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

Nicorandil improves myocardial high-energy phosphates in postinfarction porcine hearts

1. Nicorandil is a potent vasodilator combining the effects of a nitrate with an ATP-sensitive potassium channel (K(ATP)) opener. Because the postinfarct remodelled heart has increased vulnerability to subendocardial hypoperfusion, it is possible that the vasodilator effects of nicorandil could cause transmural redistribution of blood flow away from the subendocardium. Alternatively, the K(ATP) channel opening effects of nicorandil could exert a beneficial effect on mitochondrial respiration. Consequently, the present study was performed to examine the effect of nicorandil on energy metabolism in the postinfarct heart. 2. Studies were performed in swine in which myocardial infarction produced by proximal left circumflex coronary artery ligation had resulted in left ventricular remodeling. [(31)P] nuclear magnetic resonance spectroscopy (MRS) was used to examine the myocardial energy supply/demand relationship across the left ventricular wall while the transmural distribution of blood flow was examined with radioactive microspheres. Data were obtained during baseline conditions and during infusion of nicorandil (100 microg, i.v., followed an infusion of 25 microg/kg per min). 3. Nicorandil caused coronary vasodilation with a preferential increase in subepicardial flow; however, subendocardial flow also increased significantly. Nicorandil had no significant effect on the rate-pressure product or myocardial oxygen consumption. The ratio of phosphocreatine (PCr)/ATP determined with MRS was abnormally depressed in remodelled hearts (2.01 +/- 0.11, 1.85 +/- 0.10 and 1.59 +/- 0.11 for subepicardium, midwall and subendocardium, respectively) compared with normal (2.22 +/- 0.11, 2.01 +/- 0.15 and 1.80 +/- 0.09, respectively). Nicorandil had no effect on the high-energy phosphate content of normal hearts. However, nicorandil increased the PCr/ATP ratio in the subendocardium of remodelled hearts from 1.59 +/- 0.11 to 1.87 +/- 0.10 (P < 0.05). 4. Although nicorandil caused modest redistribution of blood flow away from the subendocardium of the postinfarct left ventricle, this was associated with an increase of the PCr/ATP ratio towards normal. These results suggest that nicorandil exerts a beneficial effect on energy metabolism in the subendocardium of the postinfarct remodelled left ventricle.

Effects of bimakalim on human cardiac action potentials: comparison with guinea pig and nicorandil and use-dependent study

Electrophysiologic effects of K(ATP) channel openers (KCOs) are rarely studied for tissue and species specificity, and use-dependent investigations in human tissues are lacking. We therefore investigated in vitro the concentration-dependent effects of the KCO bimakalim [from 10 nM to 10 microM, at 1,000 ms of cycle length (CL) and 37 degrees C] on human (atrium, n = 4, and ventricle, n = 6) and guinea pig (atrium, n = 7, and ventricle, n = 6) transmembrane action potential (AP). The frequency relation (from CL 1,600 to 300 ms, 31 degrees C) of human atrial AP duration 90% (APD90) shortening (10 microM vs. baseline, n = 7) also was determined. A parallel study was performed with the KCO nicorandil (from 10 nM to 1 mM, n = 3) in human atrial APs, at 31 degrees C. Resting membrane potential and maximal upstroke velocity of AP were not modified by bimakalim at maximal concentration, whereas AP amplitude was decreased in both guinea pig preparations (p < 0.05); APD90 was shortened in all tissues (p < 0.01). Median effective concentration (EC50) for APD90 shortening at 37 degrees C was 0.54 and 2.74 microM in atrial and ventricular human tissue, respectively, and 8.55 and 0.89 microM in atrial and ventricular guinea pig tissue, respectively. In human atrial tissue at 31 degrees C, EC50 with bimakalim was 0.39 microM; a much higher value was seen with nicorandil (210 microM). Bimakalim (10 microM)-induced APD90 shortening as a function of stimulation rate was greatest at longest CL. Evidence is provided for (a) species (human vs. guinea pig) and tissue (atrium vs. ventricle) differential AP sensitivity to bimakalim; (b) an approximately 500-fold higher efficacy of bimakalim versus nicorandil to shorten human atrial APD90; and (c) normal use-dependence of human atrial APD90 shortening with bimakalim at 10 microM.

Hemodynamic and hormonal responses to nicorandil in a canine model of acute ischemic heart failure: a comparison with cromakalim and nitroglycerin

The pharmacologic profiles of nicorandil in the cardiovascular system have been characterized by K-channel opening and nitrate activities. However, the effects of nicorandil on acute heart failure have yet to be elucidated. To investigate the effects of nicorandil under such pathophysiologic conditions, we administered nicorandil intravenously to dogs with acute ischemic heart failure induced by coronary embolization and compared the results with those induced by cromakalim and nitroglycerin. The heart failure in this experiment was demonstrated by a reduction of mean blood pressure (MBP) from 143+/-3 to 129+/-2 mm Hg (p < 0.01); cardiac output (CO) from 2.18+/-0.10 to 1.06+/-0.05 L/min (p < 0.01); stroke volume (SV) from 12.7+/-0.6 to 6.8+/-0.3 ml/min (p < 0.01); Vmax, an index of the contractility of the left ventricle, from 105.5+/-4.4 to 49.9+/-1.8 1/s (p < 0.01), and an increase in right atrial pressure (RAP) from 2.9+/-0.3 to 5.3+/-0.3 mm Hg (p < 0.01); left ventricular end-diastolic pressure (LVEDP) from 2.5+/-0.4 to 26.0+/-1.4 mm Hg (p < 0.01); and T, time constant of left ventricular relaxation, from 38.3+/-0.8 to 62.4+/-2.8 ms (p < 0.01). Furthermore, plasma renin activity (PRA) and plasma atrial natriuretic peptide (ANP) increased (from 1.72+/-0.29 to 5.03+/-0.68 ng AngI/ml/h, p < 0.01; from 103.9+/-5.8 to 411.5+/-29.4 pg/ml, p < 0.01, respectively), whereas brain natriuretic peptide (BNP) remained unchanged (from 23.1+/-2.2 to 26.9+/-1.4 pg/ml). Nicorandil (10-40 microg/kg/min, i.v. infusion for 20 min for each dosing) or cromakalim (0.25-1 microg/kg/min) decreased MBP, systemic vascular resistance (SVR), RAP, and LVEDP, and increased CO, SV, and Vmax. However, the reduction of RAP in cromakalim was significantly smaller than those of nicorandil and nitroglycerin in comparison at similar hypotensive doses. Nitroglycerin (2.5-10 microg/kg/min) decreased MBP, RAP, and LVEDP, and increased Vmax but did not change CO or SV. Increased plasma ANP levels, an index of cardiac filling pressure after induction of acute ischemic heart failure, were decreased significantly by cromakalim and tended to decrease by nicorandil or nitroglycerin. Plasma BNP levels and PRA were not influenced by any of these drugs. These results suggest that nicorandil produces the reduction of both preload and afterload followed by an improvement of cardiac contractility in this model. The increase in CO may be mediated mainly by the drug's K-channel opening activities and the reduction of venous tone by its nitrate properties. Nicorandil may prove to be useful in the treatment of acute ischemic heart failure.

ATP-Sensitive Potassium Channel Openers and Blockers in the Cardiovascular System: Physiology, Pharmacology, and Clinical Effects

Adenosine triphosphate (ATP)-sensitive potassium chan nels (K.ATP channels), a subclass of potassium channels activated by a low intracellular ATP concentration, have been described in various tissue types, including the heart muscle and vascular smooth muscle. In ventricu lar myocytes, activation of these channels is considered protective, because their activation caused by hypoxia or ischemia results in cell energy preservation. Activa tion of K.ATP channels in vascular smooth muscle cells causes hyperpolarization of the cell membrane, muscle cell relaxation, and vasodilation. Potassium channel openers are pharmacologic activators of K.ATP chan nels. Their protective effects on the ischemic myocar dium and their vasodilating properties have been stud ied extensively. Sulfonylurea derivatives, widely used in the treatment of noninsulin-dependent diabetes melli tus, are considered selective blockers of K.ATP channels and have been used in many experiments to show K.ATP channel involvement. This article focuses on these issues and the clinical effects and potentials of K.ATP channel modulation.

Reversal of hypothermia-induced action potential lengthening by the KATP channel agonist bimakalim in isolated guinea pig ventricular muscle

1. ATP-sensitive potassium (KATP) channel openers shorten cardiac ventricular muscle action potential duration (APD), reduce resting and developed contractile force, and have been shown to provide cardioprotection when given before, during, and after either short-term ischemia or long-term hypothermia. The authors' aim was to determine the concentration-dependent effect of the potent KATP channel opener bimakalim on transmembrane action potential changes induced by mild (27 degrees C) and moderate (20 degrees C) hypothermia in isolated guinea pig ventricular muscle. 2. Conventional microelectrode techniques were used to record action potentials (APs) in single myocytes during normothermia (37 degrees C) and hypothermia in the presence and absence of 0.1 to 30 mumol.l-1 bimakalim. 3. Hypothermia alone increased APD and depolarized the diastolic membrane potential (DMP): APD90 = 141.7 +/- 7.0 msec and DMP -86.2 +/- 1.4 mV (n = 6) at 37 degrees C versus 235.7 +/- 7.8 msec and -75.6 +/- 1.0 mV at 20 degrees C (n = 7). At 37 degrees C, bimakalim (0.1-10 mumol.l-1) shortened APD in a concentration-dependent fashion. 4. APD90 was markedly reduced from 141.7 +/- 7.0 msec without bimakalim to 9.5 +/- 2.6 msec with 10 mumol.l-1 bimakalim (n = 6); this effect was blocked by glibenclamide. DMP was hyperpolarized by bimakalim. More bimakalim was required to shorten APs during mild and moderate hypothermia. The 50% effective concentration (EC50) of bimakalim required to maximally shorten APD90 was 0.96 +/- 0.10 mumol.l-1 at 37 degrees C; this increased to 3.96 +/- 0.24 mumol.l-1 at 27 degrees C, and to 12.34 +/- 0.72 mumol.l-1 at 20 degrees C. Relative to hypothermia-induced depolarization, bimakalim hyperpolarized DMP toward drug-free values obtained at 37 degrees C. 5. These results indicate that hypothermia shifts the bimakalim concentration APD90 response curve to the right such that 13 times more bimakalim is required at 20 degrees C shorten APD by the same amount as at 37 degrees C. Bimakalim also reverses hypothermia-induced AP lengthening and tends to reverse the hypothermia-induced decrease in DMP. 6. These findings aid in our understanding of the cardioprotective effects of KATP channel openers during hypothermia.

Effects of nicorandil, an antianginal potassium channel opener, on left ventricular systolic and diastolic function in patients with chronic coronary artery disease

We compared the effects of nicorandil with nitroglycerin and nifedipine on left ventricular function. Intravenous nicorandil may be a balanced-typed vasodilator and useful in patients with left ventricular systolic and diastolic dysfunction.

Systemic, pulmonary and coronary haemodynamic actions of the novel dopamine receptor agonist in awake pigs at rest and during treadmill exercise Z1046

1. In view of the potential therapeutic application of specific dopamine receptor agonists in the treatment of hypertension and left ventricular dysfunction, we investigated the cardiovascular actions of the novel mixed D1/D2 dopamine receptor agonist Z1046 in awake pigs at rest and during treadmill exercise. 2. Thirteen swine were chronically instrumented under sterile conditions for measurement of systemic, pulmonary, and coronary haemodynamics. Regional blood flows were determined with the radioactive microsphere technique. 3. Z1046 (1, 10, 100 micrograms kg-1, i.v.) produced dose-dependent reductions in central aortic blood pressure (up to 27 +/- 3%, P < or = 0.05) in awake resting pigs which was accompanied by only minimal reflex activation of the sympathetic nervous system. The hypotensive response was principally the result of peripheral vasodilatation (system vascular resistance decreased up to 35 +/- 4%, P < or = 0.05), which was located in the cerebral, coronary, renal, mesenteric, adrenal, splenic and skeletal muscular vascular beds (vascular resistance decreased up to 30-40% after the highest dose in these beds). Only in the cerebral and mesenteric bed was the vasodilatation sufficiently large to overcome the decrease in blood pressure and result in an increased blood flow; the vasodilatation in the coronary bed was most likely due to autoregulation as neither coronary blood flow nor myocardial oxygen demand were changed significantly by Z1046. The systemic vasodilatation that was caused by the highest i.v. dose (100 micrograms kg-1) was accompanied by transient and minor increases in heart rate (15 +/- 5%, P < or = 0.05) and cardiac output (15 +/- 5%, P < or = 0.05) whereas after 10 micrograms kg-1, i.v., a slight decrease in cardiac output also contributed to the hypotension. Z1046 had no effect on pulmonary vascular resistance. 4. The systemic vasodilator responses to Z1046 (100 micrograms kg 1, i.v.) were sustained during treadmill exercise (2-4 km h-1 which produced heart rates of up to 233 +/- 10 beats min-1), but with increasing treadmill speed attenuation of the exercise-induced increase in heart rate (-11 +/- 3%, P < or = 0.05) and hence cardiac output (-10 +/- 3%, P < or = 0.05) (as stroke volume was not altered by Z1046) contributed significantly to a lower aortic blood pressure (-20 +/- 3%, P < or = 0.05) Z1046 had no effect on pulmonary vascular resistance during exercise. 5. Oral administration of Z1046 (0.5, 1.5 mg kg-1) produced a fall in central aortic blood pressure (up to 15 +/- 3%, P < or = 0.05), which developed gradually during the first 90 min and lasted up to 4 h after administration, again with negligible changes in heart rate and LVdP/dtmax. 6. Neither non-selective alpha- and beta-adrenoceptor blockade, nor selective alpha 2-adrenoceptor blockade altered the vasodilator actions of Z1046, but non-selective alpha- and beta-adrenoceptor blockade abolished the cardiac responses to dopamine receptor stimulation, suggesting that its cardiac actions were principally caused by D2-receptor-mediated inhibition of catecholamine release, whereas the vasodilator response was probably the result of vascular D1-receptor stimulation. 7. In conclusion, the novel dopamine receptor agonist Z1046 is an effective blood pressure lowering agent that elicits minimal reflex activation of the sympathetic nervous system in awake resting pigs Systemic vasodilatation was not affected by combined alpha- and beta-adrenoceptor blockade, which is consistent with a predominantly D1 receptor-dependent vasodilator mechanism. The hypotensive effect is maintained during treadmill exercise during which systemic vasodilatation and a lower cardiac output both contribute to the blood pressure lowering actions of Z1046. The cardiovascular profile of this orally active compound warrants further investigation of this class of drugs in experimental and clinical hypertension.

Selective guanylyl cyclase inhibitor reverses nitric oxide-induced vasorelaxation

Effects of a novel soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), were characterized on guanylyl cyclase activity in cytosolic fraction of COS-7 cells overexpressing the alpha 1 and beta 1 subunits of the rat soluble enzyme. ODQ was a noncompetitive inhibitor of soluble guanylyl cyclase with respect to Mn2+ or Mn(2+)-GTP and was a mixed competitive/noncompetitive inhibitor with respect to nitric oxide (NO) donation. ODQ (10 mumol/L) reduced deta nonoate-stimulated cGMP production in COS-7 cells overexpressing soluble guanylyl cyclase and in rat aortic vascular smooth muscle cells. ODQ did not inhibit particulate forms of the enzyme rat guanylyl cyclase-A, -B, or -C, did not block NO synthase, and did not auto-oxidize deta nonoate-donated NO in the presence of cells at physiological pH. Therefore, ODQ is a selective inhibitor of soluble guanylyl cyclase. Using ODQ in isolated aortic ring preparations, we tested the hypothesis that soluble guanylyl cyclase mediates vasorelaxant activity associated with NO. Phenylephrine (100 nmol/L)-precontracted, isolated rat aortas were relaxed in a concentration-dependent manner by deta nonoate (0.01 to 100 mumol/L) and nitroglycerin (0.01 to 300 mumol/L). ODQ (10 mumol/L) attenuated deta nonoate- and nitroglycerin-mediated relaxation of contracted aortas. ODQ had no effect on natriuretic peptide-, 8-bromo-cGMP-, isoproterenol-, or bimakalim-mediated aortic relaxation. These results support the hypothesis that soluble guanylyl cyclase mediates vasorelaxant activity associated with nitric oxide.