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

The effect of nicorandil on small intestinal ischemia-reperfusion injury in a canine model

BACKGROUND

It has been shown that nicorandil, which has both ATP-sensitive K+ (KATP) channel opener-like and nitrate-like properties, has an organ-protective effect in ischemia-reperfusion injury in several experimental animal models.

AIMS

We evaluate the effectiveness of nicorandil on warm ischemia-reperfusion injury of the small intestine in a canine model.

METHODS

Eighteen beagle dogs were divided into three groups: the control group (n=6); the nicorandil group (n=6), to which nicorandil was injected intravenously before the ischemia; and the glibenclamide group (n=6), to which glibenclamide, which closes the KATP channel and does not suppress the nitrate effect of nicorandil, was orally administered, and then nicorandil was injected in the same manner as in the nicorandil group. Both the superior mesenteric artery and vein were clamped for 2 h. Superior mesenteric artery blood flow, small intestinal mucosal tissue blood flow, intramucosal pH, and histopathological analyses were compared among the three groups.

RESULTS

Superior mesenteric artery blood flow, mucosal tissue blood flow and pHi after reperfusion were significantly maintained in the nicorandil in comparison with the control and the glibenclamide groups. The histopathological findings showed less severe mucosal damage after reperfusion in the nicorandil group compared with the other two groups. Between the control group and the glibenclamide group, no significant differences were observed in all those parameters.

CONCLUSION

This study suggests that nicorandil has a protective effect on small intestinal IR injury, and activation of KATP channels plays an important role in inhibiting small intestinal IR injury.

Protective Effects of Insulin during Ischemia-Reperfusion Injury in Hamster Cheek Pouch Microcirculation

OBJECTIVE

The effects of insulin (0.18 nM-0.18 microM) on reduced capillary perfusion, microvascular permeability increase and leukocyte adhesion induced by ischemia-reperfusion injury were investigated in the hamster cheek pouch microcirculation. To gain insight into the insulin's mechanism of action, the effects of its higher concentration (0.18 microM) were investigated after inhibition of tyrosine kinase (TK), nitric oxide synthase (NOS), protein kinase C (PKC), phosphatidylinositol 3-kinase and K+(ATP) channels, alone or in combination. Two concentrations for each inhibitor were used.

METHODS

Microcirculation was visualized by fluorescence microscopy. Perfused capillary length, microvascular permeability, leukocyte adhesion to venular walls, vessel diameter and capillary red blood cell velocity were assessed by computer-assisted methods. Measurements were made at baseline (B), after 30 min of ischemia (I), and after 30 min of reperfusion (R).

RESULTS

In control animals, perfused capillary length decreased by 63 +/- 5% of baseline at R. Microvascular permeability increased at I and R, while leukocyte adhesion was most pronounced in V1 postcapillary venules at R. Insulin dose-dependently preserved capillary perfusion at R (-28 +/- 6 and -15 +/- 6% of baseline), but was unable to prevent the increase in permeability at I (0.25 +/- 0.05 and 0.29 +/- 0.06 Normalized Grey Levels, NGL) and R (0.49 +/- 0.10 and 0.53 +/- 0.09 NGL), according to the concentrations. Adhesion of leukocytes was observed mostly in V3 venules at R (9 +/- 2 and 10 +/- 2/100 microm venular length, with the lower and higher concentration, respectively). Nitric oxide synthase inhibition by N(G)-nitro-L-arginine-methyl ester prior to insulin did not affect capillary perfusion at R (-18 +/- 3% of baseline with higher concentration), but prevented permeability increase (0.20 +/- 0.04 NGL, according to higher concentration) and reduced leukocyte adhesion in V3 venules at R (1.5 +/- 1.0/100 microm of venular length, with higher concentration). Blockade of K+(ATP) channels by glibenclamide prior to insulin decreased perfused capillary length at R (-58 +/- 6% of baseline with higher concentration), attenuated leakage at R (0.30 +/- 0.04 NGL, according to higher concentration) and caused leukocyte adhesion mainly in V1 venules at R (9.0 +/- 1.5/100 microm of venular length, with higher concentration). Inhibition of either TK, PKC or phosphatidylinositol 3-kinase did not affect microvascular responses to insulin. Simultaneous inhibition of TK and NOS did not increase protection.

CONCLUSIONS

Insulin prevents ischemia-reperfusion injury by promoting capillary perfusion through an apparent activation of K+(ATP) channels and increase in nitric oxide release.

Differential effects of sulphonylureas on the vasodilatory response evoked by K(ATP) channel openers

The potency of three sulphonylureas, glibenclamide, glimepiride and gliclazide in antagonizing the vasorelaxant action of openers of adenosine triphosphate (ATP)-regulated K+ channel (KATP) was studied in vivo and in vitro in micro- and macrovessels, respectively. In the hamster cheek pouch, the vasodilatation and the increase in vascular diameter and blood flow induced by diazoxide were markedly reduced by the addition of either glibenclamide or glimepiride (0.8 microm) while they were not affected by gliclazide up to 12 microm. Similarly, in rat and guinea-pig isolated aortic rings, glibenclamide, glimepiride and gliclazide reduced the vasodilator activity of cromakalim. However, the inhibitory effect of gliclazide was considerably less when compared with either glimepiride or glibenclamide. These results suggest that, in contrast to glibenclamide and glimepiride, therapeutically relevant concentrations of gliclazide do not block the vascular effects produced by KATP channel openers in various in vitro and in vivo animal models.

Characterisation of the effects of potassium channel modulating agents on mouse intestinal smooth muscle

The actions of agents which modulate ATP-sensitive potassium (K(ATP)) channels in excitable cells were investigated in an in-vitro preparation of mouse ileum from which the mucosa was removed. A range of potassium channel openers of diverse structure, cromakalim (0.1-100 microM), pinacidil (0.1-200 microM) and its analogue P1060 (0.1-200 microM), SDZ PCO400 ((-)-(3S,4R)-3,4-dihydro-3-hydroxy-2,2-dimethyl-4-(3-oxo-cyclopent-1-enyloxy)-2H-1-benzopyran-6-carbonitrile) (0.3-60 microM), caused concentration-related reduction in twitch height of electrical field stimulated ileum. P1060 and SDZ PCO400 were the most potent agents; diazoxide (0.1-100 microM) was without effect. The order of inhibitory potency, based on EC50 values (concentration of a relaxant producing 50% of the maximum inhibition of twitch) was: P1060 = SDZ PCO400 > cromakalim > pinacidil. The relaxant effect of the potassium channel openers was antagonised by the sulfonylureas glibenclamide (0.1-1.0 microM) and glipizide (3-30 microM) but the nature of the antagonism differed. Antagonism of P1060 and SDZ PCO400 by glibenclamide appeared to be competitive whereas the antagonism of relaxation induced by cromakalim and pinacidil was apparently not competitive. Both phentolamine (1-10 microM) and tolbutamide (100-300 microM) showed competitive antagonism of the actions of pinacidil while yohimbine (1-20 microM) did not antagonise relaxation and appeared to have actions at sites other than the K(ATP) channel in this preparation. The relative effectiveness of the antagonists on pinacidil-induced relaxation was found to be: glibenclamide > phentolamine > tolbutamide > yohimbine, which is in agreement with studies in other tissues. The results show that many structurally diverse potassium channel openers are potent relaxants of mouse ileum. These observations are consistent with the existence of ATP-dependent K+ channels in murine intestinal muscle which, however, differ somewhat in properties from those reported for vascular muscle and pancreatic beta-cells.

Guanylate cyclase and not ATP-dependent K(+) channels seems temperature-dependent in smooth muscle relaxation of human umbilical arteries

The effects of K(+) channel opener, nicorandil [N-(2-hydroxyethyl)-nicotinamide nitrate], on isolated human umbilical arteries were investigated at two temperatures: 37 degrees C and 25 degrees C. The purpose of this investigation was: (1) to confirm the relaxant effect of nicorandil, (2) to elucidate the influence of endothelium and temperature on nicorandil-induced relaxation, (3) to determine which of guanylate cyclase or ATP-sensitive K(+) channels was implicated in temperature-induced relaxation of smooth muscles. Rings, 3-mm-wide, were suspended in organ chambers for isometric force measurement. All solutions were aerated with 95% O(2)-5% CO(2) and maintained at 37 degrees C or 25 degrees C (cooling), pH 7.4. The presence of an intact endothelium was confirmed by immunohistochemistry. During the first set of experiments after contraction with 5-hydroxytryptamine (5-HT 10(-5) M), nicorandil (10(-9)-10(-4) M) was added to the organ chambers with controls and in with rings incubated with L-arginine, N-nitro-L-arginine (L-NNA) an inhibitor of nitric oxide (NO) synthase, [1-H-(1,2,4) oxadiazole (4,3-a) quinoxalin-1-one] (ODQ), a specific inhibitor of guanylate cyclase, or glibenclamide, an antagonist of nicorandil, all at 10(-5) M. In another set of experiments, rings were contracted with 5-HT (10(-5) M) and relaxed with 3-morpholinosydnonimine [SIN-1 (10(-9)-3x10(-5) M) or cromakalim (10(-9)-3x10(-5) M)]. Our results showed that nicorandil induced concentration-dependent relaxation. At 37 degrees C, in the control, the maximum relaxation was 90+/-5%, and 60+/-8% at 25 degrees C (P<0.01). However, the relaxation at 37 degrees C or 25 degrees C remained unchanged after pretreatment with L-arginine, L-NNA, this suggests that the same concentration of drugs used in this type of vessel does not appear to modulate the relaxant effect of nicorandil. On the other hand, we observed that the relaxant effect of SIN-1 was 72+/-5% at 37 degrees C and only 28+/-7% at 25 degrees C (P<0.01). However, relaxations with cromakalim were partly influenced by cooling. In the presence of ODQ, the nicorandil-induced relaxation observed at 37 degrees C or 25 degrees C was less than that in the control and in the rings incubated with glibenclamide. These results for human umbilical arteries indicate that cooling decreases the relaxation response of smooth muscles and that this is partly due to a decreased response to guanylate cyclase.

Pharmacological and molecular analysis of ATP-sensitive K(+) channels in the pig and human detrusor

The pharmacological and molecular properties of ATP-sensitive K(+) channels present in pig detrusor smooth muscle were investigated. In isolated pig detrusor strips, ATP-sensitive K(+) channel openers inhibited contractions elicited by low frequency field-stimulation in a concentration-dependent manner. The inhibitory effects of P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine] were attenuated by glyburide with a pA(2) value of 7.38 (slope=1.08). The potency of the inhibitory effects of the K(+) channel openers on the field-stimulated contractions correlated well with those evoked by the muscarinic receptor agonist, carbachol (r=0.93) and furthermore, to relaxation of the pre-contracted (25 mM potassium chloride, KCl) human detrusor (r=0.95). Reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed the presence of mRNA for sulfonylurea receptors SUR1 and SUR2B in both pig and human detrusor. Considering the similarities in the molecular and pharmacological profile of ATP-sensitive K(+) channels between the pig and the human detrusor, it is concluded that the pig detrusor may serve as a suitable in vitro model for the evaluation of novel K(+) channel openers with potential use in urological disorders in humans.

Effects of intravenous nicorandil on coronary circulation in humans: plasma concentration and action mechanism

We investigated the cardiovascular profile of nicorandil, an antianginal agent, in humans. Pharmacologically, nicorandil acts as both an adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channel opener and a nitrate. We examined which of these mechanistic components has a predominant vasodilatory effect at clinical doses. Fourteen patients underwent cardiac catheterization. The effects of the continuous intravenous infusion of nicorandil (12 mg/45 min) were examined in angiographically normal coronary arteries. Coronary vascular resistance was calculated from coronary artery diameter and coronary blood flow velocity measured using an intravascular Doppler catheter. We compared the hemodynamic responses to nicorandil with those to the intracoronary injection of nitroglycerin (250 microg) and papaverine (12 mg). The epicardial coronary arteries responded to nicorandil at the lowest plasma concentration examined (dilation of +14.0 +/- 3.3% at approximately 170 ng/ml), whereas dilation of the coronary resistance arteries (i.e., a decrease in coronary vascular resistance) took place only at higher concentrations (>200 ng/ml). Nitroglycerin caused no further changes in coronary artery diameter or coronary vascular resistance. Papaverine caused no further increase in coronary artery diameter, but markedly decreased coronary vascular resistance (1.6 +/- 0.3 to 0.4 +/- 0.1 mm Hg/ml/min; p < 0.05). Nicorandil significantly decreased pulmonary capillary wedge pressure (i.e., reduced cardiac preload) at a plasma level of >200 ng/ml, but did not change either systemic or pulmonary vascular resistance. Thus nicorandil preferentially dilated epicardial coronary arteries rather than coronary resistance arteries, and had a stronger effect on preload than on afterload. These changes in human coronary hemodynamics suggest that the nitrate actions of nicorandil as a coronary vasodilator predominate over those as a K(ATP) opener.

Inhibitory effects of glibenclamide on the contraction of human arterial conduits used in coronary artery bypass surgery

Glibenclamide has been shown to inhibit prostanoid-induced contraction in a number of blood vessel types. In this study, the effects of glibenclamide on the contraction of human peripheral arteries in response to both prostanoid and non-prostanoid agonists were compared and possible mechanisms of action were investigated. Segments of left internal mammary artery (LIMA) and radial artery, taken from patients undergoing coronary artery bypass graft (CABG) surgery, were mounted in organ baths containing physiological saline solution aerated with 95% O2/5% CO2 at 37 degrees C. Contractions were obtained by either the use of a thromboxane analogue (U46619), L-phenylephrine, KCl or CaCl2. The effects of glibenclamide on these contractions were observed and pEC50 values were determined after manipulation of a logistic curve-fitting equation. Concentration-dependent relaxation of U46619-contracted LIMA and radial artery was observed in the presence of glibenclamide, with calculated pEC50 values of 4.2+/-0.17 (n = 7) for LIMA and 3.26+/-0.48 (n = 5) for radial artery. Incubation of both LIMA and radial artery with glibenclamide (50 microM) caused the concentration-response curves for U46619 and L-phenylephrine to shift significantly to the right. Similarly the KCl tension relationship was caused to shift to the right. Finally, glibenclamide (100 microM) also had an inhibitory effect on Ca2+-induced tension in radial artery. These results show that the inhibitory effects of glibenclamide on human peripheral blood vessels are not restricted to prostanoid-induced contractions. Furthermore, evidence has been provided to suggest that these effects might be mediated through an interaction with voltage-sensitive Ca2+ channels.

Involvement of endothelium in relaxant action of glibenclamide on the rat mesenteric artery

The present report describes the complex effect of glibenclamide, an antidiabetic sulfonylurea agent, on the rat isolated mesenteric artery. Although glibenclamide concentration dependently reversed the relaxant effect of pinacidil, an activator of ATP-sensitive K+ channels (the concentration for half-maximum reversal effect was 0.56 microM with endothelium and 0.17 microM without endothelium), in the artery precontracted with phenylephrine (1 microM), it relaxed phenylephrine-induced sustained contraction at higher concentrations (IC50: 4.4+/-1.1 microM with endothelium and 226.1+/-44.2 microM without endothelium). The relaxant effect of glibenclamide was partially inhibited by pretreatment of the artery with either NG-nitro-L-arginine (10-100 microM) or methylene blue (1 microM). Indomethacin (10 microM) had no effect. Moreover, glibenclamide also concentration dependently (3-500 microM) reduced the sustained contraction induced by 60 mM K+ (IC50: 99.5+/-16.1 microM). The relaxation induced by glibenclamide was not affected by various putative K+ channel blockers such as charybdotoxin (100 nM), tetraethylammonium ions (1 mM), apamin (100 nM) and 4-aminopyridine (1 mM). The results indicate an involvement of the endothelium, probably of nitric oxide, in the relaxation induced by glibenclamide in the endothelium-intact rat mesenteric arteries. The inhibitory effect of glibenclamide on the high-K+-induced contraction suggests that glibenclamide may interfere with Ca2+ influx, which in turn affects intracellular Ca2+ levels in arterial smooth muscle, leading to reduction of muscle contractility. It is suggested that two distinct pharmacological effects induced by glibenclamide may be mediated through different glibenclamide binding sites, however, the data show an overlap of concentrations of glibenclamide for producing the two effects in rat isolated mesenteric arteries.

Effect of K+ channel blocking drugs and nitric oxide synthase inhibition on the response to hypoxia in rat pulmonary artery rings

1. The aims of this study were to investigate the effects of potassium (K+) channel blockers and the nitric oxide (NO) synthase inhibitor, L-nitroarginine (L-NOARG), on the response produced by acute hypoxia in rat intrapulmonary artery rings in vitro. 2. In rat phenylephrine-precontracted pulmonary artery rings, hypoxia (pO2 = 7 mmHg) induced a response which consisted of a rapidly developing initial contraction (phase 1), a transient relaxation (phase 2) and a slowly developing sustained contraction (phase 3) over 30 min. The NOS inhibitor, L-NOARG (300 microM), attenuated phase 1 and 3, and amplified phase 2 of the response to hypoxia. The voltage-gated K+ channel blocker 4-aminopyridine (4-AP) (10 mM) also abolished phase 3 and magnified phase 2 of the response to hypoxia. 3. The hypoxic response was not modified by the calcium-activated K+ channel (KCa) blockers, tetraethylammonium (TEA) (20 mM) or charybdotoxin (50 or 200 nM), nor by the ATP-dependent K+ channel (KATP), blocker, glibenclamide (10 microM). 4. L-NOARG (300 microM) and 4-AP (10 mM) also abolished carbachol-induced endothelium-dependent NO-mediated relaxation. Relaxation produced by the NO releasing agent 3-morpholino sydnonimine (SIN-1) was reduced by 4-AP (10 mM) and TEA (20 mM). 5. The data suggest that NO production is reduced during severe hypoxia in rat intrapulmonary artery rings and that this underlies the sustained phase of the hypoxic contraction. The data also suggests that 4-AP-sensitive K+ channels play an important role in the release and or action of NO, and therefore, in the response to hypoxia.

The effects of levcromakalim and pinacidil on the human internal mammary artery

The present study was undertaken to examine the effects of pinacidil and levcromakalim, two potassium, channel openers, on human internal mammary artery (HIMA) obtained from patients undergoing coronary artery bypass surgery, and to clarify the contribution of different K+ channel subtypes in pinacidil and levcromakalim action in this blood vessel. Pinacidil and levcromakalim induced a concentration-dependent relaxation of the precontracted arterial segments (pEC50 = 5.77 +/- 0.05 and 6.89 +/- 0.03, respectively), 4-Aminopyridine (3 mM), a non-selective blocker of K+ channels, induced significant shifts to the right of the concentration-response curves for pinacidil and levcromakalim. Tetraethylammonium (6 mM), charybdotoxin (0.4 microM) and apamin (0.1 microM), blockers of Ca(2+)-sensitive K+ channels, had no effect on the pinacidil- and levcromakalim-evoked relaxation. Glibenclamide (0.1-10 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels, competitively antagonized the response to levcromakalim (pKB = 7.92 +/- 0.07). In contrast, glibenclamide, in significantly higher concentrations (3-30 microM), non-competitively antagonized the response to pinacidil. High concentrations of pinacidil (> 10 microM) relaxed arterial rings bathed by a medium containing 100 mM K+ with maximum response 83 +/- 6%. Under the same conditions, the maximum levcromakalim-induced relaxation on HIMA was almost abolished (15 +/- 2%). It is concluded that pinacidil and levcromakalim do not relax the HIMA through the same subtype of K+ channel. ATP-sensitive K+ channels are probably involved in levcromakalim- but not in a pinacidil-induced relaxation in the HIMA. In addition, in pinacidil-induced relaxation of the HIMA, K+ channel-independent mechanisms seem to be involved.

In vitro and in vivo vasodilating effects of KRN4884, Ki1769 and Ki3005, pyridinecarboxamidine derivatives

The vasodilating potencies and mechanism of action of a novel pyridinecarboxamidine derivative, KRN4884 [5-amino-N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine ] were compared with those of Ki1769 [N-cyano-N'-(2-phenylethyl)-3-pyridinecarboxamidine] and Ki3005 [N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine] in rat isolated aortas and in anesthetized normotensive rats. In vitro. KRN4884 (10(-10)-10(-6) M). Ki1769 (10(-8)-10(-5) M) and Ki3005 (10(-10)-10(-6) M) produced concentration-dependent relaxations. KRN4884 was about 100- and 10-fold more potent than Ki1769 and Ki3005, respectively. The relaxant effects of these compounds were antagonized by glibenclamide. In vivo, KRN4884 (1-10 micrograms/kg, intravenously [i.v.]), Ki1769 (10-100 micrograms/kg, i.v.) and Ki3005 (3-30 micrograms/kg, i.v.) produced dose-dependent decreases in mean blood pressure with slight increases in heart rate. At 10 micrograms/kg, i.v., the hypotensive effect of KRN4884 was about the same as that of Ki3005 and about 5-fold more pronounced than that of Ki1769. The hypotensive action remained for a longer period after KRN4884 administration. In rats pre-treated with glibenclamide (20 mg/kg, i.v.), the hypotensive effect of KRN4884 was abolished. These results suggest that the effect of KRN4884 in vitro and in vivo is based on its K channel opening action and that the in vitro vasorelaxant effect of these compounds in aortic rings does not predict their relative hypotensive effect in vivo.

KRN4884, a novel K channel opener: antihypertensive effects in conscious renal hypertensive dogs

We examined the antihypertensive effects of KRN4884, 5-amino-N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarbocamidine+ ++, in normotensive dogs, a high-renin model acute renal hypertensive dog (RHD), and a low-renin model chronic RHD in the conscious state, compared with levcromakalim and nilvadipine. KRN4884 decreased mean blood pressure (MBP) at a dose of 0.1 mg/kg p.o. in normotensive dogs and both RHDs. The decrease in MBP was greater in both RHDs than in normotensive dogs, and there were no significant differences between the two RHDs. A transient increase in heart rate (HR) accompanied the increase in MBP in all three types of dogs. In the chronic RHD, KRN4884 at doses of 0.05, 0.1, and 0.2 mg/kg produced a dose-dependent decrease in MBP. The antihypertensive effect of KRN4884 (0.1 mg/kg) was similar to those of levcromakalim (0.05 mg/kg) and nilvadipine (1.0 mg/kg) in magnitude and more prolonged than those of the compounds. The tachycardia induced by KRN4884 was similar to that induced by levcromakalim and was stronger than that induced by nilvadipine. In the 15-day repeated oral-administration study, KRN4884 (0.1 mg/kg) induced sustained hypotensive effects and transient increases in HR and plasma renin activity. No tolerance to the antihypertensive effect of KRN4884 was observed during a 15-day repeated dosing period. After withdrawal of KRN4884, no rebound phenomena in MBP and HR were observed. Neither the maximal concentration nor area under the curve (AUC) of KRN4884 in plasma were changed at days 1, 8, and 15. These data indicate that KRN4884 produces a strong and persistent antihypertensive response in both low-renin and high-renin models of RHD in a conscious state, which suggests that KRN4884 may be useful as an antihypertensive agent.

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.

The pulmonary vasodilator properties of potassium channel opening drugs

1. This article reviews the effects of potassium channel opening drugs (KCOs) on blood vessels of the pulmonary circulation. KCOs are effective pulmonary vasodilators in vitro (isolated arteries and perfused lungs) and in vivo in a variety of animal species. They prevent or reverse pulmonary vasoconstriction/contraction induced by a range of vasoconstrictor spasmogens or by alveolar hypoxia. 2. The pulmonary vasorelaxant effects of the KCO drugs are blocked by glibenclamide, do not depend on the endothelium, are dependent on the vasoconstrictor spasmogen used to contract the preparations and are enhanced in preparations taken from pulmonary hypertensive rats. 3. Selectivity for pulmonary compared with systemic vessels is seen in vessels from pulmonary hypertensive rats but not in the absence of pulmonary hypertension. 4. The pulmonary vasodilatation that is induced by (a) endothelium derived hyperpolarising factor, (b) endothelin, (c) increased pulmonary blood flow or (d) prolonged, severe hypoxia is probably due to potassium efflux through the same population of potassium channels as those on which the KCOs act. 5. Acute hypoxic pulmonary vasoconstriction, and also the depolarisation seen in arteries from chronically hypoxic rats, each involve inhibition of potassium efflux through glibenclamide-insensitive potassium channels. 6. It is suggested that the KCOs warrant investigation as possible therapeutic agents in the treatment of pulmonary hypertension.

Comparison of the effects of nicorandil, pinacidil and nitroglycerin on hypoxic and hypercapnic pulmonary vasoconstriction in the isolated perfused lung of rat

1. The aims of this study were to compare in the rat isolated perfused lung preparation, the dilator actions of nicorandil, pinacidil and nitroglycerin on the hypoxic pulmonary pressure response with or without hypercapnic acidosis and to investigate the possible involvement of K channels and EDRF in these effects. 2. Isolated lungs from male Wistar rats (260-320 g) were ventilated with 21%O2 + 5%CO2 + 74%N2 (normoxia) or 5%CO2 + 95%N2 (hypoxia) and perfused with a salt solution supplemented with ficoll and gassed with 40%CO2 + 60%N2 to produce hypercapnic acidosis. Glibenclamide (1 microM), charybdotoxin (0.1 microM), NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) and methylene blue (30 microM) were used to block KATP channels, KCa channels, EDRF synthesis and guanylate cyclase, respectively. 3. Hypoxic pressure response was significantly increased by hypercapnic acidosis (+115%, P < 0.001), L-NAME (+111%, P < 0.001), methylene blue (+100%, P < 0.05) but not by glibenclamide or charybdotoxin. In contrast none of these inhibitors affected the hypoxic hypercapnic acidosis response. 4. Nicorandil, pinacidil and nitroglycerin caused relaxation during the hypoxic pressure response and hypoxic hypercapnic acidosis response. Nicorandil was more potent in the latter. Glibenclamide inhibited the relaxant effects of nicorandil and pinacidil but not those of nitroglycerin during hypoxia alone. In contrast, glibenclamide inhibited the relaxant effects of the three drugs during hypoxia + hypercapnia. Charybdotoxin inhibited the relaxant effect of pinacidil during normocapnia and hypoxia but not those of nicorandil or nitroglycerin. Methylene blue inhibited partially the dilator response to pinacidil but did not modify the effects of nitroglycerin or nicorandil. 5. It is concluded that in the rat isolated lung preparation, EDRF limits hypoxic pulmonary vasoconstriction but not hypoxic vasoconstriction potentiated by hypercapnic acidosis, whereas KATP or KCa channels are not involved in either case. Nicorandil and pinacidil dilate pulmonary vessels mainly through KATP channels but the effects of pinacidil may also involve an additional mechanism of action through KCa channels. Finally it is suggested that nitroglycerin may partly exert its relaxant effects through KATP channels.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

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.

Comparison of the effects of several potassium-channel openers on rat bladder and rat portal vein in vitro

1. The ability of several K-channel openers to inhibit KCl-induced contractions of rat bladder detrusor and spontaneous mechanical activity in rat portal vein was examined. 2. Lemakalim, pinacidil, Ro 31-6930, RP 49356, P1060 and S 0121 dose-dependently relaxed rat detrusor, precontracted with 20 mM KCl. With the exception of pinacidil, concentrations of these agents below 30 microM did not inhibit 80 mM KCl-included contractions. Pinacidil (10 microM) produced a small, but significant (P < 0.05) relaxation of 80 mM KCl-induced mechanical activity. Minoxidil sulphate and BRL 38226 produced some relaxation of 20 mM but not 80 mM KCl-induced contractions. 3. Glibenclamide (0.3-3 microM) antagonized the relaxant effects of lemakalim, pinacidil, Ro 31-6930, RP 49356, P1060 and S 0121 in a competitive manner (pA2 values 6.3-6.6). The effects of minoxidil sulphate and BRL 38226 were fully antagonized by 3 microM glibenclamide. 4. Lemakalim, pinacidil, S 0121, BRL 38226 and minoxidil sulphate were each approximately 8 times more potent as inhibitors of the spontaneous contractions of rat portal vein than KCl-induced contractions of the rat detrusor. Minoxidil sulphate was approximately 30 times more potent in the rat portal vein than in the bladder. This may indicate that either minoxidil sulphate is acting at different recognition sites in these two tissues, or that this compound has an additional mechanism of action in the portal vein. 5. With the exception of minoxidil sulphate, all the compounds tested stimulated 86Rb efflux and 42K efflux from preloaded rat detrusor strips. The stimulated 86Rb efflux was qualitatively but not quantitatively similar to the stimulated 42K efflux. Minoxidil sulphate stimulated 42K efflux from rat portal vein but not from rat bladder. 6. It is concluded that all the compounds tested cause relaxation of rat detrusor predominantly by Kchannel opening. Selectivity for bladder rather than vascular smooth muscle was not shown by any compound.

The effects of potassium channel openers and blockers on the specific binding sites for [3H]glibenclamide in rat tissues

The effects of K+ channel openers (PCOs), NIP-121, levcromakalim and nicorandil, and the blockers of the specific binding sites for [3H]glibenclamide, ATP-sensitive K+ channel blocker, were investigated in rat brain and cardiac ventricle membrane preparations. When the microsomes were incubated with [3H]glibenclamide, the specific glibenclamide binding was fully inhibited by unlabeled glibenclamide (1 microM) and apamin (100 microM). However, the specific glibenclamide binding was not influenced by excess NIP-121, levcromakalim and nicorandil, although glibenclamide antagonized the increase in the 86Rb+ efflux by PCOs. On the other hand, the binding of [3H]glibenclamide after a long pre-incubation (60 min) at 37 degrees C with NIP-121 and levcromakalim at pharmacological effective concentrations (10 nM to 1 microM) was significantly influenced. Both PCOs partially reduced both Kd and Bmax values of the specific [3H]glibenclamide binding in a concentration-dependent manner that was not regulated by GTP gamma S. The dose-effect relationships for the Bmax's of NIP-121 and levcromakalim seemed similar to those for vasorelaxation. These findings indicate that the pharmacological effect of PCO may be caused by the binding to its own specific sites but not to the specific sulfonylurea sites. The binding of PCOs may inhibit, in a negative allosteric manner the binding of sulfonylureas.

A comparison of the relaxant effects of pinacidil in rabbit renal and mesenteric artery

1. The relaxant effects of pinacidil were compared in isolated rabbit renal and mesenteric artery. 2. Pinacidil (10 nm-300 microM) relaxed renal and mesenteric arterial rings precontracted with phenylephrine with pD2 values of 5.11 +/- 0.03 and 6.27 +/- 0.04, respectively. 3. The inhibitory effect of pinacidil on the rabbit mesenteric artery was competitively antagonized by glibenclamide (1-10 microM). The calculated pKB value was 6.37 +/- 0.04. On the renal artery, glibenclamide (2-20 microM) did not significantly affect pinacidil-induced relaxation (P > 0.05). 4. Tetraethylammonium (TEA, 1-10 mM) competitively antagonized the pincaidil induced relaxation of the rabbit renal artery. The pKB value was 3.22 +/- 0.08. On the mesenteric artery TEA antagonized the effect of pinacidil in a noncompetitive manner. 5. The concentration-response curves for pinacidil on the rabbit renal and mesenteric artery were not affected by apamin (0.1 microM). 6. It is concluded that ATP-sensitive K+ channels (KATP) are not involved in pinacidil action on the rabbit renal artery. On the contrary, KATP are probably major sites of pinacidil action on the rabbit mesenteric artery.

Vasospasmolytic effect of KRN2391 on 3,4-diaminopyridine-induced rhythmic contraction of porcine coronary artery

1. In the present study, we examined the vasospasmolytic effect of KRN2391 on rhythmic contractions of porcine coronary artery caused by 3,4-diaminopyridine (3,4-DAP) compared with cromakalim and nitroglycerin. 2. KRN2391 at 10(-7) showed a tendency to prolong the cycle length and at 10(-6) M completely eliminated rhythmic contractions in all preparations. The elimination by 10(-6) M KRN2391 was antagonized by either oxyhemoglobin (10(-5) M) or glibenclamide (3 x 10(-6)) although not completely. 3. Cromakalim at 10(-5) M and nitroglycerin at 10(-7) M completely eliminated 3,4-DAP-induced rythmic contractions in all preparations. The elimination by cromakalim and nitroglycerin was completely antagonized by glibenclamide and oxyhemoglobin, respectively. 4. The present study suggests that the vasospasmolytic effect of KRN2391 on 3,4-DAP-induced rhythmic contractions is based on its nitrate action and K channel opening action.

Effects of two K+ channel openers, aprikalim and pinacidil, on hypoxic pulmonary vasoconstriction

This study investigated the effects of two K+ channel openers, aprikalim and pinacidil, on hypoxic pulmonary vasoconstriction induced in isolated rat lung perfused at constant flow. In order to evaluate the mechanism of the hypoxic vasoconstriction we also studied the effects of an inhibitor of the endothelium-derived relaxing factor (EDRF), NG-nitro-L-arginine methyl ester (100 microM), an inhibitor of the guanylate cyclase, methylene blue (30 microM), two K+ channel blockers, glibenclamide (1 microM) and tetraethylammonium (20 mM). In normoxia, NG-nitro-L-arginine methyl ester, methylene blue, glibenclamide or tetraethylammonium did not enhance significantly the baseline perfusion pressure, suggesting that neither EDRF nor K+ channels are involved in the modulation of the low basal pulmonary vascular tone. In hypoxia, aprikalim and pinacidil (0.03-3 microM) induced a concentration-dependent decrease of pulmonary pressure, exhibiting their spasmolytic effects in acute hypoxia. The hypoxic pressure response was significantly increased by NG-nitro-L-arginine methyl ester, methylene blue and tetraethylammonium, but not by glibenclamide suggesting that EDRF and K+ channels other than ATP-sensitive K+ channels are involved in the modulation of the hypoxic pressure response. The spasmolytic effects of aprikalim and pinacidil (1 microM) were not modified by NG-nitro-L-arginine methyl ester, but were partially reduced by tetraethylammonium and completely abolished by glibenclamide, suggesting that these effects are mainly but not exclusively mediated through ATP-sensitive K+ channel opening.

Pharmacological analysis of the inhibitory effects of KRN2391 on endothelin-1-induced contraction in isolated large coronary artery of the pig

1. The relaxant effect of KRN2391, N-cyano-N'-(2-nitroxyethyl)-3-pyridine-carboximidamide-mo nomethanesulfonate (with both K+ channel opener and nitrate actions), nifedipine (Ca2+ channel blocker), nitroglycerin (nitrate) and cromakalim (K+ channel opener) were investigated in isolated porcine large coronary arteries contracted by endothelin-1. These drugs inhibited endothelin-1-induced contraction in a concentration-dependent manner. 2. The relaxation induced by KRN2391 was nearly complete at their maximum effects, but nifedipine and cromakalim could not produce complete relaxation. 3. The concentration-relaxation curves for KRN2391 underwent a rightward shift in the presence of methylene blue or glibenclamide. The concentration ratios of KRN2391 calculated based on EC50 values were 2.8 and 3.7 in the presence of methylene blue and glibenclamide, respectively. 4. The concentration-relaxation curves for nitroglycerin and cromakalin underwent a rightward shift in the presence of methylene blue and glibenclamide, respectively, and the concentration ratios of nitroglycerin and cromakalim were 12.0 and 6.3. 5. These relaxant effects of KRN2391 and nitroglycerin on endothelin-1-induced contraction of porcine coronary artery were greater than those of cromakalim and nifedipine. This potent relaxant action of KRN2391 on endothelin-induced contraction is thought to be based on both a nitrate action and a K+ channel opening action.

Cardiovascular effects of KRN2391, nitroglycerin and cromakalim in dihydroergotamine-treated pithed rats

1. The effects of KRN2391 on the cardiovascular system were compared with those of nitroglycerin and cromakalim in pithed rats treated with dihydroergotamine (DHE) in order to examine the effects of these drugs on venous blood vessels. 2. DHE (100 micrograms/kg, i.v.) produced increases in mean blood pressure (MBP), cardiac output (CO) and central venous pressure (CVP) without changes in total peripheral vascular resistance (TPR) and heart rate (HR) based on venoconstriction. The DHE-treated pithed rats, nitroglycerin (30 micrograms/kg, i.v.) decreased CO and CVP whereas cromakalim (30 micrograms/kg, i.v.) produced a slight increase in CO followed by a decrease and did not affect CVP. KRN2391 (30 micrograms/kg, i.v.) produced a decrease in CVP without affecting CO. Decreases in MBP and TPR were induced by all drugs. 3. These results suggest that nitroglycerin acts predominantly as a venodilator and KRN2391 and cromakalim showed a venodilating action in addition to an arterial dilating action in DHE treated pithed rats. However, the venodilating action of KRN2391 in this condition is more potent than that of cromakalim.

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.

Comparison of the cromakalim antagonism and bradycardic actions of a series of novel alinidine analogues in the rat

Alinidine, and eight derivatives, were synthesized and tested for their ability to antagonise the actions of the K+ channel opener cromakalim in rat thoracic aorta, and for their ability to induce bradycardia in rat isolated spontaneously beating right atria. Ring segments of rat thoracic aorta were suspended in organ baths to record isometric tension. Tissues were precontracted with K+ (20 mM), and full concentration-relaxation curves constructed to cromakalim (0.01-30 microM) in the absence and presence of increasing concentrations of alinidine/derivative. The majority of the compounds tested caused rightward shifts in the cromakalim concentration-effect curves. Rat spontaneously beating right atria were suspended in organ baths to record rate of contraction. Addition of alinidine/derivative caused a concentration-dependent negative chronotropic response. In terms of structure-activity relationships, increasing the length of the N-allyl side-chain on the alinidine molecule (from 3 carbon (3C), to 5C) resulted in a significant increase in the activity of the compounds as both bradycardic agents and cromakalim antagonists. The most potent compounds in both cases (bradycardic agent and cromakalim antagonist) had no double bond in the side chain. The results suggest that the carbon side-chain influences the activity of alinidine-related compounds both as cromakalim antagonists and as bradycardic agents. However, while similar structure-activity relationships appear to apply for both effects in some instances, there was no significant correlation between the two actions of the alinidine analogues. The results suggest that the ability of alinidine-derivatives to induce bradycardia or to block K+ channels opened by cromakalim can be differentiated on the basis of structure.

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.

Antihypertensive and vasorelaxant effects on KRN2391 in spontaneously hypertensive rats

1. In conscious spontaneously hypertensive rats (SHR), the oral administration of KRN2391 (0.1-3.0 mg/kg) produced a dose-dependent decrease in blood pressure. The antihypertensive effect of KRN2391 was about 2 and 20 times more potent than those of pinacidil and nifedipine, respectively, but about 2 times less potent than that of cromakalim. 2. During oral administration of KRN2391 (0.5 and 1.0 mg/kg) once daily for 5 weeks, its antihypertensive effect did not diminish in conscious SHR. 3. In anaesthetized SHR, KRN2391 (3-100 micrograms/kg, i.v.) produced a decrease in blood pressure in a dose-dependent manner. Its antihypertensive effect was antagonized by glibenclamide (20 mg/kg, i.v.). 4. In isolated aorta obtained from SHR, KRN2391 (0.01-100 microM) produced a concentration-dependent relaxation. Its concentration-relaxation curve was shifted to the right by glibenclamide (1 microM) and methylene blue (3 microM). 5. These results indicate that the antihypertensive effect of KRN2391 in SHR is due to its direct action on vascular smooth muscle based on a K+ channel opening action and a nitrate action. In addition, KRN2391 is absorbed from the gastrointestinal tract into blood and does not induce tolerance despite possessing some nitrate action.

Effects of K+ channel openers on the vascular actions of human gamma globulin

The aim of this study was to determine if the stimulatory action of human gamma globulin on the spontaneous activity of the rat mesenteric portal vein is due to decreased K+ conductance. Glibenclamide potentiated the action of human gamma-globulin on the portal vein by 45% and on its own had a concentration- and time-dependent biphasic (increase followed by a decrease) effect on the spontaneous activity of the portal vein. Diazoxide and pinacidil both inhibited the action of human gamma-globulin on the rat mesenteric portal vein. Levcromakalim (BRL 38227) potentiated the stimulatory action of human gamma-globulin on the integrated force of the spontaneous contractions of the rat mesenteric portal vein by 40% and 49% at concentrations of 0.5 and 5 microM, respectively. These studies suggest that human gamma-globulin can act by directly modulating a K+ channel.

Differential vasodilator properties of KRN2391, cromakalim, nitroglycerin and nifedipine in rabbit isolated femoral artery and vein

1. The selectivity for artery and vein of KRN2391, cromakalim, nitroglycerin and nifedipine was examined in isolated femoral artery and vein preparations of the rabbit. 2. All drugs produced a concentration-dependent relaxation in both femoral artery and vein. 3. Nitroglycerin was more potent in femoral vein than in femoral artery at all concentrations. The EC50 value obtained in the vein was about 14 times smaller than that obtained in artery. 4. Cromakalim and nifedipine were almost equipotent on both vascular preparations. Cromakalim at the highest concentration (10(-5) M) produced 88 and 78% relaxation in femoral artery and vein, respectively. The maximum relaxation induced by nifedipine (10(-6) M) was less than 50% in both preparations. 5. KRN2391 was active at a lower concentration in the vein than in the artery and its maximum relaxation at 10(-5) M was about 90% in both preparations. 6. Glibenclamide (10(-6) M) inhibited the vasorelaxation caused by KRN2391 in both artery and vein. Methylene blue (10(-5) M) also inhibited the relaxant action of KRN2391 but this action was slight in the artery. 7. These results suggest that KRN2391 and nitroglycerin are more potent in the vein than in the artery and cromakalim and nifedipine are equipotent in both. It is considered that the relaxation induced by low concentrations of KRN2391 reflects predominantly its action as a nitrate and that at high concentrations it acts as a K+ channel opener in addition to its nitrate action. The different vascular selectivities of these drugs are thought to relate to the differences in their mechanisms of action in vascular smooth muscle.

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.

Effects of three K+ channel openers on airways and pulmonary circulation in the isolated guinea-pig lung

The antispasmodic and spasmolytic effects of levcromakalim (BRL 38227), aprikalim (RP 52891) and pinacidil were investigated in airways and pulmonary vessels of the isolated guinea-pig perfused lung. In airways, the three drugs exhibited modest antispasmodic properties, and pinacidil was more potent than levcromakalim and aprikalim against the contractions induced by carbachol (0.001-10 microM) or K+ (5-50 mM). Whereas levcromakalim and aprikalim acted only at low concentrations of K+, the rightward shift of the K+ concentration-effect curve produced by pinacidil was observed at all K+ concentrations (5-50 mM), suggesting that a mechanism of action other than K+ channel opening is involved in the effects of pinacidil. Pinacidil (0.3-100 microM) had the greatest spasmolytic effect in airways precontracted by carbachol (0.3 microM). The three K+ channel openers were equipotent against the sustained contractions of airways induced by 25 or 30 mM K+ and their spasmolytic activity was more marked against contractions induced by low rather low than high K+ concentrations. Levcromakalim and aprikalim were more effective as relaxant than as antispasmodic drugs. In K(+)-precontracted pulmonary vessels, the relaxant activity of pinacidil and levcromakalim was more pronounced than that observed in airways, suggesting a vascular selectivity of these drugs.

Evidence that imidazol(id)ine- and sulphonylurea-based antagonists of cromakalim act at different sites in the rat thoracic aorta

1. Ring segments of rat thoracic aorta were suspended in organ baths to record isometric tension. Tissues were precontracted with K+ (20 mmol/L), and full concentration-relaxation curves constructed to cromakalim (0.01-30 mumol/L) in the absence and presence of increasing concentrations of glibenclamide, glipizide, tolbutamide (the sulphonylureas), alinidine (an imidazolidine), phentolamine (an imidazoline), and chlorpromazine (the phenothiazine derivative). Whereas the active sulphonylureas, glibenclamide and glipizide, displayed classical competitive antagonism, the remaining compounds (alinidine, phentolamine and chlorpromazine) caused shifts in the cromakalim concentration-effect curves associated with a reduction in the slope and maximum response. 2. A single concentration of each antagonist was selected and the shift in the concentration-effect curve determined. The possibility that sulphonylurea and imidazol(id)ine antagonists act at different sites was tested using the concentration-ratio method for combined antagonists described by Paton and Rang (1965). The combination of alinidine and phentolamine (collectively called imidazol(id)ines) at a number of different concentrations (10-30 mumol/L) resulted in a concentration-ratio to cromakalim which was additive, suggesting a common site of action. Similar results were obtained when examining the interaction between two sulphonylurea compounds (glibenclamide and glipizide). However, the interaction between sulphonylurea (glibenclamide) and imidazol(id)ine (alinidine) produced concentration-ratios which were multiplicative, suggesting a different or additional site of action for compounds from these two groups. Results indicated that chlorpromazine was able to block cromakalim via an action at the same site where alinidine and phentolamine act.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasorelaxant mechanism of KRN2391 and nicorandil in porcine coronary arteries of different sizes

1. The relaxant mechanisms of action of KRN2391, a novel vasodilator, and nicorandil on epimyocardial coronary artery (2.5- 3.0 mm outer diameter) and mid-myocardial coronary artery (0.8-1.0 mm outer diameter) were investigated in porcine isolated coronary arteries. In addition, the vasorelaxant responses of KRN2391 and nicorandil were compared with those of nitroglycerin and cromakalim, a K+ channel opener, in epi- and mid-myocardial coronary arteries. 2. Nitroglycerin showed a more potent relaxant effect on epi-myocardial coronary arteries than on mid-myocardial coronary arteries, whereas cromakalim produced greater relaxation responses in mid-myocardial coronary arteries. There was no difference between epi- and mid-myocardial coronary arteries in terms of the relaxant effect of KRN2391 and nicorandil. 3. Relaxation induced by KRN2391 in epi- and mid-myocardial coronary arteries was inhibited by oxyhaemoglobin, a pharmacological antagonist of nitrovasodilators, and glibenclamide, a pharmacological antagonist of K+ channel opening drugs. However, the inhibitory effect of glibenclamide on KRN2391-induced relaxation was greater in mid-myocardial coronary artery than in epi-myocardial coronary artery. 4. Relaxation induced by nicorandil was inhibited by oxyhaemoglobin alone in epi-myocardial coronary arteries and by both oxyhaemoglobin and glibenclamide in mid-myocardial coronary arteries. 5. In epi- and mid-myocardial coronary arteries, relaxation induced by cromakalim was inhibited by glibenclamide but not by oxyhaemoglobin, whereas relaxation induced by nitroglycerin was inhibited by oxyhaemoglobin but not by glibenclamide. 6. These results suggest that KRN2391 and nicorandil exhibit a dual mechanism of action acting partly as a nitrate and partly as a K+ channel opener. The mechanism of action of these drugs depend on the segment of coronary artery studied. Furthermore, the dual mechanism of action of KRN2391 and nicorandil seems to contribute to the equipotent relaxant effect between epi- and mid-myocardial coronary arteries.

Mechanisms of oxygen-induced contraction of ductus arteriosus isolated from the fetal rabbit

The present study was designed to investigate the effect of O2 on intracellular Ca concentration ([Ca]i) in the ductus arteriosus and the mechanisms for O2-induced ductal contraction. The force of isometric contraction of the ring of the ductus arteriosus isolated from fetal rabbits at 30 days of gestation (term, 31 days) was measured. The ductus arteriosus was loaded with fura 2, a calcium-sensitive dye, and [Ca]i was determined from the ratio of fluorescence intensity at 340 and 380 nm excitation wavelengths. The ductus arteriosus was initially superfused with hypoxic control solutions and contraction was induced by application of oxygenated solutions. The O2-induced contraction of the ductus arteriosus was associated with increases in [Ca]i and was eliminated in the absence of extracellular calcium. An increase in [K]o from 5 to 50 mM, which causes membrane depolarization, induced ductal contraction. The calcium channel blockers verapamil, diltiazem, and nickel caused a similar inhibition of O2-induced contraction as well as KCl-induced contraction. The role of intracellular calcium stores in O2-induced ductal contraction was examined using ryanodine, an inhibitor of calcium uptake and release from the sarcoplasmic reticulum. The inhibition of O2-induced contraction by ryanodine was minimal. Infusion of glibenclamide, an inhibitor for opening the ATP-sensitive potassium channel, caused contraction of the ductus arteriosus in the hypoxic solution. Cromakalim, an opener of ATP-sensitive potassium channels, completely relaxed the contraction induced by O2. These data suggest that O2 increases [Ca]i and causes contraction in the ductus arteriosus. Application of O2 may change from anaerobic to aerobic metabolism and depolarize membrane potential by closing the ATP-sensitive potassium channel, which in turn increases calcium influx via the voltage-dependent calcium channel. Mechanisms other than the ATP-sensitive potassium channel may also be involved in the O2-induced contraction and remain to be studied.

Relaxing effect of vesnarinone (OPC-8212) on the tracheal muscle strips isolated from guinea pigs

The effect of vesnarinone (OPC-8212), an orally active positive inotropic agent was studied in tracheal muscle isolated from guinea pigs, and the mechanism of its action was analyzed. Vesnarinone (10(-6)-10(-4) M) caused a concentration-dependent relaxation of tracheal muscle pre-contracted by 10(-4) M histamine. The potency of the relaxing effect of vesnarinone was greater than that of theophylline; the pD2 values for vesnarinone and theophylline were 4.9 and 4.5, respectively. Vesnarinone reduced the high-K(+)-induced contracture of depolarized tracheal muscle non-competitively (pD'2 = 3.7). Vesnarinone at the low concentration of 3 x 10(-6) M shifted the concentration-response curve for isoproterenol in a parallel fashion to the left. Vesnarinone additively acted on the relaxing effect of isobutyl methyl xanthine. Propranolol (10(-5) M) and reserpine pre-treatment (5 mg/kg, i.p., 24 hr) had no effect on the relaxing effect of vesnarinone. These results suggested that vesnarinone elevated the intracellular cyclic AMP level via phosphodiesterase inhibition, resulting in the tracheal muscle relaxation.

Modulation of rabbit aortic Ca(2+)-activated K+ channels by pinacidil, cromakalim, and glibenclamide

Rabbit aortic smooth muscle microsomes were isolated and large-conductance Ca(2+)-activated K+ (BK) channels incorporated into planar lipid bilayers. The selectivity sequence and relative permeability ratios for monovalent cations was K+ (1.0) > Rb+ (0.68) > NH4+ (0.14) >> Na+, Cs+ (< 0.05). Application of pinacidil or cromakalim (0.05-10 microM) shifted the probability of opening (Po)-voltage relationship in the hyperpolarizing direction. The concentrations of pinacidil and cromakalim required to increase Po 50% of the maximum value at -40 mV were 0.96 +/- 0.04 and 0.52 +/- 0.03 microM, respectively. Neither pinacidil nor cromakalim altered the voltage sensitivity of the channel (11-13 mV/e-fold change in Po). Kinetic analysis of data at -40 mV demonstrated that pinacidil (1 microM) decreased the length of time the channel dwelled in its long-closed state by 50% from 173 +/- 50 to 86 +/- 19 ms. No significant change was observed for the open time constant (20 ms). Glibenclamide (10 microM) had no effect on Po of BK channels. However, glibenclamide reversed the pinacidil- or cromakalim-stimulated increase in Po of BK channels. These data suggest that both cromakalim and pinacidil increased the probability of opening of single rabbit aortic large-conductance Ca(2+)-activated K+ channels and that this channel modulation may contribute to the vasorelaxant properties of these drugs.

Inhibitory effect of cromakalim in human detrusor muscle is mediated by glibenclamide-sensitive potassium channels

The effects of cromakalim, a potassium channel activating drug, and glibenclamide, a relatively selective antagonist of ATP-sensitive potassium channels, have been investigated on isolated detrusor muscle from human bladder. Specimens of human bladder were cut into strips and suspended in an organ bath filled with modified Tyrode solution for measurement of isometric contractile force. Concentration-response curves to acetylcholine were constructed before and after pretreatment with cromakalim and cromakalim plus glibenclamide. The concentration-response curves to acetylcholine were displaced to the right, and the maximal response to acetylcholine was significantly inhibited by cromakalim in a concentration-dependent manner. The inhibitory effect of cromakalim on acetylcholine-induced contraction was significantly reduced by glibenclamide. Following sustained contraction induced by 20 mM. KCl, the cumulative addition of cromakalim to the organ bath produced a concentration-dependent relaxation. However, in strips precontracted with 60 mM. KCl, the addition of cromakalim in concentrations as high as 10(-5) M. did not induce relaxation. The relaxation induced by cromakalim in strips precontracted with 20 mM. KCl was significantly inhibited by glibenclamide. These results suggest that the inhibitory effect of cromakalim in human bladder involves activation of glibenclamide-sensitive potassium channels.

BRL 38227 (levcromakalim)-induced hyperpolarization reduces the sensitivity to Ca2+ of contractile elements in canine coronary artery

Potassium (K+) channel openers decrease intracellular free Ca2+ concentrations ([Ca2+]i by hyperpolarizing the membrane and deactivating the Ca(2+)-channels. To examine whether the hyperpolarization produced by K(+)-channel openers has other effects on the mechanical activity of vascular smooth muscle, we investigated the effects of levcromakalim (BRL 38227) on membrane potential, [Ca2+]i, as measured with fura-2, and force of contraction induced by 30 mmol/l KCl-physiological salt solution (PSS), in canine coronary arteries. BRL 38227 hyperpolarized the membrane and reduced increases in [Ca2+]i and in contractile force induced by 30 mmol/l KCl-PSS. The [Ca2+]i-contractile force curve, determined in the presence of BRL 38227, was located to the right of the control curve determined by decreasing extracellular Ca2+ concentrations ([Ca2+]o) in 30 mmol/l KCl-PSS. The [Ca2+]i-contractile force curve, determined by decreasing extracellular K+ concentrations ([K+]o), was also located to the right of that determined by decreasing [Ca2+]o in 30 mmol/l KCl-PSS. The effect of BRL 38227, a reduction in the Ca(2+)-sensitivity of contractile elements, was antagonized by the ATP-sensitive K(+)-channel blocker, glibenclamide (10(-6) or 10(-5) mol/l). These results suggest that the membrane hyperpolarization induced by BRL 38227, or the repolarization caused by reducing [K+]o, decreases the Ca(2+)-sensitivity of contractile elements of vascular smooth muscle.

A target K+ channel for the LP-805-induced hyperpolarization in smooth muscle cells of the rabbit portal vein

The resting membrane potential of smooth muscle cells of the rabbit portal vein was -51.2 mV. LP-805 (8-tert-butyl-6,7-dihydropyrrolo[3,2-e] 5-methylpyrazolo [1,5-a] pyrimidine-3-carbonitrile) hyperpolarized the membrane to -62.3 mV (10 microM) and inhibited the burst spike discharges as measured using the microelectrode method. In dispersed smooth muscle cells, LP-805 (10 microM) generated an outward-current with a maximum amplitude of 68 pA at a holding potential of -40 mV in experiments using the voltage-clamp procedure. The reversal potential of the outward current evoked by LP-805 was -82 mV and this value was close to the equilibrium potential for K+ (-80 mV) in the present ionic conditions, suggesting that LP-805 activated the K+ channel. Generation of both the hyperpolarization and the outward current by LP-805 was inhibited by glibenclamide (> or = 1 microM). Using the cell-attached and cell-free patch-clamp (in the presence of GDP) procedures, the maxi-K+ channel current (150 pS) could be recorded in the absence of LP-805; application of LP-805 additionally opened a small conductance K+ channel current (15 pS) without change in the activity of the maxi-K+ channel. The maxi-K+ channel was sensitive to charybdotoxin (0.1 microM) and to intracellular Ca2+ ([Ca2+]i) concentration. The 15 pS channel was insensitive to [Ca2+]i and charybdotoxin, but sensitive to intracellular ATP concentration. Glibenclamide (> 1 microM) inhibited the 15 pS K+ channel activated by LP-805.(ABSTRACT TRUNCATED AT 250 WORDS)

On the mechanism of inhibition of KATP channels by glibenclamide in rat ventricular myocytes

INTRODUCTION

The mechanism by which glibenclamide inhibits KATP channel activity has been examined in membrane patches from isolated rat ventricular cells.

METHODS AND RESULTS

Inside-out patches were exposed to zero, or low, [ATP] to activate KATP channels. Glibenclamide did not affect single channel conductance, but reversibly reduced channel open probability from either side of the membrane. Internal (cytoplasmic) glibenclamide inhibited with half-maximal inhibitory [glibenclamide] = 6 microM, Hill coefficient = 0.35. Complete channel inhibition was not observed, even at 300 microM [glibenclamide]. The response to step increases of internal [glibenclamide] could be resolved into two phases of channel inhibition (t1/2,fast < 1 sec, t1/2, slow = 10.5 +/- 0.9 sec, n = 8). Step decrease of [glibenclamide] caused a single resolvable phase of reactivation (t1/2 = 20.4 +/- 0.7 sec, n = 16). Channel inhibition by internal glibenclamide could be relieved by ADP, but only in the presence of Mg2+.

CONCLUSION

Glibenclamide can inhibit KATP channels from either side of the membrane, with block from one side being competitive with block from the other. Internal MgADP antagonizes the blocking action of glibenclamide. Glibenclamide inhibition of cardiac KATP channels differs quantitatively and qualitatively from the inhibition of pancreatic KATP channels.

Opposing actions of tolbutamide and glibenclamide on hypoxic pulmonary vasoconstriction

1. We show that cromakalin and diazoxide, drugs that activate ATP-sensitive potassium (KATP) channels, abolish hypoxic pulmonary vasoconstriction (HPV) of isolated, perfused rat lungs. 2. Glibenclamide, an inhibitor of these channels, does not affect HPV, but it reverses the relaxation caused by cromakalim and diazoxide. 3. Tolbutamide, which has effects similar to glibenclamide in other tissues, paradoxically abolishes HPV, an effect reversed by glibenclamide. 4. These results suggest that: (i) pulmonary vessels contain KATP channels which are normally closed and are not opened by levels of hypoxia that cause constriction, (ii) tolbutamide acts on the pulmonary vasculature by a mechanism which differs from that of glibenclamide.

Pro-arrhythmic effect of nicorandil in isolated rabbit atria and its suppression by tolbutamide and quinidine

Nicorandil, a potent vasodilator substance which exerts its effects through complex mechanisms including KATP channel activation, has so far been reported to exert antiarrhythmic but not pro-arrhythmic cardiac activity. We now examined the effects of 10(-4) M nicorandil on spontaneously active or electrically driven isolated rabbit atria. Nicorandil (a) significantly reduced the action potential duration at both 50% (by approximately 45%) and 80% (by approximately 30%) repolarization and the effective refractory period (by approximately 25%) and (b) reproducibly induced short periods of tachycardia either in normal Tyrode solution after a single extra-stimulus or in low-potassium media in the absence of extra-stimulation. Quinidine (10(-5) M) or the KATP channel inhibitor, tolbutamide (10(-5) M), suppressed the nicorandil-induced arrhythmias. It is suggested that the pro-arrhythmic effect of nicorandil results from its KATP channel opener activity and occurs essentially when the underlying conditions facilitate re-entry.

Characterization of endothelium-dependent relaxations resistant to nitro-L-arginine in the porcine coronary artery

1. Previous studies, demonstrated that endothelium-dependent relaxations which are resistant to nitro-L-arginine (an inhibitor of nitric oxide synthase) are accompanied by membrane hyperpolarization in the porcine coronary artery. The present experiments were designed to characterize further this type of endothelium-dependent relaxation in response to bradykinin by measuring isometric force in isolated rings of that artery. The experiments were performed in the presence of indomethacin to rule out vasoactive prostanoids. 2. Bradykinin induced comparable endothelium-dependent relaxations of proximal and distal rings of porcine coronary arteries contracted with prostaglandin F2 alpha in the presence of nitro-L-arginine. 3. Bradykinin and SIN 1 (a donor of nitric oxide) reduced contractions induced by prostaglandin F2 alpha in an additive fashion in the presence of nitro-L-arginine. 4. Bradykinin (in the presence of nitro-L-arginine) relaxed the tissues contracted with tetraethylammonium, prostaglandin F2 alpha, phorbol 12, 13-diacetate or endothelin, with similar pD2 values. 5. The time course of the relaxations induced by bradykinin (in the presence of nitro-L-arginine) and UK14304 (an alpha 2-adrenoceptor agonist, in the absence of the inhibitor of nitric oxide synthase) were comparable. 6. These results suggest that, in the porcine coronary artery, nitro-L-arginine-resistant relaxations (a) are distributed similarly in the proximal and distal parts of the artery, (b) contribute to inhibition of vascular smooth muscle with nitric oxide in an additive fashion, (c) occur during contractions induced by various contractile agents and (d) do not precede those mediated by nitric oxide.