Alpha-adrenergic receptors regulate human lymphocyte amiloride-sensitive sodium channels

δ ENaC: a novel divergent amiloride-inhibitable sodium channel

The fourth subunit of the epithelial sodium channel, termed delta subunit (δ ENaC), was cloned in human and monkey. Increasing evidence shows that this unique subunit and its splice variants exhibit biophysical and pharmacological properties that are divergent from those of α ENaC channels. The widespread distribution of epithelial sodium channels in both epithelial and nonepithelial tissues implies a range of physiological functions. The altered expression of SCNN1D is associated with numerous pathological conditions. Genetic studies link SCNN1D deficiency with rare genetic diseases with developmental and functional disorders in the brain, heart, and respiratory systems. Here, we review the progress of research on δ ENaC in genomics, biophysics, proteomics, physiology, pharmacology, and clinical medicine.

Cpt-cAMP activates human epithelial sodium channels via relieving self-inhibition

External Na(+) self-inhibition is an intrinsic feature of epithelial sodium channels (ENaC). Cpt-cAMP regulates heterologous guinea pig but not rat αβγ ENaC in a ligand-gated manner. We hypothesized that cpt-cAMP may eliminate the self-inhibition of human ENaC thereby open channels. Regulation of self-inhibition by this compound in oocytes was analyzed using the two-electrode voltage clamp and Ussing chamber setups. External cpt-cAMP stimulated human but not rat and murine αβγ ENaC in a dose- and external Na(+) concentration-dependent fashion. Intriguingly, cpt-cAMP activated human δβγ more potently than αβγ channels, suggesting that structural diversity in ectoloop between human α, δ, and those ENaC of other species determines the stimulating effects of cpt-cAMP. Cpt-cAMP increased the ratio of stationary and maximal currents. Mutants having abolished self-inhibition (β(ΔV348) and γ(H233R)) almost completely eliminated cpt-cAMP mediated activation of ENaC. On the other hand, mutants both enhancing self-inhibition and elevating cpt-cAMP sensitivity increased the stimulating effects of the compound. This compound, however, could not activate already fully opened channels, e.g., degenerin mutation (αβ(S520C)γ) and the proteolytically cleaved ENaC by plasmin. Cpt-cAMP activated native ENaC to the same extent as that for heterologous ENaC in human lung epithelial cells. Our data demonstrate that cpt-cAMP, a broadly used PKA activator, stimulates human αβγ and δβγ ENaC channels by relieving self-inhibition.

Effect of 3 months of doxazosin therapy on T-cell subsets in type 2 diabetic patients

Doxazosin, an alpha(1)-adrenergic receptor inhibitor, is commonly administered to patients with type 2 diabetes, hypertension and nephropathy. The impact of 3 months' doxazosin therapy on the prevalence of activated and regulatory T lymphocytes was analysed in this pilot study of men with type 2 diabetes (n = 10) who received doxazosin 4 mg/day in addition to their ongoing therapy. The prevalence of CD4(+), CD8(+), CD25(+) and CD69(+) cells at baseline and after 3 months of add-on therapy was determined. The prevalence of regulatory T-cells was detected by two different approaches: forkhead box P3 (FoxP3) positivity; and the number of CD4(+)CD25(+high) cells. During 3 months of doxazosin therapy, patients' blood pressure, blood glucose control and lipid profiles all significantly improved. Simultaneously, the prevalence of activated T-cells (CD4(+)CD69(+) and CD8(+)CD69(+) cells) decreased, whereas that of regulatory T-cells increased. These results indicate an immunomodulatory action of doxazosin in type 2 diabetic patients.

Alpha-1A adrenergic receptor activation increases inhibitory tone in CA1 hippocampus

The endogenous catecholamine norepinephrine (NE) exhibits anti-epileptic properties, however it is not well understood which adrenergic receptor (AR) mediates this effect. The aim of this study was to investigate alpha(1)-adrenergic receptor activation in region CA1 of the hippocampus, a subcortical structure often implicated in temporal lobe epilepsies. Using cell-attached and whole-cell recordings in rat hippocampal slices, we confirmed that selective alpha(1)-AR activation increases action potential firing in a subpopulation of CA1 interneurons. We found that this response is mediated via the alpha(1A)-AR subtype, initiated by sodium influx, and appears independent of second messenger signaling. In CA1 pyramidal cells, alpha(1A)-AR activation decreases activity due to increased pre-synaptic GABA and somatostatin release. Examination of post-synaptic receptor involvement revealed that while GABA(A) receptors mediate the majority of alpha(1A)-adrenergic effects on CA1 pyramidal cells, significant contributions are also made by GABA(B) and somatostatin receptors. Finally, to test whether alpha(1A)-AR activation could have potential therapeutic implications, we performed AR agonist challenges using two in vitro epileptiform models. When GABA(A) receptors were available, alpha(1A)-AR activation significantly decreased epileptiform bursting in CA1. Together, our findings directly link stimulation of the alpha(1A)-AR subtype to release of GABA and somatostatin at the single cell level and suggest that alpha(1A)-AR activation may represent one mechanism by which NE exerts anti-epileptic effects within the hippocampus.

ENaC plays a role in regulated antibody secretion by hybridomas

Hybridomas are fused immortal lymphocytes that typically secrete monoclonal antibodies to a known antigen. Hybridomas express two ionic conductances that have properties consistent with epithelial sodium channel (ENaC) and CFTR. Both ion channels are expressed by lymphocytes. Both of these channels are known to play a role in epithelial cell physiology. However, the physiological role of these channels in lymphocytes is unclear. We tested the hypothesis that ENaC plays a role in the process of regulated antibody secretion. We have been able to demonstrate that hybridomas can be provoked to acutely secrete monoclonal antibodies by a variety of agonists. Concurrently, we were able to show that these same agonists activate amiloride-sensitive sodium currents in whole cell clamped hybridomas. Inhibition of ENaC by amiloride inhibited the acute provoked antibody secretion, thereby linking ENaC to the process of acute antibody secretion. Interestingly, the concentration of amiloride necessary to completely inhibit the provoked secretion was approximately an order of magnitude higher than the concentration necessary to inhibit all of the transmembrane current. However, because amiloride is a weak base, the equilibrium concentration necessary to produce partial inhibition was precisely in accord with the K(i) for amiloride and ENaC, indicating that the inhibition was intracellular.

Nongenomic regulation of ENaC by aldosterone

Aldosterone is involved in salt and water homeostasis. The main effect is thought to involve genomic mechanisms. However, the existence of plasma membrane steroid receptors has been postulated. We used whole cell patch clamp to test the hypothesis that epithelial sodium channels (ENaC) expressed by renal collecting duct principal cells can be regulated nongenomically by aldosterone. In freshly isolated principal cells from rabbit, aldosterone (100 nM) rapidly (<2 min) increased ENaC sodium current specifically. The aldosterone-activated current was completely inhibited by amiloride. Aldosterone also activated ENaC in cells treated with the mineralocorticoid receptor blocker spiranolactone. Nongenomic activation was inhibited by inclusion of S-adenosyl-L-homocysteine in the pipette solution, which inhibits methylation reactions. Also, the nongenomic activation required 2 mM ATP supplementation in the pipette solution. Aldosterone did not activate any ENaC current in whole cell clamped rat collecting duct principal cells. These functional studies are consistent with aldosterone membrane binding studies, suggesting the presence of a plasma membrane steroid receptor that affects cellular processes by mechanisms unrelated to altered gene expression.

Immunomodulatory effect of xylazine, an alpha(2) adrenergic agonist, on rat spleen cells in culture

Xylazine is an adrenergic alpha(2) agonist, which is used in veterinary medicine as a sedative and anesthetic agent. In this work we found that xylazine administered in vivo at a dose of 2.5 mg/kg enhanced spleen cell proliferation and interleukin 2 (IL-2) production in cultures stimulated with concanavalin A (Con A), whereas doses of 10 and 25 mg/kg were inhibitory. A similar stimulatory (10 microM) and inhibitory (50-500 microM) effect on splenocyte proliferation and IL-2 production was observed in vitro. Clonidine, another alpha(2) adrenergic agonist, only had a stimulatory proliferative effect on splenocytes. Yohimbine, an alpha(2) adrenergic antagonist, abrogated the stimulatory action of both clonidine and xylazine, but not the suppressive proliferative activity of xylazine in vitro. The inhibited proliferation of splenocytes to Con A correlated with increased apoptosis of T cells. The apoptosis was not blocked by yohimbine or antibodies to Fas and Fas-L. N-Nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthase, enhanced proliferation of splenocytes to Con A, partly abrogated the inhibitory effect of xylazine in the proliferation assay, and, only at high concentration (1000 microM), partly suppressed apoptosis of lymphocytes. The enhancing effect of L-NAME on the Con A-induced proliferation of splenocytes correlated with decreased NO production. However, decreased NO production observed in cultures with xylazine was followed by both decreased lymphocyte proliferation and apoptosis. Cumulatively, these results suggest that the immunosuppressive properties of xylazine on splenocytes in vitro are due to increased apoptosis of lymphocytes, predominantly involve NO-independent pathways, and are probably independent of its action through alpha(2) adrenoreceptors.