Effects of P1060 and aprikalim on whole-cell currents in rat portal vein; inhibition by glibenclamide and phentolamine

Redox control of cardiac excitability

Reactive oxygen species (ROS) have been associated with various human diseases, and considerable attention has been paid to investigate their physiological effects. Various ROS are synthesized in the mitochondria and accumulate in the cytoplasm if the cellular antioxidant defense mechanism fails. The critical balance of this ROS synthesis and antioxidant defense systems is termed the redox system of the cell. Various cardiovascular diseases have also been affected by redox to different degrees. ROS have been indicated as both detrimental and protective, via different cellular pathways, for cardiac myocyte functions, electrophysiology, and pharmacology. Mostly, the ROS functions depend on the type and amount of ROS synthesized. While the literature clearly indicates ROS effects on cardiac contractility, their effects on cardiac excitability are relatively under appreciated. Cardiac excitability depends on the functions of various cardiac sarcolemal or mitochondrial ion channels carrying various depolarizing or repolarizing currents that also maintain cellular ionic homeostasis. ROS alter the functions of these ion channels to various degrees to determine excitability by affecting the cellular resting potential and the morphology of the cardiac action potential. Thus, redox balance regulates cardiac excitability, and under pathological regulation, may alter action potential propagation to cause arrhythmia. Understanding how redox affects cellular excitability may lead to potential prophylaxis or treatment for various arrhythmias. This review will focus on the studies of redox and cardiac excitation.

Electrophysiologic and mechanical evidence of superiority of hyperpolarizing versus depolarizing cardioplegia in protection of endothelium-derived hyperpolarizing factor–mediated endothelial function: a study in coronary resistance arteries

OBJECTIVE

The advantages of hyperpolarizing cardioplegia with potassium-channel openers versus depolarizing cardioplegia have been suggested but not demonstrated in coronary microarteries. This study examined the simultaneous electric and tonic alteration of coronary microarteries at the cellular level during and after exposure to depolarizing cardioplegia or hyperpolarizing cardioplegia, with emphasis on endothelium-derived hyperpolarizing factor-mediated relaxation and hyperpolarization.

METHODS

Porcine coronary microarteries (diameter, approximately 200-400 microm) were incubated with depolarizing cardioplegia (20 mmol/L KCl) or hyperpolarizing cardioplegia (10 micromol/L aprikalim) for 1 hour. Cellular membrane potential with a glass microelectrode in a coronary smooth muscle cell and isometric force of the muscle were simultaneously measured in a myograph.

RESULTS

Depolarizing cardioplegia incubation produced a stable contraction (from 4.9 +/- 0.3 mN to 7.3 +/- 0.4 mN) and depolarization (from -51 +/- 1 mV to -41 +/- 2 mV). In contrast, hyperpolarizing cardioplegia relaxed (from 4.8 +/- 0.3 mN to 3.5 +/- 0.3 mN) and hyperpolarized (from -51 +/- 2 mV to -56 +/- 1 mV) the smooth muscle. After exposure to depolarizing cardioplegia, the bradykinin-induced, endothelium-derived hyperpolarizing factor-mediated relaxation reduced from 66.2% +/- 5.0% to 18.4% +/- 3.7% (P <.001), and the membrane hyperpolarization reduced from 18 +/- 1 mV to 7 +/- 1 mV (P <.001) in the presence of indomethacin and N(G)-nitro-L-arginine. In contrast, hyperpolarizing cardioplegia did not affect the bradykinin-induced responses.

CONCLUSIONS

In the coronary microarteries, exposure to hyperpolarizing cardioplegia preserves whereas depolarizing cardioplegia reduces the endothelium-derived hyperpolarizing factor-mediated electric (hyperpolarization) and mechanical (relaxation) responses. Thus hyperpolarizing cardioplegia is superior to depolarizing cardioplegia in protecting the endothelial function in the coronary microcirculation.

The effects of potassium channel blockers on progesterone-induced suppression of rat portal vein contractility

The suppression of contractility of rat portal vein caused by progesterone appears to be due to the potassium (K+) channel opening effect of this hormone. The identity of the specific K+ channels involved has been investigated using a variety of K+ channel blockers. Incubation with 100 nM iberiotoxin antagonised the progesterone-induced inhibition of spontaneous and 20 mM K+-induced phasic activity of the portal vein such that the contractions resembled those of the non-progesterone, non-iberiotoxin control tissues treated with the corresponding solvent vehicles. Incubation with barium chloride (20 and 100 microM), 4-aminopyridine (1 mM), tetraethylammonium chloride (1 mM), glibenclamide (1 microM) or apamin (1 microM) did not, however, have the same antagonistic effect. These results suggest that progesterone's selective suppression of rat portal vein contractility is mediated by the opening of BKCa channels.

A KATP-channel opener as a potential treatment modality for erectile dysfunction

OBJECTIVES

To assess the effects of pinacidil (a KATP-channel opener) for the treatment of penile erectile dysfunction and to examine the role of the K+-channel in cavernosal smooth muscle contractility.

MATERIALS AND METHODS

Using a feline model, the magnitude of penile erection caused by pinacidil was compared with that caused by erectogenic drugs, e.g. acetylcholine, prostaglandin E1 (PGE1) and L-arginine. The effects of K+-channel blockers (4-aminopyridine, glibenclamide and tetraethylammonium) and pinacidil on penile erections induced by the drugs were investigated.

RESULTS

The intra-arterial injection of pinacidil caused a dose-dependent increase in intracavernosal pressure (ICP) and the increase in ICP induced by pinacidil with acetylcholine, PGE1 or L-arginine was more pronounced than with the compounds alone. Furthermore, pinacidil (1 mmol/L) effectively reversed the inhibitory effects of the K+-channel blockers on the cavernosal relaxation induced by acetylcholine, PGE1 or L-arginine (P<0.01). Notably, pinacidil induced cavernosal relaxation after injecting the drugs even in cases refractory to higher concentrations (0.1 mol/L) of the drugs (n=11, P<0.01).

CONCLUSIONS

These results suggest that pinacidil is effective in relaxing feline erectile tissue in vivo, probably via increased K+ permeability and subsequent hyperpolarization. Further comparative studies with erectogenic compounds on human erectile tissue and clinical testing are required to determine whether K+-channel openers can be used in the diagnosis and treatment of erectile dysfunction. However, pinacidil seems promising as an intracavernosal agent combined with PGE1 to produce synergistic effects.

Inhibition of potassium (KATP) channels reduces the short-circuit current response of rat colonic mucosa to acetylcholine

Intestinal secretion depends upon electrogenic chloride transport into the gut lumen, which requires maintenance of an electrically negative cell-membrane voltage. We have investigated whether secretory responses of rat colonic mucosa to acetylcholine were sensitive to inhibition of potassium channels and whether selective inhibition could indicate the nature of the channel involved. Rat colonic mucosa was set up in Ussing chambers, short-circuit current responses obtained to acetylcholine, and the sensitivity of such responses to inhibition of potassium channels was investigated. Non-selective potassium-channel blockade by barium induced concentration-dependent inhibition of responses to acetylcholine. Similar inhibitory effects were obtained using 4-aminopyridine and glibenclamide. 5-Hydroxydecanoate and phentolamine also inhibited the increase in short-circuit current. However, a combination of charybdotoxin plus apamin was without effect. We conclude that a basolateral outward movement of potassium ions is required for the secretory action of acetylcholine on rat colonic mucosa. The potassium channel involved seems to be ATP-dependent and calcium-insensitive.

Modulation of membrane currents and mechanical activity by niflumic acid in rat vascular smooth muscle

The effects of niflumic acid on whole-cell membrane currents and mechanical activity were examined in the rat portal vein. In freshly dispersed portal vein cells clamped at -60 mV in caesium (Cs+)-containing solutions, niflumic acid (1-100 microM) inhibited calcium (Ca2+)-activated chloride currents (IC1(Ca)) induced by caffeine (10 mM) and by noradrenaline (10 microM). In a potassium (K+)-containing solution and at a holding potential of - 10 mV, niflumic acid (10-100 microM) induced an outward K+ current (IK(ATP)) which was sensitive to glibenclamide (10-30 microM). At concentrations < 30 microM and at a holding potential of -2 mV, niflumic acid had no effect on the magnitude of the caffeine- or noradrenaline-stimulated current (IBK(Ca)) carried by the large conductance, Ca(2+)-sensitive K+ channel (BKCa). However, at a concentration of 100 microM, niflumic acid significantly inhibited IBK(Ca)) evoked by caffeine (10 mM) but not by NS1619 (1-(2'-hydroxy-5'-trifluoromethylphenyl)-5-trifluoromethyl-2(3 H) benzimidazolone; 20 microM). In Cs(+)-containing solutions, niflumic acid (10-100 microM) did not inhibit voltage-sensitive Ca2+ currents. In intact portal veins, niflumic acid (1-300 microM) inhibited spontaneous mechanical activity, an action which was partially antagonised by glibenclamide (1-10 microM), and contractions produced by noradrenaline (10 microM), an effect which was glibenclamide-insensitive. It is concluded that inhibition of ICl(Ca) and stimulation of IK(ATP) both contribute to the mechano-inhibitory actions of niflumic acid in the rat portal vein.

Pharmacological characterization of the inwardly-rectifying current in the smooth muscle cells of the rat bladder

1. In freshly-isolated single cells of the rat bladder detrusor, outwardly-rectifying and inwardly-rectifying membrane currents were identified by the whole-cell voltage-clamp technique. 2. The inwardly-rectifying current (IIR) exhibited features of a cation current permeable to both K+ and Na+ but it was unaffected by changes in extracellular Ca2+. It had an activation threshold close to -60 mV and an estimated reversal potential of -29 mV. 3. IIR activated slowly with a voltage-sensitive time-constant of 69 ms at -140 mV and 209 ms at -100 mV but did not exhibit time-dependent inactivation. 4. IIR was unaffected by tetraethylammonium (up to 20 mM) but it was reduced by extracellular Ba2+ (1 mM) and by extracellular Cs+ (1 mM). 5. IIR was reduced by terikalant (100 microM) and markedly inhibited by ciclazindol (100 microM) although at these concentrations, both agents also reduced outward currents. 6. IIR was inhibited by ZD7288 (10-100 microM) in a concentration-dependent manner. At concentrations up to 30 microM, ZD7288 did not reduce the magnitude of outward currents but these were inhibited by 100 microM ZD7288. 7. In strips of bladder detrusor, spontaneous mechanical activity was increased by ZD7288 (0.3-100 microM) and by ciclazindol (0.3-100 microM) but was unaffected by glibenclamide (1-10 microM). 8. It is concluded that IIR closely resembles the hyperpolarization-activated current Ih, previously described in the smooth muscle of rabbit jejunum and in a variety of other cell types. This current may play an important role in modulating detrusor excitability but this could not be confirmed using the inhibitors ZD7288 and ciclazindol.

The novel cardioprotective agent BMS-180448 activates a potassium conductance in cardiac and vascular smooth muscle

The goal of the present study was to further characterize the effects of the novel cardioprotective agent BMS-180448 on potassium fluxes in cardiac and vascular smooth muscle. Exposure of voltage-clamped guinea pig ventricular myocytes to BMS-180448 (300 microM) produced an inhibition of IK followed by the delayed (5.5 +/- 0.5 min) activation of a large time-independent potassium current. At 100 microM, BMS-180448 produced only inhibition of IK. The BMS-180448 activated current was refractory to block by 30 microM glyburide but was largely inhibited by 100 microM alinidine (84 +/- 6% inhibition at +40 mV). Cromakalim (100 microM)-activated currents were fully inhibited by 3 microM glyburide and 79 +/- 4% blocked by 100 microM alinidine. The current responses to BMS-180448 were unaffected by the inclusion of 10 mM UDP (100 microM ATP) in the pipette. BMS-180448 also produced a concentration-dependent increase in 86Rb efflux from aortic strips; efflux responses were increased in low calcium medium and fully antagonized by 3 microM glyburide. Thus, BMS-180448 activates a potassium conductance in both cardiac and smooth muscle. The glyburide sensitivity of the BMS-180448-induced increase in 86Rb efflux from the aortic preparations suggests that this drug activates IKATP in vascular smooth muscle. Moreover, the observation that BMS-180448 (100 microM) partially inhibits the effects of cromakalim in ventricular muscle cells suggests that these drugs interact, directly or indirectly, with a common site in cardiac muscle.

The effects of pharmacological modulation of KATP on the guinea-pig isolated diaphragm

The purpose of the present study was to investigate the functional consequences of KATP modulation in the normal and the metabolically inhibited guinea-pig isolated diaphragm using the K+ channel openers cromakalim, pinacidil, RP49356 (N-methyl-2-(3-pyridil)-tetrahydrothiopyran-2-carbothiami de-1-oxide) and ZM260384 (2-(2,2-bis(difluoromethyl)-6-nitro-3,4-dihydro-2H-1,4-benzoxazine -4-yl)pyridine-N-oxide) and the K+ channel inhibitors glibenclamide, phentolamine and ciclazindol. All K+ channel openers accelerated the decline in function induced by intermittent tetanic contractions following metabolic inhibition and delayed the development of contracture. Cromakalim also improved the recovery of twitch tension following 10 min intermittent tetanic stimulation in the hypoxic guinea-pig diaphragm preparation. Of the K+ channel inhibitors tested, only ciclazindol, at the highest concentration tested (10 microM), significantly delayed the decline in tetanic tension following metabolic inhibition in the guinea-pig isolated diaphragm. None of the inhibitors significantly accelerated the development of contracture. All inhibitors however, antagonised the actions of the K+ channel opener, cromakalim. The results indicate that opening of KATP can accelerate the decline in function following metabolic inhibition in the guinea-pig isolated diaphragm. In the absence of K+ channel openers however, KATP does not appear to contribute to this decline under the conditions of the present study.

Investigation of the effects of 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) on membrane currents in rat portal vein

1. The effects of 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) were investigated on evoked and spontaneous currents in freshly-isolated cells from the rat portal vein by use of conventional whole-cell recording and perforated-patch techniques. 2. At a holding potential of -60 mV in potassium-free, caesium-containing solutions, NPPB (10 microM) inhibited calcium (Ca)-sensitive chloride currents (ICl(Ca)) evoked by caffeine (10 mM) and by noradrenaline (10 microM) by 58% and 96%, respectively. 3. At a holding potential of -2 mV in potassium (K)-containing solutions, NPPB (10 microM) inhibited charybdotoxin-sensitive K-currents (IBK(Ca)) induced by noradrenaline (10 microM) and acetylcholine (10 microM) by approximately 90%. In contrast, IBK(Ca) induced by caffeine (10 mM) was unaffected in the presence of NPPB (10 microM). Conversely, IBK(Ca) elicited by caffeine (2 mM) was reduced by approximately 50% whereas IBK(Ca) evoked by noradrenaline (50 microM) was not significantly inhibited by NPPB. 4. In K-containing solutions, NPPB (10 microM) abolished spontaneous transient outward currents (STOCs) and induced a slowly-developing outward K-current. Bath application of glibenclamide (10 microM) abolished the outward current but did not antagonize the inhibitory effects of NPPB on STOCs or on IBK(Ca) evoked by noradrenaline. 5. In caesium-containing solutions, NPPB (30 microM) inhibited voltage-sensitive Ca-currents. 6. In Ca-free, K-containing solutions and in the presence of glibenclamide (5 microM), IBK(Ca) induced by 20 microM NS1619 was enhanced by NPPB (10 microM). 7. It is concluded that NPPB inhibits agonist-induced ICl(Ca) in rat portal vein smooth muscle. However, this agent also inhibits agonist-evoked IBK(Ca) and STOCs. Moreover, NPPB inhibits voltage-sensitive Ca-currents and stimulates a glibenclamide-sensitive K-current and IBK(Ca). The effects of this agent on evoked ICl(Ca) and IBK(Ca) and on STOCs probably involves an inhibitory action on intracellular Ca-stores.

Effects of BRL55834 in rat portal vein and bovine trachea: evidence for the induction of a glibenclamide-resistant, ATP-sensitive potassium current

1. The effects of the benzopyran K-channel opener, BRL55834, on mechanical activity in bovine trachealis and rat portal vein were studied together with membrane currents in freshly-isolated single cells derived from these tissues. 2. BRL55834 (3 nM-1 microM) produced a concentration-dependent relaxation of bovine trachealis precontracted with 100 microM histamine and reduced the spontaneous mechanical activity of rat portal veins, effects which were antagonized by glibenclamide (1-10 microM) but were not reversible on washing. In contrast, charybdotoxin (250 nM) did not modify the spasmolytic effect of BRL55834 in bovine trachealis. 3. BRL55834 (10 nM-10 microM) did not relax segments of bovine trachealis precontracted with 80 mM KCl. 4. In some freshly-isolated single cells from bovine trachealis held at -10 mV, BRL55834 (3 microM) induced a time-independent outward K-current which was partially resistant to inhibition by glibenclamide (10 microM). In other cells, a very noisy, outwardly-rectifying and charybdotoxin-sensitive current developed in the presence of BRL55834 (3 microM) and in time-matched control cells. 5. In freshly-isolated single cells from rat portal vein held at -10 mV, BRL55834 (3 microM) induced a time- and calcium-independent outward K-current which was partially resistant (approximately 25% inhibition at +40 mV) to subsequent inhibition by glibenclamide (10 microM). In contrast, levcromakalim induced a time-independent outward K-current which was completely inhibited by glibenclamide 10 microM. 6. With the non-hydrolysable ATP analogue, AMP-PCP (5 mM), in the pipette, the ability of BRL55834 to induce a time-independent K-current in portal vein cells was markedly reduced (approximately 80% inhibition at +40 mV) whereas the effects of 10 microM levcromakalim were totally inhibited. 7. The glibenclamide-resistant current component induced by BRL55834 was totally inhibited by phentolamine (100 microM), a concentration that had no effect on the peak current (IBK(Ca)) induced by NS1619 (33 microM). 8. Stationary fluctuation analysis of the noise associated with the glibenclamide-insensitive K-current induced by BRL55834 in rat portal vein cells indicated that the unitary current flowing through the underlying channels was 0.26 pA at -10 mV, a value inconsistent with the involvement of BKCa. 9. It is concluded that the relaxations of both bovine trachea and rat portal vein produced by BRL55834 are associated with the opening of K-channels. These are probably identical to the ATP-sensitive K-channel opened by levcromakalim, although the involvement of an additional K-channel cannot be excluded. The reduced sensitivity of the BRL55834-induced changes to glibenclamide and toAMP-PCP may result from avid binding of BRL55834 to its site of action.

Pharmacology of the potassium channel openers

The potassium-channel openers comprise a large number of molecules that can be classified into three basic groups: (1) agents like levcromakalim that open a small-conductance (10-30 pS) glibenclamide-sensitive K+ channel currently known as the ATP-sensitive K+ channel, KATP; (2) hybrid molecules, such as nicorandil, that open KATP channels and that also activate the enzyme-soluble guanylate cyclase; (3) molecules like dehydrosaponin 1 that open the large-conductance (100-150 pS) calcium-dependent K+ channel, BKCa. K(+)-channel openers in groups 1 and 2 are most potent on smooth muscle, but KATP channels in cardiac muscle, neurones and the pancreatic beta cell are also affected. In vivo, moderate to high doses produce a fall in diastolic pressure with reflex tachycardia; low doses may exert selective dilator effects on specific vascular beds with little effect on systemic pressure. In vitro, all smooth muscles are relaxed with loss of spontaneous electric and mechanical activity; hyperpolarization to the region of EK is often observed. These effects can be antagonized by glibenclamide and also by imidazolines and guanidines, such as phentolamine, guanethidine, and antazoline, agents that also inhibit the smooth muscle delayed rectifier channel, KV. The mode and site of action of the group 1 and 2 K(+)-channel openers is the subject of intense study. Irrespective of their specific mode of action, the K(+)-channel openers, especially the hybrid molecules such as nicorandil, constitute a novel and promising approach to the treatment of cardiovascular disease.

Inhibition of delayed rectifier K(+)-current by levcromakalim in single intestinal smooth muscle cells: effects of cations and dependence on K(+)-flux

1. Whole-cell voltage-clamp recordings were made from single smooth muscle cells isolated from the longitudinal layer of the guinea-pig small intestine. 2. Levcromakalim ((-)Ckm) inhibited delayed rectifier K-current (IK(DR)) and induced a voltage-independent K-current (IK(-Ckm)). Both effects were inhibited similarly by glibenclamide. In some cells, however, IK(-Ckm) could be induced without any effect on IK(DR). 3. Ba2+ caused a voltage-dependent block of IK(-Ckm). The IC50 was 0.2 mM at -40 mV (6 cells), but at 0 mV 2 mM Ba2+ caused only a 26 +/- 7% inhibition (n = 5). Ba2+ had much less effect on IK(DR), 2 mM Ba2+ having no inhibitory effect on current elicited by depolarization to -30 mV (n = 6) or 0 mV (n = 5). 4. Low concentrations of Zn2+ blocked IK(-Ckm) while having little effect on IK(DR). Zn2+ (40 microM) caused a 77 +/- 1% reduction of IK(-Ckm) at -30 mV (n = 4) but IK(DR) was inhibited by only 10 +/- 3% at the same voltage (n = 4). 5. Inward current amplitudes were compared in 135 mM Rb+ and 135 mM K+ bath solutions. (-)Ckm-activated Rb(+)-current was only 4% of the K(+)-current, whereas delayed rectifier Rb(+)-current was larger than K(+)-current. 6. (-)Ckm did not inhibit IK(DR) if IK(-Ckm) was blocked. In the presence of 2 mM Ba2+ or 135 mM Rb+, (-)Ckm did not induce current nor did it inhibit the delayed rectifier. When [Rb+]o was 25 mM and [K+]J was 130 mM, (-)Ckm elicited outward current and inhibited outward delayed rectifier current (at voltages positive of the reversal potential) but it did not elicit inward current or inhibit inward delayed rectifier current (at voltages negative of the reversal potential).7. These experiments indicate that (-)Ckm-activated K channels are more sensitive to inhibition by Ba2+and Zn2+ and pass inward Rb+ current less well than delayed rectifier K channels. They also suggest that (-)Ckm does not modulate delayed rectifier K channels directly or via an intermediate protein but that the inhibitory effect of (-)Ckm on IK(DR) arises as a consequence of K+-flux through (-)Ckm activated K channels.

Ion channel modulation by NS 1619, the putative BKCa channel opener, in vascular smooth muscle

1. The effects of NS 1619, the putative BKCa channel opener, were investigated on rat intact portal veins and on single smooth muscle cells enzymatically separated from the same tissue. 2. Under whole-cell patch clamp conditions with K-rich pipettes, exposure of single cells held at -10 mV to NS 1619 (10-33 microM) induced a noisy, outward current which reached a maximum (33 microM NS 1619; mean 35.8 +/- 17 pA, n = 8) within about 6 min. 3. On stepping to test potentials (range -50 to +50 mV) from a holding potential of -10 mV, the NS 1619-induced noisy current exhibited time-dependent activation and marked outward rectification. 4. The stimulation of outward currents by NS 1619 at -10 mV was independent of the presence of Ca2+ in the bath or pipette solutions but was antagonized by either charybdotoxin (250 nM) or penitrem A (100 nM) in the bath solution. 5. Stationary fluctuation analysis of the noisy current induced by NS 1619 at -10 mV yielded a value of 70 +/- 8 pS (n = 4) (under the quasi-physiological conditions of the experiment) for the unitary conductance of the channel involved. 6. At -10 mV, NS 1619 (10-33 microM) rapidly inhibited spontaneous transient outward currents. 7. With a holding potential of -90 mV, NS 1619 (10-33 microM) produced a reduction of outward currents evoked by depolarizing steps to +50 mV, an effect associated with marked inhibition of the delayed rectifier current, IK(V). 8. NS 1619 (3-100 microM) produced a concentration-dependent inhibition of spontaneous activity in rat portal vein characterized by a reduction in the amplitude and duration of the tension waves. This inhibition was slightly potentiated in the presence of either charybdotoxin (250 nM) or penitrem A (1 microM). NS 1619 also totally inhibited contractions of rat aorta induced by KCl (both 20 mM and 80 mM). 9. Under whole-cell recording conditions and using Cs-rich pipettes, Ca-currents evoked in portal vein cells by stepping from a holding potential of - 90 mV to test potentials in the range - 30 to + 50 mV were totally inhibited in the presence of 33 JAM NS 1619.10. NS 1619 (33 JAM) inhibited the induction of IK(ATP) by levcromakalim (10 JAM).11. It is concluded that NS 1619 activates the large conductance, Ca2+-sensitive channel, BKca and over the same concentration range it inhibits both KV and L-type Ca-channels. The observed NS 1619-induced mechanical inhibition in rat portal vein and aorta seems most likely to be due to the observed inhibition of Ca-currents.

Levcromakalim-induced modulation of membrane potassium currents, intracellular calcium and mechanical activity in rat mesenteric artery

In freshly-dispersed cells from rat mesenteric artery, levcromakalim (1 and 10 microM) induced a non-inactivating potassium current (IKCO), an event which was associated with increased current noise. IKCO was fully inhibited in the presence of 10 microM glibenclamide. Stationary fluctuation analysis of the current noise associated with IKCO induced by levcromakalim at a holding potential of -10 mV indicated that the unitary conductance of the underlying K-channels was 10.2 pS at 0 mV under the quasi-physiological conditions of the experiment. In isolated arterioles both levcromakalim (10 nM-10 microM) and nifedipine (10 nM-10 microM) each elicited full, concentration-dependent, parallel reductions of the increases in [Ca2+]i (assessed using fura-2) and tension induced by 10 microM noradrenaline. However, the effects of both drugs on KCl-induced increases in tension and in [Ca2+]i, did not follow a simple relationship. Levcromakalim relaxed KCl- and noradrenaline-induced sustained contractions with a similar potency. This was in contrast to nifedipine which was approximately 20 times more potent against KCl-induced contractions. It is concluded that levcromakalim relaxes rat mesenteric arterioles primarily by the opening of a small conductance, glibenclamide-sensitive K-channel. An additional action of levcromakalim is suggested by its relative inability to suppress the increase in [Ca2+]i produced by 30 mM K(+)-PSS.

ATP-sensitive K+ channel opener acts as a potent Cl- channel inhibitor in vascular smooth muscle cells

We describe the activation of a K+ current and inhibition of a Cl- current by a cyanoguanidine activator of ATP-sensitive K+ channels (KATP) in the smooth muscle cell line A10. The efficacy of U83757, an analogue of pinacidil, as an activator of KATP was confirmed in single channel experiments on isolated ventricular myocytes. The effects of U83757 were examined in the clonal smooth muscle cell line A10 using voltage-sensitive dyes and digital fluorescent imaging techniques. Exposure of A10 cells to U83757 (10 nM to 1 microM) produced a rapid membrane hyperpolarization as monitored by the membrane potential-sensitive dye bis-oxonol ([diBAC4(3)], 5 microM). The U83757-induced hyperpolarization was antagonized by glyburide and tetrapropylammonium (TPrA) but not by tetraethlyl-ammonium (TEA) or charybdotoxin (ChTX). The molecular basis of the observed hyperpolarization was studied in whole-cell, voltage-clamp experiments. Exposure of voltage-clamped cells to U83757 (300 nM to 300 microM) produced a hyperpolarizing shift in the zero current potential; however, the hyperpolarizing shift in reversal potential was associated with either an increase or decrease in membrane conductance. In solutions where EK = -82 mV and ECl = 0 mV, the reversal potential of the U83757-sensitive current was approximately -70 mV in those experiments where an increase in membrane conductance was observed. In experiments in which a decrease in conductance was observed, the reversal potential of the U83757-sensitive current was approximately 0 mV, suggesting that U83757 might be acting as a Cl- channel blocker as well as a K+ channel opener. In experiments in which Cl- current activation was specifically brought about by cellular swelling and performed in solutions where Cl- was the major permeant ion, U83757 (300 nM to 300 microM) produced a dose-dependent current inhibition. Taken together these results (i) demonstrate the presence of a K(+)-selective current which is sensitive to KATP channel openers in A10 cells and (ii) indicate that the hyperpolarizing effects of K+ channel openers in vascular smooth muscle may be due to both the inhibition of Cl- currents as well as the activation of a K(+)-selective current.

Antagonism of levcromakalim by imidazoline- and guanidine-derivatives in rat portal vein: involvement of the delayed rectifier

1. In rat whole portal veins, guanabenz (100 nM to 10 microM) and antazoline (100 nM to 100 microM) each increased the amplitude, frequency and duration of spontaneous contractions. In addition, guanabenz (30 microM) and antazoline (30 microM) each antagonized the ability of levcromakalim (3 nM to 10 microM) to inhibit the spontaneous contractions of this tissue. 2. Whole-cell voltage-clamp recordings were made from freshly-isolated rat portal vein cells dispersed by a collagenase/pronase enzyme treatment. The ability of several agents (antazoline, cirazoline, clonidine, guanabenz and phentolamine, each containing an imidazoline or guanidine moiety), to modulate potassium (K) currents and to inhibit the actions of levcromakalim was investigated. 3. Antazoline, cirazoline, clonidine, guanabenz and phentolamine (each at a concentration of 30 microM) had little effect on control non-inactivating currents but inhibited the delayed-rectifier current, IK(V). 4. Levcromakalim (1 microM) induced a non-inactivating current, IK(ATP), and also inhibited the delayed rectifier current, IK(V). 5. Glibenclamide (1 microM) had no effect on control delayed rectifier or non-inactivating currents, but it inhibited the simultaneous induction of IK(ATP) and reduction of IK(V) produced by levcromakalim (1 microM). 6. Antazoline, cirazoline, clonidine and guanabenz (each at a concentration of 30 microM) prevented the induction of IK(ATP) by levcromakalim (1 microM). Phentolamine (30 microM) and clonidine (30 microM) each inhibited the IK(ATP) generated by levcromakalim (1 microM). 7. It is concluded that a variety of agents which possess either an imidazoline (antazoline, cirazoline, clonidine and phentolamine) or a guanidine (guanabenz) moiety within their structure inhibit the delayed rectifier current, IK(V). This action may thus be mediated via a so-called non-adrenoceptor imidazoline binding site. Furthermore, the ability of these ligands to inhibit IK(V) and to antagonize both the induction of IK(ATP) and the vasorelaxation produced by levcromakalim is consistent with the view that the channel (KATP) which underlies IK(ATP) is a voltage-insensitive state of the delayed rectifier K-channel (Kv).

Induction of a glibenclamide-sensitive K-current by modification of a delayed rectifier channel in rat portal vein in insulinoma cells

In insulinoma cells (RINm5F), the glibenclamide-sensitive K-current (IK(ATP)) which developed spontaneously or after exposure to levcromakalim or to butanedione monoxime was always accompanied by a reduction in the delayed rectifier current (IK(V)). At potentials over which IK(V) was fully activated, the total outward current remained constant. In rat portal vein, the delayed rectifier channel inhibitor, margatoxin, reduced the combined induction of IK(ATP) and inhibition of IK(V) by levcromakalim. These data suggest that the ATP-sensitive K-channel, K(ATP), is a voltage-insensitive state of the delayed rectifier, KV.

Levcromakalim may induce a voltage-independent K-current in rat portal veins by modifying the gating properties of the delayed rectifier

1. Smooth muscle cells of the rat portal vein were dispersed by enzymatic treatment and recordings of whole-cell currents under calcium-free conditions were made by the voltage-clamp technique. The effects of the potassium (K)-channel opener, levcromakalim, on K-currents were compared with those of agents which modify protein phosphorylation. 2. Levcromakalim (1-10 microM) added to the extracellular (bath) fluid caused the development of a non-inactivating current (IK(ATP)) and simultaneously inhibited the delayed rectifier current (IK(V)) in a concentration-dependent manner. On prolonged exposure to levcromakalim (10 microM), IK(ATP) declined and IK(V) was further diminished. 3. Addition to the pipette (intracellular) solution of the selective inhibitor of protein kinase C, calphostin C, itself had no effect on K-currents and did not modify the induction of IK(ATP) or the simultaneous inhibition of IK(V) produced by 1 microM levcromakalim. 4. Addition of the protein kinase inhibitor (PKI(6-22)amide, 1 microM) to the pipette solution caused the production of a glibenclamide-sensitive, non-inactivating current and inhibited IK(V). 5. In an assay system, levcromakalim (10 microM) did not inhibit the activity of purified protein kinase A (Type 1 or Type 2). 6. Addition to the pipette solution of the phosphatase inhibitor, okadaic acid (1 microM), did not itself modify K-currents and had little effect on the simultaneous induction of IK(ATP) and inhibition of IK(V) by levcromakalim (1 microM). 7. When the pipette solution contained 1 mM MgATP (but was depleted of substrates for ATP production), a non-inactivating, glibenclamide-sensitive K-current developed spontaneously in 5 out of 11 cells with the simultaneous reduction of IK(V). In 3 of the 6 remaining cells, addition of the dephosphorylating agent, butanedione monoxime (5 mM) to the bath inhibited IK(V) and stimulated a glibenclamide-sensitive non-inactivating current. 8. Depletion of intracellular Mg2+ slightly enhanced IK(V). Under these conditions, levcromakalim (1 microM and 10 microM) did not significantly induce IK(ATP) or inhibit IK(V). 9. It is concluded that the effects of levcromakalim on K-currents can be mimicked by procedures designed to reduce channel phosphorylation. The results are consistent with the view that levcromkalim dephosphorylates the delayed rectifier channel, KV, which becomes converted into a voltage-independent, non-inactivating form known as KATP. The possible mechanisms which underlie this interconversion are discussed.

Do the K+ channel openers relax smooth muscle by opening K+ channels?

During the past decade, a group of chemically heterogeneous compounds known as the K+ channel openers has emerged. These compounds open a certain class of K+ channels (ATP-sensitive K+ channels) in the sarcolemma of vascular smooth muscle cells, which leads to hyperpolarization of the cell membrane and relaxation of the tissue. The mechanisms by which hyperpolarization affects smooth muscle contraction and contractility can thus be examined. Hyperpolarization induced by these K+ channel openers prevents Ca2+ entry through voltage-operated Ca2+ channels. Surprisingly, and by mechanisms not yet defined, hyperpolarization of the cell also reduces agonist-induced accumulation of inositol 1,4,5-trisphosphate (and consequently, Ca2+ mobilization from intracellular stores), and the Ca2+ sensitivity of the contractile apparatus. In addition, recent evidence reviewed here by Ulrich Quast suggests that the K+ channel openers possess further mechanisms of vasorelaxation not linked to the opening of plasmalemmal K+ channels.

Effects of rubidium on responses to potassium channel openers in rat isolated aorta

1. In a physiological salt solution (PSS) in which potassium (K) was replaced by rubidium (Rb), segments of rat aorta precontracted with 20 mM RbCl were fully relaxed by K-channel openers with an order of potency levcromakalim > cromakalim > aprikalim > RP 49356. These relaxations were inhibited by glibenclamide. 2. Segments of rat aorta bathed in normal PSS and precontracted with 20 mM KCl were also relaxed by these K-channel openers with an order of potency levcromakalim > cromakalim > aprikalim > RP 49356. These relaxations were glibenclamide-sensitive. However, the absolute potencies of the K-channel openers were approximately four times greater in normal PSS than in RbPSS. 3. In RbPSS, minoxidil sulphate relaxed segments of aorta precontracted with 20 mM RbCl by approximately 20% whereas in normal PSS it fully relaxed those contracted with 20 mM KCl. 4. In RbPSS, levcromakalim-induced relaxation of aortic segments precontracted with 20 mM RbCl was initially well-maintained but then faded by approximately 60% of the initial relaxation to a new, stable level. Subsequent exposure to RP 49356 or to higher concentrations of levcromakalim was without further relaxant effect. Similar changes were observed when RP 49356 was the initial relaxant and tissues were exposed to either RP 49356 or levcromakalim. In normal PSS, levcromakalim- or RP 49356-induced relaxation of contractions produced by 20 mM KCl was well-maintained. 5. In RbPSS, minoxidil sulphate-induced relaxation of aortic segments precontracted with 20 mM RbCl was well-maintained. Subsequent exposure to either levcromakalim or to RP 49356 produced further tissue relaxation. 6. In RbPSS, levcromakalim produced no detectable increase in either 86Rb- or 42K-efflux from rat aortic strips. In normal PSS, a significant increase in the exchange of both isotopes was detected.7. Levcromakalim hyperpolarized segments of rat aorta bathed both in normal PSS and after depolarization by the addition of 20 mM KCI. Exposure to RbPSS depolarized the tissue and under these conditions, levcromakalim had no effect on membrane potential.8. In Rb- and normal PSS, levcromakalim produced a similar degree of inhibition of the refilling of then or adrenaline-sensitive Ca store.9. It is concluded that millimolar concentrations of Rb inhibit the plasmalemmal ATP-sensitive K-channels (KATP) which are the target of the K-channel openers. The relaxant actions of the K-channel openers in both normal and Rb-PSS and the inhibition of these effects by glibenclamide may reflect a functional interaction between these agents at ATP-binding sites associated with both KATP and with intracellular structures including Ca stores.