The gut microbial metabolite imidazole propionate inhibits metformin action.

Imidazole propionate inhibits metformin action in a manner dependent on a p38γ-Akt-AMPK axis.

RIPK1 Promotes Energy Sensing by the mTORC1 Pathway.

The mechanisms of cellular energy sensing and AMPK-mediated mTORC1 inhibition are not fully delineated. Here, we discover that RIPK1 promotes mTORC1 inhibition during energetic stress. RIPK1 is involved in mediating the interaction between AMPK and TSC2 and facilitate TSC2 phosphorylation at Ser1387. RIPK1 loss results in a high basal mTORC1 activity that drives defective lysosomes in cells and mice, leading to accumulation of RIPK3 and CASP8 and sensitization to cell death. RIPK1-deficient cells are unable to cope with energetic stress and are vulnerable to low glucose levels and metformin. Inhibition of mTORC1 rescues the lysosomal defects and vulnerability to energetic stress and prolongs the survival of RIPK1-deficient neonatal mice. Thus, RIPK1 plays an important role in the cellular response to low energy levels and mediates AMPK-mTORC1 signaling. These findings shed light on the regulation of mTORC1 during energetic stress and unveil a point of crosstalk between pro-survival and pro-death pathways.

Actualización del efecto de los antihiperglicemiantes en la función renal en diabetes mellitus tipo 2 / Update of the effect of antihyperglycemic agents on renal function in type 2 diabetes mellitus

Resumen La diabetes mellitus tipo 2 es una de las enfermedades metabólicas que afecta a diferentes órganos, uno en el cual es el riñón. Una de las principales complicaciones microvasculares es la nefropatía diabética, siendo la principal causa de insuficiencia renal crónica a nivel mundial. De ahí la importancia de las recomendaciones en la utilización o no de los fármacos antihiperglicemiantes, basadas en sus efectos beneficiosos a nivel de la función renal en relación con la tasa de filtración glomerular estimada y la relación albumina/creatinina en pacientes con diabetes mellitus tipo 2 y enfermedad renal. En estudios recientes se han evaluado antihiperglicemiantes con un impacto beneficioso a nivel de desenlaces cardiovascular y renal. En el presente artículo se revisan las acciones y los efectos de los diferentes grupos de medicamentos como la metformina, los inhibidores de la dipeptidil peptidasa 4, los agonistas de la GLP-1, tiazolidinedionas, sulfonilureas, inhibidores del cotransportador de sodio-glucosa tipo 2 e insulinas en la función renal en cuanto a las dosis de cada fármaco, tanto el uso de dosis establecidas, disminución de la dosis o el no uso del medicamento con base en el empeoramiento de la tasa de filtración glomerular estimada. Con respecto a la metodología aplicada para el desarrollo del artículo, se seleccionó artículos a partir de palabras claves como diabetes mellitus tipo 2, antihiperglicemiantes en la función renal, tasa de filtración glomerular estimada y relación albumina/creatinina; se emplearon artículos de revistas reconocidas que no superaran 5 años en su publicación, sin embargo, se utilizaron artículos que superaran este tiempo, dado que aportaban datos importantes para el artículo de revisión.

Abstract Type 2 diabetes mellitus is one of the metabolic diseases that affects different organs, one of which is the kidney. One of the main microvascular complications is diabetic nephropathy, being the main cause of chronic renal failure worldwide. Hence the importance of recommendations on the use or non-use of antihyperglycemic drugs based on their beneficial effects on kidney function, expressed by the estimated glomerular filtration rate and the albumin / creatinine ratio in patients with type 2 diabetes mellitus and kidney disease. Recent studies have shown antihyperglycemic agents with beneficial impact in cardiovascular and renal endpoints. In the present article we will review the actions and effects of different groups of drugs such as metformin, inhibitors of dipeptidyl peptidase 4, GLP-1 agonists, thiazolidinediones, sulfonylureas, SGLT-2 inhibitors and insulins in renal function in relation to the doses of each drug, both the use of established doses, reduction of the dose or non-use of the drug based on the worsening of the estimated glomerular filtration rate. With respect to the methodology applied for the development of the article, a selection of articles was made based on key words such as type 2 diabetes mellitus, antihyperglycemic agents in renal function, estimated glomerular filtration rate and albumin/ creatinine ratio. Prestigious journal articles were used with less than 5 years since its publication, however articles that exceed this time were used as they provided important data in the review article.

Inhibitory Interaction Potential of 22 Antituberculosis Drugs on Organic Anion and Cation Transporters of the SLC22A Family.

Twenty-two currently marketed antituberculosis drugs were comprehensively evaluated for their inhibitory effect on organic anionic transporter (OAT)- and organic cation transporter (OCT)-mediated uptake using stably transfected HEK293 cells in vitro We observed moderate to strong inhibitory effects on OAT1- and OAT3-mediated para-aminohippurate (PAH) uptake and OCT1- and OCT2-mediated N-methyl-4-phenylpylidinium acetate (MPP+) uptake. Ciprofloxacin, linezolid, para-aminosalicylic acid (PAS), and rifampin were observed to have strong inhibitory effects, with the concentrations for a 50% inhibitory effect (IC50s) being 35.1, 31.1, 37.6, and 48.1 µM, respectively, for OAT1 and >100, 21.9, 24.6, and 30.2 µM, respectively, for OAT3. Similarly, pyrazinamide, rifabutin, and levofloxacin were observed to have inhibitory effects, with IC50 values being 36.5, 42.7, and 30.3 µM, respectively, for OCT1 and with the IC50 value for PAS being 94.2 µM for OCT2. In addition, we used zidovudine and metformin as clinically prescribed substrates of OATs and OCTs, respectively, and zidovudine and metformin uptake was also strongly inhibited by the antituberculosis drugs. Among the tested drugs, the highest drug-drug interaction (DDI) indexes were found for PAS, which were 9.3 to 13.9 for OAT1 and 12.0 to 17.7 for OAT3, and linezolid, which were 1.18 to 2.15 for OAT1 and 1.7 to 3.01 for OAT3. Similarly, the DDI indexes of pyrazinamide and levofloxacin were 0.57 and 0.30, respectively, for OCT1, and the DDI index of PAS was 3.8 for OCT2, suggesting a stronger possibility (DDI index value cutoff, >0.1) of in vivo DDIs. This is the first comprehensive report of the inhibitory potential of anti-TB drugs on OAT- and OCT-mediated uptake of prototype and clinically prescribed substrate drugs in vitro, providing an ability to predict DDIs between anti-TB drugs and other coprescribed drugs in clinical studies in vivo.

Sildenafil does not enhance but rather attenuates vasorelaxant effects of antidiabetic agents.

Type 2 diabetic men commonly experience erectile dysfunction for which phosphodiesterase-5 (PDE5) inhibitors like sildenafil (Viagra) are often recommended. By preventing degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle, these inhibitors also enhance arterial vasorelaxant effects of nitric oxide donors (which stimulate cGMP synthesis). In the present work, we confirmed this enhancing effect after co-administration of sildenafil with nitroprusside to freshly-isolated rat tail arterial tissues. However, in the same tissues we also observed that sildenafil does not enhance but rather attenuates vasorelaxant effects of three commonly-used antidiabetic drugs, i.e. the biguanide metformin and the thiazolidinediones pioglitazone and rosiglitazone. Indeed, sildenafil completely blocked vasorelaxant effects of low concentrations of these drugs. In addition, we found that this same novel anti-vasorelaxant interaction of sildenafil with these agents was abolished by either 1) omitting extracellular glucose or 2) inhibiting specific smooth muscle glycolytic pathways; pathways known to preferentially utilize extracellular glucose to fuel certain adenosine triphosphate (ATP)-dependent ion transporters: e.g. ATP-sensitive K channels, sarcoplasmic reticulum Ca-ATPase, plasma membrane Ca-ATPase and Na/K-ATPase. Accordingly, we suspect that altered activity of one or more of these ion transporters mediates the observed attenuating (anti-vasorelaxant) interaction of sildenafil with the antidiabetic drugs. The present results are relevant because hypertension is so common and difficult to control in Type 2 diabetes. The present data suggest that sildenafil might interfere with the known antihypertensive potential of metformin and the thiazolidinediones. However, they do not suggest that it will interact with them to cause life-threatening episodes of severe hypotension, as can occur when it is co-administered with nitrates.

[SGLT-2 inhibitors: diabetes treatment by glycosuria; literature review on the effect of dapagliflozin]. / SGLT-2-remmers: glucosurie als diabetesbehandeling; literatuurstudie naar het effect van dapagliflozine.

OBJECTIVE: To describe the efficacy and safety of dapagliflozin, the first sodium-glucose co-transporter-2 (SGLT-2) inhibitor for the treatment of diabetes mellitus type 2 (DM2) to be registered in the Netherlands. DESIGN: Literature review. METHOD: We searched the Medline database for articles on the use of dapagliflozin in patients with DM2. We included randomised studies with a minimum duration of 12 weeks and systematic reviews published up to 19 October 2012. Two assessors selected the articles on the basis of title, abstract and if necessary, the complete text. RESULTS: Eleven articles were suitable for analysis. On comparison with placebo, the use of dapagliflozin gave a drop in HbA1c-value of approximately 0.5-0.8 percentage points (6-9 mmol/mol). The body weight of patients who used dapagliflozin dropped between 1.0-2.4 kg on comparison with the placebo and metformin control groups. Urinary tract infections occurred twice as often and genital infections three to four times more often. There were no data on the effect on micro- and macrovascular complications or on mortality. CONCLUSION: Dapagliflozin regulates the blood glucose levels less effectively than currently used medications, although the small number of studies that compare dapagliflozin with metformin or glipizide show no differences in the drop in HbA1c between the study groups. Dapagliflozin use leads to minor, clinically non-relevant weight loss. There are as yet no data on its long term efficacy and safety.

Metformin inhibits vascular calcification in female rat aortic smooth muscle cells via the AMPK-eNOS-NO pathway.

Metformin exhibits diverse protective effects against diabetic complications, such as bone loss. Here, we investigated the effect of metformin on vascular calcification, another type 2 diabetes complication. In female rat aortic smooth muscle cells (RASMCs), we observed that metformin significantly alleviated ß-glycerophosphate-induced Ca deposition and alkaline phosphatase activity, corresponding with reduced expression of some specific genes in osteoblast-like cells, including Runx2 and bone morphogenetic protein-2, and positive effects on α-actin expression, a specific marker of smooth muscle cells. Mechanistic analysis showed that phosphorylation levels of both AMP-activated protein kinase (AMPK) and endothelial nitric oxide synthase (eNOS) were increased with NO overproduction. After inhibition of either AMPK or eNOS with the pharmacologic inhibitors, compound C or Nω-Nitro-L-arginine methyl ester, NO production was lowered and metformin-meditated vascular protection against ß-glycerophosphate-induced Ca deposition was removed. Our results support that metformin prevents vascular calcification via AMPK-eNOS-NO pathway.

L-Carnitine for the treatment of a calcium channel blocker and metformin poisoning.

INTRODUCTION: The object of the current communication is to discuss the theory and the evidence for the use of L-carnitine in calcium channel blocker and metformin poisonings. CASE REPORT: A 68-year-old male known for hypertension and type II diabetes was admitted to the critical care unit of a community hospital following an overdose of amlodipine and metformin. The patient was intubated, ventilated, and hemodynamically supported with vasopressors. Despite calcium, glucagon, high-dose insulin (HDI), and lipid emulsion for calcium channel blocker and bicarbonate for metabolic acidosis, the patient remained hemodynamically unstable. The patient was considered too unstable to initiate continuous renal replacement therapy; and without access to extracorporeal life support, the administration of L-carnitine was administered as a last resort. One hour after L-carnitine, the norepinephrine requirements started to decrease, the patient began to improve and was subsequently extubated successfully without apparent sequelae in less than 4 days. DISCUSSION: L-Carnitine combined with HDI may have helped with the calcium channel blocker (CCB) poisoning by decreasing insulin resistance, promoting intracellular glucose transport, facilitating the metabolism of free fatty acids, and increasing calcium channel sensitivity. It may have also stimulated oxidative utilization of glucose instead of converting pyruvate into lactate and contributed to decrease lactate production with metformin poisoning.

Metformin inhibits advanced glycation end products (AGEs)-induced renal tubular cell injury by suppressing reactive oxygen species generation via reducing receptor for AGEs (RAGE) expression.

Advanced glycation end products (AGEs) and their receptor (RAGE) play a role in tubulointerstitial damage in diabetic nephropathy. Recently, metformin has been shown to ameliorate tubular injury both in cell culture and diabetic animal model. However, effects of metformin on AGEs-induced tubular cell apoptosis and damage remain unknown. We examined here whether and how metformin could block the AGEs-RAGE-elicited tubular cell injury in vitro. Gene expression level was evaluated by real-time reverse-transcription polymerase chain reactions. Reactive oxygen species (ROS) generation was measured with dihydroethidium staining. Apoptosis was evaluated by DNA fragmentation and annexin V expression level. AGEs upregulated RAGE mRNA levels and subsequently increased ROS generation and intercellular adhesion molecule-1, monocyte chemoattractant protein-1 and transforming growth factor-ß gene expression in human renal proximal tubular cells, all of which were significantly blocked by the treatment of 0.01 and 0.1 mM metformin. Compound C, an inhibitor of AMP-activated protein kinase significantly blocked the effects of metformin on RAGE gene expression and ROS generation in AGEs-exposed tubular cells. Furthermore, metformin dose-dependently inhibited the AGEs-induced apoptotic cell death of tubular cells; 1 mM metformin completely suppressed the pro-apoptotic effects of AGEs in 2 different assay systems. Our present study suggests that metformin could inhibit the AGEs-induced apoptosis and inflammatory and fibrotic reactions in tubular cells probably by reducing ROS generation via suppression of RAGE expression through AMP-activated protein kinase activation. Metformin may protect against tubular cell injury in diabetic nephropathy by blocking the AGEs-RAGE-ROS axis.

Effects of a MATE protein inhibitor, pyrimethamine, on the renal elimination of metformin at oral microdose and at therapeutic dose in healthy subjects.

A microdose study of metformin was conducted to investigate the predictability of drug-drug interactions at the therapeutic dose (ThD). Healthy subjects received a microdose (100 µg) or ThD (250 mg) of metformin orally, with or without a potent and competitive multidrug and toxin extrusion (MATE) inhibitor, pyrimethamine (50 mg, p.o.), in a crossover fashion. Pyrimethamine significantly reduced the renal clearance of metformin by 23 and 35% at the microdose and ThD, respectively. At ThD, but not at microdose, it caused significant increases in the maximum concentration (C(max)) and area under the plasma concentration-time curve (AUC) of metformin (142 and 139% of control values, respectively). Human canalicular membrane vesicles showed pyrimethamine-inhibitable metformin uptake. Pyrimethamine did not affect plasma lactate/pyruvate after ThD of metformin but significantly reduced the renal clearance of creatinine, thereby causing elevation of plasma creatinine level. This microdose study quantitatively predicted a drug-drug interaction involving the renal clearance of metformin at ThD by pyrimethamine. Pyrimethamine is a useful in vivo inhibitor of MATE proteins.

Interaction of glibenclamide and metformin at the level of translation in pancreatic ß cells.

Sulfonylurea and metformin are used in the treatment of diabetes. Their chronic effects on ß cells are not well known. We have shown that sustained exposure of rat ß cells to glibenclamide increased their protein synthesis activity, while metformin caused an inhibition. The effect of glibenclamide was attributed to an activation of translation factors. This study examines whether both drugs interact at the level of protein translation in ß cells. Purified rat ß cells were cultured with and without glibenclamide and metformin before measurement of protein and insulin synthesis, abundance of (phosphorylated) translation factors, and cell viability. A 24 h exposure to metformin stimulated AMP-activated protein kinase (AMPK), suppressed activation of translation factors- both the mammalian target of rapamycin (mTOR; also known as mechanistic target of rapamycin, MTOR)-dependent ones (eukaryotic initiation factor 4E-binding protein 1 and ribosomal protein S6) and the mTOR-independent eukaryotic elongation factor 2-, and inhibited protein synthesis; a 72 h exposure resulted in 50% dead cells. These effects were counteracted by addition of glibenclamide, the action of which was blocked by the mTOR inhibitor rapamycin and the protein kinase A (PKA) inhibitor Rp-8-Br-cAMPs. In conclusion, metformin activates AMPK in ß cells leading to suppression of protein translation through mTOR-dependent and -independent signaling. Glibenclamide antagonizes these metformin effects through activation of mTOR- and PKA-dependent signaling pathways.

The transmembrane transport of metformin by osteoblasts from rat mandible.

Previous studies have demonstrated that metformin, one of systemic antihyperglycemic drugs, can slow bone loss caused by diabetes mellitus and has an osteogenic action on osteoblasts in vitro. It is tempting to speculate that metformin would be transported into bone tissues around dental implant by topical administration to improve the bone-implant contact in diabetic patients. In this study, the osteoblasts from rat mandible were cultured with 5.5 mM (control) or 16.5 mM d-glucose, then the uptake of metformin by osteoblasts was detected with high performance liquid chromatography (HPLC). Rat organic cation transporter (rOct) expression was characterized by immunocytochemistry, RT-PCR and Western blotting. It was found that, the uptake of metformin was saturable, Na(+)-dependent, affected by extracellular pH and inhibited by both phenformin and cimetidine (an inhibitor of Octs). rOct1 but no rOct2 was expressed extensively in osteoblasts and the protein level of rOct1 could be up-regulated by metformin. The uptake of metformin and phosphorylated-rOct1 at hyperglycaemic cell culture (16.5 mM d-glucose) significantly increased versus 5.5 mM control (p < 0.05). In conclusion, rat osteoblasts have the ability to transport the metformin intra-cellularly, the uptake of metformin by osteoblasts is a secondary active transportation mediated by rOct1 and high-glucose can improve the uptake of metformin by osteoblasts through phosphorylation of rOct1. The current results suggest that metformin could be used for dental implant topically in type 2 diabetic patients to increase the bone formation, therefore, to enhance the success rate of dental implants clinically.

Effect of genetic variation in the organic cation transporter 1 (OCT1) on metformin action.

Metformin is among the most widely prescribed drugs for the treatment of type 2 diabetes. Organic cation transporter 1 (OCT1) plays a role in the hepatic uptake of metformin, but its role in the therapeutic effects of the drug, which involve activation of AMP-activated protein kinase (AMPK), is unknown. Recent studies have shown that human OCT1 is highly polymorphic. We investigated whether OCT1 plays a role in the action of metformin and whether individuals with OCT1 polymorphisms have reduced response to the drug. In mouse hepatocytes, deletion of Oct1 resulted in a reduction in the effects of metformin on AMPK phosphorylation and gluconeogenesis. In Oct1-deficient mice the glucose-lowering effects of metformin were completely abolished. Seven nonsynonymous polymorphisms of OCT1 that exhibited reduced uptake of metformin were identified. Notably, OCT1-420del (allele frequency of about 20% in white Americans), previously shown to have normal activity for model substrates, had reduced activity for metformin. In clinical studies, the effects of metformin in glucose tolerance tests were significantly lower in individuals carrying reduced function polymorphisms of OCT1. Collectively, the data indicate that OCT1 is important for metformin therapeutic action and that genetic variation in OCT1 may contribute to variation in response to the drug.

Effects of metformin on intestinal 5-hydroxytryptamine (5-HT) release and on 5-HT3 receptors.

Nearly 30% of patients treated with metformin experience gastrointestinal side effects. Since release of 5-hydroxytryptamine (5-HT) from the intestine is associated with nausea, vomiting, and diarrhea, we examined whether metformin induces 5-HT release from the intestinal mucosa. In 40% of tissue biopsy specimens of human duodenal mucosa, metformin (1, 10, and 30 microM) caused an increase in 5-HT outflow by 35, 70, and 98%, respectively. Peak increases in 5-HT outflow were observed after 10-15 min exposure to metformin, returning to baseline levels after 25 min. Tetrodotoxin (1 microM) reduced by about 50% the metformin-evoked increase in 5-HT outflow (P<0.05). Metformin-evoked release was not affected by scopolamine + hexamethonium, propranolol, the 5-HT3 receptor antagonist dolasetron, naloxone, or the NK1 receptor antagonist L703606. In the presence of tetrodotoxin (1 microM), somatostatin (1 microM) further reduced metformin-induced 5-HT release by 15-20%. In view of the 5-HT releasing effects of selective 5-HT3 receptor agonists to which metformin (N-N-dimethylbiguanide) is structurally related, we investigated whether metformin directly interacts with 5-HT3 receptors. Receptor binding (inhibition of [3H]-GR65630 binding) and agonist effects (stimulation of [14C]-guanidinium influx) at 5-HT3 receptors were studied in murine neuroblastoma N1E-115 cells, which express functional 5-HT3 receptors. Metformin up to 0.3 mM failed to inhibit [3H]-GR65630 binding and to modify displacement of [3H]-GR65630 binding induced by 5-HT. 5-HT (3 microM) stimulated the influx of [14C]-guanidinium in intact N1E-115 cells. Metformin up to 1 mM failed to modify basal influx, 5-HT-induced influx, and 5-HT+ substance P-induced influx of [14C]-guanidinium. Our results indicate that metformin induces 5-HT3 receptor-independent release of 5-HT from human duodenal mucosa via neuronal and non-neuronal mechanisms. Part of the gastrointestinal side effects observed during treatment with metformin could, thus, be produced by the release of 5-HT and other neurotransmitter substances within the duodenal mucosa.

Acute vasorelaxant effects of metformin and attenuation by stimulation of sympathetic agonist release.

We recently discovered 1) that intravenous injection of the antidiabetic drug metformin in the rat rapidly reduces arterial pressure elevations maintained by the alpha-adrenoceptor agonist phenylephrine (PE) and 2) that direct administration of metformin to isolated rat tail arterial tissue rings rapidly relaxes PE-induced contractions. To further characterize this potential direct vasodilator action, we examined effects of metformin on contractions induced not only by PE but also by norepinephrine (NE) and by nonadrenergic agonists (5-hydroxytryptamine, 5HT; arginine vasopressin, AVP). Also, because the rat tail artery contains abundant adrenergic nerve endings we conducted these tests not only in arterial rings with nerve endings intact but in rings in which they had been removed by pretreatment with 6-hydroxydopamine. In intact rings, metformin at levels from approximately 0.2 to 20 mmol/L rapidly relaxed half-maximal contractions induced by PE and NE similarly and to a markedly greater degree than contractions induced by 5-HT (p<0.05). Metformin did not relax AVP-induced contractions. In addition, removal of adrenergic nerve endings facilitated metformin's relaxant effects (p<0.05). Thus, the acute vasodilator action of metformin appears 1) to be selectively more powerful on arterial smooth muscle contractions induced adrenergically versus nonadrenergically and 2) to be buffered by a possible metformin-induced release of endogenous NE from adrenergic nerve endings. Such results were not seen during relaxation produced by either the calcium channel inhibitor nifedipine or the nitrovasodilator nitroprusside suggesting that metformin's effects are mediated by other mechanisms.

4-Aminopyridine antagonizes the acute relaxant action of metformin on adrenergic contraction in the ventral tail artery of the rat.

The ability of metformin (MF) to acutely relax phenylephrine (PE)-induced contraction in the isolated rat tail artery is reported to be accompanied by repolarization of the arterial smooth muscle cell (SMC) membranes. These membranes contain potassium (K) channels which if opened could mediate such repolarization and resultant relaxation. We have shown that the acute relaxation of rat tail arterial tissue rings by graded levels of MF > or = 0.24 mmol/L is markedly antagonized by a high concentration of tetraethylammonium (TEA; 10 mmol/L) which nonselectively inhibits nearly all K channels. Thus, we tested effects of more selective inhibitors of K channels in the same tissue. We also tested MF for relaxation of contractions induced by high levels of extracellular K. To avoid confounding variables, we also conducted these tests in arterial rings in which endothelium and sympathetic nerve endings had been removed. In the absence of K channel inhibition, half-maximal PE-induced contractions were rapidly relaxed by all levels of MF with an EC50 of 1.7+/-0.2 mmol/L (n=8 rings). 1 mmol/L 4-aminopyridine (4AP) which only inhibits voltage-operated and ATP-sensitive K channels markedly antagonized this relaxation, shifting the EC50 for MF to 7.5+/-0.7 mmol/L (n=8; p < 0.05). TEA at 1 mmol/L (which only inhibits calcium-activated K channels), barium at 20 micromol/L (which only inhibits inward rectifier K channels) and glyburide at 5 micromol/L (which only inhibits ATP-sensitive K channels) did not alter this relaxation. Finally, MF failed to relax contractions produced by elevations of extracellular K to levels high enough to abolish the K gradient across arterial SMC membranes. Thus, acute relaxation of rat tail arterial smooth muscle by MF may be dependent on the transmembrane K gradient and mediated at least in part by specific activation of K efflux through 4AP-sensitive voltage-dependent K channels in arterial SMC membranes.