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

Measuring T-Type Calcium Channel Currents in Isolated Vascular Smooth Muscle Cells

Patch clamp electrophysiology is a powerful tool that has been important in isolating and characterizing the ion channels that govern cellular excitability under physiological and pathophysiological conditions. The ability to enzymatically dissociate blood vessels and acutely isolate vascular smooth muscle cells has enabled the application of patch clamp electrophysiology to the identification of diverse voltage dependent ion channels that ultimately control vasoconstriction and vasodilation. Since intraluminal pressure results in depolarization of vascular smooth muscle, the channels that control the voltage dependent influx of extracellular calcium are of particular interest. This chapter describes methods for isolating smooth muscle cells from resistance vessels, and for recording, isolating, and characterizing voltage dependent calcium channel currents, using patch clamp electrophysiological and pharmacological protocols.

Smooth muscle Ca(2+) -activated and voltage-gated K+ channels modulate conducted dilation in rat isolated small mesenteric arteries

OBJECTIVE

  To assess the influence of blocking smooth muscle large conductance Ca(2+) -activated K+ channels and voltage-gated K+ channels on the conducted dilation to ACh and isoproterenol.

MATERIALS AND METHODS

  Rat mesenteric arteries were isolated with a bifurcation, triple-cannulated, pressurized and imaged using confocal microscopy. Phenylephrine was added to the superfusate to generate tone, and agonists perfused into a sidebranch to evoke local dilation and subsequent conducted dilation into the feed artery.

RESULTS

  Both ACh- and isoproterenol-stimulated local and conducted dilation with similar magnitudes of decay with distance along the feed artery (2000μm: ∼15% maximum dilation). The gap junction uncoupler carbenoxolone prevented both conducted dilation and intercellular spread of dye through gap junctions. IbTx, TEA or 4-AP, blockers of large conductance Ca(2+) -activated K+ channels and voltage-gated K+ channels, did not affect conducted dilation to either agonist. A combination of either IbTx or TEA with 4-AP markedly improved the extent of conducted dilation to both agonists (2000μm: >50% maximum dilation). The enhanced conducted dilation was reflected in the hyperpolarization to ACh (2000μm: Control, 4±1 mV, n = 3; TEA with 4-AP, 14±3mV, n=4), and was dependent on the endothelium.

CONCLUSIONS

  These data show that activated BK(Ca) and K(V) -channels serve to reduce the effectiveness of conducted dilation.

Development of an image-based system for measurement of membrane potential, intracellular Ca(2+) and contraction in arteriolar smooth muscle cells

OBJECTIVE

Changes in smooth muscle cell (SMC) membrane potential (Em) are critical to vasomotor responses. As a fluorescent indicator approach would lessen limitations of glass electrodes in contracting preparations, we aimed to develop a Forster (or fluorescence) resonance energy transfer (FRET)-based measurement for Em.

METHODS

The FRET pair used in this study (donor CC2-DMPE [excitation 405 nm] and acceptor DisBAC(4) (3)) provide rapid measurements at a sensitivity not achievable with many ratiometric indicators. The method also combined measurement of changes in Ca(2+) (i) using fluo-4 and excitation at 490 nm.

RESULTS

After establishing loading conditions, a linear relationship was demonstrated between Em and fluorescence signal in FRET dye-loaded HEK cells held under voltage clamp. Over the voltage range from -70 to +30 mV, slope (of FRET signal vs. voltage, m) = 0.49 ± 0.07, r(2)  = 0.96 ± 0.025. Similar data were obtained in cerebral artery SMCs, slope (m) = 0.30 ± 0.02, r(2)  = 0.98 ± 0.02. Change in FRET emission ratio over the holding potential of -70 to +30 mV was 41.7 ± 4.9% for HEK cells and 30.0 ± 2.3% for arterial SMCs. The FRET signal was also shown to be modulated by KCl-induced depolarization in a concentration-dependent manner. Further, in isolated arterial SMCs, KCl-induced depolarization (60 mM) measurements occurred with increased fluo-4 fluorescence emission (62 ± 9%) and contraction (-27 ± 4.2%).

CONCLUSIONS

The data support the FRET-based approach for measuring changes in Em in arterial SMCs. Further, image-based measurements of Em can be combined with analysis of temporal changes in Ca(2+) (i) and contraction.

Direct stimulation of K(ATP) channels by exogenous and endogenous hydrogen sulfide in vascular smooth muscle cells

ATP-sensitive K+ (K(ATP)) channels in vascular smooth muscle cells (VSMC) are important targets for endogenous metabolic regulation and exogenous drug therapy. H2S, as a novel gasotransmitter, has been shown to relax rat aortic tissues via opening of K(ATP) channels. However, interaction of H2S, exogenous-applied or endogenous-produced, with K(ATP) channels in resistance artery VSMC has not been delineated. In the present study, using the whole-cell and single-channel patch-clamp technique, we demonstrated that exogenous H2S activated K(ATP) channels and hyperpolarized cell membrane in rat mesenteric artery VSMC. H2S enhanced the amplitude of whole-cell K(ATP) currents with an EC50 value of 116 +/- 8.3 microM and increased the open probability of single K(ATP) channels. H2S hyperpolarized membrane potentials by -12 mV in nystatin-perforated VSMC. Furthermore, inhibition of endogenous H2S production with D,L-propargylglycine (PPG) reduced whole-cell K(ATP) currents. PPG alone had no effect on unitary K(ATP) channel currents in cell-free membrane patches. In addition, effects of H2S on K(ATP) channels and membrane potentials were independent of cGMP-mediated phosphorylation. This study demonstrated modulation of K(ATP) channel activity by exogenous and endogenous H2S in resistance artery VSMC, thus helping elucidate cardiovascular functions of this endogenous gas.

Increased Na+ intake during gestation in rats is associated with enhanced vascular reactivity and alterations of K+ and Ca2+ function

Gestation is associated with decreased blood pressure and resistance to the effects of vasoconstrictor agents. A recent study showed that pregnant rats, on increased sodium intake, present physiological changes that resemble those observed in preeclampsia. We investigated the effects of sodium supplementation on reactivity and on potassium and Ca2+ channel activity in blood vessels during gestation. Sodium supplements, 0.9% or 1.8% NaCl as drinking water, were given to nonpregnant and pregnant rats for 7 days (last week of gestation). Reactivity to phenylephrine (PE), KCl, arginine vasopressin (AVP), and tetraethylammonium (TEA) was measured in aortic rings under modulation of potassium and calcium channels. TEA, a nonselective K+ channel inhibitor, induced concentration-dependent responses in aortic rings from nonpregnant but not in those from pregnant rats. The response to TEA was restored in rings from pregnant rats after preincubation with 10 mmol/l KCl. Sodium supplementation did not affect the response to TEA in the aortas of pregnant animals. After sodium supplementation, maximum responses to PE and AVP were decreased and increased in aortic rings from nonpregnant and pregnant rats, respectively. Cromakalim (an ATP-sensitive K+ channel activator)-induced inhibition of the responses to the three vasoconstrictors was more striking in aorta from nonpregnant than pregnant rats on regular diet, whereas it produced similar inhibition in tissues from both groups of animals on 0.9% and 1.8% NaCl. NS-1619 (a Ca2+-sensitive K+ activator) elicited heightened effects in the aortas of pregnant animals receiving 0.9% NaCl supplementation. Nifedipine (a Ca2+ channel blocker) caused greater inhibition of the contractile responses in tissues from nonpregnant rats on regular diet, and its action was increased in pregnant rats on sodium-supplemented diets. These data demonstrate that augmented sodium intake during gestation in the rat is linked with the reversal of gestational-associated resistance to vasopressors and indicate that this is an experimental model showing some features of gestational hypertension.

Vasorelaxant effects of the potassium channel opener SR 47063 on the isolated human saphenous vein and rat aorta

The vasorelaxant effects of SR 47063 (4-(2-cyanimino-1, 2-dihydropyrid-1-yl)-2,2-dimethyl-6-nitr ochromene), a new K(+)-channel opener structurally related to levcromakalim, were examined in isolated human saphenous vein (HSV) and rat aorta (RA). HSV or RA rings were precontracted with either KCl or noradrenaline and cumulative relaxant concentration-response curves were obtained for SR 47063 (0.1 nM to 1 microM) in the presence or absence of 3 microM glibenclamide. SR 47063 potently relaxed HSV and RA precontracted with 20 mM (but not 60 mM) KCl or 10 microM noradrenaline in a concentration-dependent manner, showing slightly greater activity in the aorta. The potency of the effect of SR 47063 on HSV and RA was 12- and 58-fold greater, respectively, than that reported for the structurally related K(+)-channel opener levcromakalim. The vasorelaxant action of SR 47063 in both blood vessels was strongly inhibited by 3 microM glibenclamide, consistent with a mechanism of action involving ATP-dependent K(+)-channels.

Inhibitory action of niflumic acid on noradrenaline- and 5-hydroxytryptamine-induced pressor responses in the isolated mesenteric vascular bed of the rat

1. The effects of niflumic acid, an inhibitor of calcium-activated chloride currents, were compared with the actions of the calcium channel blocker nifedipine on noradrenaline- and 5-hydroxytryptamine (5-HT)-induced pressor responses of the rat perfused isolated mesenteric vascular bed. 2. Bolus injections of noradrenaline (1 and 10 nmol) increased the perfusion pressure in a dose-dependent manner. Nifedipine (1 microM) inhibited the increase in pressure produced by 1 nmol noradrenaline by 31 +/- 5%. Niflumic acid (10 and 30 microM) also inhibited the noradrenaline-induced increase in perfusion pressure and 30 microM niflumic acid reduced the pressor response to 1 nmol noradrenaline by 34 +/- 6%. 3. The increases in perfusion elicited by 5-HT (0.3 and 3 nmol) were reduced by niflumic acid (10 and 30 microM) in a concentration-dependent manner and 30 microM niflumic acid inhibited responses to 0.3 and 3 nmol 5-HT by, respectively, 49 +/- 8% and 50 +/- 7%. Nifedipine (1 microM) decreased the pressor response to 3 nmol 5-HT by 44 +/- 9%. 4. In the presence of a combination of 30 microM niflumic acid and 1 microM nifedipine the inhibition of the pressor effects of noradrenaline (10 nmol) and 5-HT (3 nmol) was not significantly greater than with niflumic acid (30 microM) alone. Thus the effects of niflumic acid and nifedipine were not additive. 5. In Ca-free conditions the transient contractions induced by 5-HT (3 nmol) were not reduced by 30 microM niflumic acid, suggesting that this agent does not inhibit calcium release from the intracellular store or the binding of 5-HT to its receptor. 6. Niflumic acid 30 microM did not inhibit the pressor responses induced by KCl (20 and 60 mumol) which were markedly reduced by 1 microM nifedipine. In addition, 1 microM levcromakalim decreased pressor responses produced by 20 mumol KCl. These data suggest that niflumic acid does not block directly calcium channels or activate potassium channels. 7. It is concluded that niflumic acid selectively reduces a component of noradrenaline- and 5-HT-induced pressor responses by inhibiting a mechanism which leads to the opening of voltage-gated calcium channels. Our data suggest that the Ca(2+)-activated chloride conductance may play a pivotal role in the activation of voltage-gated calcium channels in agonist-induced constriction of resistance blood vessels.

Potassium channel openers prevent potassium-induced calcium loading of cardiac cells: possible implications in cardioplegia

Hyperkalemic solutions that are used as cardioplegic agents, while effective in inducing electromechanical arrest, are only partially cardioprotective, and ventricular dysfunction has been observed. The underlying pathophysiology of cardioplegia-associated ventricular dysfunction is complex and not fully understood, but it could be related, in part, to intracellular Ca2+ loading induced by high K+ concentrations present in cardioplegic solutions. Yet no effective cytoprotective means against possible intracellular Ca2+ loading, under these conditions, has been described. Recently, potassium channel openers, which open adenosine triphosphate-sensitive K+ channels, have been reported to possess cardioprotective properties under global ischemic conditions. However, it is not known whether these novel agents could prevent intracellular Ca2+ loading that could occur during cardioplegia. Intracellular Ca2+ was monitored in ventricular myocytes, loaded with the Ca(2+)-sensitive fluorescent probe Fluo-3AM, using epifluorescent digital imaging and laser confocal microscopy. Exposure of a myocyte to a 16 mmol/L concentration of K+, a concentration of K+ commonly used in cardioplegic solutions, induced a nonhomogeneous increase in intracellular Ca2+. Potassium channel opening drugs, such as aprikalim or nicorandil, effectively prevented these solutions from increasing intracellular Ca2+. The preventive effect of potassium channel opening drugs was antagonized by glyburide, a selective blocker of adenosine triphosphate-sensitive K+ channels. This study demonstrates, at the single cardiac cell level, that solutions containing a 16 mmol/L concentration of K+ promote intracellular Ca2+ loading, which can be prevented by potassium channel opening drugs. Therefore, potassium channel opening drugs should be considered to prevent intracellular Ca2+ loading associated with the use of cardioplegic solutions.

Differential vasorelaxant effects of levcromakalim and P1060 in the isolated KCl- and RbCl-precontracted human saphenous vein: possible involvement of intracellular Ca2+ stores

The influence of rubidium-substituted physiological salt solution (Rb-PSS) on the relaxant effects of K+ channel openers was investigated in the human saphenous vein. In tissues precontracted with 20 mM KCl (in K-PSS) levcromakalim and P1060 produced complete, sustained relaxations. However, in Rb-PSS (containing 20 mM RbCl) these effects were inhibited and, although complete relaxations still occurred, were transient. When caffeine was applied at the beginning of this fade of levcromakalim-induced relaxation in Rb-PSS its contractile effect was potentiated. Similarly, the contraction to noradrenaline was potentiated when applied at the beginning of this fade of levcromakalim-induced relaxation, whereas this response was attenuated in control tissues bathed in 20 mM KCl (in K-PSS). Our results show that the relaxant effects of K+ channel openers in human saphenous vein are inhibited in Rb-PSS, in agreement with previous studies in animal tissue, and suggest that an increased Ca2+ uptake into intracellular stores may be contributory to vasorelaxation.

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.

Activation by intracellular GDP, metabolic inhibition and pinacidil of a glibenclamide-sensitive K-channel in smooth muscle cells of rat mesenteric artery

1. Single-channel recordings were made from cell-attached and isolated patches, and whole-cell currents were recorded under voltage clamp from single smooth muscle cells obtained by enzymic digestion of a small branch of the rat mesenteric artery. 2. In single voltage-clamped cells 1 mM uridine diphosphate (UDP) or guanidine diphosphate (GDP) added to the pipette solution, or pinacidil (100 microM) a K-channel opener (KCO) applied in the bathing solution, evoked an outward current of up to 100pA which was blocked by glibenclamide (10 microM). In single cells from which recordings were made by the 'perforated patch' (nystatin pipette) technique, metabolic inhibition by 1 mM NaCN and 10 mM 2-deoxy-glucose also evoked a similar glibenclamide-sensitive current. 3. Single K-channel activity was observed in cell-attached patches only infrequently unless the metabolism of the cell was inhibited, whereupon channel activity blocked by glibenclamide was seen; pinacidil applied to the cell evoked similar glibenclamide-sensitive channel activity. If the patch was pulled off the cell to form an isolated inside-out patch, similar glibenclamide-sensitive single-channel currents were observed in the presence of UDP and/or pinacidil to those seen in cell-attached mode; channel conductance was 20 pS (60:130 K-gradient) and openings showed no voltage-dependence and noisy inward currents, typical of the nucleoside diphosphate (NDP) activated K-channel (KNDP) seen previously in rabbit portal vein. 4. Formation of an isolated inside-out patch into an ATP-free solution did not increase the probability of channel opening which declined with time even when some single-channel activity had occurred in the cell-attached mode before detachment. However, application of 1 mM UDP or GDP, but not ATP, to inside-out patches evoked single-channel activity. Application of ATP-free solution to isolated patches, previously exposed to ATP and in which channel activity had been seen, did not evoke channel activity. 5. It is concluded that small conductance K-channels (KNDP) open in smooth muscle cells from this small artery in response to UDP or GDP acting from the inside, or pinacidil acting from the outside; the same channels open during inhibition of metabolism presumably mainly due to the rise in nucleoside diphosphates, but a fall in the ATP concentration on the inside of the channel did not by itself evoke channel activity. Failure to respond to a fall in ATP concentration upon formation of an inside-out patch could not be due to dephosphorylation of the channel because sometimes it had been active previously during cell-attached recording. NDPs, instead of ATP, are more important regulators of KNDP channels. It is suggested that the KNDP is the main target K-channel for KCOs.

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).

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.