The effects of caffeine on ATP-sensitive K(+) channels in smooth muscle cells from pig urethra

Role of Various Potassium Channels in Caffeine-induced Aortic Relaxation in Rats

Background:

Studies done on caffeine-induced changes in aortic rings have demonstrated inconclusive results. Moreover, the role of various potassium channels in caffeine-induced effects has not been explored so far. The present in vitro study was designed to explore the direct effects of caffeine on rat aortic rings and the role of various potassium channels in those changes/effects.

Materials and Methods:

This study was carried out in College of Medicine, University of Dammam. Aortic rings obtained from Sprague Dawley rats were mounted in the organ bath. Tension in the aortic rings was measured with an isometric force transducer and recorded with a PowerLab data-acquisition system. Aortic rings in relaxed and contractile state were exposed to caffeine and various potassium channel blockers (glyburide, 4-aminopyridine, or tetraethylammonium).

Results:

Caffeine produced significant relaxation of isolated aortic rings (baseline tension: 1.26 ± 0.30 g, tension after adding cumulative concentrations of caffeine: 1.12 ± 0.31 g, P < 0.05) in the absence or presence of norepinephrine (NE) (tension induced by NE: 1.06 ± 0.37 g, tension after adding cumulative concentrations of caffeine: 1.01 ± 0.36 g, P < 0.05). Caffeine's vasodilatory effects were, however, blocked in aortic rings pretreated with different types of potassium channel blockers such as 4-aminopyridine (tension induced by NE: 1.52 ± 0.41 g, tension after adding cumulative concentrations of caffeine: 1.50 ± 0.37 g, P > 0.05), glyburide (tension induced by NE: 0.82 ± 0.35 g, tension after adding cumulative concentrations of caffeine: 0.79 ± 0.42 g, P > 0.05), and tetraethylammonium (tension induced by NE: 0.68 ± 0.34 g, tension after adding cumulative concentrations of caffeine: 0.67 ± 0.33 g, P > 0.05).

Conclusion:

Caffeine causes significant dilation of aortic rings, and this vasodilatory effect may involve ATP-dependent, calcium-mediated, or voltage-dependent potassium channels.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Inhibitory effects of SKF96365 on the activities of K(+) channels in mouse small intestinal smooth muscle cells

In order to investigate the effects of SKF96365 (SKF), which is a non-selective cationic channel blocker, on K⁺ channel currents, we recorded currents through ATP sensitive K⁺ (I_KATP), voltage-gated K⁺ (I_Kv) and Ca²⁺ activated K⁺ channels (I_BK) in the absence and presence of SKF in single small intestinal myocytes of mice with patch-clamp techniques. SKF (10 µM) reversibly abolished I_KATP that was induced by cromakalim (10 µM), which is a selective ATP sensitive K⁺ channel opener. These inhibitory effects were induced in a concentration-dependent and voltage-independent manner. The 50% inhibitory concentration (IC₅₀) was 0.85 µM, which was obviously lower than that reported for the muscarinic cationic current. In addition, SKF (1 µM ≈ the IC₅₀ value in I_KATP suppression) reversibly inhibited the I_Kv that was induced by repetitive depolarizing pulses from −80 to 20 mV. However, the extent of the inhibitory effects was only ~30%. In contrast, SKF (1 µM) had no significant effects on spontaneous transient I_BK and caffeine-induced I_BK. These results indicated that SKF inhibited ATP sensitive K⁺ channels and voltage-gated K⁺ channels, with the ATP sensitive K⁺ channels being more sensitive than the voltage-gated K⁺ channels. These inhibitory effects on K⁺ channels should be considered when SKF is used as a cationic channel blocker.

Ion channels of the mammalian urethra

The mammalian urethra is a muscular tube responsible for ensuring that urine remains in the urinary bladder until urination. In order to prevent involuntary urine leakage, the urethral musculature must be capable of constricting the urethral lumen to an extent that exceeds bladder intravesicular pressure during the urine-filling phase. The main challenge in anti-incontinence treatments involves selectively-controlling the excitability of the smooth muscles in the lower urinary tract. Almost all strategies to battle urinary incontinence involve targeting the bladder and as a result, this tissue has been the focus for the majority of research and development efforts. There is now increasing recognition of the value of targeting the urethral musculature in the treatment and management of urinary incontinence. Newly-identified and characterized ion channels and pathways in the smooth muscle of the urethra provides a range of potential therapeutic targets for the treatment of urinary incontinence. This review provides a summary of the current state of knowledge of the ion channels discovered in urethral smooth muscle cells that regulate their excitability.

Habitual coffee intake, genetic polymorphisms, and type 2 diabetes

BACKGROUND

The association between coffee intake and type 2 diabetes may be modulated by common genetic variation.

OBJECTIVE

The purpose of this study was to examine the association between habitual coffee intake and the risk of type 2 diabetes and to determine whether this association varied by genetic polymorphisms related to type 2 diabetes in Korean adults.

DESIGN AND METHODS

A population-based cohort study over a follow-up of 4 years was conducted. A total of 4077 Korean men and women aged 40-69 years with a normal glucose level at baseline were included. Coffee intake was assessed using a validated food frequency questionnaire, and incident type 2 diabetes or prediabetes was defined by oral glucose tolerance test or fasting blood glucose test. The genomic DNA samples were genotyped with the Affymetrix Genome-Wide Human SNP Array 5.0, and nine single-nucleotide polymorphisms related to type 2 diabetes in East Asian populations were extracted.

RESULTS

A total of 120 cases of type 2 diabetes and 1128 cases of prediabetes were identified. After adjustment for potential confounding factors, we observed an inverse association, but without any clear linear trend, between coffee intake and the combined risk of type 2 diabetes and prediabetes. We found that inverse associations between habitual coffee intake and the combined risk of type 2 diabetes and prediabetes were limited to those with the T-allele (GT/TT) of rs4402960 in IGF2BP2, those with the G-allele (GG/GC) of rs7754840 in CDKAL1, or those with CC of rs5215 in KCNJ11.

CONCLUSION

We found a lower risk of prediabetes and type 2 diabetes combined with coffee intake among individuals with the GT/TT of IGF2BP2 rs4402960, GG/GC of CDKAL1 rs7754840, or CC of KCNJ11 rs5215, which are known to be related to type 2 diabetes in East Asians.

Caffeine inhibits erythrocyte membrane derangement by antioxidant activity and by blocking caspase 3 activation

The aim of this research was to investigate the effect of caffeine on band 3 (the anion exchanger protein), haemoglobin function, caspase 3 activation and glucose-6-phosphate metabolism during the oxygenation-deoxygenation cycle in human red blood cells. A particular attention has been given to the antioxidant activity by using in vitro antioxidant models. Caffeine crosses the erythrocyte membrane and interacts with the two extreme conformational states of haemoglobin (the T and the R-state within the framework of the simple two states allosteric model) with different binding affinities. By promoting the high affinity state (R-state), the caffeine-haemoglobin interaction does enhance the pentose phosphate pathway. This is of benefit for red blood cells since it leads to an increase of NADPH availability. Moreover, caffeine effect on band 3, mediated by haemoglobin, results in an extreme increase of the anion exchange, particularly in oxygenated erythrocytes. This enhances the transport of the endogenously produced CO(2) thereby avoiding the production of dangerous secondary radicals (carbonate and nitrogen dioxide) which are harmful to the cellular membrane. Furthermore caffeine destabilizes the haeme-protein interactions within the haemoglobin molecule and triggers the production of superoxide and met-haemoglobin. However this damaging effect is almost balanced by the surprising scavenger action of the alkaloid with respect to the hydroxyl radical. These experimental findings are supported by in silico docking and molecular dynamics studies and by what we may call the "caspase silence"; in fact, there is no evidence of any caspase 3 activity enhancement; this is likely due to the promotion of positive metabolic conditions which result in an increase of the cellular reducing power.

Expression of ATP-sensitive potassium channels in human pregnant myometrium

BACKGROUND

Potassium channels play critical roles in the regulation of cell membrane potential, which is central to the excitability of myometrium. The ATP-sensitive potassium (KATP) channel is one of the most abundant potassium channels in myometrium. The objectives of this study were to investigate the protein expression of KATP channel in human myometrium and determine the levels of KATP channel in lower and upper segmental myometrium before and after onset of labour.

METHODS

Both lower segmental (LS) and upper segmental (US) myometrial biopsies were collected at cesarean section from pregnant women not-in-labour (TNL) or in-labour (TL) at term. Protein expression level and cellular localization of four KATP channel subunits in US and LS myometrium were determined by Western blot analysis and immunohistochemistry, respectively. The contractile activity of myometrial strip was measured under isometric conditions.

RESULTS

Four KATP channel subunits, namely Kir6.1, Kir6.2, SUR1 and SUR2B were identified in pregnant myometrium. While found in vascular myocytes, these subunits appear to be preferentially expressed in myometrial myocytes. Diazoxide, a KATP channel opener, inhibited the spontaneous contractility of pregnant myometrium, suggesting that the KATP channels are functional in human pregnant myometrium. Diazoxide was less potent in TL strips than that in TNL strips. Interestingly, expression of SUR1 was greater in TL than TNL tissues, although no differences were found for SUR2B in these two tissues. For both lower and upper segmental myometrium, Kir6.1 and Kir6.2 were less in TL compared with TNL tissues.

CONCLUSIONS

Functional KATP channels are expressed in human pregnant myometrium. Down-regulation of Kir6.1 and Kir6.2 expression in myometrium may contribute to the enhanced uterine contractility associated with the onset of labour.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Molecular biology of K(ATP) channels and implications for health and disease

The ATP-sensitive potassium (K(ATP)) channel is expressed in most excitable tissues and plays a critical role in numerous physiological processes by coupling intracellular energetics to electrical activity. The channel is comprised of four Kir6.x subunits associated with four regulatory sulfonylurea receptors (SUR). Intracellular ATP acts on Kir6.x to inhibit channel activity, while MgADP stimulates channel activity through SUR. Changes in the cytosolic [ATP] to [ADP] ratio thus determine channel activity. Multiple mutations in Kir6.x and SUR genes have implicated K(ATP) channels in various diseases ranging from diabetes and hyperinsulinism to cardiac arrhythmias and cardiovascular disease. Continuing studies of channel physiology and pathology will bring new insights to the molecular basis of K(ATP) channel function, leading to a better understanding of the role that K(ATP) channels play in both health and disease.

ATP-sensitive K+ channels in pig urethral smooth muscle cells are heteromultimers of Kir6.1 and Kir6.2

The inwardly rectifying properties and molecular basis of ATP-sensitive K(+) channels (K(ATP) channels) have now been established for several cell types. However, these aspects of nonvascular smooth muscle K(ATP) channels still remain to be defined. In this study, we investigated the molecular basis of the pore of K(ATP) channels of pig urethral smooth muscle cells through a comparative study of the inwardly rectifying properties, conductance, and regulation by PKC of native and homo- and heteroconcatemeric recombinant Kir6.x channels coexpressed with sulfonylurea receptor subunit SUR2B in human embryonic kidney (HEK) 293 cells by the patch-clamp technique (conventional whole-cell and cell-attached modes). In conventional whole-cell clamp recordings, levcromakalim (> or = 1 microM) caused a concentration-dependent increase in current that demonstrated strong inward rectification at positive membrane potentials. In cell-attached mode, the unitary amplitude of levcromakalim-induced native and recombinant heteroconcatemeric Kir6.1-Kir6.2 K(ATP) channels also showed strong inward rectification at positive membrane potentials. Phorbol 12,13-dibutyrate, but not the inactive phorbol ester, 4alpha-phorbol 12,13-didecanoate, enhanced the activity of native and heteroconcatemeric K(ATP) channels at -50 mV. The conductance of the native channels at approximately 43 pS was consistent with that of heteroconcatemeric channels with a pore-forming subunit composition of (Kir6.1)(3)-(Kir6.2). RT-PCR analysis revealed the expression of Kir6.1 and Kir6.2 transcripts in pig urethral myocytes. Our findings provide the first evidence that the predominant K(ATP) channel expressed in pig urethral smooth muscle possesses a unique, heteromeric pore structure that differs from the homomeric Kir6.1 channels of vascular myocytes and is responsible for the differences in inward rectification, conductance, and PKC regulation exhibited by the channels in these smooth muscle cell types.

Dual regulation of the ATP-sensitive potassium channel by caffeine

ATP-sensitive potassium (K(ATP)) channels couple cellular metabolic status to changes in membrane electrical properties. Caffeine (1,2,7-trimethylxanthine) has been shown to inhibit several ion channels; however, how caffeine regulates K(ATP) channels was not well understood. By performing single-channel recordings in the cell-attached configuration, we found that bath application of caffeine significantly enhanced the currents of Kir6.2/SUR1 channels, a neuronal/pancreatic K(ATP) channel isoform, expressed in transfected human embryonic kidney (HEK)293 cells in a concentration-dependent manner. Application of nonselective and selective phosphodiesterase (PDE) inhibitors led to significant enhancement of Kir6.2/SUR1 channel currents. Moreover, the stimulatory action of caffeine was significantly attenuated by KT5823, a specific PKG inhibitor, and, to a weaker extent, by BAPTA/AM, a membrane-permeable Ca(2+) chelator, but not by H-89, a selective PKA inhibitor. Furthermore, the stimulatory effect was completely abrogated when KT5823 and BAPTA/AM were co-applied with caffeine. In contrast, the activity of Kir6.2/SUR1 channels was decreased rather than increased by caffeine in cell-free inside-out patches, while tetrameric Kir6.2LRKR368/369/370/371AAAA channels were suppressed regardless of patch configurations. Caffeine also enhanced the single-channel currents of recombinant Kir6.2/SUR2B channels, a nonvascular smooth muscle K(ATP) channel isoform, although the increase was smaller. Moreover, bidirectional effects of caffeine were reproduced on the K(ATP) channel present in the Cambridge rat insulinoma G1 (CRI-G1) cell line. Taken together, our data suggest that caffeine exerts dual regulation on the function of K(ATP) channels: an inhibitory regulation that acts directly on Kir6.2 or some closely associated regulatory protein(s), and a sulfonylurea receptor (SUR)-dependent stimulatory regulation that requires cGMP-PKG and intracellular Ca(2+)-dependent signaling.

Physiological roles of ATP-sensitive K+ channels in smooth muscle

Potassium channels that are inhibited by intracellular ATP (ATP(i)) were first identified in ventricular myocytes, and are referred to as ATP-sensitive K+ channels (i.e. K(ATP) channels). Subsequently, K+ channels with similar characteristics have been demonstrated in many other tissues (pancreatic beta-cells, skeletal muscle, central neurones, smooth muscle). Approximately one decade ago, K(ATP) channels were cloned and were found to be composed of at least two subunits: an inwardly rectifying K+ channel six family (Kir6.x) that forms the ion conducting pore and a modulatory sulphonylurea receptor (SUR) that accounts for several pharmacological properties. Various types of native K(ATP) channels have been identified in a number of visceral and vascular smooth muscles in single-channel recordings. However, little attention has been paid to the molecular properties of the subunits in K(ATP) channels and it is important to determine the relative expression of K(ATP) channel components which give rise to native K(ATP) channels in smooth muscle. The aim of this review is to briefly discuss the current knowledge available for K(ATP) channels with the main interest in the molecular basis of native K(ATP) channels, and to discuss their possible linkage with physiological functions in smooth muscle.

Inhibition of human TREK-1 channels by caffeine and theophylline

Caffeine (1,3,7-trimethylxanthine) and theophylline (1,3-dimethylxanthine) are used for therapeutic purposes and can cause life-threatening convulsive seizures due to systemic toxicity. The mechanisms for the epileptogenicity of caffeine and theophylline are not clear. TWIK-related K(+) channels (TREK-1) are highly expressed in the human central nervous system and have a major role in the control of neuronal excitability by regulating the resting membrane potential. In view of their physiological significance, inhibition of TREK-1 channels may be implicated in caffeine- and theophylline-induced seizures. We thus investigated, using whole-cell patch-clamp technique, modulation of hTREK-1 channels expressed in Chinese hamster ovary (CHO) cells by caffeine and theophylline. Caffeine and theophylline produced reversible inhibition of TREK-1 channels in a concentration-dependent manner. The half-maximal inhibitory concentrations (IC(50)) for caffeine and theophylline were 377+/-54microM and 486+/-76microM, respectively. Caffeine and theophylline depolarized the membrane potential of CHO(TREK-1) cells in a reversible and concentration-dependent manner. Inhibition by caffeine (5mM) and theophylline (2mM) was attenuated in TREK-1 channels with mutation of the PKA consensus sequence at serine 348, suggesting the involvement of cAMP/PKA pathway in the inhibitory process. Inhibition of TREK-1 channels and consequent membrane depolarization may contribute to the convulsive seizures induced by toxic levels of caffeine and theophylline.

Multiple effects of mefenamic acid on K(+) currents in smooth muscle cells from pig proximal urethra

The effects of mefenamic acid on both membrane potential and K+ currents in pig urethral myocytes were investigated using patch-clamp techniques (conventional whole-cell, cell-attached, outside-out and inside-out configuration). In the current-clamp mode, mefenamic acid caused a concentration-dependent hyperpolarization, which was inhibited by preapplication of 1 microm glibenclamide. In the voltage-clamp mode, mefenamic acid induced an outward current that was blocked by glibenclamide even in the presence of iberiotoxin (IbTX, 300 nm) at -50 mV. ATP-sensitive K+ channels (KATP channels) could be activated in the same patch by mefenamic acid and levcromakalim, with the same unitary amplitude and the similar opening gating at -50 mV in cell-attached configuration. In outside-out recording, external application of mefenamic acid activated intracellular Ca2+-activated IbTX-sensitive large-conductance K+ channels (BKCa channels). Mefenamic acid (<or=30 microm) activated spontaneous transient outward currents (STOCs). In contrast, mefenamic acid (>or=100 microm) increased sustained outward currents, diminishing the activity of STOCs. Over the whole voltage range, mefenamic acid caused opposite effects on the membrane currents in the absence and presence of 5 microm glibenclamide. In the presence of 10 mm 4-aminopyridine (4-AP), mefenamic acid only increased the outward currents. These results indicate that mefenamic acid increases the channel activities of two distinct types of K+ channels (i.e. BKCa channels and KATP channels) and decreased 4-AP-sensitive K+ channels in pig urethral myocytes.

Caffeine analogs: effects on ryanodine-sensitive calcium-release channels and GABAA receptors

1. Caffeine at 0.3-10 mM enhanced the binding of [3H]ryanodine to calcium-release channels of rabbit muscle sarcoplasmic reticulum. A variety of other xanthines were as efficacious as caffeine or nearly so, but none appeared markedly more potent. 2. Caffeine at 1 mM markedly inhibited binding of [3H]diazepam to GABAA receptors in rat cerebral cortical membranes. 3. Other xanthines also inhibited binding with certain dimethylpropargylxanthines being nearly fivefold more potent than caffeine. 4. Caffeine at 1 mM stimulated binding of [35S]TBPS to GABAA receptors as did certain other xanthines. 5. The dimethylpropargylxanthines had little effect. 1,3-Dipropyl-8-cyclopentylxanthine at 100 microM had no effect on [3H]diazepam binding, but markedly inhibited [35S]TBPS binding. 6. Structure-activity relationships for xanthines do differ for calcium-release channels and and for different sites on GABAA receptors, but no highly selective lead compounds were identified.

ATP-sensitive K+ channels of vascular smooth muscle cells

ATP-sensitive potassium channels (K(ATP)) of vascular smooth muscle cells represent potential therapeutic targets for control of abnormal vascular contractility. The biophysical properties, regulation and pharmacology of these channels have received intense scrutiny during the past twenty years, however, the molecular basis of vascular K(ATP) channels remains ill-defined. This review summarizes the recent advancements made in our understanding of the molecular composition of vascular K(ATP) channels with a focus on the evidence that hetero-octameric complexes of Kir6.1 and SUR2B subunits constitute the vascular K(ATP) subtype responsible for control of arterial diameter by vasoactive agonists.

Modification of ATP-sensitive K+ channels by proteolysis in smooth muscle cells from pig urethra

Patch-clamp experiments have been performed to investigate the effects of endoproteases (such as trypsin, carboxypeptidase B) on both membrane currents and unitary currents in isolated smooth muscle cells from pig proximal urethra (conventional whole-cell configuration, cell-attached configuration, and inside-out patches). Application of either trypsin (1 mg/mL) or carboxypeptidase B (0.1 mg/mL) to the intracellular surface of the excised membrane patches stimulated the activity of a 2.1 pA K+ channel (in symmetrical 140 mM K+ conditions) at a holding potential of -50 mV. The trypsin-induced K+ channels in inside-out configuration exhibited the same amplitude and similar channel opening kinetics to the levcromakalim-induced ATP-sensitive K+ channel (i.e. K ATP channel) in cell-attached patches of the same membrane; however, the sensitivity of the channels to glibenclamide was greatly reduced after the trypsin-treatment. The activity of the trypsin-induced K+ channel was reversibly inhibited by cibenzoline in an inside-out configuration (Ki = 5 microM). It is concluded that trypsin and carboxypeptidase B reactivate the channel with an intact pore activity but the different pharmacological properties of the channels may reflect some change in the conformation in channel proteins after proteolysis.

The molecular composition of K(ATP) channels in human pulmonary artery smooth muscle cells and their modulation by growth

Multiple types of ATP-sensitive potassium (K(ATP)) channels have been described in smooth muscle, including those inhibited by ATP and those activated by nucleotide diphosphate (K(NDP)). The molecular identities of these channels have been proposed to be SUR2B/Kir6.2 and SUR2B/Kir6.1, respectively. However, subunit expression is largely unknown in vascular muscle, and the native channel has not been reported previously in human tissue. We used the patch-clamp technique to examine K(ATP) channel properties in cultured human pulmonary artery smooth muscle cells (HPASMC). Under physiological recording conditions, levcromakalim (10 microM) hyperpolarized cells (approximately 25-30 mV) and activated a glibenclamide-sensitive, background K(+) current, which was smaller in proliferating cells. Lowering ATP from 1 to 0.1 mM significantly enhanced responses to levcromakalim in HPASMC but not in HEK-293 cells stably transfected with SUR2B/Kir6.1. In both cell types, levcromakalim activated a 28-29 pS channel, which, upon patch excision, required the presence of nucleotide diphosphate for significant openings. Transcripts for SUR2B and Kir6.1, but not Kir6.2, were found by reverse transcription-polymerase chain reaction in HPASMC and in rat pulmonary arterial tissue. We conclude that K(ATP) channels are expressed in human pulmonary artery, and whereas data are consistent with the presence of nucleotide diphosphate-activated potassium channels, native whole-cell regulation cannot be reconstituted fully in heterologous expression systems.

The effects of flecainide on ATP-sensitive K(+) channels in pig urethral myocytes

The effects of the antiarrhythmic drug flecainide on levcromakalim-induced hyperpolarization, macroscopic and unitary K(+) currents in pig urethra were investigated using patch-clamp techniques. The effects of flecainide were also examined on currents in inside-out patches of COS7 cells expressing carboxy terminus truncated inwardly rectifying K(+) channel (Kir6.2) subunits (i.e. Kir6.2DeltaC36) which form ATP-sensitive K(+) channels (K(ATP) channels). In current-clamp mode, application of flecainide (> or =100 microM) caused a significant depolarization after the membrane potential had been hyperpolarized by levcromakalim. In voltage-clamp experiments, the levcromakalim-induced outward current was suppressed by 300 microM flecainide in quasi-physiological K(+) conditions (K(i)=51 microM). In contrast, approximately 20% of the levcromakalim-induced inward current still remained even after application of 300 microM flecainide in symmetrical 140 mM K(+) conditions (K(i)=51 microM). In contrast, approximately 20% of the levcromakalim-induced inwar=126 microM). In cell-attached configuration, the channel activity of the levcromakalim-induced K(ATP) channels was reversibly inhibited by flecainide (> or =30 microM) at -50 mV. Their activity was also suppressed by either disopyramide or cibenzoline. Flecainide reversibly inhibited the channel activity of Kir6.2DeltaC36 expressed in COS7 cells using inside-out configuration. Inhibitory effects of flecainide on the levcromakalim-induced currents became more potent when the value of external pH increased, although this slightly reduced the proportion of drug molecules carrying a positive charge. These results suggest that flecainide inhibits channel activity through blocking the pore site of the K(ATP) channel in pig urethra.

Dual action of ZD6169, a novel K(+) channel opener, on ATP-sensitive K(+) channels in pig urethral myocytes

1. The effects of ZD6169, a novel K(+) channel opener, on both membrane and unitary currents in pig urethra were investigated using patch-clamp techniques. Its effect was also examined on currents in inside-out patches of COS7 cells expressing carboxy terminus truncated inwardly rectifying K(+) channel (Kir6.2) subunits (Kir6.2C36) which form ATP-sensitive K(+) channels (K(ATP) channels). 2. In current-clamp mode, ZD6169 (< or = 10 microM) induced a concentration-dependent membrane hyperpolarization. Higher concentrations (> or = 30 microM) caused a transient membrane hyperpolarization, followed by a gradual membrane depolarization. On removal of ZD6169, an after hyperpolarization was observed. 3. In conventional voltage-clamp configuration, at -50 mV in symmetrical 140 mM K(+) conditions, ZD6169 (100 microM) caused a transient inward current which gradually decayed. Removal of ZD6169 evoked a much larger amplitude K(+) current with a similar time course. 4. ZD6169 produced an inward glibenclamide-sensitive K(+) current, demonstrating a bell-shaped concentration-response relationship. 5. In cell-attached configuration in symmetrical 140 mM K(+) conditions, ZD6169 (< or = 30 microM) activated an K(ATP) channel which was reversibly suppressed by application of glibenclamide. In contrast, ZD6169 (100 microM) inhibited the activity of the levcromakalim-induced K(ATP) channels. 6. ZD6169 (100 microM) had no significant effect on the channel activity of Kir6.2C36 in inside-out configuration, although cibenzoline greatly suppressed the channel activity. 7. These results demonstrate that ZD6169 possesses a dual effect on the activity of the K(ATP) channel; activating at low concentration and inhibiting at higher concentration.