Diacylglycerols stimulate short-circuit current across frog skin by increasing apical Na+ permeability

Role of PKCalpha in feedback regulation of Na(+) transport in an electrically tight epithelium

It has long been known that Na(+) channels in electrically tight epithelia are regulated by homeostatic mechanisms that maintain a steady state and allow new levels of transport to be sustained in hormonally challenged cells. Little is known about the potential pathways involved in these processes. In addition to short-term effect, recent evidence also indicates the involvement of PKC in the long-term regulation of the epithelial Na(+) channel (ENaC) at the protein level (40). To determine whether stimulation of ENaC involves feedback regulation of PKC levels, we utilized Western blot analysis to determine the distribution of PKC isoforms in polarized A6 epithelia. We found the presence of PKC isoforms in the conventional (alpha and gamma), novel (delta, eta, and epsilon), and atypical (iota, lambda, and zeta) groups. Steady-state stimulation of Na(+) transport with aldosterone was accompanied by a specific decrease of PKCalpha protein levels in both the cytoplasmic and membrane fractions. Similarly, overnight treatment with an uncharged amiloride analog (CDPC), a procedure that through feedback regulation causes a stimulation of Na(+) transport, also decreased PKCalpha levels. These effects were additive, indicating separate mechanisms that converge at the level of PKCalpha. These effects were not accompanied by changes of PKCalpha mRNA levels as determined by Northern blot analysis. We propose that this may represent a novel regulatory feedback mechanism necessary for sustaining an increase of Na(+) transport.

Protein kinase C isoform antagonism controls BNaC2 (ASIC1) function

We explored the involvement of protein kinase C (PKC) and its isoforms in the regulation of BNaC2. Reverse transcriptase PCR evaluation of PKC isoform expression at the level of mRNA revealed the presence of alpha and epsilon/epsilon' in all glioma cell lines analyzed; most, but not all cell lines expressed delta and zeta. No messages were found for the betaI and betaII isotypes of PKC in the tumor cells. Normal astrocytes expressed beta but not gamma. The essential features of these results were confirmed at the protein level by Western analysis. This disproportionate pattern of PKC isoform expression in glioma cell lines was further echoed in the functional effects of these PKC isoforms on BNaC2 activity in bilayers. PKC holoenzyme or the combination of PKCbetaI and PKCbetaII isoforms inhibited BNaC2. Neither PKCepsilon nor PKCzeta or their combination had any effect on BNaC2 activity in bilayers. The inhibitory effect of the PKCbetaI and PKCbetaII mixture on BNaC2 activity was abolished by a 5-fold excess of a PKCepsilon and PKCzeta combination. PKC holoenzymes, PKCbetaI, PKCbetaII, PKCdelta, PKCepsilon, and PKCzeta phosphorylated BNaC2 in vitro. In patch clamp experiments, the combination of PKCbetaI and PKCbetaII inhibited the basally activated inward Na(+) conductance. The variable expression of the PKC isotypes and their functional antagonism in regulating BNaC2 activity support the idea that the participation of multiple PKC isotypes contributes to the overall activity of BNaC2.

Specific and nonspecific effects of protein kinase C on the epithelial Na (+) channel

The Xenopus oocyte expression system was used to explore the mechanisms of inhibition of the cloned rat epithelial Na(+) channel (rENaC) by PKC (Awayda, M.S., I.I. Ismailov, B.K. Berdiev, C.M. Fuller, and D.J. Benos. 1996. J. Gen. Physiol. 108:49-65) and to determine whether human ENaC exhibits similar regulation. Effects of PKC activation on membrane and/or channel trafficking were determined using impedance analysis as an indirect measure of membrane area. hENaC-expressing oocytes exhibited an appreciable activation by hyperpolarizing voltages. This activation could be fit with a single exponential, described by a time constant (tau) and a magnitude (DeltaI (V)). A similar but smaller magnitude of activation was also observed in oocytes expressing rENaC. This activation likely corresponds to the previously described effect of hyperpolarizing voltage on gating of the native Na(+) channel (Palmer, L.G., and G. Frindt. 1996. J. Gen. Physiol. 107:35-45). Stimulation of PKC with 100 nM PMA decreased DeltaI(V) in hENaC-expressing oocytes to a plateau at 57.1 +/- 4.9% (n = 6) of baseline values at 20 min. Similar effects were observed in rENaC-expressing oocytes. PMA decreased the amiloride-sensitive hENaC slope conductance (g(Na)) to 21.7 +/- 7.2% (n = 6) of baseline values at 30 min. This decrease was similar to that previously reported for rENaC. This decrease of g (Na) was attributed to a decrease of membrane capacitance (C (m)), as well as the specific conductance (g(m)/C(m )). The effects on g(m)/C(m) reached a plateau within 15 min, at approximately 60% of baseline values. This decrease is likely due to the specific ability of PKC to inhibit ENaC. On the other hand, the decrease of C(m) was unrelated to ENaC and is likely an effect of PKC on membrane trafficking, as it was observed in ENaC-expressing as well as control oocytes. At lower PMA concentrations (0.5 nM), smaller changes of C(m) were observed in rENaC- and hENaC-expressing oocytes, and were preceded by larger changes of g(m ) and by changes of g(m)/C(m), indicating specific effects on ENaC. These findings indicate that PKC exhibits multiple and specific effects on ENaC, as well as nonspecific effects on membrane trafficking. Moreover, these findings provide the electrophysiological basis for assessing channel-specific effects of PKC in the Xenopus oocyte expression system.

Cloning of a bovine renal epithelial Na+ channel subunit

A bovine homologue of the rat and human epithelial Na+ channel subunits, alpha-rENaC and alpha-hENaC, was cloned. The cDNA clone, termed alpha-bENaC, was isolated from a bovine renal papillary collecting duct cDNA expression library. The bovine cDNA is 3,584 base pairs (bp) long, has an open reading frame of 2,094 bp encoding a 697-amino acid protein, and is 75-85% homologous to its rat and human counterparts. In vitro translation of the transcribed cRNA yields an 80-kDa polypeptide and one at 92 kDa in the presence of pancreatic microsomes. The clone exhibits consensus sequences for N-linked glycosylation and for phosphorylation by protein kinase C, but not for protein kinase A. After expression in Xenopus laevis oocytes, a small amiloride-sensitive Na+ conductance that exhibited inward rectification and a reversal potential greater than +30 mV, consistent with the predicted equilibrium potential for Na+, was identified. The expressed alpha-bENaC-associated Na+ current was not responsive to elevations in adenosine 3',5'-cyclic monophosphate but could be stimulated by phorbol 12-myristate 13-acetate, an activator of protein kinase C. alpha-bENaC also formed amiloride-sensitive chimeric channels when coexpressed with the rat beta- and gamma-ENaC subunits in Xenopus oocytes. alpha-bENaC therefore represents a novel isoform of a growing family of epithelial Na+ channels.

Second-messenger regulation of sodium transport in mammalian airway epithelia

1. Sodium absorption is the dominant ion transport process in conducting airways and is a major factor regulating the composition of airway surface liquid. However, little is known about the control of airway sodium transport by intracellular regulatory pathways. 2. In sheep tracheae and human bronchi mounted in Ussing chambers under short circuit conditions, the sodium current can be isolated by pretreating tissues with acetazolamide (100 microM) to inhibit bicarbonate secretion, bumetanide (100 microM) to inhibit chloride secretion and phloridzin (200 microM) to inhibit sodium-glucose cotransport. This sodium current consists of amiloride-sensitive (57%) and amiloride-insensitive (43%) components. 3. The regulation of the isolated sodium current by three second messenger pathways was studied using the calcium ionophore A23187 to elevate intracellular calcium, a combination of forskolin and the phosphodiesterase inhibitor zardaverine to elevate intracellular cyclic AMP, and the phorbol ester 12,13-phorbol dibutyrate (PDB) to stimulate protein kinase C. 4. In sheep trachea, A23187 produces a dose-related inhibition of the sodium current with maximal effect (38% of ISC) at 10 microM and IC50 1 microM. This response affects both the amiloride-sensitive and insensitive components of the sodium current and is not altered by prior stimulation of protein kinase C or elevation of intracellular cyclic AMP. In human bronchi, A23187 (10 microM) produced a significantly greater inhibition of ISC (68%), a response which was unaffected by prior treatment with PDB or forskolin-zardaverine. 5. In sheep trachea, stimulation of protein kinase C with PDB produced a dose-related inhibition of ISC maximal (56% of ISC) at 50 nM (IC50 7 nM). This response was abolished by amiloride (100 microM) pretreatment suggesting a selective effect on the amiloride-sensitive component of the sodium current. The response was not altered by prior elevation of intracellular calcium or cyclic AMP. PDB (10 nM) caused a similar inhibition of ISC in human bronchi (43%). The effect of PKC stimulation following pretreatment with A23187 was diminished in human bronchi. Elevating intracellular cyclic AMP did not alter this response. 6. Addition of forskolin (1 microM) together with the phosphodiesterase inhibitor zardaverine (100 microM) produced a mean 35-fold increase in intracellular cyclic AMP in sheep trachea. This was associated with a small, but significant, 6% transient increase in ISC followed by a significant 4% fall. Neither effect could be abolished by amiloride pretreatment. In human bronchi, a small decrease in ISC which could not be distinguished from that occurring in controls was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca(2+)-independent form of protein kinase C may regulate Na+ transport across frog skin

Activators of protein kinase C (PKC) stimulate Na+ transport (JNa) across frog skin. We have examined the effect of Ca2+ on PKC stimulation of JNa. Both the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and the diacyl-glycerol sn-1,2-dioctanoylglycerol (DiC8) were used as PKC activators. Blocking Ca2+ entry into the cytosol (either from external or internal stores) reduced the subsequent natriferic effect of the PKC activators. This negative interaction did not simply reflect saturation of activation of the apical Na+ channels, since the stimulations produced by blocking Ca2+ entry and adding cyclic AMP were simply additive. The Ca2+ dependence of the natriferic effect could have reflected either a direct action of cytosolic Ca2+ on PKC or an indirect action on the final receptor site (the Na+ channel). To distinguish between these possibilities, the TPA- and phospholipid-dependent kinase activity of broken-cell preparations was assayed. The kinase activity was not stimulated by physiological levels of Ca2+, and in fact was inhibited at millimolar concentrations of Ca2+. We conclude that the effects of Ca2+ on the natriferic response to PKC activators are indirect. Reducing cytosolic uptake of Ca2+ may have stimulated Na+ transport by a chemical modification of the apical channels observed in other tight epithelia. The usual stimulation of Na+ transport produced by PKC activators in frog skin may reflect the operation of a nonconventional form of PKC. This enzyme is Ca2+ independent and seems related to the nPKC or PKC epsilon observed in other systems.

Effect of 12-O-tetradecanoyl phorbol 13-acetate on solute transport and production of cAMP in isolated frog skin

In the present study we have examined the action of the phorbol diester tetradecanoyl phorbol acetate, an activator of protein kinase C, on the transepithelial transport of sodium, chloride and water and the production of cAMP in the isolated frog skin epithelium (Rana esculenta). Addition of tetradecanoyl phorbol acetate to the mucosal solution resulted initially in an increase in the short-circuit current, which was followed by a progressive decrease. If the short-circuit current was first activated by addition of the antidiuretic hormone, arginine vasotocin, then the addition of tetradecanoyl phorbol acetate resulted only in a pronounced inhibition. The changes in the short-circuit current were the result of changes in the active influx of Na+. The effect of tetradecanoyl phorbol acetate on the intracellular potential measured under short-circuited conditions (Vscc) was time-dependent. Just after addition of tetradecanoyl phorbol acetate to the mucosal solution, Vscc depolarized; this was followed by a slight hyperpolarization, after which Vscc continued to decline. The inhibition of the Na+ transport by tetradecanoyl phorbol acetate was associated with a decline in the response to the antidiuretic hormone (arginine vasotocin), but the ability of arginine vasotocin to increase the cellular level of cAMP and to stimulate the osmotic water flow was not affected by the presence of tetradecanoyl phorbol acetate. In skin halves in which the short-circuit current was stimulated with arginine vasotocin, addition of tetradecanoyl phorbol acetate resulted in a dose-dependent inhibition of the short-circuit current, but only minor changes in Vscc were observed. The results presented suggest that the addition of tetradecanoyl phorbol acetate to the isolated frog skin first increases and then decreases the arginine vasotocin-sensitive sodium permeability of the apical membrane. This might be due to a stimulating effect of tetradecanoyl phorbol acetate on both the activation and deactivation (turnover) of the sodium channels.

Interactions of TPA and insulin on Na+ transport across frog skin

The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) activates protein kinase C (PKC) and produces an early stimulation of Na+ transport across frog skin. The ionic basis for this stimulation was studied with combined transepithelial and intracellular electrical measurements. In an initial series of experiments, TPA approximately doubled the amiloride-sensitive short-circuit current (ISC), apical Na+ permeability (PapNa), and apical membrane conductance without affecting the basolateral membrane conductance. The apical effects led to a marked depolarization of the short-circuited skin and a small increase in intracellular Na+ concentration. TPAs increase of PapNa was sufficient to explain the stimulation of basolateral Na+ transport when both the voltage and substrate dependence of the pump were taken into account. After the early stimulation, TPA later depressed ISC. Added at this point (congruent to 1-2 h after TPA administration), insulin had no effect on ISC, whereas a partial response to vasopressin was still observed. Measured either early or late after TPA addition, the phorbol ester reduced insulin binding by congruent to 40%. Insofar as 60% of the specific binding is retained, the abolishment of insulin's natriferic response is unlikely to result from the TPA-induced reduction in hormonal binding. The data provide further support for the concept that activation of PKC produces an early stimulation of Na+ transport by increasing apical Na+ permeability, and that part of insulin's natriferic effect may be mediated by PKC activation.

Insulin and phorbol ester stimulate conductive Na+ transport through a common pathway

Insulin stimulates Na+ transport across frog skin, toad urinary bladder, and the distal renal nephron. This stimulation reflects an increase in apical membrane Na+ permeability and a stimulation of the basolateral membrane Na,K-exchange pump. Considerable indirect evidence has suggested that the apical natriferic effect of insulin is mediated by activation of protein kinase C. However, no direct information has been available documenting that insulin and protein kinase C indeed share a common pathway in stimulating Na+ transport across frog skin. In the present work, we have studied the interaction of insulin and phorbol 12-myristate 13-acetate (PMA), a documented activator of protein kinase C. Preincubation of skins with 1,2-dioctanoylglycerol, another activator of protein kinase C, increases baseline Na+ transport and reduces the subsequent natriferic response to PMA. Preincubation with PMA markedly reduces the subsequent natriferic action of insulin. This effect does not appear to primarily reflect PMA-induced internalization of insulin receptors. The insulin receptors are localized on the basolateral surface of frog skin, but the application of PMA to this surface is much less effective than mucosal treatment in reducing the response to insulin. Preincubation with D-sphingosine, an inhibitor of protein kinase C, also reduces the natriferic action of insulin. The current results provide documentation that insulin and protein kinase C share a common pathway in stimulating Na+ transport across frog skin. The data are consistent with the concept that the natriferic effect of insulin on frog skin is, at least in part, mediated by activation of protein kinase C.