Anorectic drugs and vascular disease: the role of voltage-gated K+ channels

Inhibition of voltage-dependent K+ channels in rabbit coronary arterial smooth muscle cells by the atypical antipsychotic agent sertindole

The regulation of membrane potential and the contractility of vascular smooth muscle cells (VSMCs) by voltage-dependent K+ (Kv) potassium channels are well-established. In this study, native VSMCs from rabbit coronary arteries were used to investigate the inhibitory effect of sertindole, an atypical antipsychotic agent, on Kv channels. Sertindole induced dose-dependent inhibition of Kv channels, with an IC50 of 3.13 ± 0.72 μM. Although sertindole did not cause a change in the steady-state activation curve, it did lead to a negative shift in the steady-state inactivation curve. The application of 1- or 2-Hz train pulses failed to alter the sertindole-induced inhibition of Kv channels, suggesting use-independent effects of the drug. The inhibitory response to sertindole was significantly diminished by pretreatment with a Kv1.5 inhibitor but not by Kv2.1 and Kv7 subtype inhibitors. These findings demonstrate the sertindole dose-dependent and use-independent inhibition of vascular Kv channels (mainly the Kv1.5 subtype) through a mechanism that involves altering steady-state inactivation curves. Therefore, the use of sertindole as an antipsychotic drug may have adverse effects on the cardiovascular system.

Serotonin contracts the rat mesenteric artery by inhibiting 4-aminopyridine-sensitive Kv channels via the 5-HT2A receptor and Src tyrosine kinase

Serotonin (5-hydroxytryptamine (5-HT)) is a neurotransmitter that regulates a variety of functions in the nervous, gastrointestinal and cardiovascular systems. Despite such importance, 5-HT signaling pathways are not entirely clear. We demonstrated previously that 4-aminopyridine (4-AP)-sensitive voltage-gated K⁺ (Kv) channels determine the resting membrane potential of arterial smooth muscle cells and that the Kv channels are inhibited by 5-HT, which depolarizes the membranes. Therefore, we hypothesized that 5-HT contracts arteries by inhibiting Kv channels. Here we studied 5-HT signaling and the detailed role of Kv currents in rat mesenteric arteries using patch-clamp and isometric tension measurements. Our data showed that inhibiting 4-AP-sensitive Kv channels contracted arterial rings, whereas inhibiting Ca²⁺-activated K⁺, inward rectifier K⁺ and ATP-sensitive K⁺ channels had little effect on arterial contraction, indicating a central role of Kv channels in the regulation of resting arterial tone. 5-HT-induced arterial contraction decreased significantly in the presence of high KCl or the voltage-gated Ca²⁺ channel (VGCC) inhibitor nifedipine, indicating that membrane depolarization and the consequent activation of VGCCs mediate the 5-HT-induced vasoconstriction. The effects of 5-HT on Kv currents and arterial contraction were markedly prevented by the 5-HT_2A receptor antagonists ketanserin and spiperone. Consistently, α-methyl 5-HT, a 5-HT₂ receptor agonist, mimicked the 5-HT action on Kv channels. Pretreatment with a Src tyrosine kinase inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, prevented both the 5-HT-mediated vasoconstriction and Kv current inhibition. Our data suggest that 4-AP-sensitive Kv channels are the primary regulator of the resting tone in rat mesenteric arteries. 5-HT constricts the arteries by inhibiting Kv channels via the 5-HT_2A receptor and Src tyrosine kinase pathway.

Ion channel pharmacology

Because ion channels are involved in many cellular processes, drugs acting on ion channels have long been used for the treatment of many diseases, especially those affecting electrically excitable tissues. The present review discusses the pharmacology of voltage-gated and neurotransmitter-gated ion channels involved in neurologic diseases, with emphasis on neurologic channelopathies. With the discovery of ion channelopathies, the therapeutic value of many basic drugs targeting ion channels has been confirmed. The understanding of the genotype-phenotype relationship has highlighted possible action mechanisms of other empirically used drugs. Moreover, other ion channels have been pinpointed as potential new drug targets. With regards to therapy of channelopathies, experimental investigations of the intimate drug-channel interactions have demonstrated that channel mutations can either increase or decrease affinity for the drug, modifying its potential therapeutic effect. Together with the discovery of channel gene polymorphisms that may affect drug pharmacodynamics, these findings highlight the need for pharmacogenetic research to allow identification of drugs with more specific effects on channel isoforms or mutants, to increase efficacy and reduce side effects. With a greater understanding of channel genetics, structure, and function, together with the identification of novel primary and secondary channelopathies, the number of ion channel drugs for neurologic channelopathies will increase substantially.

Ketamine blocks voltage-gated K(+) channels and causes membrane depolarization in rat mesenteric artery myocytes

Clinical doses of ketamine typically increase blood pressure, heart rate, and cardiac output. However, the precise mechanism by which ketamine produces these cardiovascular effects remains unclear. The voltage-gated K(+) (K(V)) channel is the major regulator of resting membrane potential (E (m)) and vascular tone in many arteries. Therefore, we sought to evaluate the effects of ketamine on K(V) currents using the standard whole-cell patch clamp recordings in single myocytes, enzymatically dispersed from rat mesenteric arteries. Ketamine [(+/-)-racemic mixture] inhibited K(V) currents reversibly and concentration dependently with a K ( d ) of 566.7 +/- 32.3 microM and Hill coefficient of 0.75 +/- 0.03. The inhibition of K(V) currents by ketamine was voltage independent, and the time courses of channel activation and inactivation were little affected. The effects of ketamine on steady-state activation and inactivation curves were also minimal. Use-dependent inhibition was not observed either. S(+)-ketamine inhibited K(V) currents with similar potency and efficacy as the racemic mixture. The average resting E (m) in rat mesenteric artery myocytes was -44.1 +/- 4.2 mV, and both racemic and S(+)-ketamine induced depolarization of E (m) (15.8 +/- 3.6 and 24.3 +/- 5.0 mV at 100 microM, respectively). We conclude that ketamine induces E (m) depolarization in vascular myocytes by blocking K(V) channels in a state-independent manner, which may contribute to the increased vascular tone and blood pressure produced by this drug under a clinical setting.

Genetics and mediators in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an uncommon disorder of the pulmonary vasculature characterized by remodeling of the smallest pulmonary arteries, leading to a progressive increase in pulmonary vascular resistance. Various forms of PAH exist, including familial (FPAH) and idiopathic (IPAH) forms and associated conditions. FPAH transmits as an autosomal dominant trait that exhibits genetic anticipation but also markedly reduced penetrance (20%). The primary genetic defect of FPAH, identifiable in more than 70% of cases of FPAH, is a mutation in the gene encoding bone morphogenetic protein receptor type 2 (BMPR2), a member of the transforming growth factor beta superfamily. The true prevalence of BMPR2 mutations in IPAH is unknown, with reports ranging from 10% to 40% of patients. The cause of the variable phenotypic expression of PAH among carriers of mutated BMPR2 genes and patients is unclear, and likely related to environmental and genetic modifiers of disease not yet fully elucidated. Although BMPR2-related pathways seem to be pivotal, many other mediator pathways participate in the pathogenesis of different forms of PAH and are being actively investigated, both independently and in combination. As understanding of the molecular basis of this devastating disease improves, opportunities for earlier diagnosis, additional therapeutic regimens, and perhaps disease prevention will emerge.

Serotonin-induced ion channel modulations in mesenteric artery myocytes from normotensive and DOCA-salt hypertensive rats

Although serotonin (5-hydroxytryptamine, 5-HT) has been found to be a potent vasoconstrictor, a pivotal role of 5-HT in the control of appetite and mood control by the modulation of neuronal synapse has also been proposed. Selective 5-HT reuptake inhibitors (SSRIs) are frequently used to suppress appetite and treat depressive disorder, and the target protein of SSRIs is the 5-HT transporter (5-HTT) in the neuronal synapse. However, SSRIs may increase the free 5-HT concentration in circulating blood because platelets and vascular smooth muscles express functional 5-HTT. In addition, enhanced vasoactive action of 5-HT and alterations in 5-HT receptor subtypes have been reported in some types of hypertension. Therefore, we can infer that the use of drugs such as SSRIs in some hypertensive patients is potentially risky. Altered functional expression of ion channels in vascular smooth muscle is suggested to be a mechanism for the enhanced vasoconstriction by vasoactive agonists, including 5-HT. In this brief review, we compared the electrophysiological properties of mesenteric artery myocytes and their modulation by 5-HT between sham-operated control and deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

Gene therapy for obesity

A sedentary life-style and an environment of caloric abundance have contributed to the recent rise in the incidence of obesity. Treating such a complex disease requires an understanding of its underlying molecular mechanisms that has only recently become possible with the sequencing of the human genome and the mapping of hundreds of genes associated with increased risk of obesity. With few safe and efficacious weight-maintenance drugs available on the market, gene therapy offers an alternative long-term treatment modality. Still in its infancy, gene therapy for obesity is poised for significant progress, due in large part to a wide variety of available gene targets and the development of novel systems to control gene expression. Coincidently, novel vectors are being developed based on adeno-associated virus providing efficient and sustained expression of a transgene in metabolically important tissues. These advances are driving the development of gene therapy as a viable therapeutic option in treating obesity and its associated disorders.

Effects of dexfenfluramine on serotonin levels of mice ileum, contractility, glutathione and malondialdehyde level

Dexfenfluramine is one of the anorectic drugs that suppresses food intake which acts via inhibition of reuptake of serotonin into brain terminal. Gastrointestinal tract is the main source of peripheral serotonin which is involved in the regulation of gastrointestinal motility. During the use of anorectic drugs, the antioxidant defence is affected especially by reactive oxygen species. The purpose of this study to search: The effect of dexfenfluramine on serotonin levels of ileum and the effect of dexfenfluramine on ileal contractility and oxidative stress.

MATERIALS AND METHODS

Twenty-two adult male Swiss-albino mice were divided two groups (1) Control, (2) Dexfenfluramine treated (i.p. twice a day 0.2 mg kg(-1) in 0.2 ml saline solution for 7 days). Animal body weights were recorded at the beginning and at the end of the experimental period. Ileum tissues contractile responses to different concentrations of KCl and acethycholine were recorded on polygraph. In the meantime ileal tissue malondialdehyde, a product of lipid peroxidation, and glutathione, endogenous antioxidant levels were assessed by spectrophotometric methods. Ileal tissue serotonin level determined by immunohistochemical method. Body weights decrease and ileal contractile response of acethycholine increased significantly by dexfenfluramine treatment. Meanwhile, ileum glutathione levels decreased and malondialdehyde levels increased in dexfenfluramine treated group. Immunohistochemical detection showed that ileal serotonin levels increased by dexfenfluramine treatments. As a conclusion, there is a relationship between increased ileal contractility and oxidant status in dexfenfluramine treated animals. These effects can be related by increased serotonin levels which is induced by dexfenfluramine in ileum.

Phobic memory and somatic vulnerabilities in anorexia nervosa: a necessary unity?

Anorexia nervosa is a clinically significant illness that may be associated with permanent medical complications involving almost every organ system. The paper raises a question whether some of them are associated with premorbid vulnerability such as subcellular ion channel abnormalities ('channelopathy') that determines the clinical expression of the bodily response to self-imposed malnutrition. Aberrant channels emerge as a tempting, if rather speculative alternative to the notion of cognitively-driven neurotransmitter modulation deficit in anorexia nervosa. The concept of channelopathies is in keeping with some characteristics of anorexia nervosa, such as a genetically-based predisposition to hypophagia, early onset, cardiac abnormalities, an appetite-enhancing efficacy of some antiepileptic drugs, and others. The purpose of this article is to stimulate further basic research of ion channel biophysics in relation to restrictive anorexia.

Myogenic regulation of arterial diameter: role of potassium channels with a focus on delayed rectifier potassium current

The phenomenon of myogenic constriction of arterial resistance vessels in response to increased intraluminal pressure has been known for over 100 years, yet our understanding of the molecular mechanisms involved remains incomplete. The focus of this paper concerns the potassium (K+) channels that provide a negative feedback control of the myogenic depolarization of vascular smooth muscle cells that is provoked by elevations in intraluminal pressure, and specifically, the contribution of delayed rectifier (KDR) channels. Our knowledge of the important role played by KDR channels, as well as their molecular identity and acute modulation via changes in gating, has increased dramatically in recent years. Several lines of evidence point to a crucial contribution by heteromultimeric KV1 subunit-containing KDR channels in the control of arterial diameter and myogenic reactivity, but other members of the KV superfamily are also expressed by vascular myocytes, and less is known concerning their specific functions. The effect of pharmacological modulation of KDR channels is discussed, with particular reference to the actions of anorexinogens on KV1- and KV2-containing KDR channels. Finally, the need for a greater understanding of the mechanisms that control KDR channel gene expression is stressed in light of evidence indicating that there is a reduced expression of KDR channels in diseases associated with abnormal myogenic reactivity and vascular remodelling.Key words: resistance arteries, myogenic response, potassium channels, delayed rectifier K+ current, KV channels, KV1, KV2.

Sustained peripheral expression of transgene adiponectin offsets the development of diet-induced obesity in rats

Adiponectin (Acrp30) is a physiologically active polypeptide hormone secreted by adipose tissue that shows insulin-sensitizing, antiinflammatory, and antiatherogenic properties. In humans, Acrp30 levels are inversely related to the degree of adiposity. In the current study, we tested the long-term weight-reducing and insulin-enhancing effects of Acrp30 cDNA delivered peripherally by a viral vector. To this end, we have generated a series of recombinant adeno-associated virus vectors of serotypes 1 and 5 encoding mouse Acrp30 cDNAs. The long-term expression of recombinant adeno-associated virus-Acrp30 vectors was tested after intramuscular or intraportal injection in female Sprague-Dawley rats with diet-induced obesity. We show that a single peripheral injection of 10(12) physical particles of Acrp30-encoding vectors resulted in sustained (up to 280 days) significant reduction in body weight, concomitant with the reduction in daily food intake. Acrp30 treatment resulted in higher peripheral insulin sensitivity measured by the i.p. glucose tolerance test in fasted animals. Ectopic expression of the Acrp30 transgene resulted in modulation of hepatic gluconeogenesis and lipogenesis, as demonstrated by the reduction of the expression of two key genes: PEPCK (phosphoenolpyruvate carboxykinase) and SREBP-1c (sterol regulatory element-binding protein 1c) in the liver. These data show successful peripheral therapy in a clinically relevant model for human obesity and insulin resistance.