Differential effects of estrogen and progesterone on potassium channels expressed in Xenopus oocytes

Cycling matters: Sex hormone regulation of vascular potassium channels

Sex hormones and the reproductive cycle (estrus in rodents and menstrual in humans) have a known impact on arterial function. In spite of this, sex hormones and the estrus/menstrual cycle are often neglected experimental factors in vascular basic preclinical scientific research. Recent research by our own laboratory indicates that cyclical changes in serum concentrations of sex -hormones across the rat estrus cycle, primary estradiol, have significant consequences for the subcellular trafficking and function of KV. Vascular potassium channels, including KV, are essential components of vascular reactivity. Our study represents a small part of a growing field of literature aimed at determining the role of sex hormones in regulating arterial ion channel function. This review covers key findings describing the current understanding of sex hormone regulation of vascular potassium channels, with a focus on KV channels. Further, we highlight areas of research where the estrus cycle should be considered in future studies to determine the consequences of physiological oscillations in concentrations of sex hormones on vascular potassium channel function.

Estrogenic Modulation of Ionic Channels, Pumps and Exchangers in Airway Smooth Muscle

To preserve ionic homeostasis (primarily Ca²⁺, K⁺, Na⁺, and Cl^-), in the airway smooth muscle (ASM) numerous transporters (channels, exchangers, and pumps) regulate the influx and efflux of these ions. Many of intracellular processes depend on continuous ionic permeation, including exocytosis, contraction, metabolism, transcription, fecundation, proliferation, and apoptosis. These mechanisms are precisely regulated, for instance, through hormonal activity. The lipophilic nature of steroidal hormones allows their free transit into the cell where, in most cases, they occupy their cognate receptor to generate genomic actions. In the sense, estrogens can stimulate development, proliferation, migration, and survival of target cells, including in lung physiology. Non-genomic actions on the other hand do not imply estrogen's intracellular receptor occupation, nor do they initiate transcription and are mostly immediate to the stimulus. Among estrogen's non genomic responses regulation of calcium homeostasis and contraction and relaxation processes play paramount roles in ASM. On the other hand, disruption of calcium homeostasis has been closely associated with some ASM pathological mechanism. Thus, this paper intends to summarize the effects of estrogen on ionic handling proteins in ASM. The considerable diversity, range and power of estrogens regulates ionic homeostasis through genomic and non-genomic mechanisms.

The modulation of potassium channels by estrogens facilitates neuroprotection

Estrogens, the sex hormones, have the potential to govern multiple cellular functions, such as proliferation, apoptosis, differentiation, and homeostasis, and to exert numerous beneficial influences for the cardiovascular system, nervous system, and bones in genomic and/or non-genomic ways. Converging evidence indicates that estrogens serve a crucial role in counteracting neurodegeneration and ischemic injury; they are thereby being considered as a potent neuroprotectant for preventing neurological diseases such as Alzheimer's disease and stroke. The underlying mechanism of neuroprotective effects conferred by estrogens is thought to be complex and multifactorial, and it remains obscure. It is well established that the K+ channels broadly expressed in a variety of neural subtypes determine the essential physiological features of neuronal excitability, and dysfunction of these channels is closely associated with diverse brain deficits, such as ataxia and epilepsy. A growing body of evidence supports a neuroprotective role of K+ channels in malfunctions of nervous tissues, with the channels even being a therapeutic target in clinical trials. As multitarget steroid hormones, estrogens also regulate the activity of distinct K+ channels to generate varying biological actions, and accumulated data delineate that some aspects of estrogen-mediated neuroprotection may arise from the impact on multiple K+ channels, including Kv, BK, KATP, and K2P channels. The response of these K+ channels after acute or chronic exposure to estrogens may oppose pathological abnormality in nervous cells, which serves to extend our understanding of these phenomena.

PGE2 receptors in detrusor muscle: Drugging the undruggable for urgency

Overactive bladder (OAB) syndrome is a prevalent condition of the lower urinary tract that causes symptoms, such as urinary frequency, urinary urgency, urge incontinence, and nocturia, and disproportionately affects women and the elderly. Current medications for OAB merely provide symptomatic relief with considerable limitations, as they are no more than moderately effective, not to mention that they may cause substantial adverse effects. Identifying novel molecular targets to facilitate the development of new medical therapies with higher efficacy and safety for OAB is in an urgent unmet need. Although the molecular mechanisms underlying the pathophysiology of OAB largely remain elusive and are likely multifactorial, mounting evidence from preclinical studies over the past decade reveals that the pro-inflammatory pathways engaging cyclooxygenases and their prostanoid products, particularly the prostaglandin E2 (PGE₂), may play essential roles in the progression of OAB. The goals of this review are to summarize recent progresses in our knowledge on the pathogenic roles of PGE₂ in the OAB and to provide new mechanistic insights into the signaling pathways transduced by its four G-protein-coupled receptors (GPCRs), i.e., EP1-EP4, in the overactive detrusor smooth muscle. We also discuss the feasibility of targeting these GPCRs as an emerging strategy to treat OAB with better therapeutic specificity than the current medications.

Renal and colonic potassium transporters in the pregnant rat

Gestational potassium retention, most of which occurs during late pregnancy, is essential for fetal development. The purpose of this study was to examine mechanisms underlying changes in potassium handling by the kidney and colon in pregnancy. We found that potassium intake and renal excretion increased in late pregnancy while fecal potassium excretion remained unchanged and that pregnant rats exhibited net potassium retention. By quantitative PCR we found markedly increased H+-K+-ATPase type 2 (HKA2) mRNA expression in the cortex and outer medullary of late pregnant vs. virgin. Renal outer medullary potassium channel (ROMK) mRNA was unchanged in the cortex, but apical ROMK abundance (by immunofluorescence) was decreased in pregnant vs. virgin in the distal convoluted tubule (DCT) and connecting tubule (CNT). Big potassium-α (BKα) channel-α protein abundance in intercalated cells in the cortex and outer medullary collecting ducts (by immunohistochemistry) fell in late pregnancy. In the distal colon we found increased HKA2 mRNA and protein abundance (Western blot) and decreased BKα protein with no observed changes in mRNA. Therefore, the potassium retention of pregnancy is likely to be due to increased collecting duct potassium reabsorption (via increased HKA2), decreased potassium secretion (via decreased ROMK and BK), as well as increased colonic reabsorption via HKA2.

Exploring the neural mechanisms of finasteride: a proteomic analysis in the nucleus accumbens

The enzyme 5α-reductase (5αR) catalyzes the conversion of progesterone and testosterone into neuroactive steroids implicated in a wide array of behavioral functions. The prototypical 5αR inhibitor, finasteride (FIN), is clinically approved for the treatment of androgenic alopecia and benign prostatic hyperplasia. Recent evidence has shown that FIN, albeit generally well tolerated, can induce untoward psychological effects in a subset of patients; furthermore, this drug may have therapeutic efficacy for a number of different neuropsychiatric conditions, ranging from Tourette syndrome to schizophrenia. In rat models of these conditions, FIN has been shown to block the effects of dopamine receptors in the nucleus accumbens (NAcc), a key terminal of the dopamine mesolimbic system. The biological underpinnings of these effects, however, remain mostly elusive. To elucidate the neurochemical networks that may be responsible for the behavioral effects of FIN, we evaluated the proteomic profile of the NAcc following acute (100mg/kg, IP) and subchronic (7 days; 100mg/kg/day, IP) treatment with this drug, in comparison with vehicle treatment (n=5/group). Two-dimensional electrophoresis (2-DE) analysis coupled to mass spectrometry revealed significant changes in the expression of nine proteins (CRMP2, PSMD1, STX18, KCNC3, CYP255, GABRP, GABT, PRPS1, CYP2B3), which were further analyzed by ontological classification (PANTHER). These results point to a number of novel potential chemical targets of FIN, and may help elucidate the underpinnings of FIN's behavioral effects and therapeutic potential for neuropsychiatric disorders.

Rapid estrogen actions on ion channels: A survey in search for mechanisms

A survey of nearly two hundred reports shows that rapid estrogenic actions can be detected across a range of kinds of estrogens, a range of doses, on a wide range of tissue, cell and ion channel types. Striking is the fact that preparations of estrogenic agents that do not permeate the cell membrane almost always mimic the actions of the estrogenic agents that do permeate the membrane. All kinds of estrogens, ranging from natural ones, through receptor modulators, endocrine disruptors, phytoestrogens, agonists, and antagonists to novel G-1 and STX, have been reported to be effective. For actions on specific types of ion channels, the possibility of opposing actions, in different cases, is the rule, not the exception. With this variety there is no single, specific action mechanism for estrogens per se, although in some cases estrogens can act directly or via some signaling pathways to affect ion channels. We infer that estrogens can bind a large number of substrates/receptors at the membrane surface. As against the variety of subsequent routes of action, this initial step of the estrogen's binding action is the key.

Oxidative Stress and Maxi Calcium-Activated Potassium (BK) Channels

All cells contain ion channels in their outer (plasma) and inner (organelle) membranes. Ion channels, similar to other proteins, are targets of oxidative impact, which modulates ion fluxes across membranes. Subsequently, these ion currents affect electrical excitability, such as action potential discharge (in neurons, muscle, and receptor cells), alteration of the membrane resting potential, synaptic transmission, hormone secretion, muscle contraction or coordination of the cell cycle. In this chapter we summarize effects of oxidative stress and redox mechanisms on some ion channels, in particular on maxi calcium-activated potassium (BK) channels which play an outstanding role in a plethora of physiological and pathophysiological functions in almost all cells and tissues. We first elaborate on some general features of ion channel structure and function and then summarize effects of oxidative alterations of ion channels and their functional consequences.

Pharmacological characterization of the mechanisms involved in the vasorelaxation induced by progesterone and 17β-estradiol on isolated canine basilar and internal carotid arteries

Progesterone and 17β-estradiol induce vasorelaxation through non-genomic mechanisms in several isolated blood vessels; however, no study has systematically evaluated the mechanisms involved in the relaxation induced by 17β-estradiol and progesterone in the canine basilar and internal carotid arteries that play a key role in cerebral circulation. Thus, relaxant effects of progesterone and 17β-estradiol on KCl- and/or PGF2α-pre-contracted arterial rings were investigated in absence or presence of several antagonists/inhibitors/blockers; the effect on the contractile responses to CaCl2 was also determined. In both arteries progesterone (5.6-180 μM) and 17β-estradiol (1.8-180 μM): (1) produced concentration-dependent relaxations of KCl- or PGF2α-pre-contracted arterial rings; (2) the relaxations were unaffected by actinomycin D (10 μM), cycloheximide (10 μM), SQ 22,536 (100 μM) or ODQ (30 μM), potassium channel blockers and ICI 182,780 (only for 17β-estradiol). In the basilar artery the vasorelaxation induced by 17β-estradiol was slightly blocked by tetraethylammonium (10mM) and glibenclamide (KATP; 10 μM). In both arteries, progesterone (10-100 μM), 17β-estradiol (3.1-31 μM) and nifedipine (0.01-1 μM) produced a concentration-dependent blockade of the contraction to CaCl2 (10 μM-10mM). These results suggest that progesterone and 17β-estradiol produced relaxation in the basilar and internal carotid arteries by blockade of L-type voltage dependent Ca(2+) channel but not by genomic mechanisms or production of cAMP/cGMP. Potassium channels did not play a role in the relaxation to progesterone in both arteries or in the effect of 17β-estradiol in the internal carotid artery; meanwhile KATP channels play a minor role on the effect of 17β-estradiol in the basilar artery.

Functional insights into modulation of BKCa channel activity to alter myometrial contractility

The large-conductance voltage- and Ca(2+)-activated K(+) channel (BKCa) is an important regulator of membrane excitability in a wide variety of cells and tissues. In myometrial smooth muscle, activation of BKCa plays essential roles in buffering contractility to maintain uterine quiescence during pregnancy and in the transition to a more contractile state at the onset of labor. Multiple mechanisms of modulation have been described to alter BKCa channel activity, expression, and cellular localization. In the myometrium, BKCa is regulated by alternative splicing, protein targeting to the plasma membrane, compartmentation in membrane microdomains, and posttranslational modifications. In addition, interaction with auxiliary proteins (i.e., β1- and β2-subunits), association with G-protein coupled receptor signaling pathways, such as those activated by adrenergic and oxytocin receptors, and hormonal regulation provide further mechanisms of variable modulation of BKCa channel function in myometrial smooth muscle. Here, we provide an overview of these mechanisms of BKCa channel modulation and provide a context for them in relation to myometrial function.

Effects of estradiol on voltage-gated potassium channels in mouse dorsal root ganglion neurons

Voltage-gated potassium channels are regulators of membrane potentials, action potential shape, firing adaptation, and neuronal excitability in excitable tissues including in the primary sensory neurons of dorsal root ganglion (DRG). In this study, using the whole-cell patch-clamp technique, the effect of estradiol (E2) on voltage-gated total outward potassium currents, the component currents transient "A-type" current (I A) currents, and "delayed rectifier type" (I KDR) currents in isolated mouse DRG neurons was examined. We found that the extracellularly applied 17β-E2 inhibited voltage-gated total outward potassium currents; the effects were rapid, reversible, and concentration-dependent. Moreover, the membrane impermeable E2-BSA was as efficacious as 17β-E2, whereas 17α-E2 had no effect. 17β-E2-stimulated decrease in the potassium current was unaffected by treatment with ICI 182780 (classic estrogen receptor antagonist), actinomycin D (RNA synthesis inhibitor), or cycloheximide (protein synthesis inhibitor). We also found that I A and I KDR were decreased after 17β-E2 application. 17β-E2 significantly shifted the activation curve for I A and I KDR channels in the hyperpolarizing direction. In conclusion, our results demonstrate that E2 inhibited voltage-gated K(+) channels in mouse DRG neurons through a membrane ER-activated non-genomic pathway.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

Progesterone inhibits L-type calcium currents in gallbladder smooth muscle cells

BACKGROUND AND AIM

The incidence of gallbladder stones is higher in women during pregnancy than in men. Progesterone can inhibit gallbladder motility and facilitate gallstone formation. However, the ionic mechanisms have not been fully illuminated. This study sought to investigate the effects of progesterone on L-type calcium currents and voltage-dependent potassium currents in gallbladder smooth muscle cells.

METHODS

Gallbladder smooth muscle cells were isolated by enzymatic digestion from adult guinea pigs. Ionic currents were recorded by the whole-cell patch clamp method.

RESULTS

Progesterone inhibited L-type calcium currents in a concentration-dependent manner. The characteristic of current-voltage curve was not significantly altered. The amplitude of calcium currents was gradually suppressed, reached a steady-state level within 4-6 min, and restored partly after washout. In the presence of protein kinase A (PKA) inhibitor, Rp-cAMP, the inhibitory effect induced by progesterone was apparently attenuated, whereas forskolin, a direct activator of adenylate cyclase, could suppress L-type calcium channel. However, progesterone did not significantly affect voltage-dependent potassium currents.

CONCLUSIONS

Progesterone inhibits L-type calcium channel by cAMP/PKA pathway in gallbladder smooth muscle cells. This may be an important mechanism for the gallbladder hypomotility induced by progesterone.

Hormonal regulation of cardiac KCNE2 gene expression

The KCNE2 gene encodes a single transmembrane domain protein that modulates a variety of K+ channel functions in various tissues. Here we show that cardiac KCNE2 transcript levels are approximately 10-fold upregulated at the end of pregnancy. This upregulation was mimicked by 17-beta estradiol but not by 5alpha-dihydrotestosterone treatments in ovariectomized mice. To investigate the mechanism of KCNE2 transcriptional regulation by estrogen, we experimentally identified KCNE2 transcription start sites, delineated its gene structure and characterized its promoter region. Estrogen treatment stimulated KCNE2 promoter activity in a dose-dependent manner and ICI 182,780 blocked estrogen stimulation. A direct genomic mechanism was demonstrated by (i) the loss of estrogen responsiveness in the presence of a DNA-binding domain mutant estrogen receptor alpha or mutant KCNE2 ERE and (ii) binding of ERalpha to the KCNE2 ERE. These findings show that a genomic mechanism of estrogen action alters KCNE2 expression, which may have important physiological implications.