and X-33 after apical or basolateral application. The stimulation of G(Cl), which occurs only in the presence of Cl(-) in the mucosal solution, is caused by a shift of the voltage sensitivity to lower clamp potentials and an increase of the maximally activated level. Furosemide reversed both the shift of sensitivity and the increase in magnitude. These patterns are fundamentally different from those seen after application of membrane-permeant, nonmetabolized analogs of cAMP, and they indicate that the xanthines stimulate G(Cl) directly. This notion is strengthened by the lack of influence on intracellular cAMP content, which is consistent with the observations in CHO and Calu-3 cells. We propose that the xanthine derivatives increase the voltage sensitivity of a regulative component in the conductive Cl(-) pathway across amphibian skin.

Mitochondria-rich cells as experimental model in studies of epithelial chloride channels

The mitochondria-rich (mr) cell of amphibian skin epithelium is differentiated as a highly specialised pathway for passive transepithelial transport of chloride. The apical membrane of mr cells expresses several types of Cl(-) channels, of which the function of only two types has been studied in detail. (i) One type of channel is gated by voltage and external chloride concentration. This intriguing type of regulation leads to opening of channels only if [Cl(-)](o) is in the millimolar range and if the electrical potential is of a polarity that secures an inwardly directed net flux of this ion. Reversible voltage activations of the conductance proceed with long time constants, which depend on V in such a way that the rate of conductance activation increases when V is clamped at more negative values (serosal bath grounded). The gating seems to involve processes that are dependent on F-actin localised in the submembrane domain in the neck region of the flask-shaped mr cell. (ii) The other identified Cl(-) pathway of mr cells is mediated by small-conductance apical CFTR chloride channels as concluded from its activation via beta-adrenergic receptors, ion selectivity, genistein stimulation and inhibition by glibenclamide. bbCFTR has been cloned, and immunostaining has shown that the gene product is selectively expressed in mr cells. There is cross-talk between the two pathways in the sense that activation of the conductance of the mr cell by voltage clamping excludes activation via receptor occupation, and vice versa. The mechanism of this cross-talk is unknown.

The route of passive chloride movement across amphibian skin: localization and regulatory mechanisms

Transepithelial Cl(-) conductance (G(Cl)) in amphibian skin can be activated in several species by serosa positive potentials. Mitochondria-rich cells (MRC) or tight junctions (TJ) between the epithelial cells are possible sites for this pathway. The properties and the techniques used to investigate this pathway are reviewed in the present paper. In situ techniques are preferable, since specific properties of the MRC are apparently not maintained in isolated cells. Volume measurements and electronprobe microanalysis of intracellular ions suggest the localization of voltage-activated G(Cl) to MRC. G(Cl) correlates poorly with the density of MRC. The vibrating voltage probe allows quantitative correlation of the local Cl(-) current through morphologically identified structures and the transepithelial Cl(-) current. Our analysis shows that 80% of the voltage-activated Cl(-) current is accounted for by current through MRC or their immediate vicinity. The activation patterns of this current and the inhibition by the alpha(1)-adrenergic agonist, epinephrine, conform to those of the transepithelial current. However, less than 20% of the MRC are active at a certain moment and the activity is spontaneously variable with time. The molecular nature of this pathway, physiological control mechanisms and their relation to the temporal activity of MRC remain to be studied.

Selective inhibition of Cl(-) conductance in toad skin by blockers of Cl(-) channels and transporters

We compared the effects exerted by two classes of Cl(-) transport inhibitors on a Cl(-)-selective, passive anion transport route across the skin of Bufo viridis, the conductance (G(Cl)) of which can be activated by transepithelial voltage perturbation or high cAMP at short circuit. Inhibitors of antiporters (erythrosine, eosin) or cotransporters (furosemide) reduced voltage-activated G(Cl) with IC(50) of 6 +/- 1, 54 +/- 12, and 607 +/- 125 microM, respectively; they had no effect on the cAMP-induced G(Cl). The voltage for half-maximal activation of G(Cl) (V(50)) increased compared with controls, but effects on the maximal G(Cl) at more positive clamp potentials were small. Cl(-) channel blockers from the diphenylamino-2-carboxylic acid (DPC) family [dichloro-DPC, niflumic acid, flufenamic acid, and 5-nitro-2-(3-phenylpropylamino)benzoic acid] reduced the voltage-activated G(Cl) with IC(50) of 8.3 +/- 1.2, 10.5 +/- 0.6, 16.5 +/- 3.4, and 36.5 +/- 11.4 microM, respectively, and also inhibited the cAMP-induced G(Cl), albeit with slightly larger IC(50). V(50) was not significantly changed compared with controls; the maximal G(Cl) was strongly reduced. We conclude that the pathway for Cl(-) is composed of the conductive pore proper, which is blocked by the derivatives of DPC, and a separate, voltage-sensitive regulator, which is influenced by blockers of cotransporters or antiporters. This influence is partly overcome by increasing the clamp potential and removed by high concentrations of cAMP, which renders the pathway insensitive to voltage.

Trypsin inhibits voltage-activated chloride conductance of toad skin

The effect of trypsin on the voltage-activated chloride conductance (GCl) of toad skin was investigated. Serosal application of > 0.1 mg ml-1 trypsin decreased the voltage-activated GCl without notable delay. The maximal inhibition to 38% of the control values, exerted within 15 min, was in some experiments partly or completely reversible. Chymotrypsin had much lower effect than trypsin. Mucosal application of trypsin did not have any effect. Trypsin did neither interfere with the conductive pathway opened by supramaximal concentrations of cAMP nor with the inhibitory effect of epinephrine on the voltage-activated GCl. The effect of trypsin required influx of Ca2+ from the extracellular space. It is concluded that protease-activated receptors or trypsin-sensitive proteins in the basolateral membrane of toad skin epithelial cells interfere with regulative steps involved in the voltage-activation of GCl. This may be harmful for the segregation of epithelial cells using this enzyme.

Structural basis for specificity and potency of xanthine derivatives as activators of the CFTR chloride channel

1. On the basis of their structure, we compared the ability of 35 xanthine derivatives to activate the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel stably expressed in chinese hamster ovary (CHO) cells using the cell-attached patch clamp and iodide efflux techniques. 2. Activation of CFTR channels was obtained with 3-mono, 1,3-di or 1,3,7-tri-substituted alkyl xanthine derivatives (enprofylline, theophylline, aminophylline, IBMX, DPMX and pentoxifylline). By contrast, xanthine derivatives substituted at the C8- or N9-position failed to open CFTR channels. 3. The CFTR chloride channel activity was blocked by glibenclamide (100 microM) but not by DIDS (100 microM). 4. Activation of CFTR by xanthines was not mimicked by the calcium ionophore A23187, adenosine, UTP, ATP or the specific phosphodiesterase inhibitors rolipram, Ro 20-1724 and milrinone. In addition, we found no correlation between the effect of xanthines on CFTR and on the cellular cyclic AMP or ATP levels. 5. We then synthesized a series of 3,7-dimethyl-1-alkyl xanthine derivatives; among them, 3,7-dimethyl-1-propyl xanthine and 3,7-dimethyl-1-isobutyl xanthine both activated CFTR channels without increasing the intracellular cyclic AMP level, while the structurally related 3,7-dimethyl-1-(2-propenyl) xanthine and 3,7-dimethyl-1-(oxiranyl methyl) xanthine were inactive. 6. Our findings delineate a novel function for xanthine compounds and identify the molecular features that enable xanthine activation of CFTR. These results may be useful in the development of new molecules for studying the pharmacology of chloride channels.

The influence of luminal pH on transport of neutral and charged dipeptides by rat small intestine, in vitro

Four hydrolysis-resistant dipeptides (D-phenylalanyl-L-alanine, D-phenylalanyl-L-glutamine, D-phenylalanyl-L-glutamate and D-phenylalanyl-L-lysine) were synthesized to investigate the effects of net charge on transmural dipeptide transport by isolated jejunal loops of rat small intestine. At a luminal pH of 7.4 and a concentration of 1 mM the two dipeptides with a net charge of -1 and +1 were transported at substantially slower rates (18 +/- 1.3 and 8.4 +/- 1.3 nmol min(-1)(g dry wt.)(-1), respectively) than neutral D-phenylalanyl-L-alanine and D-phenylalanyl-L-glutamine (87 +/- 0.2 and 197 +/- 14 nmol min(-1)(g dry wt.)(-1), respectively). We investigated the effects of luminal pH on dipeptide transport by varying the NaHCO3 content of Krebs Ringer perfusate equilibrated with 95% 02/5% CO2. The pH changes did not affect water transport, but serosal glucose appearance increased significantly at pH 6.8. Transmural transport of D-phenylalanyl-L-alanine and D-phenylalanyl-L-glutamine at pH 6.8 was stimulated (P < 0.01) by 61% and 49%, respectively, whereas the lower pH increased the rate for negatively charged D-phenylalanyl-L-glutamate by 306% (P < 0.01) and decreased that for positively charged D-phenylalanyl-L-lysine by 46% (P < 0.05). Increasing luminal pH to 8.0 inhibited D-phenylalanyl-L-alanine transport by 60%, whereas D-phenylalanyl-L-lysine transport was 60% faster.

Heterogeneity of chloride channels in the apical membrane of isolated mitochondria-rich cells from toad skin

The isolated epithelium of toad skin was disintegrated into single cells by treatment with collagenase and trypsine. Chloride channels of cell-attached and excised inside-out apical membrane-patches of mitochondria-rich cells were studied by the patch-clamp technique. The major population of Cl- channels constituted small 7-pS linear channels in symmetrical solutions (125 mM Cl-). In cell-attached and inside-out patches the single channel i/V-relationship could be described by electrodiffusion of Cl- with a Goldmann-Hodgkin-Katz permeability of, PCl = 1.2 x 10(-14) - 2.6 x 10(-14) cm3. s-1. The channel exhibited voltage-independent activity and could be activated by cAMP. This channel is a likely candidate for mediating the well known cAMP-induced transepithelial Cl- conductance of the amphibian skin epithelium. Another population of Cl- channels exhibited large, highly variable conductances (upper limit conductances, 150-550 pS) and could be activated by membrane depolarization. A group of intermediate-sized Cl(-)-channels included: (a) channels (mean conductance, 30 pS) with linear or slightly outwardly rectifying i/V-relationships and activity occurring in distinct "bursts," (b) channels (conductance-range, 10-27 pS) with marked depolarization-induced activity, and (c) channels with unresolvable kinetics. The variance of current fluctuations of such "noisy" patches exhibited a minimum close to the equilibrium-potential for Cl-. With channels occurring in only 38% of sealed patches and an even lower frequency of voltage-activated channels, the chloride conductance of the apical membrane of mitochondria-rich cells did not match quantitatively that previously estimated from macroscopic Ussing-chamber experiments. From a qualitative point of view, however, we have succeeded in demonstrating the existence of Cl-channels in the apical membrane with features comparable to macroscopic predictions, i.e., activation of channel gating by cAMP and, in a few patches, also by membrane depolarization.