The Structure-Activity Relationship of KRN2391 as an N-K Hybrid

Effect of KRN4884 on lipid metabolism in hyperlipidemic rats

1. KRN4884 is a novel pyridinecarboxamidine type potassium channel opener. 2. To determine whether KRN4884 affects lipid metabolism, we investigated its effects on serum lipid levels by using two types of hyperlipidemic rats: genetically hyperlipidemic obese Zucker rats and diet-induced hyperlipidemic rats fed a high fat diet. KRN4884 dose dependently decreased systolic blood pressure in Zucker rats. 3. Oral administration of KRN4884 (1-10 mg/(kg day) for 14 days dose dependently reduced serum triglyceride levels in Zucker rats. The reductions in serum triglyceride were associated with reductions in triglyceride in chylomicron and very low density lipoprotein. 4. KRN4884 produced no change in serum insulin and glucose levels in Zucker rats. 5. KRN4884 exhibited a similar triglyceride lowering effect in diet-induced hyperlipidemic rats. 6. These results suggest that KRN4884 has a beneficial effect on serum triglyceride levels as well as a hypotensive effect.

Effects of the potassium channel openers KRN4884 and levcromakalim on the contraction of rat aorta induced by A23187, compared with nifedipine

We examined the different vasodilatory effects of the K+ channel openers levcromakalim and 5-amino-N- [2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine (KRN4884), and the Ca2+ channel blocker nifedipine in the rat aorta. KRN 4884 (10(-10)-10(-5) M) and nifedipine (10(-10)-10(-5) M) produced concentration-dependent relaxation in the rat aorta precontracted by 25 mM KCl. The K+ channel blocker glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 but did not influence nifedipine-induced relaxation. KRN4884 had almost no effect on contraction induced by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl. These results indicate that KRN4884 is a K+ channel opener. We investigated the relaxant effects of KRN4884 (10(-10)-10(-5) M), levcromakalim (10(-9)-10(-5) M) and nifedipine (10(-9)-10(-5) M) on A23187 (1 microM)-induced contraction. KRN4884 and levcromakalim had a potent relaxant effect but nifedipine only a weak effect on the smooth muscle contracted by A23187. Glibenclamide (1 microM) inhibited the relaxation induced by KRN4884 and levcromakalim, but did not influence the nifedipine-induced relaxation. KRN4884 (1 microM) produced a larger relaxation of A23187-induced contraction but had little effect on the increase in intracellular [Ca2+] induced by A23187. These results suggest that KRN4884 is a specific K+ channel opener and its vasodilating mechanisms involve not only deactivation of Ca2+ channels but also a decrease in the Ca2+ sensitivity of contractile elements.

Structure-activity relationship of a novel K+ channel opener, KRN4884, and related compounds in porcine coronary artery

1. KRN4884 (5-amino-N-[2-(2-chlorophenyl) ethyl]-N'-cyano-3-pyridinecarboxamidine), Ki3005 (5-deamino KRN4884), Ki5624 (2-dechloro KRN4884) and Ki1769 (5-deamino-2-dechloro KRN4884) produced concentration-dependent relaxations in isolated porcine coronary arteries contracted by 25 mM KC1. The order of relaxant potency was KRN4884 > Ki3005 > Ki5624 > Ki1769. 2. The relaxation induced by these compounds was antagonized by glibenclamide; they had almost no effect on coronary arteries contracted by 60 mM KC1. 3. The present results suggest that these pyridinecarboxamidine derivatives have vasodilating ability based on a K+ channel opening action, and that both the amino groups in the pyridine nucleus and the chlorine atom in the benzene nucleus in pyridinecarboxamidine are important for their potency as a K+ channel opener.

Effects of KRN2391, a novel vasodilator acting as a nitrate and a K+ channel opener, on the rabbit lower urinary tract

1. The relaxant effect of KRN2391, suggested to act both as a nitrate and a K+ channel opener, was investigated in the rabbit lower urinary tract and compared with the effects of the NO-donor SIN-1 and the K+ channel opener levcromakalim. 2. KRN2391 10(-4) M was able to relax precontracted urethral preparations by 87 +/- 4%. Corresponding values for levcromakalim 10(-4) M and SIN-1 10(-4) M were 58 +/- 8% and 103 +/- 2%, respectively. The -logEC50 values for KRN2391, SIN-1 and levcromakalim were 6.0 +/- 0.1, 4.9 +/- 0.2 and 5.8 +/- 0.2. The relaxant effect of KRN2391 on the bladder was small (29 +/- 3%). 3. The levels of cyclic GMP in the urethral preparations were significantly increased after administration of KRN2391 10(-4) M and SIN-1 10(-4) M, but not after levcromakalim 10(-4) M, the levels measured being 9.9 +/- 2.2, 20.9 +/- 5.1, and 5.2 +/- 1.0, compared to the control value, 3.7 +/- 0.5 pmol/mg protein. The levels of cyclic AMP were, however, not changed. 4. The relaxations, caused by KRN2391 in the urethral preparations, were accompanied by a hyperpolarization (14 +/- 4 mV) of the membrane potential. 5. Methylene blue 3 x 10(-5) M and glibenclamide 10(-5) M significantly reduced the relaxant effect of KRN2391 in the urethral smooth muscle. 6. We suggest that in the rabbit lower urinary tract, KRN2391 acts mainly as an NO-donor.

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.

Comparative analysis of vasodilating mechanisms of Ki1769, Ki3315 and KRN2391, pyridinecarboximidamide derivatives, in porcine isolated coronary artery

1. The vasodilating mechanisms of pyridinecarboximidamide derivatives which have a nitroxyl group (KRN2391), a phenyl group (Ki1769) or a hydroxyl group (Ki3315) were studied in porcine isolated coronary artery. 2. KRN2391 (10(-6) M) increased cyclic GMP formation but did not affect intracellular cyclic AMP level. Ki1769 (10(-5) M) and Ki3315 (10(-3) M) had no effect on intracellular cyclic GMP and cyclic AMP levels. 3. Despite producing submaximal relaxation at KRN2391 (10(-6) M) and nitroglycerin (10(-6) M), the increase in cyclic GMP caused by KRN2391 was lower than that caused by nitroglycerin. 4. Methylene blue (10(-5) M) inhibited KRN2391- and nitroglycerin-induced relaxations but did not affect Ki1769- and Ki3315-induced relaxation. 5. Glibenclamide (10(-6) M) inhibited KRN2391-, Ki1769- and Ki3315-induced relaxation but did not affect nitroglycerin-induced relaxation. 6. These results suggest that the nitroxyl group of KRN239 contributes to its nitrate action and the pyridinecarboximidamide moiety plays an important role of developing a K channel opening action.

Pharmacological properties of KRN2391, a novel vasodilator of the nitrate-potassium channel opener hybrid type

1. The pharmacological properties of KRN2391 in animal experiments are reviewed. 2. The vasodilating mechanism of KRN2391 is based on both a nitrate action and a K channel opening action, and whether KRN2391 acts as a nitrate and/or a K channel opener depends on the type and the segment of blood vessels. 3. KRN2391 causes a preferential increase in coronary blood flow in anesthetized dogs. 4. KRN2391 produces an increase in oxygen supply to the heart and a decrease in myocardial oxygen consumption in anesthetized dogs. 5. KRN2391 shows antiangial effects in various anginal models of rats and cardioprotective effects in perfused rat hearts. 6. KRN2391 does not develop self-tolerance or cross-tolerance between KRN2391 and other nitrates in coronary dilating and vasodepressor effects. 7. The pharmacological properties of KRN2391 are thought to be beneficial for the treatment of patients with ischemic heart disease.

A possibility that the ATP-sensitive potassium channel in coronary artery has a high-affinity internal binding site for tetraalkylammonium

The functionally responsible sites for the blocking action of tetraalkylammonium ions (TAAs) in ATP-sensitive K+ (KATP) channels opened by levcromakalim were estimated in canine coronary artery. Tetraethylammonium (TEA) and tetrabutylammonium (TBA) inhibited the levcromakalim-induced relaxation in a noncompetitive manner. Analyses of the noncompetitive antagonism revealed that the binding constant of TBA was about 900 times lower than that of TEA, although the reported affinity of TBA for the internal binding site in various K+ channels was only 10 times higher than that of TEA. TBA is much more lipid-soluble and permeable through membranes than TEA. Thus, TBA blocks KATP channels by binding to a possible high-affinity internal site for TAAs, whereas TEA seems to bind to the external site.

Mechanism of action of KRN2391 in canine coronary vascular bed

The present studies were performed to clarify the mechanism of action of KRN2391 in various sizes of canine coronary artery. We used the responses of isolated large and small coronary arteries and the changes in coronary blood flow (CBF) as indicators reflecting the responses of conductive arteries and resistive arterioles, respectively. In isolated small coronary artery, the effect of KRN2391 (10(-8)-10(-5) M) was antagonized by either methylene blue or glibenclamide. In isolated large coronary artery, the vasorelaxant effect of KRN2391 (10(-8)-10(-5) M) and nicorandil (10(-7)-10(-4) M) were antagonized by methylene blue (10(-5) M) but not by glibenclamide (10(-6) M). The relaxant effect of cromakalim was antagonized by glibenclamide but not by methylene blue in isolated large coronary artery. Intracoronary arterial injection of KRN2391, nicorandil or cromakalim produced an increase in CBF dose-dependently. Glibenclamide (5 mg/kg, i.v.) attenuated the increase in CBF caused by KRN2391, nicorandil and cromakalim. ED20, the dose that produced an increase in CBF by 20 ml/min, increased about 5-fold for KRN2391 and nicorandil and about 12-fold for cromakalim after administration of glibenclamide. These results suggest that the mechanism of action of KRN2391 and nicorandil depends on the segment of coronary arteries; i.e., they show a nitrate action alone in large coronary artery, and a K-channel opening action in addition to a nitrate action as the size of the coronary artery decreases.

Vasorelaxant action of Ki1769, a new pyridinecarboximidamide, in isolated porcine coronary artery

The characteristics of KRN2391 (N-cyano-N'-(2-nitroxyethyl)-3-pyridinecarboximidamide monomethansulfonate) and its phenethyl and 2-hydroxyethyl derivatives (Ki1769 and Ki3315) were studied in isolated porcine coronary arteries. KRN2391, Ki1769 and Ki3315 produced concentration-dependent relaxation of coronary arteries contracted by 25 mM KCl and the order of relaxant potency was KRN2391 > Ki1769 > Ki3315. At the maximum effect, KRN2391 produced nearly complete relaxation but Ki1769 produced about 66% relaxation. The maximum effect of Ki3315 could not be obtained because of its solubility. The relaxation induced by KRN2391 was antagonized by glibenclamide and methylene blue but relaxations caused by Ki1769 and Ki3315 were antagonized by glibenclamide alone. The antagonistic effect of glibenclamide on Ki1769- and Ki3315-induced relaxations was more potent than that on KRN2391-induced relaxation. KRN2391 induced relaxation of coronary arteries contracted by 40 mM KCl in a concentration-dependent manner but the effect of KRN2391 was smaller against 40 mM KCl-induced contractions than against 25 mM KCl-induced contractions. Ki1769 had almost no effect on coronary arteries contracted by 40 mM KCl. These results suggest that pyridinecarboximidamide derivatives which do not possess a nitroxyl group have vasodilating ability based on a K+ channel opening action.

Hyperpolarization induced by K+ channel openers inhibits Ca2+ influx and Ca2+ release in coronary artery

The vasodilating mechanisms of the K+ channel openers--cromakalim, pinacidil, nicorandil, KRN2391, and Ki4032--were examined by measurement of the cytoplasmic Ca2+ concentration ([Ca2+]i) using the fura-2 method in canine or porcine coronary arterial smooth muscle. The five K+ channel openers all produced a reduction of [Ca2+]i in 5 and 30 mM KCl physiological salt solution (PSS), the effects of which were antagonized by tetrabutylammonium (TBA) or glibenclamide, but failed to affect [Ca2+]i in 45 and 90 mM MCl-PSS. Cromakalim and Ki4032 only partially inhibited the 30 mM KCl-induced contractures, whereas pinacidil, nicorandil, and KRN2391 nearly abolished contractions produced by high KCl-PSS. The increased [Ca2+]i and force produced by a thromboxane A2 analogue, U46619, were inhibited by K+ channel openers and verapamil. In the absence of extracellular Ca2+, U46619 induced a transient increase in [Ca2+]i with a contraction, which is effectively inhibited by cromakalim and Ki4032. Their inhibitory effects were blocked by TBA and counteracted by 20 mM KCl-induced depolarization. Cromakalim and Ki4032 did not affect caffeine-induced Ca2+ release. Cromakalim reduced U46619-induced IP3 production and TBA blocked this inhibitory effect. Thus, cromakalim and Ki4032 are more specific K+ channel openers than pinacidil, nicorandil, and KRN2391. The vasodilation related with a reduction of [Ca2+]i produced by K+ channel openers is due to the hyperpolarization of the plasma membrane resulting in not only the closure of voltage-dependent Ca2+ channels but also inhibition of the production of IP3 and Ca2+ release from intracellular stores related to stimulation of the thromboxane A2 receptor.