Characterization of a novel K+ channel activator, EMD 52962, in electrophysiological and pharmacological experiments

Effects of the KATP channel opener bimakalim on coronary blood flow, monophasic action potential duration, and infarct size in dogs

BACKGROUND

The major purpose of the present study was to determine the effect of the potassium channel opener bimakalim, administered intracoronary only during the initial 10 minutes of ischemia, on myocardial infarct size in anesthetized dogs. A second aim was to test the possibility that bimakalim mediates its cardioprotective effects by accelerating the rate of myocyte action potential shortening during early ischemia. A third aim was to determine the relative potency of bimakalim to open coronary vascular ATP-regulated potassium (KATP) channels versus myocyte KATP channels.

METHODS AND RESULTS

Barbital-anesthetized open-chest dogs were used. In the initial studies, bimakalim (0.1 to 10 micrograms/min) was infused into the left anterior descending coronary artery (LAD), and changes in coronary blood flow and monophasic action potential duration (MAPD) were used as indexes of coronary vascular KATP channel and myocyte KATP channel activity, respectively. In subsequent infarct studies, dogs were subjected to 60 minutes of LAD occlusion followed by 4 hours of reperfusion. Based on preliminary studies, two doses of bimakalim that did not shorten MAPD during nonischemic conditions (0.1 and 0.3 microgram/min) and one that markedly shortened MAPD during nonischemic conditions (3.0 micrograms/min) or an equal volume of vehicle were infused into the LAD during the initial 10 minutes of coronary artery occlusion. Transmural myocardial blood flow was measured at 5 and 30 minutes of occlusion by the radioactive microsphere technique, and infarct size was determined at the end of 4 hours of reperfusion by triphenyltetrazolium staining. The monophasic action potential duration at 50% repolarization (MAPD50) was measured by an epicardial probe placed in the center of the ischemic area. Bimakalim had an approximately 10-fold greater affinity for the coronary vascular than the myocardial KATP channel (ED50 coronary, approximately 0.3 microgram/min; ED50 myocyte, approximately 3.0 micrograms/min). Three doses of bimakalim (0.1, 0.3, and 3.0 micrograms/min) all markedly reduced infarct size expressed as percent of the area at risk (12.6 +/- 3.3%, 14.5 +/- 2.2%, and 14.2 +/- 5.3%, respectively, versus 27.2 +/- 5.7% in controls) to nearly equal extents. Subsequently, we found that the two higher doses of bimakalim (0.3 and 3.0 micrograms/min) markedly accelerated yet the 0.1-microgram/min dose of bimakalim did not significantly affect the ischemia-related shortening of the action potential during the initial 5 minutes of occlusion. In addition, 0.1 and 0.3 microgram/min bimakalim did not increase the incidence of ventricular fibrillation during the 60 minutes of occlusion (0 of 7 and 0 of 8 dogs, respectively), whereas 3.0 micrograms/min bimakalim had a profibrillatory effect (6 of 6) compared with the control group (1 of 8). There were no significant differences between groups in systemic hemodynamics, myocardial oxygen demand, ischemic bed size, or collateral blood flow to the ischemic region.

CONCLUSIONS

The results of the present study clearly reveal that a small dose (0.1 or 0.3 microgram/min) of the KATP channel opener bimakalim administered only during the initial 10-minute period of ischemia markedly reduced myocardial infarct size to an extent equal to that of a higher profibrillatory dose in barbital-anesthetized dogs. These data also suggest that bimakalim and other potassium channel openers may partially exert their cardioprotective effects by accelerating KATP channel activation during early ischemia as evidenced by an enhanced rate of ischemic myocyte action potential shortening. However, the results also suggest that other cellular mechanisms may be involved in mediating the cardioprotection produced by a low dose of bimakalim (0.1 microgram/min) because it did not significantly accelerate the shortening of the action potential duration, yet it had an efficacy to redu

BACKGROUND

The major purpose of the present study was to de

Cardiovascular effects of the novel potassium channel opener bimakalim in conscious pigs after myocardial infarction: a comparative study with nicorandil

The benzopyran-derivative bimakalim is an ATP-dependent potassium channel activator that has been shown to be a potent arterial vasodilator in anesthetized pigs. In the present study we evaluated the cardiovascular profile of bimakalim in normal conscious animals and conscious animals with chronic left ventricular dysfunction and compared the results to those obtained with nicorandil. In normal conscious pigs, bimakalim (37.5-300 ng/kg/min, n = 6) and nicorandil (10-80 micrograms/kg/min, n = 8) increased cardiac output from 2.7 +/- 0.1 l/min to 3.8 +/- 0.2 l/min and from 2.7 +/- 0.1 l/min to 3.9 +/- 0.3 l/min (both p less than .05) due to increases in heart rate (up to 62 +/- 14% and 74 +/- 9%, respectively, both p less than .05). The mean arterial blood pressure decreased gradually from 104 +/- 4 mmHg to 91 +/- 5 mmHg with bimakalim and from 98 +/- 3 mmHg to 84 +/- 5 mmHg with nicorandil (both p less than .05), due to similar decreases in systemic vascular resistance. LVdP/dtmax also increased with both drugs (up to 48 +/- 11% and 69 +/- 7%, respectively, p less than .05), but left ventricular end-diastolic pressure remained unchanged with bimakalim, while it gradually decreased from 9 +/- 1 mmHg to 5 +/- 1 mmHg (p less than .05) with nicorandil. In pigs with a 3- to 4-week-old myocardial infarction, the vasodilator responses to bimakalim (n = 8) and nicorandil (n = 9) were not affected, but the increases in heart rate and LVdP/dtmax were attenuated compared to the effects in the normal animals.(ABSTRACT TRUNCATED AT 250 WORDS)

EMD 52692 (bimakalim), a new potassium channel opener, attenuates luminol-enhanced chemiluminescence and superoxide anion radical formation by zymosan-activated polymorphonuclear leukocytes

We investigated the relationship of potassium channel activation on modulation of oxidative respiratory bursts in canine neutrophils. Generation of superoxide anion radicals in opsonized zymosan-activated cells was determined using the technique of ferricytochrome c reduction. Preincubation of cells with the selective potassium channel opener, EMD 52692 (1-100 microM), attenuated superoxide anion radical production. Furthermore, EMD 52692 also produced a concentration-dependent inhibition of luminol-enhanced chemiluminescence by activated neutrophils. Glyburide, a selective antagonist of ATP-sensitive potassium channels, prevented the modulatory effect of EMD 52692 on both superoxide anion generation and luminol-enhanced chemiluminescence. The results suggest that ATP-sensitive potassium channels may play a significant role in regulating oxygen-derived free radical production in neutrophil-induced tissue injury.

Two different types of potassium channels in human skeletal muscle activated by potassium channel openers

The inside-out patch clamp technique was used to record the effects of K+ channel openers (EMD 52692, RP 49356 and Cromakalim) on single channel currents in membrane blebs of human skeletal muscle. Two types of K+ channels were activated by these drugs: an ATP-sensitive K+ channel which was inhibited by 3 mM ATP and 5 microM Glibenclamide and an ATP insensitive K+ channel. The open probability of both types was strongly increased by K+ channel openers. Glibenclamide antagonized the action of the K+ channel openers.

Haemodynamic profile of the potassium channel activator EMD 52692 in anaesthetized pigs

1. The systemic and regional haemodynamic effects of the potassium channel activator EMD 52692 or its solvent were investigated after intravenous and after intracoronary administration in anaesthetized pigs. 2. Consecutive intravenous 10 min infusions of EMD 52692 (0.15, 0.30, 0.60, 1.20 micrograms kg-1 min-1; n = 7) dose-dependently decreased mean arterial blood pressure by up to 50%. This was entirely due to peripheral vasodilatation, since cardiac output did not change. Heart rate increased by up to 50%, while left ventricular end diastolic pressure decreased dose-dependently from 6 +/- 1 mmHg to 3 +/- 1 mmHg (P less than 0.05), and stroke volume decreased from 30 +/- 2 ml to 21 +/- 2 ml (P less than 0.05). Left ventricular dP/dtmax was not affected. 3. Although cardiac output did not change, EMD 52692 caused a redistribution of blood flow from the arteriovenous anastomoses to the capillary channels. Blood flow to the adrenals, small intestine, stomach, bladder, spleen and brain increased, while renal blood flow decreased and blood flow to several muscle groups and skin were not altered. Vascular conductance was increased dose-dependently in all organs, except for the kidneys, where after the initial increase, vascular conductance returned to baseline with the highest dose. Particularly striking were the effects on the vasculature of the brain. With the highest dose of EMD 52692 blood flow more than doubled, while vascular conductance increased four fold. 4. Transmural myocardial blood flow increased slightly, which was entirely due to an increase in subepicardial blood flow. Myocardial O2-consumption and segment length shortening were not significantly affected. 5. After consecutive 10 min intracoronary infusions (0.0095, 0.019, 0.0375 and 0.075 microgram kg-1 min-1; n = 7) into the left anterior descending coronary artery (LADCA), mean arterial blood pressure was maintained with the lowest two doses, but decreased by up to 15% with the higher doses, whereas heart rate increased by up to 24%. Blood flow to the LADCA-perfused myocardium doubled with the highest dose, the subepicardium benefitting the most. Coronary venous O2-saturation increased dose-dependently from 23 +/- 2% to 60 +/- 4%, while myocardial O2-consumption of the LADCA-perfused myocardium was not affected by the drug. 6. It is concluded that EMD 52692 is a potent vasodilator, with particularly pronounced effects on vasculature of the brain. Its selectivity for vascular smooth muscle cells exceeds that for the myocytes, since with doses that are much higher than those of potential clinical interest no negative inotropic effects were observed. The compound primarily dilates arteries but some venodilatation may also occur.

K+ channel openers suppress myotonic activity of human skeletal muscle in vitro

Isolated fibre bundles from myotonic human skeletal muscle showed after-contractions and spontaneous mechanical activity. The K+ channel openers cromakalim (10-100 mumols/l) and EMD 52962 (1-10 mumols/l) completely suppressed these abnormalities in mechanical activity. Voltage-clamp experiments revealed that cromakalim (100 mumols/l) increased the membrane K+ conductance of isolated, non-myotonic human skeletal muscle fibres 4-fold; Cl- conductance was not altered. The data show that myotonia is suppressed by an increase in in membrane K+ conductance.