Hyperpolarizing vasodilators activate ATP-sensitive K+ channels in arterial smooth muscle

Vasodilators are used clinically for the treatment of hypertension and heart failure. The effects of some vasodilators seem to be mediated by membrane hyperpolarization. The molecular basis of this hyperpolarization has been investigated by examining the properties of single K+ channels in arterial smooth muscle cells. The presence of adenosine triphosphate (ATP)-sensitive K+ channels in these cells was demonstrated at the single channel level. These channels were opened by the hyperpolarizing vasodilator cromakalim and inhibited by the ATP-sensitive K+ channel blocker glibenclamide. Furthermore, in arterial rings the vasorelaxing actions of the drugs diazoxide, cromakalim, and pinacidil and the hyperpolarizing actions of vasoactive intestinal polypeptide and acetylcholine were blocked by inhibitors of the ATP-sensitive K+ channels, suggesting that all these agents may act through a common pathway in smooth muscle by opening ATP-sensitive K+ channels.

Sulphonylurea receptor 2B and Kir6.1 form a sulphonylurea-sensitive but ATP-insensitive K+ channel

1. We analysed the K+ channel composed of the sulphonylurea receptor 2B (SUR2B) and an inwardly rectifying K+ channel subunit Kir6.1 coexpressed in a mammalian cell line, HEK293T, with the patch clamp technique. 2. In the cell-attached configuration, K+ channel openers (pinacidil and nicorandil) activated approximately 33 pS K+ channels (approximately 145 mM external K+), which were inhibited by the sulphonylurea glibenclamide. 3. Although SUR2B forms an ATP-sensitive K+ channel with Kir6.2, whose amino acid sequence is approximately 70% homologous with that of Kir6.1, the K+ channel composed of SUR2B and Kir6.1 surprisingly did not spontaneously open on patch excision in the absence of intracellular ATP. 4. In inside-out patches, uridine diphosphate and guanosine diphosphate induced channel activity, which was inhibited by glibenclamide but not ATP. Intracellular ATP on its own activated the channels. K+ channel openers and intracellular nucleotides synergistically activated the channel. 5. Therefore, the K+ channel composed of SUR2B and Kir6.1 is not a classical ATP-sensitive K+ channel but closely resembles the nucleotide diphosphate-dependent K+ channel in vascular smooth muscle cells.

ATP-regulated K+ channels protect the myocardium against ischemia/reperfusion damage

The role of ATP-regulated K+ channels in protecting the myocardium against ischemia/reperfusion damage was explored using glibenclamide and pinacidil to block and activate the channels, respectively. Electrical and mechanical activity of arterially perfused guinea pig right ventricular walls was recorded simultaneously via an intracellular microelectrode and a force transducer. The preparations were subjected to either 1) 20 minutes of no-flow ischemia with or without glibenclamide (1 and 10 microM) followed by reperfusion, or 2) 30 minutes of no-flow ischemia with or without pinacidil (1 and 10 microM) followed by reperfusion. No-flow ischemia for 20 minutes produced changes in electrical and mechanical activity that were completely reversed on reperfusion; resting membrane potential declined by 13 +/- 1.2 mV, action potential duration at 90% repolarization (APD90) decreased by 62%, and developed tension fell by greater than 95%, but resting tension did not change significantly. Glibenclamide (10 microM) had no effect on activity during normal perfusion, but during ischemia, resting membrane potential fell slightly further (17 +/- 1.8 mV) and APD90 declined by only 24%. Developed tension declined more slowly and to a lesser extent, but resting tension rose significantly between 10 and 20 minutes of ischemia. Reperfusion of glibenclamide-treated tissues elicited arrhythmias (extrasystoles and tachycardia), and the preparations failed to recover mechanical function. Glibenclamide at 1 microM produced qualitatively similar effects, albeit less severe. After 30 minutes of no-flow ischemia in untreated tissues, resting tension increased by approximately 130% during the no-flow period. Reperfusion caused arrhythmias (extrasystoles, tachyarrhythmias, and fibrillation) and failed to restore resting or developed tension to preischemic levels. Pinacidil at 1 microM did not affect electrical or contractile function, but at 10 microM it had a negative inotropic effect, decreasing APD90 and developed tension by 5% and 18%, respectively. Both concentrations of the drug caused a faster and greater decline in APD90 during the no-flow period. Resting tension did not change during 30 minutes of no-flow ischemia in the presence of pinacidil, and reperfusion led to 85% and complete recovery of electrical and mechanical activity at 1 and 10 microM, respectively. The data indicate that glibenclamide enhances whereas pinacidil reduces myocardial damage caused by ischemia/reperfusion. The results are consistent with the hypothesis that activation of ATP-regulated K+ channels during ischemia is an important adaptive mechanism for protecting the myocardium when blood flow to the tissue is compromised.

Membrane hyperpolarization inhibits agonist-induced synthesis of inositol 1,4,5-trisphosphate in rabbit mesenteric artery

1. Effects of membrane hyperpolarization induced by pinacidil on Ca2+ mobilization induced by noradrenaline (NA) were investigated by measuring intracellular Ca2+ concentration ([Ca2+]i), isometric tension, membrane potential and production of inositol 1,4,5-trisphosphate (IP3) in smooth muscle cells of the rabbit mesenteric artery. 2. Pinacidil (0.1-10 microM) concentration dependently hyperpolarized the smooth muscle membrane with a reduction in membrane resistance. Glibenclamide (1 microM) blocked the membrane hyperpolarization induced by 1 microM-pinacidil. NA (10 microM) depolarized the smooth muscle membrane with associated oscillations. Pinacidil (1 microM) inhibited this response and glibenclamide (1 microM) prevented the action of pinacidil on both the NA-induced events. 3. In thin smooth muscle strips, 10 microM-NA produced a large phasic and a subsequent small tonic increase in [Ca2+]i with associated oscillations. These changes in [Ca2+]i seemed to be coincident with phasic, tonic and oscillatory contractions, respectively. Pinacidil (0.1-1 microM) inhibited the increases in [Ca2+]i and in tension induced by NA, but not by 128 mM-K+. Glibenclamide inhibited these actions of pinacidil. Pinacidil (1 microM) also inhibited the contraction induced by 10 microM-NA in strips treated with A23187 (which functionally removes cellular Ca2+ storage sites), suggesting that membrane hyperpolarization inhibits Ca2+ influxes activated by NA. 4. In Ca2(+)-free solution containing 2 mM-EGTA, NA (10 microM) transiently increased [Ca2+]i, tension and synthesis of IP3. Pinacidil (over 0.1 microM) inhibited the increases in [Ca2+]i, tension and synthesis of IP3 induced by 10 microM-NA in Ca2(+)-free solution containing 5.9 mM-K+, but not in a similar solution containing 40 or 128 mM-K+. Glibenclamide (1 microM) inhibited these actions of pinacidil. These inhibitory actions of pinacidil were still observed in solutions containing low Na+ or low Cl-. These results suggest that pinacidil inhibits NA-induced Ca2+ release from storage sites through an inhibition of IP3 synthesis resulting from its membrane hyperpolarizing action. 5. In beta-escin-treated skinned strips, NA (10 microM) or IP3 (20 microM) increased Ca2+ in Ca2(+)-free solution containing 50 microM-EGTA and 3 microM-guanosine triphosphate (GTP) after brief application of 0.3 microM-Ca2+, suggesting Ca2+ is released from intracellular storage sites. Heparin (500 micrograms/ml, an inhibitor of the IP3 receptor), but not pinacidil (1 microM) or glibenclamide (1 microM), inhibited the Ca2+ release from storage sites induced by NA or IP3. These results suggest that membrane hyperpolarization is essential for the inhibitory action of pinacidil on the NA-induced Ca2(+)-releasing mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning and functional expression of a rat heart KATP channel

Potassium channels that are ATP-sensitive (KATP) couple membrane potential to the metabolic status of the cell. KATP channels are inhibited by intracellular ATP and are stimulated by intracellular nucleotide diphosphates. KATP channels are important regulators of secretory processes and muscle contraction, and are targets for therapeutic treatment of type II diabetes by the inhibitory sulphonylureas and for hypertension by activators such as pinacidil. In cardiac tissue, KATP channels are central regulators of post-ischaemic cardioprotection. Electrophysiological and pharmacological characteristics vary among KATP channels recorded from diverse tissues suggesting extensive molecular heterogeneity. A complementary DNA encoding a KATP channel was isolated from rat heart using the polymerase chain reaction. We report here that the expressed channels possess all of the essential features of native cardiac KATP channels, including sensitivity to intracellular nucleotides. In addition the cloned channels are activated by the potassium channel opener, pinacidil, but are not inhibited by the sulphonylurea, glibenclamide.

Mechanosensitive gating of atrial ATP-sensitive potassium channels

Cell-attached and inside-out excised-patch recording techniques were used to search for mechanosensitive ion channels in neonatal and adult rat atrial myocytes. A channel activated by negative pressure applied to the patch, with a single-channel conductance of 52 pS in symmetric potassium solutions, was frequently observed. This channel has been identified as the atrial ATP-sensitive potassium (KATP) channel on the basis of its potassium selectivity, as well as its inhibition by ATP or tolbutamide in the inside-out excised patch. Mechanosensitive modulation of the KATP channel has not previously been reported. In the presence of 1 mM ATP, 10-50 microM pinacidil (a specific KATP channel agonist) does not significantly increase basal KATP channel activity; however, these concentrations of pinacidil potentiated the mechanosensitive modulation of the KATP channel. A hypotonic swelling protocol (a mechanical stimulus) was used in an effort to determine whether mechanosensitive modulation of this channel can generate significant whole-cell currents. Under perforated-patch whole-cell recording conditions, superfusion of atrial myocytes with a 240 mosm/kg solution (control solution, 290 mosm/kg) stimulated whole-cell currents with a magnitude similar to those activated by 10 microM pinacidil. These results demonstrate that the gating of the atrial KATP channel is mechanosensitive and suggest that mechanosensitive modulation may be an additional and significant mechanism, modulating channel activity under both physiological and pathological conditions.

K+ channel openers prevent global ischemia-induced expression of c-fos, c-jun, heat shock protein, and amyloid beta-protein precursor genes and neuronal death in rat hippocampus

Transient global forebrain ischemia induces in rat brain a large increase of expression of the immediate early genes c-fos and c-jun and of the mRNAs for the 70-kDa heat-shock protein and for the form of the amyloid beta-protein precursor including the Kunitz-type protease-inhibitor domain. At 24 hr after ischemia, this increased expression is particularly observed in regions that are vulnerable to the deleterious effects of ischemia, such as pyramidal cells of the CA1 field in the hippocampus. In an attempt to find conditions which prevent the deleterious effects of ischemia, representatives of three different classes of K+ channel openers, (-)-cromakalim, nicorandil, and pinacidil, were administered both before ischemia and during the reperfusion period. This treatment totally blocked the ischemia-induced expression of the different genes. In addition it markedly protected neuronal cells against degeneration. The mechanism of the neuroprotective effects involves the opening of ATP-sensitive K+ channels since glipizide, a specific blocker of that type of channel, abolished the beneficial effects of K+ channel openers. The various classes of K+ channel openers seem to deserve attention as potential drugs for cerebral ischemia.

The natural history of medication compliance in a drug trial: limitations of pill counts

To assess medication compliance over time, we prospectively performed pill counts among 121 ambulatory hypertensive subjects for less than or equal to 12 months. Prescribed regimens consisted of pinacidil or hydralazine administered four times a day and of secondary drugs administered up to twice daily. Surreptitious pill counts occurred every 1 to 12 weeks. Among a middle-aged subject group that had been selected for high rates of compliance, we observed mean compliance rates that approximated 100%. We noted marked intrasubject and intersubject variability for any one medication, between medications, and over time. From baseline blood pressures (+/- SE) of 155.5 +/- 1.9/97.3 +/- 1.0 mm Hg, subsequent mean blood pressures varied by compliance subgroup: "hypocompliers" (less than 80%), 151.3/91.0 mm Hg; "hypercompliers" (greater than or equal to 120%), 147.6/91.4 mm Hg; and "eucompliers" (80% to 119%), 143.3/88.5 mm Hg (systolic blood pressure: F1,52 = -220.9, NS; diastolic blood pressure: F1,52 = -121.4, NS). We concluded that weekly pill counts indicated marked intersubject and intrasubject variability, obscured by long-term averages; that compliance lapses appeared to be random; and that excessive medication-taking was the most consistent with "pill dumping."

Guanosine diphosphate activates an adenosine 5'-triphosphate-sensitive K+ channel in the rabbit portal vein

1. Properties of the pinacidil-sensitive K+ channel in the smooth muscle of the rabbit portal vein were investigated using cell-attached and inside- and outside-out patch clamp techniques. 2. In the cell-attached patch configuration, a K+ channel with a unitary conductance of 150 pS could be recorded when physiological salt solution (PSS) was in the pipette and high-K+ solution was in the bath. Tetraethylammonium (TEA; less than 1 mM) and charybdotoxin (CTX; greater than 50 nM) inhibited the 150 pS K+ channel from the outside of the membrane. This channel was activated by an increase in the concentrations of intracellular Ca2+ but not by pinacidil (less than or equal to 500 microM). 3. In the cell-attached patch configuration, bath application of pinacidil (greater than 3 microM) activated a K+ channel (ATP-sensitive K+ channel) with a unitary conductance of 15 pS and the enhancing action of pinacidil was blocked by glibenclamide. However, in the cell-free patch configuration, pinacidil (100 microM) failed to open the 15 pS K+ channel. With pinacidil in the pipette, the 15 pS K+ channel was completely inactivated within 5 s of the excision of the membrane. Opening of the 15 pS K+ channel also disappeared after saponin treatment (50 micrograms/ml). 4. In the cell-free patch configuration, application of guanosine 5'-diphosphate (GDP; greater than 100 microM) re-activated the inactivated 15 pS K+ channel only when pinacidil was present either in the pipette or bath. GDP increased the mean open time and open probability of the 15 pS K+ channel in a concentration-dependent manner. Simultaneous application of MgCl2 (less than or equal to 1 mM) with GDP did not modify the GDP-induced activation. Neither GDP nor GTP (1 mM) had any effect on the 150 pS K+ channel. 5. Guanosine 5'-triphosphate (GTP; 1 mM) activated the 15 pS K+ channel to a lesser extent that did GDP. Other guanine nucleotides (guanosine 5'-monophosphate, GMP, 1 mM; guanosine 5'-O-(3-thiotriphosphate), GTP gamma S, 100 microM; and guanosine 5'-O-(2-thiodiphosphate), GDP beta S, 1 mM) failed to activate the 15 pS K+ channel. However, GDP beta S, but not GMP or GTP gamma S, inhibited this channel when it was activated by 1 mM-GDP. 6. In the presence of pinacidil, adenosine 5'-triphosphate (ATP; greater than or equal to 10 microM) inhibited the ATP-sensitive K+ channel when it was activated by 1 mM-GDP.(ABSTRACT TRUNCATED AT 400 WORDS)

Synergistic modulation of ATP-sensitive K+ currents by protein kinase C and adenosine. Implications for ischemic preconditioning

Ischemic preconditioning has been shown to involve the activation of adenosine receptors, protein kinase C (PKC), and ATP-sensitive K+ (K ATP) channels. We investigated the effects of PKC activation and adenosine on K(ATP) current (I KATP) and action potentials in isolated rabbit ventricular myocytes. Responses to pinacidil (100 to 400 micromol/L), an opener of K(ATP) channels, were markedly increased by preexposure to the PKC activator phorbol 12-myristate 13-acetate (PMA, 100 nmol/L). I(KATP) measured at 0 mV was increased by PMA pretreatment from 0.55 +/- 0.32 to 3.25 +/- 0.47 nA (n=6, P < .01). We next determined whether PKC activation abbreviates the time required to turn on I(KATP) developed after an average of 15.1 +/- 2.4 minutes (n=8). Ten-minute pretreatment with PMA alone (PMA+MI) did not significantly alter this latency (11.9 +/- 2.0 minutes, n=8). Since adenosine receptor activation has been shown to play an important role in the preconditioning response, two groups of myocytes were studied with adenosine (10 micromol/L) included during MI. Without PMA, adenosine alone (MI+Ado) did not affect the latency to develop I(KATP) (12.3 +/- 1.5 minutes, n=8). However, if cells were pretreated with PMA and then subjected to MI in the presence of adenosine (PMA+MI+Ado), the latency was greatly shortened to 5.5 +/- 1.6 minutes (n=8;P < .02 versus MI, PMA+MI, and MI+Ado groups). This effect could not be reproduced by an inactive phorbol but was completely abolished by the adenosine receptor antagonist 8-(p-sulfophenyl)-theophylline. The opening of K(ATP) channels may be cardioprotective because of the abbreviation of action potential duration (APD) during ischemia. Therefore, we tested whether PKC activation could modify the time course of APD shortening during MI. Consistent with the ionic current measurements, PMA pretreatment significantly accelerated APD shortening, but only when adenosine (10 micromol/L) was included during MI. The effects were not attributable to accelerated ATP consumption: PMA pretreatment did not alter the time required to induce rigor during MI, whether or not adenosine was included. Our results indicate that PKC activation increases the I(KATP) Induced by pinacidil or by MI. The latter effect requires concomitant adenosine receptor activation. The synergistic modulation of I(KATP) by PKC and adenosine provides an explicit basis for current paradigms of ischemic preconditioning.

The protective effects of cromakalim and pinacidil on reperfusion function and infarct size in isolated perfused rat hearts and anesthetized dogs

The direct myocardial protective effects of intracoronary infusions of cromakalim and pinacidil were determined in an anesthetized canine model of coronary occlusion and reperfusion. The left circumflex coronary artery was occluded for 90 minutes and reperfused for 5 hours, at which time the infarct size was determined. Cromakalim (0.1 micrograms/kg/min) or pinacidil (0.09 micrograms/kg/min) were infused into the left circumflex coronary artery starting 10 minutes preischemia. Cromakalim significantly reduced infarct size as a percent of the left ventricular area at risk (25 +/- 5%) compared with vehicle controls (55 +/- 7%). Pinacidil did not reduce infarct size at an equimolar dose, but at the higher dose also significantly reduced infarct size. Collateral blood flow was not significantly altered by either drug, though reperfusion flow was significantly higher in cromakalim-treated animals, particularly in the subepicardial region. When the same dose of cromakalim was given starting 2 minutes before the initiation of reperfusion, no significant beneficial effect of cromakalim was observed. In another study, isolated buffer-perfused rat hearts were subjected to 25 minutes of global ischemia and 30 minutes of reperfusion. These hearts were treated with 7 microM cromakalim, either starting 10 minutes before ischemia or only during reperfusion, and its effect on reperfusion function and LDH release were determined. Cromakalim pretreatment (both when given throughout the experiment and when not present in the reperfusion buffer) resulted in significant improvements in the reperfusion function. Reperfusion contracture and LDH were also significantly reduced with this treatment. When given only during reperfusion, cromakalim did not reduce the severity of ischemia when compared with vehicle controls. Thus, both cromakalim and pinacidil reduce ischemic/reperfusion injury, though the timing of treatment may be important.

ATP-sensitive K+ channels in rat aorta and brain microvascular endothelial cells

The endothelium plays an important role in the modulation of vascular tone and blood cell activation. Extensive work has demonstrated that the release of endothelium-derived relaxing factor (EDRF) from the endothelium is evoked by a number of physical and chemical stimuli requiring Ca2+. Because endothelial cells do not express voltage-dependent Ca2+ channels, Ca2+ influxes following receptor activation may be facilitated by cell hyperpolarizations mediated by the activation of K+ conductances. There has been recent interest in the role of ATP-sensitive K+ channels (KATP) suggesting that KATP may play a role in the regulation of blood flow. We have investigated the electrophysiological properties of an ATP-sensitive K+ conductance in whole cell and membrane patches from rat aorta and brain microvascular endothelial cells. Whole cell as well as single-channel currents were increased by either intracellular dialysis of ATP or application of glucose-free/NaCN (2 mM) solutions. Both currents were reversibly blocked by glibenclamide (1-100 microM). The KATP channel opener pinacidil (30 microM) caused activation of an outward current in the presence of physiological intracellular ATP concentrations. In inside-out patches, 10 microM-1 mM ATP invariably caused a dramatic decrease in channel activity. We conclude that both rat aorta and brain microvascular endothelial cells express KATP channels. KATP may play a role in the regulation of endothelial cell resting potential during impaired energy supply and therefore modulate EDRF release and thus cerebral blood flow.

Evidence that the mechanism of the inhibitory action of pinacidil in rat and guinea-pig smooth muscle differs from that of glyceryl trinitrate

The effects of pinacidil have been compared with those of glyceryl trinitrate (GTN) using the aorta and portal vein of the rat and the trachealis and taenia caeci of the guinea-pig. In aorta, both pinacidil and GTN inhibited responses to noradrenaline and showed some selective inhibition of contractions to 20 mM K+. Responses to 80 mM K+ were little affected. In trachealis, both pinacidil and GTN inhibited spontaneous tone and selectively relaxed spasms to 20 mM K+. Responses to 80 mM K+ were unaffected. In portal vein, pinacidil completely inhibited spontaneous electrical and mechanical activity. GTN reduced the amplitude of tension waves and extracellularly-recorded discharges, but increased the frequency of spontaneous electrical and mechanical activity. In portal vein, pinacidil inhibited contractions to noradrenaline and selectively inhibited responses to 20 mM K+. GTN had little inhibitory effect on responses to either noradrenaline or K+. In portal veins loaded with 86Rb as a K+-marker, pinacidil significantly increased the 86Rb efflux rate coefficient whilst GTN had no effect on 86Rb exchange. In taenia caeci, both pinacidil and GTN inhibited the spontaneous tone of the preparation. These inhibitory effects were not antagonized by apamin. It is concluded that pinacidil and GTN do not share a common relaxant mechanism. Evidence has been obtained that pinacidil exerts its inhibitory effects by the opening of apamin-insensitive, 86Rb-permeable K+ channels.

Influence of ATP-sensitive potassium channel modulators on ischemia-induced fibrillation in isolated rat hearts

We have confirmed the findings of Kantor and colleagues that ischemia-induced fibrillation in isolated Langendorff-perfused rat hearts can be prevented by glyburide, a blocker of ATP-dependent K channels. These data suggest that block of ATP-dependent K current [IK(ATP)] is a novel antiarrhythmic mechanism. This hypothesis was further tested by evaluating the effects of another sulfonylurea IK(ATP) blocker, tolbutamide (1 mM) and two agents known to activate these channels in cardiac tissue, BRL 34915 (10 microM) and pinacidil (30 microM). Similar to glyburide, tolbutamide was also effective in preventing fibrillation in this isolated rat heart model. The IK(ATP) activators enhanced the rate of tachycardia and shortened the time required for the hearts to develop fibrillation. Coadministration of glyburide with either IK(ATP) activator prevented their effects. It is concluded that activation of IK(ATP) during global ischemia contributes to the development of fibrillation in the perfused rat heart model.

Potassium channel openers act through an activation of ATP-sensitive K+ channels in guinea-pig cardiac myocytes

In a previous article (Escande et al. 1988a), we have shown that cromakalim (BRL 34915), a potassium channel opener (PCO), is a potent activator of ATP-sensitive K+ channels in cardiac cells. In the present article, the influence on K+ channels of two other potassium channel openers chemically unrelated to cromakalim, RP 49356 and pinacidil, has been investigated in patch-clamped isolated cardiac myocytes. In the whole-cell configuration, K+ currents were recorded in the presence of 50 microM TTX and 3 microM nitrendipine or 3 mM cobalt. Like cromakalim, RP 49356 or pinacidil activated a time-independent outward current at 33-35 degrees C but not at 19-21 degrees C, which showed little voltage-dependency in the potential range -60 to +60 mV. Its amplitude was a function of the agonist concentration, e.g. it was 2.1 +/- 0.4 nA at +60 mV with 30 microM RP 49356 and 4.3 +/- 0.8 nA with 300 microM. In control conditions, glibenclamide, a blocker of K+-ATP channels in pancreatic and heart cells, affected neither the inward rectifier, iK1, nor the delayed K+ current, iK. At 3 microM, glibenclamide fully prevented the effects of 300 microM RP 49356 or pinacidil. At lower concentrations, glibenclamide partially counteracted the activation by PCOs of a K+ current. In the cell-attached configuration, externally applied RP 49356 or pinacidil caused opening of large channels which reversed around O mV in a high K+ external medium. In inside-out patches, both RP 49356 or pinacidil activated K+-ATP channels by increasing the time period for which the channels remained in the open state. It is concluded that, like cromakalim, RP 49356 and pinacidil are potent activators of K+-ATP channels in cardiac myocytes.

Profibrillatory actions of pinacidil in a conscious canine model of sudden coronary death

Pinacidil is one of a number of new antihypertensive agents possessing an action that involves an enhanced potassium efflux in cardiac and vascular smooth muscle. An associated feature of pinacidil is a shortening of the cardiac action potential duration, which may constitute a potentially proarrhythmic effect. The present study evaluated pinacidil (0.3 mg/kg/h i.v. for 6 h) on the postinfarcted canine heart in a subset of dogs unresponsive to programmed electrical stimulation during the subacute phase of anterior myocardial infarction, and known to be at low risk of ventricular fibrillation in response to acute posterolateral ischemia. Results were compared with a comparable control group of vehicle-treated, noninducible animals. Nonsustained ventricular tachyarrhythmia developed in 2 of 15 pinacidil-treated animals as compared to the initiation of ventricular tachycardia in 1 of 16 postinfarcted hearts (p = 0.96) in the control group. Thus, pinacidil did not alter the responsiveness of the postinfarcted heart with respect to the electrical induction of tachyarrhythmias. The subsequent development of an acute ischemic event at a site remote from the previous myocardial infarction was associated with a greater incidence of ventricular fibrillation within 1 h from the onset of ischemia in the pinacidil-treated animals (9/15; 60%) as compared to the control group (1/15; 6.7%; p = 0.007). The 24-h cumulative mortality, likewise, was greater in the pinacidil-treated group [13/15 (87%)] as compared to the vehicle-treated control group 3/15; 20%; p = 0.001. Significant cardiovascular and electrophysiologic effects of pinacidil included an increase in heart rate (124 +/- 6-143 +/- 10 beats/min, p less than 0.05) and reductions in the refractory periods of normal (178 +/- 2-166 +/- 4 ms, p less than 0.05) and peri-infarcted (170 +/- 5-185 +/- 5 ms, p less than 0.01) myocardial regions. It is concluded that pinacidil does not alter the responsiveness of the postinfarcted heart to programmed electrical stimulation. However, in the presence of a superimposed acute ischemic event, pinacidil increases the potential for the development of ventricular fibrillation in a subset of postinfarcted animals that otherwise show a low risk with respect to the development of lethal arrhythmias. It is hypothesized that the increased tendency to develop ventricular fibrillation is associated with the pinacidil-induced reduction in the ventricular refractory period. This conclusion is consistent with the known ability of pinacidil to enhance potassium efflux during myocardial repolarization and to decrease the duration of the action potential.

Activators of potassium channels enhance calcium influx into endothelial cells as a consequence of potassium currents

Ca2+ influx into stimulated endothelial cells is attenuated by depolarization. We hypothesized that Ca2+ influx is driven by the membrane potential and may be enhanced by hyperpolarizing drugs like activators of K+ channels. Therefore we studied the effects of pinacidil, cromakalim, and cicletanine on membrane currents and on the intracellular free calcium concentration ([Ca2+]i) in cultured endothelial cells from porcine aorta. In patch-clamped cells, pinacidil (1 mumol/l) and cromakalim (1 mumol/l) elicited outward currents carried by K+ and significantly prolonged the Ca2(+)-dependent K+ currents induced by bradykinin and ATP. Peak currents in response to bradykinin were not affected. In cells loaded with the fluorescent Ca2+ indicator indo-1 and prestimulated with thimerosal, pinacidil (0.1-1 mumol/l elicited long-lasting increases in [Ca2+]i from 100 +/- 10 to 550 +/- 110 nmol/l. These effects were completely abolished in a medium containing 90 mmol/l K+. Similar results were obtained with cromakalim. Likewise, in cells stimulated with bradykinin, pinacidil raised [Ca2+]i when applied during the decline of [Ca2+]i after the initial peak. Cicletanine elicited K+ currents in resting and attenuated K+ currents in bradykinin-stimulated cells. It elevated [Ca2+]i even in the absence of extracellular Ca2+ and in K(+)-rich medium. Hence, the effects of cicletanine cannot be explained by direct actions on K+ channels. However, our studies demonstrate that pinacidil and cromakalim elevate [Ca2+]i secondary to their activation of K+ channels by inducing hyperpolarization and augmenting the driving force for potential-dependent Ca2+ influx. In this way, the two drugs may promote Ca2(+)-dependent formation of endothelium-derived relaxing factor.

Clinical pharmacology of potassium channel openers

Opening of K+ channels in cell membranes with resulting increase in K+ conductance, shifts the membrane potential in a hyperpolarizing direction towards the K+ equilibrium potential. Hyperpolarization reduces the opening probability of ion channels involved in membrane depolarization and excitation is reduced. K+ channel openers are believed to hyperpolarize smooth muscle cells by a direct action on the cell membrane. The best known members of the group are cromakalim, nicorandil and pinacidil, but several new compounds are being evaluated. In addition, it has recently been shown that also clinically well-known drugs like, e.g. diazoxide and minoxidil exhibit K+ channel opening properties. Nicorandil and new compounds containing nitro groups have a dual mechanism of action, also activating guanylate cyclase, an effect that contributes to their cardiovascular effect profile. K+ channel openers have a wide range of effects. Some of their properties and actions are summarized, and their present applications and/or potential for future application, in e.g. hypertension, angina pectoris, asthma, bladder instability, and several other disorders are discussed. It is concluded that K+ channel openning represents an interesting pharmacological principle with many potential clinical applications. However, most available drugs do not seem to have a sufficient tissue selectivity to be useful therapeutic alternatives. Before the potential of the new members of the group on clinical trials can be properly evaluated, clinical experiences are needed.

Direct effects of diazoxide on mitochondria in pancreatic B-cells and on isolated liver mitochondria

1. The direct effects of diazoxide on mitochondrial membrane potential, Ca2+ transport, oxygen consumption and ATP generation were investigated in mouse pancreatic B-cells and rat liver mitochondria. 2. Diazoxide, at concentrations commonly used to open adenosine 5'-triphosphate (ATP)-dependent K+-channels (K(ATP) channels) in pancreatic B-cells (100 to 1000 microM), decreased mitochondrial membrane potential in mouse intact perifused B-cells, as evidenced by an increase of rhodamine 123 fluorescence. This reversible decrease of membrane potential occurred at non-stimulating (5 mM) and stimulating (20 mM) glucose concentrations. 3. A decrease of mitochondrial membrane potential in perifused B-cells was also caused by pinacidil, but no effect could be seen with levcromakalim (500 microM each). 4. Measurements by a tetraphenylphosphonium-sensitive electrode of the membrane potential of rat isolated liver mitochondria confirmed that diazoxide decreased mitochondrial membrane potential by a direct action. Pretreatment with glibenclamide (2 microM) did not antagonize the effects of diazoxide. 5. In Fura 2-loaded B-cells perifused with the Ca2+ channel blocker, D 600, a moderate, reversible increase of intracellular Ca2+ concentration could be seen in response to 500 microM diazoxide. This intracellular Ca2+ mobilization may be due to mitochondrial Ca2+ release, since the reduction of membrane potential of isolated liver mitochondria by diazoxide was accompanied by an accelerated release of Ca2+ stored in the mitochondria. 6. In the presence of 500 microM diazoxide, ATP content of pancreatic islets incubated in 20 mM glucose for 30 min was significantly decreased by 29%. However, insulin secretion from mouse perifused islets induced by 40 mM K+ in the presence of 10 mM glucose was not inhibited by 500 microM diazoxide, suggesting that the energy-dependent processes of insulin secretion distal to Ca2+ influx were not affected by diazoxide at this concentration. 7. The effects of diazoxide on oxygen consumption and ATP production of liver mitochondria varied depending on the respiratory substrates (5 mM succinate, 10 mM alpha-ketoisocaproic acid, 2 mM tetramethyl phenylenediamine plus 5 mM ascorbic acid), indicating an inhibition of respiratory chain complex II. Pinacidil, but not levcromakalim, inhibited alpha-ketoisocaproic acid-fuelled ATP production. 8. In conclusion, diazoxide directly affects mitochondrial energy metabolism, which may be of relevance for stimulus-secretion coupling in pancreatic B-cells.

Antiarrhythmic effects of potassium channel openers in rhythm abnormalities related to delayed repolarization

BACKGROUND

Earlier observations have indicated that repolarization-delaying agents may, under certain circumstances, have the propensity to induce polymorphous ventricular tachyarrhythmias (PVTs) (i.e., torsade de pointes). We have studied whether the potassium channel opener pinacidil and two of its pyridylcyanoguanidine analogues (P1075 and P1188) have any antiarrhythmic effects on clofilium-induced PVTs and triggered responses in rabbits in vivo and in vitro.

METHODS AND RESULTS

Anesthetized rabbits were pretreated with propranolol (2 mumol/kg i.v.) and subsequently given a concomitant intravenous infusion of clofilium (63 nmol/kg/min for maximally 15 minutes) and the alpha 1-agonist methoxamine (70 nmol/kg/min). In vehicle-pretreated rabbits (n = 19), clofilium invariably induced PVTs, which closely resembled torsade de pointes and were preceded by a marked prolongation of the QTU interval (27 +/- 2.4%, p less than 0.001). In a separate group of seven rabbits in which monophasic action potentials were recorded from the left ventricular endocardium, the tachyarrhythmia was preceded by deflections consistent with early afterdepolarizations (EADs) of the plateau repolarization phase of the monophasic action potentials. Intravenous administration of the pyridylcyanoguanidines in doses reducing mean arterial blood pressure by 25 or 50 mm Hg, respectively, was associated with a dose-dependent attenuation in the occurrence of clofilium-induced PVTs. In the pinacidil-pretreated rabbits (0.41 mumol/kg or 1.86 mumol/kg i.v.), the occurrence of PVTs was reduced from seven of seven rabbits to five of six and to three of seven rabbits (p = 0.035 versus vehicle-pretreated controls), respectively. In rabbits pretreated with the low dose of P1075 (0.01 mumol/kg i.v.), PVT occurrence was reduced from six of six rabbits to two of six rabbits (p = 0.030), whereas in six rabbits given the high dose of P1075 (0.13 mumol/kg), no PVTs appeared (p = 0.001). When the sulfonylurea glibenclamide (10 mumol/kg i.v.) was administered to rabbits before P1075 (0.13 mumol/kg) was infused, clofilium induced PVTs in five of six rabbits (not significantly different from the incidence in the vehicle-pretreated rabbits). Pretreatment with P1188 (4.36 mumol/kg or 11.88 mumol/kg i.v.) caused a reduction in the occurrence of PVT from six of six rabbits to five of six and to none of six rabbits (p = 0.001), respectively. In the six animals pretreated with the high dose of P1188 in which no clofilium-induced arrhythmias were elicited, glibenclamide (20 mumol/kg i.v.) was injected after the entire dose of clofilium had been administered. In these rabbits, premature ventricular systoles and PVTs appeared within a few minutes in five and four of the animals, respectively. In contrast to the pyridylcyanoguanidines, diltiazem pretreatment (0.9 mumol/kg i.v., decreasing arterial pressure by 50 mm Hg) did not attenuate PVT occurrence (five of six rabbits). Acute administration of P1075 (0.13 mumol/kg) during recurrent attacks of PVTs abruptly regularized the rhythm in 12 of 13 animals and diminished EADs observed in monophasic action potentials recorded from the left ventricular endocardium. In in vitro experiments, action potentials were simultaneously recorded from rabbit Purkinje fibers and ventricular muscle cells. Clofilium markedly prolonged action potential duration in Purkinje fibers but not in ventricular muscle cells, and eventually, bradycardia-dependent EADs and triggered activity were elicited. P1075 completely abolished EADs and triggered activity in all (six of six) experiments. Glibenclamide antagonized the suppressive effect of P1075; hence, EADs and triggered responses reappeared and resembled those present before P1075.

CONCLUSIONS

These results suggest that ATP-sensitive potassium channel activat

BACKGROUND

Earlier observations have indicated that repolarization-delaying agents may, under certain circumstances, have the propensity to induce polymorp

Blockade of ATP-sensitive potassium channels increases infarct size but does not prevent preconditioning in rabbit hearts

Ischemic preconditioning renders the heart resistant to infarction by an unknown mechanism. This study tests whether preconditioning may be working through activation of ATP-sensitive potassium channels. If that were the case, then blockade of the channels should eliminate preconditioning's protection, and activation of these channels should mimic it. Thirty minutes of regional coronary ischemia followed by 3 hours of reperfusion caused 38.0 +/- 3.7% of the risk zone to become infarcted in control rabbits. Preconditioning with 5-minute ischemia followed by a 10-minute reperfusion before the 30-minute insult caused only 8.8 +/- 2.1% infarction, which was a reduction of 29.2% in infarct size by preconditioning (p < 0.01 versus control value). Pretreatment with the potassium channel blocker glibenclamide at three different concentrations significantly elevated infarct size in the nonpreconditioned hearts at all doses. Preconditioning, however, continued to limit infarct size by an amount not different from that seen in the control group at all doses of glibenclamide. Pinacidil, a potassium channel agonist, given before a 30-minute ischemic insult resulted in infarct sizes no different from that seen in nonpreconditioned control rabbits. We conclude that ATP-sensitive potassium channels are not involved in preconditioning in the rabbit heart; however, blocking those channels does exacerbate ischemia.

Increased myogenic tone and diminished responsiveness to ATP-sensitive K+ channel openers in cerebral arteries from diabetic rats

Diabetes mellitus has profound adverse effects on vascular and, in particular, endothelial function. Although pressure-induced constriction ("myogenic tone") is a major contributor to the regulation of blood flow, little is known about the effects of diabetes on this response. Diabetes has been shown to diminish the dilation of cerebral arteries to synthetic ATP-sensitive K+ (KATP) channel openers. In this study, we explored the effects of diabetes induced in rats by streptozotocin on cerebral artery (250 to 300 microns) myogenic tone and on vasodilations to the synthetic KATP channel openers pinacidil and levcromakalim. Elevation of intravascular pressure caused a graded membrane potential depolarization and constriction, which was greater in arteries from diabetic rats compared with normal rats (at 60 mm Hg, 5 mV more depolarized and 22 microns more constricted). Pressurized arteries (at 60 mm Hg) from diabetic rats were 5- to 15-fold less sensitive to pinacidil and levcromakalim than were control arteries (EC50 values for pinacidil and levcromakalim were 1.4 and 0.6 mumol/L, respectively, in diabetic animals and 0.3 and 0.04, respectively, in control animals; P < .05). Removal of the endothelium or addition of a NO synthase inhibitor, NG-nitro-L-arginine (LNNA), in control arteries decreased the sensitivity to KATP channel openers and depolarized and constricted control arteries to levels similar to those observed in arteries from diabetic animals. Sodium nitroprusside caused a membrane potential hyperpolarization and enhanced the response to pinacidil in arteries from diabetic animals. Removal of the endothelium or LNNA had little effect on the apparent KATP channel opener sensitivity, the membrane potential, and pressure-induced constrictions of arteries from diabetic animals. The results are consistent with the hypothesis that this type of diabetes leads to a decrease in tonic NO release from the endothelium, which in turn causes membrane potential depolarization and vasoconstriction, resulting in a diminished response to KATP channel openers.

Glibenclamide is a competitive antagonist of cromakalim, pinacidil and RP 49356 in guinea-pig pulmonary artery

The relaxant effect of cromakalim (BRL 34915), pinacidil and RP 49356 (N-methyl-2-(3-pyridyl)-tetrahydro-thiopyran-2-carbothioamide-1-ox ide) on the sustained contractions induced by 20 mM KCl were compared with the effects of nicorandil. The preparation used was vascular smooth muscle of phenoxybenzamine-treated pulmonary artery rings from reserpinized guinea-pigs. Cromakalim, pinacidil, RP 49356 and nicorandil relaxed the tissues with -log EC50 values of 6.78, 6.12, 6.02 and 5.46, respectively. The inhibitory effect of cromakalim, pinacidil and RP 49356, but not of nicorandil, was competitively antagonized by glibenclamide (10(-7)-3 X 10(-6) M), yielding uniform pA2 values of 7.17-7.22 against all three relaxant drugs. The order of potency of other K+ channel blocking agents for the inhibition of vasorelaxation by cromakalim, pinacidil and RP 49356 was procaine greater than 4-aminopyridine greater than tetraethylammonium. The mainly competitive type of inhibition of the RP 49356-induced response was more comparable to that with pinacidil than with cromakalim. The relaxation caused by nicorandil was only attenuated by a high concentration of 4-aminopyridine or tetraethylammonium but was markedly antagonized by methylene blue (3 X 10(-6)-10(-5) M) and potentiated by M & B 22948 (3 X 10(-6)-10(-5) M). These results suggest that the vascular relaxation caused in guinea-pig pulmonary artery by cromakalim, pinacidil and RP 49356 is mediated through the same glibenclamide-sensitive K+ channel whereas the major mechanism for the effect of nicorandil seems to involve stimulation of guanylate cyclase.