Potassium channel modulation in rat portal vein by ATP depletion: a comparison with the effects of levcromakalim (BRL 38227)

Venous Vasomotion

Veins exhibit spontaneous contractile activity, a phenomenon generally termed vasomotion. This is mediated by spontaneous rhythmical contractions of mural cells (i.e. smooth muscle cells (SMCs) or pericytes) in the wall of the vessel. Vasomotion occurs through interconnected oscillators within and between mural cells, entraining their cycles. Pharmacological studies indicate that a key oscillator underlying vasomotion is the rhythmical calcium ion (Ca²⁺) release-refill cycle of Ca²⁺ stores. This occurs through opening of inositol 1,4,5-trisphosphate receptor (IP₃R)- and/or ryanodine receptor (RyR)-operated Ca²⁺ release channels in the sarcoplasmic/endoplasmic (SR/ER) reticulum and refilling by the SR/ER reticulum Ca²⁺ATPase (SERCA). Released Ca²⁺ from stores near the plasma membrane diffuse through the cytosol to open Ca²⁺-activated chloride (Cl^-) channels, this generating inward current through an efflux of Cl^-. The resultant depolarisation leads to the opening of voltage-dependent Ca²⁺ channels and possibly increased production of IP₃, which through Ca²⁺-induced Ca²⁺ release (CICR) of IP₃Rs and/or RyRs and IP₃R-mediated Ca²⁺ release provide a means by which store oscillators entrain their activity. Intercellular entrainment normally involves current flow through gap junctions that interconnect mural cells and in many cases this is aided by additional connectivity through the endothelium. Once entrainment has occurred the substantial Ca²⁺ entry that results from the near-synchronous depolarisations leads to rhythmical contractions of the mural cells, this often leading to vessel constriction. The basis for venous/venular vasomotion has yet to be fully delineated but could improve both venous drainage and capillary/venular absorption of blood plasma-associated fluids.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

The gasotransmitter role of hydrogen sulfide

A novel concept of "gasotransmitter" arrived recently. Gasotransmitters are small molecules of endogenous gases with important physiological functions. Their production and metabolism are enzymatically regulated, and their effects are not dependent on specific membrane receptors. Following the identification of nitric oxide and carbon monoxide as gasotransmitters, hydrogen sulfide (H(2)S) may be qualified as the third gasotransmitter. Recent studies have shown that H(2)S is generated from vascular smooth muscle cells (SMCs), catalyzed by specific H(2)S-generating enzyme. At physiologically relevant concentrations, H(2)S relaxes vascular tissues, an effect mediated by the activation of ATP-sensitive K(+) (K(ATP)) channels in vascular SMCs. H(2)S directly alters the activity of K(ATP) channels without the involvement of second messengers. Furthermore, the endogenous production of H(2)S in the cardiovascular system is likely regulated by nitric oxide, whereas the vasorelaxant effect of nitric oxide is inhibited by H(2)S. It is anticipated that future studies will better reveal the molecular mechanisms underlying the effect of H(2)S on K(ATP) channel proteins, the interaction of H(2)S and other gasotransmitters in cardiovascular system, the endogenous stimulators and inhibitors of H(2)S metabolism, the role of H(2)S in the regulation of heart function, and the abnormal H(2)S production and action under various pathophysiological conditions.

Inotropic responses to human gene 2 (B29) relaxin in a rat model of myocardial infarction (MI): effect of pertussis toxin

Relaxin produces powerful inotropic and chronotropic responses in isolated atria. The effect of relaxin has been examined in a rat model of cardiac failure, induced by myocardial infarction (MI). Maximum inotropic responses to isoprenaline (sham 5.4+/-0.3 mN; MI 2.6+/-0.3 mN; P<0.001) and relaxin (sham 5.1+/-0.6 mN; MI 2.8+/-0.5 mN; P=0.013) were reduced in left atria following MI. No change in chronotropic responsiveness was observed in right atria. Pertussis toxin (PTX) treatment restored inotropic responses to isoprenaline (sham 5.5+/-1.3 mN; MI 5.8+/-1.0 mN; P=0.850) but not to relaxin. Instead, PTX reduced inotropic responses to relaxin in sham animals to the same level seen in the MI group (sham 3.2+/-1.7 mN; MI 2.8+/-0.6 mN; P=0.847). In right atria, PTX treatment did not affect the maximum chronotropic response to isoprenaline, but reduced responses to relaxin in both sham and MI animals. R3 relaxin and relaxin receptor (LGR7) mRNA was present in atria and left ventricle (LV) from sham and MI animals. R3 relaxin mRNA expression was increased in atria but not LV from MI animals. LGR7 mRNA expression was reduced in atria and LV from MI animals. PTX treatment in unoperated rats increased chronotropic responses (vehicle 184.3+/-5.3 beats min(-1); PTX 211.3+/-9.5 beats min(-1); P=0.029) and produced a rightward shift in the concentration-response curve to isoprenaline in left atria. PTX reduced inotropic (vehicle 3.3+/-0.7 mN; PTX 0.8+/-0.2 mN; P=0.005) and chronotropic (vehicle 130.2+/-8.1 beats min(-1); PTX 90.6+/-11.1 beats min(-1); P=0.012) responses to relaxin. 6 In left atria, relaxin produced a small increase in cAMP compared to those produced by isoprenaline and forskolin. However, PTX treatment significantly reduced relaxin-, isoprenaline- and forskolin-stimulated cAMP accumulation. Cardiac failure in MI animals caused a reduced inotropic response to both relaxin and (-)-isoprenaline. In non-MI animals, PTX treatment also reduced inotropic responses to relaxin. Differences between responses to (-)-isoprenaline and relaxin can be explained by changes in coupling efficiency occurring at the level of adenylate cyclase.

Impaired isoproterenol-induced hyperpolarization in isolated mesenteric arteries of aged rats

Stimulation of vascular beta-adrenoceptors leads to membrane hyperpolarization, presumably via the beta-adrenoceptor/G(s) protein/adenylate cyclase signaling cascade; the ionic mechanisms of this phenomenon remain unclear. beta-Adrenoceptor-mediated vascular relaxation is impaired with aging; however, little is known concerning whether beta-adrenoceptor-mediated hyperpolarization is altered with aging. We sought to determine the ionic mechanisms of isoproterenol-induced hyperpolarization in the rat mesenteric resistance artery, as well as the age-related changes in isoproterenol-induced hyperpolarization and their underlying mechanisms. Isoproterenol-induced hyperpolarization was inhibited by high-K(+) solution and glibenclamide (10(-6) mol/L), an inhibitor of ATP-sensitive K(+) channels (K(ATP)), but not by apamin, iberiotoxin, or charybdotoxin, inhibitors of Ca(2+)-activated K(+) channels. Isoproterenol-induced hyperpolarization was markedly less in aged rats (>/=24 months) than in adults rats (12 to 20 weeks) (3x10(-6) mol/L; -3.1 versus -9.9 mV; P<0.001; n=8 to 9). Cholera toxin (10(-9) g/mL), an activator of G(s), evoked hyperpolarization only in adult rats. Hyperpolarization to forskolin, a direct activator of adenylate cyclase, was also reduced to some extent in aged rats (10(-5) mol/L; -8.8 versus -13 mV; P<0.05; n=6), whereas hyperpolarization to levcromakalim, a K(ATP) opener, was comparable in both groups. These findings suggest that isoproterenol elicits hyperpolarization via an opening of K(ATP) in the rat resistance artery and that isoproterenol-induced hyperpolarization is attenuated in aged rats mainly because of a defective coupling of beta-adrenoceptors to adenylate cyclase and partly because of a defect at the level of adenylate cyclase, but not because of an alteration of K(ATP) per se.

Levcromakalim decreases cytoplasmic Ca2+ and vascular tone in basilar artery of SAH model dogs

We investigated the effects of levcromakalim, a K+ channel opener, on [Ca2+]i and the contractile force of basilar arteries obtained from normal dogs and subarachnoid hemorrhage (SAH) dogs. The responsiveness to serotonin was increased more in the SAH group than in the control group. Levcromakalim decreased the resting [Ca2+]i and force more profoundly than did a Ca2+ channel blocker, nicardipine, and these effects were more prominent in the SAH group than in the control group. Levcromakalim diminished the increases in [Ca2+]i and contractile force induced by serotonin more profoundly than nicardipine did, and these effects were equal in both groups. The effects of levcromakalim were not inhibited by iberiotoxin but were antagonized completely by glibenclamide. These results suggest that levcromakalim-induced opening of adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channels reduces [Ca2+]i more effectively than does nicardipine and that levcromakalim exerts the vasodilator effects under the condition in which large conductance Ca2+-activated K+ (BK) channels are blocked with iberiotoxin. Consequently, K+ channel openers like levcromakalim may be useful drug candidates to treat delayed cerebral vasospasm after SAH.

K+ channel modulation in arterial smooth muscle

Potassium channels play an essential role in the membrane potential of arterial smooth muscle, and also in regulating contractile tone. Four types of K+ channel have been described in vascular smooth muscle: Voltage-activated K+ channels (Kv) are encoded by the Kv gene family, Ca(2+)-activated K+ channels (BKCa) are encoded by the slo gene, inward rectifiers (KIR) by Kir2.0, and ATP-sensitive K+ channels (KATP) by Kir6.0 and sulphonylurea receptor genes. In smooth muscle, the channel subunit genes reported to be expressed are: Kv1.0, Kv1.2, Kv1.4-1.6, Kv2.1, Kv9.3, Kv beta 1-beta 4, slo alpha and beta, Kir2.1, Kir6.2, and SUR1 and SUR2. Arterial K+ channels are modulated by physiological vasodilators, which increase K+ channel activity, and vasoconstrictors, which decrease it. Several vasodilators acting at receptors linked to cAMP-dependent protein kinase activate KATP channels. These include adenosine, calcitonin gene-related peptide, and beta-adrenoceptor agonists. beta-adrenoceptors can also activate BKCa and Kv channels. Several vasoconstrictors that activate protein kinase C inhibit KATP channels, and inhibition of BKCa and Kv channels through PKC has also been described. Activators of cGMP-dependent protein kinase, in particular NO, activate BKCa channels, and possibly KATP channels. Hypoxia leads to activation of KATP channels, and activation of BKCa channels has also been reported. Hypoxic pulmonary vasoconstriction involves inhibition of Kv channels. Vasodilation to increased external K+ involves KIR channels. Endothelium-derived hyperpolarizing factor activates K+ channels that are not yet clearly defined. Such K+ channel modulations, through their effects on membrane potential and contractile tone, make important contributions to the regulation of blood flow.

Comparison of relaxin receptors in rat isolated atria and uterus by use of synthetic and native relaxin analogues

1. The receptors for relaxin in the rat atria and uterus were investigated and compared by use of a series of synthetic and native relaxin analogues. The assays used were the positive chronotropic and inotropic effects in rat spontaneously beating, isolated right atrium and electrically driven left atrium and the relaxation of K+ precontracted uterine smooth muscle. 2. Relaxin analogues with an intact A- and B-chain were active in producing powerful chronotropic and inotropic effects in the rat isolated atria at nanomolar concentrations. Single-chain analogues and structural homologues of relaxin such as human insulin and sheep insulin-like growth factor I had no agonist action and did not antagonize the effect of the B29 form of human gene 2 relaxin. 3. Shortening the B-chain carboxyl terminal of human gene 1 (B2-29) relaxin to B2-26 reduced the activity of the peptide and removal of another 2 amino acid residues (B2-24) abolished the activity. This suggests that the B-chain length may be important for determination of the activity of relaxin. More detailed studies are needed to determine the effect of progressive amino acid removal on the structure and the bioactivity of relaxin. 4. Porcine prorelaxin was as active as porcine relaxin on a molar basis, suggesting that the presence of the intact C-peptide did not affect the binding of the prorelaxin to the receptor to produce functional responses. 5. Relaxin caused relaxation of uterine longitudinal and circular smooth muscle precontracted with 40 mM K+. The pEC50 values for human gene 2 and porcine relaxins were lower than those in the atrial assay, but rat relaxin had similar pEC50 values in both atrial and uterine assays. Rat relaxin was significantly less potent than either human gene 2 or porcine relaxin in the atrial assay, but in the uterine assay they were equipotent. The results suggest that the relaxin receptor or the signalling pathway in rat atria may differ from that in the uterus.

Multiple types of ion channels in cavernous smooth muscle

The excitation process of the smooth muscle of corpus cavernosum is important to the erectile process. single-cell electrophysiology performed during the last decade has given deep insights into the different current components participating in that process. However, the existence of a tremendous amount of literature might be confusing for the nonspecialist in the field of smooth-muscle excitation and, especially, in the excitation of corpus cavernosum. In a compact form, this paper gives an overview on significant ionic currents as well as their ability to change the membrane potential and to enhance or inhibit calcium influx for the modulation of smooth-muscle tone.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

ATP-sensitive potassium channels in isolated rat aorta during physiologic, hypoxic, and low-glucose conditions

In arterial smooth muscle, adenosine triphosphate (ATP)-sensitive potassium channels are the targets of a variety of synthetic and endogenous vasodilators. In this study, we evaluated the influence of glibenclamide, an ATP-sensitive K(+)-channel blocker, on various vasodilator responses, including those by levcromakalim under hypoxic and low-glucose conditions in isolated rat aortic rings. The concentration-response curves induced by methacholine and sodium nitroprusside (after precontraction with 1 microM phenylephrine) were not affected by glibenclamide. Glibenclamide influenced neither the adenosine- nor the iloprost- (a stable prostacyclin) induced vasodilator effects. Glibenclamide caused a concentration-dependent rightward shift of the concentration-response curves of levcromakalim. The vascular tone induced by phenylephrine was not affected under low-glucose conditions, whereas hypoxia caused a decrease in the phenylephrine-induced contraction when compared with that under normal circumstances. Under all conditions, glibenclamide did not influence the phenylephrine-induced increase in vascular tone. Under low-glucose and hypoxic conditions, the concentration-response curves for levcromakalim showed a significantly less steep slope than under normal conditions, and higher concentrations of glibenclamide were necessary to inhibit the vasodilator response induced by levcromakalim under these experimental conditions adopted to mimic pathologic conditions. In conclusion, methacholine, sodium nitroprusside, adenosine, and iloprost appear not to induce vasodilation in the rat aorta by glibenclamide-sensitive K+ channels, whereas hypoxia and low-glucose levels cause an impaired function of the glibenclamide-sensitive K+ channels.

ATP-sensitive K+ channels in the kidney

ATP-sensitive K+ channels (KATP channels) form a link between the metabolic state of the cell and the permeability of the cell membrane for K+ which, in turn, is a major determinant of cell membrane potential. KATP channels are found in many different cell types. Their regulation by ATP and other nucleotides and their modulation by other cellular factors such as pH and kinase activity varies widely and is fine-tuned for the function that these channels have to fulfill. In most excitable tissues they are closed and open when cell metabolism is impaired; thereby the cell is clamped in the resting state which saves ATP and helps to preserve the structural integrity of the cell. There are, however, notable exceptions from this rule; in pancreatic beta-cells, certain neurons and some vascular beds, these channels are open during the normal functioning of the cell. In the renal tubular system, KATP channels are found in the proximal tubule, the thick ascending limb of Henle's loop and the cortical collecting duct. Under physiological conditions, these channels have a high open probability and play an important role in the reabsorption of electrolytes and solutes as well as in K+ homeostasis. The physiological role of their nucleotide sensitivity is not entirely clear; one consequence is the coupling of channel activity to the activity of the Na-K-ATPase (pump-leak coupling), resulting in coordinated vectorial transport. In ischemia, however, the reduced ATP/ADP ratio would increase the open probability of the KATP channels independently from pump activity; this is particularly dangerous in the proximal tubule, where 60 to 70% of the glomerular ultrafiltrate is reabsorbed. The pharmacology of KATP channels is well developed including the sulphonylureas as standard blockers and the structurally heterogeneous family of channel openers. Blockers and openers, exemplified by glibenclamide and levcromakalim, show a wide spectrum of affinities towards the different types of KATP channels. Recent cloning efforts have solved the mystery about the structure of the channel: the KATP channels in the pancreatic beta-cell and in the principal cell of the renal cortical collecting duct are heteromultimers, composed of an inwardly rectifying K+ channel and sulphonylurea binding subunit(s) with unknown stoichiometry. The proteins making up the KATP channel in these two cell types are different (though homologous), explaining the physiological and pharmacological differences between these channel subtypes.

Modulation of vascular KATP channels in hypothyroidism

The role of vascular KATP channels in hypothyroidism-induced decrease in myogenic activity of rat portal vein was examined by using pharmacologically relevant concentrations of K+ channel ligands. As compared to controls, a significant decrease in the myogenic tone and noradrenaline (10(-9)-10(-5) M)-induced contractions was observed in portal veins from hypothyroid rats. In both euthyroid and hypothyroid states, pinacidil (10(-9)-10(-5) M) and cromakalim (10(-9)-10(-5) M) caused concentration-related inhibition of the myogenic tone (frequency and amplitude). However, hypothyroidism caused a leftward shift in the concentration-response curves of the K+ channel openers with a corresponding decrease in their IC50 values both in the absence and presence of the KATP channel blocker, glibenclamide (10(-7) M). Further, concentration-dependent increase in the frequency of myogenic tone by glibenclamide (10(-8)-3 x 10(-6) M) was greater in tissues from hypothyroid rats (EC50 = 2.07 x 10(-7) M; 95% CL, 1.06-4.05 x 10(-7) M) in comparison to controls (EC50 = 8.07 x 10(-7) M; 95% CL, 0.53-1.22 x 10(-6) M). These results suggest that a decrease in the myogenic tone of rat portal vein may possibly be related to an enhanced opening of the KATP channels in hypothyroidism.

Effects of potassium channel modulators cromakalim, tetraethylammonium and glibenclamide on the contractility of the isolated human ureter

PURPOSE

Experiments were performed to assess the effect of the potassium channel modulators cromakalim, tetraethylammonium (TEA) and glibenclamide on the contractility of isolated human ureteric rings.

MATERIALS AND METHODS

Segments of human distal ureter obtained from kidney donors (leftovers) were cut into rings and suspended in an organ bath filled with modified Tyrode solution for measurement of isometric contractile force. The ureter was stimulated electrically or with KCl, and the contractile activity recorded on a polygraph.

RESULTS

The amplitude of the contraction induced by electrical stimulation was not changed by glibenclamide but was enhanced by tetraethylammonium. The resting tension of the ureter was not changed by either potassium channel inhibitor. Cromakalim did not change the resting tension of the human ureter per se but induced a concentration-dependent inhibition of the contractions induced by electrical stimulation. This inhibitory effect of cromakalim was not changed by tetraethylammonium but was inhibited by glibenclamide. A phasic and tonic contractile response in the isolated human ureteric ring was induced by 60 mM. KCl. The phasic contractions were abolished by cromakalim whereas the tonic contractions were unaffected. Following sustained contraction induced by 25 mM. KCl, the cumulative addition of cromakalim to the organ bath produced a concentration-dependent relaxation. However, in rings precontracted with 60 mM. KCl, cromakalim at a concentration as high as 10(-5) M. did not induce relaxation. The cromakalim-induced relaxation of rings precontracted with 25 mM. KCl was significantly inhibited by glibenclamide.

CONCLUSION

These results suggest that potassium channels are important in the control of human ureter contractility and that potassium channel openers may be an alternative therapeutic indication in the treatment of human ureteric colic.

The pulmonary vasodilator properties of potassium channel opening drugs

1. This article reviews the effects of potassium channel opening drugs (KCOs) on blood vessels of the pulmonary circulation. KCOs are effective pulmonary vasodilators in vitro (isolated arteries and perfused lungs) and in vivo in a variety of animal species. They prevent or reverse pulmonary vasoconstriction/contraction induced by a range of vasoconstrictor spasmogens or by alveolar hypoxia. 2. The pulmonary vasorelaxant effects of the KCO drugs are blocked by glibenclamide, do not depend on the endothelium, are dependent on the vasoconstrictor spasmogen used to contract the preparations and are enhanced in preparations taken from pulmonary hypertensive rats. 3. Selectivity for pulmonary compared with systemic vessels is seen in vessels from pulmonary hypertensive rats but not in the absence of pulmonary hypertension. 4. The pulmonary vasodilatation that is induced by (a) endothelium derived hyperpolarising factor, (b) endothelin, (c) increased pulmonary blood flow or (d) prolonged, severe hypoxia is probably due to potassium efflux through the same population of potassium channels as those on which the KCOs act. 5. Acute hypoxic pulmonary vasoconstriction, and also the depolarisation seen in arteries from chronically hypoxic rats, each involve inhibition of potassium efflux through glibenclamide-insensitive potassium channels. 6. It is suggested that the KCOs warrant investigation as possible therapeutic agents in the treatment of pulmonary hypertension.

Activation of potassium currents by inhibitors of calcium-activated chloride conductance in rabbit portal vein smooth muscle cells

1. The conventional whole-cell recording technique was used to study the effects of the chloride channel inhibitors ethacrynic acid, anthracene-9-carboxylic acid (A-9-C) and indanyloxyacetic acid (IAA) on membrane currents in rabbit portal vein smooth muscle cells at a holding potential of 0 mV. 2. Using a pipette solution that contained 1 x 10(-4) M 1,2-bis (2-aminophenoxy)-ethane-N,N,N,N,-tetraacetic acid (BAPTA) and a normal bathing solution the addition of ethacrynic acid (2 x 10(-4) M to 1 x 10(-3) M) inhibited spontaneous transient outward currents (STOCs) and evoked a concentration-dependent current at a holding potential of 0 mV. A similar current was activated by IAA (5 x 10(-4) M to 1 x 10(-3) M) but not by A-9-C (1-5 x 10(-3) M) at a holding potential of 0 mV. 3. The amplitude of the current evoked by ethacrynic acid and IAA was linearly related to potential between -30 and 0 mV and displayed outward rectification at positive potentials. The current induced by A-9-C was evident only at potentials positive to +20 mV. 4. Glibenclamide (1 x 10(-5) M) abolished the current evoked by ethacrynic acid and IAA at potentials negative to +10 mV and partially inhibited the current positive to +10 mV. The glibenclamide-insensitive current at positive potentials was completely inhibited by 1 x 10(-3) M TEA. The A-9-C-evoked current was insensitive to glibenclamide and abolished by 1 x 10(-3) M TEA. 5. The glibenclamide-sensitive current activated by ethacrynic acid was not sustained and declined to control levels in the continued presence of ethacrynic acid. However, the outwardly rectifying current recorded at +50 mV was well maintained over the same period. 6. Outwardly rectifying currents evoked by ethacrynic acid and A-9-C were observed with a pipette solution containing 1 x 10(-2) M BAPTA in cells bathed in Ca-free extracellular solution containing 5 x 10(-4) M BAPTA and 1 x 10(-5) M cyclopiazonic acid. 7. It is concluded that all three chloride-channel blockers activated an outwardly rectifying, TEA-sensitive current. Moreover, ethacrynic acid and IAA evoked an additional glibenclamide-sensitive current which was present at all potentials between -30 and +50 mV.

In vitro hypoxia on rat pulmonary artery: effects on contractions to spasmogens and role of KATP channels

The effect of in vitro hypoxia for 1 h on concentration-response curves to vasoconstrictor spasmogens was examined in rat isolated pulmonary arteries. Hypoxia, like levcromakalim (KATP channel opener), did not affect contractions to endothelin-1 but attenuated contractions to U46619 ((1,5,5,)-hydroxy-11 alpha, 9 alpha-epoxymethano) prosta 5Z, 13E-dienoic acid; thromboxane-mimetic), angiotensin II, noradrenaline and 5-hydroxytryptamine. The attenuation was prevented by glybenclamide. In pre-contracted arteries, subsequent exposure to hypoxia caused a response consisting of four phases (transient relaxation due to endothelium-derived nitric oxide; transient contraction; slow relaxation; sustained contraction). Glybenclamide, if added before hypoxia, did not eliminate either of the relaxant phases but, if added during the sustained contractile phase, caused further contraction. We conclude that exposure of pulmonary arteries to prolonged hypoxia causes KATP channels to open, as in systemic arteries; this diminishes contractions to some, but not all, vasoconstrictor spasmogens. The data suggest that endothelin-1, unlike other vasoconstrictors, would remain a highly effective pulmonary vasoconstrictor under severe hypoxic conditions.

Two types of ATP-sensitive potassium channels in rat portal vein smooth muscle cells

1. Single-channel recordings were made from single, enzymatically isolated smooth muscle cells of rat portal vein by the patch-clamp technique. 2. Unitary potassium currents were identified through two types of K-channels with conductances in 60:130 mM K-gradient of 50 and 22 pS; these are referred to as LK and MK channels respectively. 3. The LK channels became extremely active if isolated patches were created into nucleotide-free solution; activity was inhibited by ATP applied to the inner surface of the patch with a half maximal inhibition (Ki) of 11-23 microM. Channel activity declined and disappeared with time and could be regenerated by a brief application of Mg-ATP or a nucleoside diphosphate such as UDP (in the presence of Mg). LK channel activity was rarely stimulated by levcromakalim and not by pinacidil (K-channel openers, KCOs) but was blocked by glibenclamide. 4. Activity of MK channels declined if isolated patches were created into nucleotide free solution; activity reappeared if UDP or ATP alone (in the presence of Mg) was applied; pinacidil or levcromakalim in the presence of ATP or UDP further increased channel activity which was blocked by glibenclamide. 5. The LK channel inhibited by ATPi is very similar in its conductance and other properties to the KATP channel described in tissues other than smooth muscle, in its conductance and properties the MK channel resembles the KNDP channel we have previous described as present in other smooth muscles and opening in responses to KCOs.

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.

Effects of potassium channel openers in isolated human cerebral arteries

The objective of this study was to compare the relaxant effects of the K+ channel openers pinacidil and lemakalim in isolated human pial arteries with the effects of the dihydropyridines nifedipine and nimodipine and the prostacyclin analog iloprost. Relaxation was measured in vessels contracted by 40 mmol/L K+. In contrast to the potent and consistent relaxant effects of nifedipine, nimodipine, and iloprost, the potency of pinacidil and lemakalim proved to be highly variable and inversely correlated with the onset velocity of the preceding contractions of K+ as well as with the endothelium-dependent relaxation of carbachol. Thus, in contrast to dihydropyridines and iloprost, pinacidil and lemakalim selectively elicited potent relaxations in those arteries that exhibited signs of altered vascular wall functions.

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.

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.

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.

ATP-sensitive K+ channels mediate alpha 2D-adrenergic receptor contraction of arteriolar smooth muscle and reversal of contraction by hypoxia

Evidence in rat skeletal muscle suggests that local metabolic control of blood flow is facilitated by the reliance on alpha 2D-adrenergic receptors (ARs) for constriction of arterioles, together with the strong sensitivity of this constriction to inhibition by hypoxia. The present study examined the role of ATP-sensitive K+ (KATP) channels in the selective interaction between alpha 2D-ARs and hypoxia. Arterioles from rat cremaster muscle that possess both alpha 1D (alpha 1A/D)- and alpha 2D-AR subtypes were microcannulated, pressurized, and isolated in a tissue bath for measurement of changes in lumen diameter. Three studies first examined whether stimulation of alpha 2D- and alpha 1D-ARs involves inhibition of the KATP channel. Concentration-dependent constriction by the KATP antagonists glibenclamide (GLB, 0.01 to 10 mumol/L) and disopyramide (0.001 to 1 mmol/L) were abolished during alpha 2D stimulation but unaffected during alpha 1D stimulation. Activation of the KATP channel by cromakalim inhibited alpha 2D constriction with greater potency than alpha 1D (EC50, 7.0 +/- 0.2 versus 6.3 +/- 0.1). Finally, GLB (0.5 mumol/L) abolished dose-dependent alpha 2D constriction, whereas alpha 1D was unaffected. These data suggest that alpha 2D but not alpha 1D stimulation is "coupled" with closure of the KATP channel, leading to depolarization and contraction of vascular smooth muscle. In a second series, hypoxic (PO2, 6 mm Hg) inhibition of intrinsic smooth muscle tone was completely reversed by 0.1 mumol/L GLB, concentration-dependent GLB constriction was enhanced during hypoxia, and hypoxia reversed GLB constriction. These data confirm reports by others that hypoxia potentiates the activation of KATP channels, leading to hyperpolarization and relaxation. Finally, GLB constriction, which was abolished by concomitant alpha 2D stimulation, was completely restored by simultaneous activation of KATP channels with hypoxia. These findings suggest that the sensitivity of alpha 2D-AR constriction to inhibition by hypoxia arises through "antagonistic coupling" between these two stimuli, by which the alpha 2D-AR inhibits and hypoxia activates KATP channels.

Comparison of the cromakalim antagonism and bradycardic actions of a series of novel alinidine analogues in the rat

Alinidine, and eight derivatives, were synthesized and tested for their ability to antagonise the actions of the K+ channel opener cromakalim in rat thoracic aorta, and for their ability to induce bradycardia in rat isolated spontaneously beating right atria. Ring segments of rat thoracic aorta were suspended in organ baths to record isometric tension. Tissues were precontracted with K+ (20 mM), and full concentration-relaxation curves constructed to cromakalim (0.01-30 microM) in the absence and presence of increasing concentrations of alinidine/derivative. The majority of the compounds tested caused rightward shifts in the cromakalim concentration-effect curves. Rat spontaneously beating right atria were suspended in organ baths to record rate of contraction. Addition of alinidine/derivative caused a concentration-dependent negative chronotropic response. In terms of structure-activity relationships, increasing the length of the N-allyl side-chain on the alinidine molecule (from 3 carbon (3C), to 5C) resulted in a significant increase in the activity of the compounds as both bradycardic agents and cromakalim antagonists. The most potent compounds in both cases (bradycardic agent and cromakalim antagonist) had no double bond in the side chain. The results suggest that the carbon side-chain influences the activity of alinidine-related compounds both as cromakalim antagonists and as bradycardic agents. However, while similar structure-activity relationships appear to apply for both effects in some instances, there was no significant correlation between the two actions of the alinidine analogues. The results suggest that the ability of alinidine-derivatives to induce bradycardia or to block K+ channels opened by cromakalim can be differentiated on the basis of structure.

Properties of the ATP-sensitive K+ current activated by levcromakalim in isolated pulmonary arterial myocytes

Tension and patch clamp recording techniques were used to investigate the relaxation of rabbit pulmonary artery and the properties of the K+ current activated by levcromakalim in isolated myocytes. Under whole-cell voltage clamp, holding at -60 mV in symmetrical 139 mM K+, levcromakalim (10 microM) induced a noisy inward current of -116 +/- 19 pA (n = 13) which developed over 1 to 2 min. This current could be blocked by either glibenclamide (10 microM) or phencyclidine (5-50 microM) and was unaffected when extracellular Ca2+ was removed. Both these drugs inhibited the levcromakalim-induced relaxation of muscle strips precontracted with 20 mM [K+]o. Application of voltage ramps in symmetrical 139 mM K+ confirmed that the levcromakalim-induced current was carried by K+ ions and was weakly voltage dependent over the potential range from -100 to +40 mV. The unitary current amplitude and density of the channels underlying the levcromakalim-activated whole-cell K+ current was estimated from the noise in the current record. We estimate that levcromakalim caused activation of around 300 channels per cell, with a single channel current of 1.1 pA, corresponding to a slope conductance of about 19 pS. Furthermore, cells dialyzed with an ATP-free pipette solution developed a large noisy inward current at -60 mV, which could subsequently be blocked by flash photolysis of caged ATP. Analysis of the noise associated with this current indicated that the single channel amplitude underlying the ATP-blocked current was 1.4 pA, a value similar to that estimated for the levcromakalim-induced current.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxia inhibits myogenic reactivity of renal afferent arterioles by activating ATP-sensitive K+ channels

Recent findings implicate K+ channels as important modulators of myogenic tone and possible mediators of the vasodilatory effects of hypoxia. In the present report, we examined the effects of hypoxia on myogenic vasoconstriction of renal afferent arterioles. Using the in vitro perfused hydronephrotic rat kidney model, we observed precisely graded decreases in arteriolar diameter when renal perfusion pressure was increased. Normal myogenic reactivity was observed over PO2 levels of 150 to 80 mm Hg. Reducing PO2 to 60, 40, and 30 mm Hg resulted in a significant progressive inhibition of myogenic reactivity. At approximately 20 mm Hg, myogenic vasoconstriction was essentially abolished, whereas the vasoconstriction induced by 30 mmol/L KCl was unaffected. The addition of 1.0 mumol/L glibenclamide completely restored myogenic vasoconstriction during hypoxia. In contrast, 1.0 mmol/L tetraethylammonium did not alter the effects of hypoxia. To investigate the relation between hypoxia-induced vasodilation and smooth muscle oxidative phosphorylation, we monitored changes in arteriolar levels of reduced NADH during exposure to hypoxia. Arterioles preconstricted by elevated pressure were optically isolated for simultaneous monitoring of vessel diameter and NADH fluorescence (360-nm excitation, 450-nm emission). Reducing perfusate PO2 from 150 to 20 mm Hg resulted in progressive loss of myogenic tone with no change in arteriolar NADH. These findings indicate that lowering PO2 within a physiological range attenuates myogenic reactivity of the renal afferent arteriole by causing the activation of ATP-sensitive K+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of K+ channel openers on the vascular actions of human gamma globulin

The aim of this study was to determine if the stimulatory action of human gamma globulin on the spontaneous activity of the rat mesenteric portal vein is due to decreased K+ conductance. Glibenclamide potentiated the action of human gamma-globulin on the portal vein by 45% and on its own had a concentration- and time-dependent biphasic (increase followed by a decrease) effect on the spontaneous activity of the portal vein. Diazoxide and pinacidil both inhibited the action of human gamma-globulin on the rat mesenteric portal vein. Levcromakalim (BRL 38227) potentiated the stimulatory action of human gamma-globulin on the integrated force of the spontaneous contractions of the rat mesenteric portal vein by 40% and 49% at concentrations of 0.5 and 5 microM, respectively. These studies suggest that human gamma-globulin can act by directly modulating a K+ channel.

The role of the membrane potential of endothelial and smooth muscle cells in the regulation of coronary blood flow

In the mammalian heart the supply of oxygen and energy-rich substrates through the coronary arterioles is continuously adapted to the variations of cardiac work. The coronary resistance arteries and the surrounding myocardium form a functional unit with multiple interactions between coronary endothelial cells, smooth muscle cells, perivascular nerves, and cardiac muscle cells. We describe the mechanisms underlying the electrical and chemical communication between the different cell types, the ionic channels contributing to the resting potential of endothelial and smooth muscle cells, and the mechanisms responsible for modulation of the resting potential. The main conclusion of our analysis is that the membrane potential of coronary endothelial and smooth muscle cells is one of the major determinants of coronary blood flow, and that modulation of the membrane potential provides a way to dilate or constrict coronary resistance arteries. It is proposed that the membrane potential of the myo-endothelial regulatory unit, i.e., of the endothelial cells and the underlying smooth muscle cells in the terminal arterioles, may function as an integrator of the numerous local and global vasodilator and constrictor signals that provide for the adaptation of coronary blood flow to the metabolic demands of the heart.

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.

K channel activation by nucleotide diphosphates and its inhibition by glibenclamide in vascular smooth muscle cells

1. Whole-cell and inside-out patch recordings were made from single smooth muscle cells that had been isolated enzymatically and mechanically from the rabbit portal vein. 2. In whole-cells the inclusion in the recording pipette solution of nucleotide diphosphates (NDPs), but not tri- or monophosphates, induced a K-current that developed gradually over 5 to 15 min. Intracellular 1 mM guanosine 5'-diphosphate (GDP) induced a slowly developing outward K-current at -37 mV that reached a maximum on average of 72 +/- 4 pA (n = 40). Half maximal effect was estimated to occur with about 0.2 mM GDP. Except for ADP, other NDPs had comparable effects. At 0.1 mM, ADP was equivalent to GDP but at higher concentration ADP was less effective. ADP induced its maximum effect at 1 mM but had almost no effect at 10 mM. 3. In 14% of inside-out patches exposed to 1 mM GDP at the intracellular surface, characteristic K channel activity was observed which showed long (> 1 s) bursts of openings separated by longer closed periods. The current-voltage relationship for the channel was linear in a 60 mM:130 mM K-gradient and the unitary conductance was 24 pS. 4. Glibenclamide applied via the extracellular solution was found to be a potent inhibitor of GDP-induced K-current (IK(GDP)) in the whole-cell. The Kd was 25 nM and the inhibition was fully reversible on wash-out. 5. IK(GDP) was not evoked if Mg ions were absent from the pipette solution. In contrast the omission of extracellular Mg ions had no effect on outward or inward IK(GDP). 6. Inclusion of 1 mM ATP in the recording pipette solution reduced IK(GDP) and also attenuated its decline during long (25 min) recordings. 7. When perforated-patch whole-cell recording was used, metabolic poisoning with cyanide and 2-deoxy-D-glucose induced a glibenclamide-sensitive K-current. This current was not observed when conventional whole-cell recording was used. Possible reasons for this difference are discussed. 8. These K channels appear similar to ATP-sensitive K channels but we refer to them as nucleotide diphosphate-dependent K channels (KNDP) to emphasise what seems to be a primary role for nucleotide diphosphates in their regulation.

Differential effects of acetylcholine, nitric oxide and levcromakalim on smooth muscle membrane potential and tone in the rabbit basilar artery

1. Endothelium-dependent hyperpolarization of smooth muscle cells in isolated, pre-contracted segments of rabbit basilar artery in response to acetylcholine (100 microM) was abolished in the presence of glibenclamide (10 microM). 2. Acetylcholine-evoked relaxation was unaffected by either glibenclamide or 65 mM potassium chloride, indicating that the change in membrane potential did not form an essential component of relaxation and that high concentrations of potassium did not inhibit the release or action of endothelium-derived relaxing factor in this vessel. 3. Saturated solutions of nitric oxide (NO) gas in solution (150 microM), which evoked maximal relaxation of arterial segments pre-contracted and depolarized by noradrenaline (10-100 microM), did not alter the membrane potential of either unstimulated or depolarized smooth muscle cells. 4. The potassium channel opener levcromakalim, evoked concentration-dependent relaxation and hyperpolarization in pre-constricted smooth muscle cells. The threshold concentrations for hyperpolarization and relaxation, the EC50 values and the maximally effective concentration of levcromakalim (around 30 nM, 150 nM and 10 microM, respectively) were not significantly different, and both components of the response were inhibited by glibenclamide (10 microM), indicating a close coupling between the two responses. 5. In the presence of 65 mM potassium chloride, the hyperpolarization to levcromakalim was abolished, while a small relaxation (25 +/- 4%) persisted, indicating an additional mechanism for relaxation to this agent. 6. These results show that different mechanisms underlie the relaxant action of potassium channel openers, NO and endothelium-derived factors in cerebral arteries and provide further evidence that in the basilar artery, in contrast to some other vessels, endothelium-dependent hyperpolarization to acetylcholine is not important for smooth muscle relaxation.

Single channel and whole-cell K-currents evoked by levcromakalim in smooth muscle cells from the rabbit portal vein

1. Single channel and whole-cell current recordings were made from single smooth muscle cells isolated from the rabbit portal vein. 2. Application of 10 microM levcromakalim ((-)-Ckm) to single cells held with pipettes containing 1 mM GDP induced a K-current (IK(Ckm)) which occurred in addition to the current caused by GDP alone (IK(GDP)) and averaged 135 pA at -37 mV. We have investigated whether the same K channels underlie the GDP- and Ckm-induced K-currents. 3. If 1 mM GDP was in the pipette but Mg ions were omitted the effect of GDP was absent and IK(Ckm) averaged only 10 pA, suggesting that the action of (-)-Ckm was Mg-dependent. 4. Intracellular ATP was not observed to have much effect on IK(-Ckm). Loading of cells with 10 mM ATP from the recording pipette had no significant effect and flash photolysis of caged-ATP loaded into cells from the pipette, estimated to release about 1 mM free ATP, also had no effect on IK(-Ckm). 5. Bath-applied glibenclamide inhibited IK(-Ckm) with an IC50 of 200 nM, a value 8 times higher than that found for inhibition of IK(GDP). The delayed rectifier K-current (IK(DR)) was also inhibited by glibenclamide but at higher concentrations (IC50 100 microM). Bath-applied tetraethylammonium ions (TEA) inhibited IK(-Ckm) and IK(GDP) to the same extent (IC50 about 7 mM). 6. In inside-out patch recordings (- )-Ckm (10 microM) applied to the intracellular surface of the membrane potentiated the opening of K channels already stimulated by I mM GDP and all of the channel activity was abolished by 10 microM glibenclamide. The unitary conductance of the channels was 24lpS in a 60 mM: 130 mM K-gradient.7. We suggest that (-)-Ckm may hyperpolarize and relax smooth muscle cells by opening KNDP, a class of small conductance K channels that are related to the ATP-sensitive K channels seen in other tissues.

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.

The individual enantiomers of cis-cromakalim possess K+ channel opening activity

Cromakalim (BRL 34915), a racemic trans-3,4-disubstituted benzopyran, is the prototype of a novel group of vasorelaxants which act by opening K+ channels in the cell membrane, the K+ channel openers. The enantiomers of cis-cromakalim were synthesized and their biological activity compared to that of the enantiomers of (trans-)cromakalim. Both the (+)-(3R,4R) enantiomer and its (-)-(3S,4S) antipode inhibited binding of the K+ channel opener [3H]P1075 in strips of rat aorta with pKi values of 5.4 and 5.2, respectively. They relaxed noradrenaline-induced contractions of rat isolated aorta under control conditions with pD2 values of 5.7 and 5.2; their vasorelaxant potency was greatly diminished under depolarized conditions (KCl = 55 mmol/l). Both compounds increased the permeability of the cell membrane for K+ as suggested by their ability to stimulate 86Rb+ efflux from rat aortic strips. The vasorelaxant and the 86Rb+ efflux-stimulating effects of the compounds were inhibited by the sulfonylurea, glibenclamide. These results show that the enantiomers of cis-cromakalim are genuine K+ channel openers. The (R,R) enantiomer is 50 times weaker than the (-)-(3S,4R) enantiomer of cromakalim (= levcromakalim, BRL 38227) but 3 times more potent than the (+)-(3R,4S) enantiomer. These data highlight the importance of the stereochemistry at both the 3 and 4 position of the benzopyran ring.

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

1 Smooth muscle cells of the rat portal vein were dispersed by enzymatic treatment and recordings of whole-cell currents were made by the voltage-clamp technique. The effects of the potassium (K) channel openers, P1060 (0.3-10 microM) and aprikalim (3-30 microM) on these currents were investigated. Antagonism of these agents by glibenclamide and phentolamine was also studied. 2 When cells were clamped at -10 mV, P1060 (1 microM) and aprikalim (3 microM) each induced a slowly-developing K-current (IKCO), the noise of which gradually increased. The rate of onset of IKCO was greater for P1060 than for aprikalim. Current-voltage plots showed that P1060 and aprikalim each caused an approximately 25 mV negative shift of the reversal potential at zero current. 3 P1060 (1 microM) and aprikalim (3 microM) each inhibited the slowly activating, slowly inactivating delayed rectifier current, ITO. 4 Addition of MgATP (5 mM) to the recording pipette inhibited the generation of IKCO by P1060 (1 microM) and reduced the accompanying inhibition of ITO. 5 Stationary fluctuation analysis of the current noise associated with IKCO induced by P1060 (1 microM) or aprikalim (3 microM) at a holding potential of -10 mV indicated that the unitary conductance of the underlying K-channels was 10.5 pS at 0 mV under the quasi-physiological conditions of the experiment. 6 In the absence of K-channel openers, neither phentolamine (30-100 microM) nor glibenclamide (1 microM) affected the magnitude of control non-inactivating currents. However, phentolamine (30-100 microM), but not glibenclamide (1 microM) inhibited the control delayed rectifier current ITO. 7. After induction of IKCO by P1060 (1 microM) or aprikalim (3 microM), subsequent exposure to glibenclamide(1 microM) or phentolamine (30 microM) inhibited this current. After aprikalim-induced reduction of ITO had developed, subsequent exposure to glibenclamide was able partially to reverse the inhibition of ITO whereas phentolamine was without effect. Pre-exposure to glibenclamide (1 microM) prevented both the generation of IKCO by aprikalim (3 microM) and the inhibitory effect of this agent on ITO.8. It is concluded that P1060 and aprikalim each induce the current IKCO by opening the same small conductance, ATP-sensitive K-channel (KATP), an effect which can be inhibited by glibenclamide orphentolamine. The opening of KATP by both P1060 and aprikalim probably involves competition between these agents and ATP for the ATP-control site associated with the channel. Inhibition of the delayed rectifier current, ITO, by P1060 and aprikalim was glibenclamide-sensitive and may be caused by the induction of a state of run-down in the channel which underlies this current.