Inhibition of K+ transport through Na+, K+-ATPase by capsazepine: role of membrane span 10 of the α-subunit in the modulation of ion gating

Quercetin-3-O-β-D-glucopyranoside-rich fraction demonstrated efficacy against infectious, secretory, and osmotic models of diarrhoeal rats

BACKGROUND

The prevalence of diarrhoea remains high despite efforts by governments and NGOs to reverse trend. This study investigated the antidiarrhoeal activity and mechanism of Spondias mombin leaf fraction rich in quercetin-3-O-β-D-glucopyranoside (Q3G-RF) because of the acclaimed therapeutic efficacy. Secretory, osmotic, and infectious diarrhoea models using castor oil, magnesium sulphate, and Shigella flexneri respectively were evaluated at the doses of 100, 200, and 400 mg/kg in Wistar rats. Enteropathy was induced with castor oil and magnesium sulphate, while gastrointestinal motility was determined with charcoal meal.

RESULTS

Findings showed no mortality after 14 days of experimental period and no significant changes in behaviour, food, and water consumption. Relative to control, Q3G-RF inhibited the three models of diarrhoea, enteropathy, and gastrointestinal motility; bacterial colonies were reduced by Q3G-RF, while it improved the relative body weight of the animals. Q3G-RF also increased the intestinal concentration/activity of glucose, total protein, and Na⁺-K⁺ ATPase but reduced the concentration of TNF-α, PGE₂, IL-1β, nitric oxide, Na⁺, K⁺, and Cl^- in the diarrhoeal models. The intestinal fluid level of K⁺, Na⁺, and Cl^- was significantly decreased by Q3G-RF in the enteropathy model. Length of the small intestine in the motility model was also increased by Q3G-RF, while peristaltic index and inhibition of peristalsis were reduced.

CONCLUSION

Overall, quercetin-3-O-β-D-glucopyranoside from Spondias mombin leaves demonstrated efficacy against infectious, secretory, and osmotic form of diarrhoeal and further justified its traditional use in the treatment of diarrhoea due to its antimotility, antisecretory, and antimicrobial properties by mechanism related to enhanced Na⁺-K⁺ ATPase, repressed nitric oxide, and suppressed prostaglandins.

Transient receptor potential vanilloid 1 (TRPV1)-independent actions of capsaicin on cellular excitability and ion transport

Capsaicin is a naturally occurring alkaloid derived from chili pepper that is responsible for its hot pungent taste. Capsaicin is known to exert multiple pharmacological actions, including analgesia, anticancer, anti-inflammatory, antiobesity, and antioxidant effects. The transient receptor potential vanilloid subfamily member 1 (TRPV1) is the main receptor mediating the majority of the capsaicin effects. However, numerous studies suggest that the TRPV1 receptor is not the only target for capsaicin. An increasing number of studies indicates that capsaicin, at low to mid µM ranges, not only indirectly through TRPV1-mediated Ca²⁺ increases, but also directly modulates the functions of voltage-gated Na⁺ , K⁺ , and Ca²⁺ channels, as well as ligand-gated ion channels and other ion transporters and enzymes involved in cellular excitability. These TRPV1-independent effects are mediated by alterations of the biophysical properties of the lipid membrane and subsequent modulation of the functional properties of ion channels and by direct binding of capsaicin to the channels. The present study, for the first time, systematically categorizes this diverse range of non-TRPV1 targets and discusses cellular and molecular mechanisms mediating TRPV1-independent effects of capsaicin in excitable, as well as nonexcitable cells.

Antiarrhythmic calcium channel blocker verapamil inhibits trek currents in sympathetic neurons

Background and Purpose: Verapamil, a drug widely used in certain cardiac pathologies, exert its therapeutic effect mainly through the blockade of cardiac L-type calcium channels. However, we also know that both voltage-dependent and certain potassium channels are blocked by verapamil. Because sympathetic neurons of the superior cervical ganglion (SCG) are known to express a good variety of potassium currents, and to finely tune cardiac activity, we speculated that the effect of verapamil on these SCG potassium channels could explain part of the therapeutic action of this drug. To address this question, we decided to study, the effects of verapamil on three different potassium currents observed in SCG neurons: delayed rectifier, A-type and TREK (a subfamily of K2P channels) currents. We also investigated the effect of verapamil on the electrical behavior of sympathetic SCG neurons.

Experimental Approach: We employed the Patch-Clamp technique to mouse SCG neurons in culture.

Key Results: We found that verapamil depolarizes of the resting membrane potential of SCG neurons. Moreover, we demonstrated that this drug also inhibits A-type potassium currents. Finally, and most importantly, we revealed that the current driven through TREK channels is also inhibited in the presence of verapamil.

Conclusion and Implications: We have shown that verapamil causes a clear alteration of excitability in sympathetic nerve cells. This fact undoubtedly leads to an alteration of the sympathetic-parasympathetic balance which may affect cardiac function. Therefore, we propose that these possible peripheral alterations in the autonomic system should be taken into consideration in the prescription of this drug.

Role of TRPV1 channels on glycogen synthase kinase-3β and oxidative stress in ouabain-induced bipolar disease

Bipolar disorder (BD) is a multifactorial chronic and refractory disease characterized by manic, depressive, and mixed mood episodes. Although epidemiological, and pathophysiological studies demonstrated a strong correlation between bipolar disorder and oxidative stress, precise etiology is still missing. Recent studies suggested the possible role of transient receptor potential channels (TRP) in the BD but, current knowledge is limited. Therefore, the current study investigates the possible role of TRPV1 in the ouabain-induced model of BD. The model was created with intracerebroventricular single dose ouabain (10^-3 M) administration. Animals were treated with capsaicin, capsazepine, and lithium for seven days. Mania and depressive-like states were investigated with open-field, sucrose preference, and elevated plus maze tests. Oxidative stress was assessed by measuring total antioxidant and oxidant states, spectrophotometrically. The phosphorylation Glycogen synthase kinase-3β (GSK-3β) evaluated by western blotting. Our results demonstrated that capsaicin dose-dependently inhibited the ouabain-induced hyperlocomotion and depression. Although capsazepine exacerbated behavioral impairment, it did not show a significant effect on the antioxidant and oxidant states, and the effects of capsazepine on behaviors were abolished by combination with capsaicin. Additionally, capsaicin potently prevented the ouabain-induced decrease in GSK-3β phosphorylation. In contrast, capsazepine potentiated ouabain-induced decrease in GSK-3β phosphorylation and combination with capsaicin, suppressed the effect of capsazepine on GSK-3β phosphorylation. The effects of TRPV1 activation on oxidative stress and mania-like behaviors in the ouabain-induced BD model might be regulated by GSK-3β phosphorylation.

Capsaicin-Sensitive Sensory Nerves and the TRPV1 Ion Channel in Cardiac Physiology and Pathologies

Cardiovascular diseases, including coronary artery disease, ischemic heart diseases such as acute myocardial infarction and postischemic heart failure, heart failure of other etiologies, and cardiac arrhythmias, belong to the leading causes of death. Activation of capsaicin-sensitive sensory nerves by the transient receptor potential vanilloid 1 (TRPV1) capsaicin receptor and other receptors, as well as neuropeptide mediators released from them upon stimulation, play important physiological regulatory roles. Capsaicin-sensitive sensory nerves also contribute to the development and progression of some cardiac diseases, as well as to mechanisms of endogenous stress adaptation leading to cardioprotection. In this review, we summarize the role of capsaicin-sensitive afferents and the TRPV1 ion channel in physiological and pathophysiological functions of the heart based mainly on experimental results and show their diagnostic or therapeutic potentials. Although the actions of several other channels or receptors expressed on cardiac sensory afferents and the effects of TRPV1 channel activation on different non-neural cell types in the heart are not precisely known, most data suggest that stimulation of the TRPV1-expressing sensory nerves or stimulation/overexpression of TRPV1 channels have beneficial effects in cardiac diseases.

Distinct effects of Q925 mutation on intracellular and extracellular Na+ and K+ binding to the Na+, K+-ATPase

Three Na⁺ sites are defined in the Na⁺-bound crystal structure of Na⁺, K⁺-ATPase. Sites I and II overlap with two K⁺ sites in the K⁺-bound structure, whereas site III is unique and Na⁺ specific. A glutamine in transmembrane helix M8 (Q925) appears from the crystal structures to coordinate Na⁺ at site III, but does not contribute to K⁺ coordination at sites I and II. Here we address the functional role of Q925 in the various conformational states of Na⁺, K⁺-ATPase by examining the mutants Q925A/G/E/N/L/I/Y. We characterized these mutants both enzymatically and electrophysiologically, thereby revealing their Na⁺ and K⁺ binding properties. Remarkably, Q925 substitutions had minor effects on Na⁺ binding from the intracellular side of the membrane - in fact, mutations Q925A and Q925G increased the apparent Na⁺ affinity - but caused dramatic reductions of the binding of K⁺ as well as Na⁺ from the extracellular side of the membrane. These results provide insight into the changes taking place in the Na⁺-binding sites, when they are transformed from intracellular- to extracellular-facing orientation in relation to the ion translocation process, and demonstrate the interaction between sites III and I and a possible gating function of Q925 in the release of Na⁺ at the extracellular side.

Arginine substitution of a cysteine in transmembrane helix M8 converts Na+,K+-ATPase to an electroneutral pump similar to H+,K+-ATPase

Na⁺,K⁺-ATPase and H⁺,K⁺-ATPase are electrogenic and nonelectrogenic ion pumps, respectively. The underlying structural basis for this difference has not been established, and it has not been revealed how the H⁺,K⁺-ATPase avoids binding of Na⁺ at the site corresponding to the Na⁺-specific site of the Na⁺,K⁺-ATPase (site III). In this study, we addressed these questions by using site-directed mutagenesis in combination with enzymatic, transport, and electrophysiological functional measurements. Replacement of the cysteine C932 in transmembrane helix M8 of Na⁺,K⁺-ATPase with arginine, present in the H⁺,K⁺-ATPase at the corresponding position, converted the normal 3Na⁺:2K⁺:1ATP stoichiometry of the Na⁺,K⁺-ATPase to electroneutral 2Na⁺:2K⁺:1ATP stoichiometry similar to the electroneutral transport mode of the H⁺,K⁺-ATPase. The electroneutral C932R mutant of the Na⁺,K⁺-ATPase retained a wild-type-like enzyme turnover rate for ATP hydrolysis and rate of cellular K⁺ uptake. Only a relatively minor reduction of apparent Na⁺ affinity for activation of phosphorylation from ATP was observed for C932R, whereas replacement of C932 with leucine or phenylalanine, the latter of a size comparable to arginine, led to spectacular reductions of apparent Na⁺ affinity without changing the electrogenicity. From these results, in combination with structural considerations, it appears that the guanidine⁺ group of the M8 arginine replaces Na⁺ at the third site, thus preventing Na⁺ binding there, although allowing Na⁺ to bind at the two other sites and become transported. Hence, in the H⁺,K⁺-ATPase, the ability of the M8 arginine to donate an internal cation binding at the third site is decisive for the electroneutral transport mode of this pump.

Ion Pathways in the Na+/K+-ATPase

Na⁺/K⁺-ATPase (NKA) is an essential cation pump protein responsible for the maintenance of the sodium and potassium gradients across the plasma membrane. Recently published high-resolution structures revealed amino acids forming the cation binding sites (CBS) in the transmembrane domain and variable position of the domains in the cytoplasmic headpiece. Here we report molecular dynamic simulations of the human NKA α1β1 isoform embedded into DOPC bilayer. We have analyzed the NKA conformational changes in the presence of Na⁺- or K⁺-cations in the CBS, for various combinations of the cytoplasmic ligands, and the two major enzyme conformations in the 100 ns runs (more than 2.5 μs of simulations in total). We identified two novel cytoplasmic pathways along the pairs of transmembrane helices TM3/TM7 or TM6/TM9 that allow hydration of the CBS or transport of cations from/to the bulk. These findings can provide a structural explanation for previous mutagenesis studies, where mutation of residues that are distal from the CBS resulted in the alteration of the enzyme affinity to the transported cations or change in the enzyme activity.

K+ congeners that do not compromise Na+ activation of the Na+,K+-ATPase: hydration of the ion binding cavity likely controls ion selectivity

The Na(+),K(+)-ATPase is essential for ionic homeostasis in animal cells. The dephosphoenzyme contains Na(+) selective inward facing sites, whereas the phosphoenzyme contains K(+) selective outward facing sites. Under normal physiological conditions, K(+) inhibits cytoplasmic Na(+) activation of the enzyme. Acetamidinium (Acet(+)) and formamidinium (Form(+)) have been shown to permeate the pump through the outward facing sites. Here, we show that these cations, unlike K(+), are unable to enter the inward facing sites in the dephosphorylated enzyme. Consistently, the organic cations exhibited little to no antagonism to cytoplasmic Na(+) activation. Na(+),K(+)-ATPase structures revealed a previously undescribed rotamer transition of the hydroxymethyl side chain of the absolutely conserved Thr(772) of the α-subunit. The side chain contributes its hydroxyl to Na(+) in site I in the E1 form and rotates to contribute its methyl group toward K(+) in the E2 form. Molecular dynamics simulations to the E1·AlF4 (-)·ADP·3Na(+) structure indicated that 1) bound organic cations differentially distorted the ion binding sites, 2) the hydroxymethyl of Thr(772) rotates to stabilize bound Form(+) through water molecules, and 3) the rotamer transition is mediated by water traffic into the ion binding cavity. Accordingly, dehydration induced by osmotic stress enhanced the interaction of the congeners with the outward facing sites and profoundly modified the organization of membrane domains of the α-subunit. These results assign a catalytic role for water in pump function, and shed light on a backbone-independent but a conformation-dependent switch between H-bond and dispersion contact as part of the catalytic mechanism of the Na(+),K(+)-ATPase.