In vitro studies on the mode of action of pinacidil

Nicorandil elevates tissue cGMP levels in a nitric-oxide-independent manner

The K(+) channel opener nicorandil is a hybrid compound that contains nitrate in its structure. It has been reported that nicorandil can relax vascular tissue in vitro via a mechanism that involves activation of K(ATP) channels and stimulation of soluble guanylyl cyclase. However, it is not known whether the increase of cGMP levels occurs through an elevation of nitric oxide (NO). The aim of the present study was to determine whether NO release was a direct effect of nicorandil. We reported here that nicorandil did not generate NO using ozone chemiluminescence detection methods in human or rat liver microsomes (P450-rich fractions) with addition of NADPH. However, nicorandil elevated cGMP levels in rat liver, aorta, and human coronary smooth muscle cells in vitro. The elevation was not inhibited by the NO trapping agent carboxy-2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (carboxy-PTIO). These results suggest that nicorandil elevates cGMP without NO generation.

Modulation of endothelium-dependent hyperpolarization and relaxation to acetylcholine in rat mesenteric artery by cytochrome P450 enzyme activity

Acetylcholine (ACh) induced hyperpolarization and relaxation in rat mesenteric arteries contracted with norepinephrine, as indicated from studies with simultaneous microelectrode and tension recordings. We tested whether the hyperpolarization to ACh was modified by induction and depletion of cytochrome P450 enzymes. Enzyme induction by treating the animals with 3-methylcholanthrene and beta-naphthoflavone for 3 days resulted in a significant increase in the endothelium-dependent hyperpolarization to a maximum of 22.7 +/- 1.0 mV from 13.9 +/- 0.4 mV in arteries from untreated animals. Enzyme depletion by treating the animals with CoCl2 for 2 days resulted in a significant reduction in the maximum hyperpolarization to 9.9 +/- 0.7 mV. When NO synthesis was inhibited by N omega-nitro-L-arginine, the relaxation was correlated to hyperpolarization. The N omega-nitro-L-arginine-resistant responses were significantly inhibited by clotrimazole. The relaxation to ACh was not altered by enzyme induction but was significantly reduced by enzyme depletion. In KCI-contracted arteries, modification of cytochrome P450 enzyme activity had no significant effect on the relaxation to ACh. Similarly, hyperpolarization and relaxation to pinacidil were not significantly affected. These results suggest that the hyperpolarization response to ACh is closely regulated by cytochrome P450-dependent enzymes.

Effects of pinacidil on arterial and venous resistances and mean circulatory filling pressure in rats

1. The effects of the potassium channel opener, pinacidil, on mean arterial pressure (MAP), mean circulatory filling pressure (MCFP), total peripheral resistance (TPR), cardiac output (CO) and resistance to venous return (Rv) were studied in rats. 2. In pentobarbitone-anaesthetized rats given mecamylamine (ganglionic blocker, 3.7 micrograms kg-1) and noradrenaline (1.5 micrograms kg-1 min-1) to suppress autonomic reflexes, pinacidil (60 and 180 micrograms kg-1 min-1), relative to the vehicle, reduced MAP and TPR in a dose-dependent manner but did not significantly alter CO, MCFP or RV. 3. Pinacidil (10-300 micrograms kg-1 min-1) caused similar increases in MCFP, an inverse index of venous compliance, and similar dose-dependent reductions in mean arterial pressure (MAP) in conscious, intact rats and rats infused with the ganglionic blocker, hexamethonium (150 micrograms kg-1 min-1). In rats with vasomotor tone elevated by the infusion of noradrenaline (1.5 micrograms kg-1 min-1), pinacidil caused markedly greater depressor responses but did not significantly alter MCFP. 4. Our results show that pinacidil is an efficacious vasodilator of arterial resistance blood vessels but has little venodilator activity.

Modifications by chronic intermittent hypoxia and drug treatment on skeletal muscle metabolism

The energy metabolism was evaluated in gastrocnemius muscle from 3-month-old rats subjected to either mild or severe 4-week intermittent normobaric hypoxia. Furthermore, 4-week treatment with CNS-acting drugs, namely, alpha-adrenergic (delta-yohimbine), vasodilator (papaverine, pinacidil), or oxygen-increasing (almitrine) agents was performed. The muscular concentration of the following metabolites was evaluated: glycogen, glucose, glucose 6-phosphate, pyruvate, lactate, lactate-to-pyruvate ratio; citrate, alpha-ketoglutarate, succinate, malate; aspartate, glutamate, alanine; ammonia; ATP, ADP, AMP, creatine phosphate. Furthermore the Vmax of the following muscular enzymes was evaluated: hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase; citrate synthase, malate dehydrogenase; total NADH cytochrome c reductase; cytochrome oxidase. The adaptation to chronic intermittent normobaric mild or severe hypoxia induced alterations of the components in the anaerobic glycolytic pathway [as supported by the increased activity of lactate dehydrogenase and/or hexokinase, resulting in the decreased glycolytic substrate concentration consistent with the increased lactate production and lactate-to-pyruvate ratio] and in the mitochondrial mechanism [as supported by the decreased activity of malate dehydrogenase and/or citrate synthase resulting in the decreased concentration of some key components in the tricarboxylic acid cycle]. The effect of the concomitant pharmacological treatment suggests that the action of CNS-acting drugs could be also related to their direct influence on the muscular biochemical mechanisms linked to energy transduction.

Stereoselective disposition and metabolism of pinacidil in rat

1. An hplc method has been established for the determination of the enantiomer ratios of pinacidil and pinacidil-pyridine-N-oxide (M-1), using a beta-cyclodextrin-containing mobile phase. 2. Shortly after the administration of racemic pinacidil (2 mg/kg, i.v. or oral) to rat, the enantiomer ((+)/(-)) ratios of pinacidil in plasma were about 1 (i.v.) or > 1 (oral), whereas those of M-1 were < 1; both ratios increased with time. The (+)/(-) ratios of M-1 and pinacidil in 0.24 h urine samples were 0.65 (i.v.), 0.63 (oral), and 2.09 (i.v.), and 1.56 (oral), respectively, in male rat, and 0.76 (i.v.), 0.77 (oral) and 1.79 (i.v.), and 1.48 (oral), respectively, in female rat. 3. No isomerization of (+)- or (-)-pinacidil was observed during incubation with liver slices, and there was no stereoselectivity in the protein binding of the drug. An (-)-enantiomer-rich M-1, however, was produced after incubation of (+/-)-pinacidil with liver slices. The N-oxidation of (-)-pinacidil exhibited a higher stereoselectivity in males than females (the (+)/(-) ratio of M-1 in male, 0.55; female, 0.68). 4. These results indicated that the stereoselective disposition of pinacidil in rat was due, in large part, to the stereoselective N-oxidation of this drug in the liver.

Propranolol antagonizes coronary artery relaxation by a potassium channel opener

Coronary artery preparations from cattle hearts responded with stable contractions to the thromboxane A2 analogue, U 46619. These contractions were progressively reduced by increasing concentrations of the prototypical potassium channel opener pinacidil (3.8 x 10(-8) to 1.1 x 10(-4) M). Pinacidil-induced relaxations were antagonized significantly by d,l-propranolol (1.2 x 10(-6) to 1.2 x 10(-5) M). Forskolin-induced relaxations of coronary preparations were also antagonized by d,l-propranolol, but those to nitroprusside were not. d-Propranolol also antagonized relaxations to pinacidil but only when used in higher concentrations than the I-isomer. Nadolol and metoprolol, two other beta receptor antagonists with differing profiles of action, also antagonized to some extent the vasodilator action of pinacidil. The known potassium channel antagonist, glibenclamide, shifted the concentration-relaxation curve for pinacidil to the right, but d,l-propranolol produced an additional antagonistic effect in the presence of glibenclamide. Relaxations of contracted tracheal ring preparations of guinea pig by pinacidil, however, were not antagonized by d,l-propranolol, suggesting specificity for vascular tissue. Isoproterenol increased significantly the cyclic AMP levels in coronary tissue, but pinacidil had no such effect, ruling out an adrenergic component to pinacidil action. Pinacidil increased the efflux of 86Rb in isolated coronary preparations, and this effect was blunted by propranolol. It is concluded that beta receptor antagonists inhibit relaxations to a potassium channel opener by a mechanism independent of beta adrenergic receptors and that this effect may have therapeutic implications.

Urinary metabolites of pinacidil. II. Species difference in the metabolism of pinacidil

1. Pinacidil was given orally to rabbit (10 mg/kg), dog (10 mg/kg), monkey (10 mg/kg) and mouse (150 mg/kg), the urinary metabolites were separated by h.p.l.c. and their structures determined by mass spectrometry. 2. Three new metabolites, namely, omega-hydroxy-pinacidil-O-glucuronide (M-8), pinacidil-pyridine-N-oxide-O-glucuronide (M-9) and pinacidil-pyridine-N-glucuronide (M-10) were isolated from rabbit urine, and one new metabolite, namely, pinacidil-pyridine-phenolic-glucuronide (M-11) was isolated from mouse urine. 3. M-9 is a unique glucuronide because the glucuronic acid is linked to the oxygen of pyridine-N-oxide. 4. In man, similar to rat, dog and mouse, pinacidil-pyridine-N-oxide (M-1) was the main urinary metabolite, with pyridine-N-oxidation being the major metabolic pathway. On the other hand, M-8 and M-9 were the most abundant metabolites in monkey and rabbit urine, respectively. Therefore, rat, dog and mouse have similar metabolism of pinacidil to man, but monkey and rabbit are significantly different in their metabolism of the drug.

Urinary metabolites of pinacidil: I. Isolation and identification of the metabolites in rat urine

1. The urinary metabolites of pinacidil administered orally to rats, were analysed by h.p.l.c. Seven metabolites were isolated from rat urine following fractionation on an HP-20 column and purified by t.l.c. 2. The major metabolite was pinacidil-pyridine-N-oxide (M-1). omega-Hydroxypinacidil (M-2) and dealkylpinacidil (M-3) were less abundant metabolites, and minor metabolites were the M-2-aldehyde, in which an intramolecular ring had formed (M-4), M-2-pyridine-N-oxide (M-5), carbamoyl-pinacidil (M-7) and M-7-pyridine-N-oxide (M-6). 3. Metabolic pathways of pinacidil in rats were postulated from the isolated metabolites.

Arterial vasodilating profile and biological effects of pinacidil in healthy volunteers

1. The effects of pinacidil (25 mg, sustained release formulation) a) on systemic (arterial pressure, cardiac output) and regional (brachial and carotid arteries' diameters and flows) haemodynamics (pulsed Doppler techniques), b) on sympathetic (plasma noradrenaline) and renin-angiotensin (plasma renin activity) systems, and c) on atrial natriuretic factor have been investigated and compared with those of a placebo during the 12 h period following oral administration in a randomized, double-blind and cross-over study performed in six healthy volunteers. Simultaneously, the plasma levels of pinacidil and of its active metabolite, pinacidil N-oxide, were determined. 2. As compared with placebo, pinacidil decreased systemic vascular resistance and arterial blood pressure but cardiac output was not modified. 3. Pinacidil significantly increased brachial and carotid arteries' diameters (by 7 and 8% respectively) and flows (by 60 and 17% respectively) and decreased forearm vascular resistance (by 43%). Thus, pinacidil dilates both large and small arteries, increases large vessels' compliance and redistributes blood flow towards the muscular vascular bed. These effects peaked at 4 h and their duration at the brachial level was 8 h. 4. Pinacidil administration resulted in a stimulation of both sympathetic (increases in heart rate and plasma noradrenaline) and renin-angiotensin systems, and induced a transient increase in atrial natriuretic factor. 5. The duration of pinacidil haemodynamic effects at the brachial level is consistent with the pharmacokinetic data which show that pinacidil and pinacidil N-oxide plasma levels almost plateaued between 3 and 8, and 2 and 8 h respectively after oral administration of the sustained release formulation used.

Long term haemodynamic effects of pinacidil and hydralazine in arterial hypertension

Eight patients with a diastolic blood pressure greater than or equal to 100mm Hg when treated with a diuretic and a beta-blocker participated in a randomised crossover study comparing the haemodynamic effects of adjunctive therapy with pinacidil or hydralazine. The vasodilator dose was increased until the diastolic blood pressure was less than 90mm Hg or the maximum dosage, hydralazine 100mg twice daily, or pinacidil 50mg twice daily, was reached. Treatment continued for 3 to 6 months and a haemodynamic study was performed. After washout, the patients received the alternative treatment. In the upright position, during supine rest and during isometric as well as dynamic exercise, pinacidil lowered blood pressure more effectively than hydralazine. No differences between the 2 treatments were found in heart rate, stroke index, cardiac index, end systolic wall stress or glomerular filtration rate. Pulmonary mean and wedge pressure were lower during treatment with pinacidil. Forearm blood flow was higher and forearm vascular resistance lower during treatment with pinacidil. Cardiac contractility, judged from the systolic time interval ratio PEP: LVET, was lower during treatment with pinacidil compared with hydralazine. The median daily dose of pinacidil was 50mg and that of hydralazine 200mg. It was also noted that during long term treatment, pinacidil seemed more effective in reducing blood pressure than hydralazine.