Mechanical and ionic response of rat aorta to diazoxide

Altered KATP Channel Subunits Expression and Vascular Reactivity in Spontaneously Hypertensive Rats With Age

ATP-sensitive potassium (KATP) channels link membrane excitability to metabolic state to regulate a series of biological activities including the vascular tone. However, their ability to influence hypertension is controversial. Here we aim to investigate possible alteration of KATP channel in vascular smooth muscles (VSMs) during hypertension development process. In this study, we used 16-week-old spontaneously hypertensive rats (SHRs), 49-week-old SHRs, and their age-matched Wistar-Kyoto rats to study the expression of VSM KATP subunits at the mRNA and protein level and the function of VSM KATP by observing the relaxation reactivity of isolated aorta rings to KATP modulators. We found that the expression of VSM KATP subunits Kir6.1 and sulfonylurea receptor (SUR2B) decreased during hypertension. Moreover, the expression of SUR2B and Kir6.1 in 49-week-old SHRs decreased much more than that in 16-week-old SHRs. Furthermore, the aorta rings of 49-week-old SHRs showed lower reactivity to diazoxide than 16-week-old SHRs. This study suggests that KATP channels in VSM subunits Kir6.1 and SUR2B contribute to modify the functionality of this channel in hypertension with age.

Synthesis, characterization and biological evaluation of benzothiazoles and tetrahydrobenzothiazoles bearing urea or thiourea moieties as vasorelaxants and inhibitors of the insulin releasing process

A series of 1,3-benzothiazoles (series I) and 4,5,6,7-tetrahydro-1,3-benzothiazoles (series II) bearing an urea or a thiourea moiety at the 2-position were synthesized and tested as myorelaxants and inhibitors of insulin secretion. Several compounds (i.e. 13u and 13v) from series I showed a marked myorelaxant activity. Benzothiazoles bearing a strong electron withdrawing group (NO2, CN) at the 6-position and an alkyl group linked to the urea or the thiourea function at the 2-position were found to be the most potent compounds. The weak vasorelaxant activity of series II compounds evidenced the necessity of the presence of a complete aromatic heterocyclic system. The myorelaxant activity of some active compounds was reduced when measured on aorta rings precontracted by 80 mM KCl or by 30 mM KCl in the presence of 10 μM glibenclamide, suggesting the involvement of KATP channels in the vasorelaxant effect. Some compounds of series I tested on rat pancreatic islets provoked a marked inhibition of insulin secretion, among which 13a exhibited a clear tissue selectivity for pancreatic β-cells.

Influence of the alkylsulfonylamino substituent located at the 6-position of 2,2-dimethylchromans structurally related to cromakalim: from potassium channel openers to calcium entry blockers?

The present study described the synthesis of original R/S-6-alkylsulfonylamino-3,4-dihydro-2,2-dimethyl-2H-1-benzopyrans bearing a 3- or 4-substituted phenylthiourea or phenylurea moiety at the 4-position. Their biological effects were evaluated both on insulin-secreting and smooth muscle cells and were compared to those of reference KATP channel activators such as (±)-cromakalim, diazoxide and previously synthesized cromakalim analogues. The study aimed at exploring the influence of the introduction of an alkylsulfonylamino substituent at the 6-position of 2,2-dimethylchromans in order to improve biological activity, tissue selectivity but also hydrophilicity of dihydrobenzopyran derivatives. Several compounds were found to be equipotent or even more potent than (±)-cromakalim and diazoxide at inhibiting the insulin releasing process. Most of the newly synthesized and more hydrophilic dihydrobenzopyrans also exhibited a marked vasorelaxant activity although they were less potent than (±)-cromakalim. Additional pharmacological and radioisotopic investigations suggested that R/S-N-3-chlorophenyl-N'-(3,4-dihydro-6-methylsulfonylamino-2,2-dimethyl-2H-1-benzopyran-4-yl)thiourea (21) did not act as a potassium channel opener but rather as a Ca(2+) entry blocker.

Multiplicity of effectors of the cardioprotective agent, diazoxide

Diazoxide has been identified over the past 50years to have a number of physiological effects, including lowering the blood pressure and rectifying hypoglycemia. Today it is used clinically to treat these conditions. More recently, another important mode of action emerged: diazoxide has powerful protective properties against cardiac ischemia. The heart has intrinsic protective mechanisms against ischemia injury; one of which is ischemic preconditioning. Diazoxide mimics ischemic preconditioning. The purpose of this treatise is to review the literature in an attempt to identify the many effectors of diazoxide and discuss how they may contribute to diazoxide's cardioprotective properties. Particular emphasis is placed on the concentration ranges in which diazoxide affects its different targets and how this compares with the concentrations commonly used to study cardioprotection. It is concluded that diazoxide may have several potential effectors that may potentially contribute to cardioprotection, including KATP channels in the pancreas, smooth muscle, endothelium, neurons and the mitochondrial inner membrane. Diazoxide may also affect other ion channels and ATPases and may directly regulate mitochondrial energetics. It is possible that the success of diazoxide lies in this promiscuity and that the compound acts to rebalance multiple physiological processes during cardiac ischemia.

Impact of the nature of the substituent at the 3-position of 4H-1,2,4-benzothiadiazine 1,1-dioxides on their opening activity toward ATP-sensitive potassium channels

The synthesis of diversely substituted 3-isopropoxy-, 3-isopropylsulfanyl-, 3-isopropylsulfinyl-, and 3-isobutyl-4H-1,2,4-benzothiadiazine 1,1-dioxides is described. Their activity on pancreatic β-cells (inhibitory effect on the insulin releasing process) and on vascular and uterine smooth muscle tissues (myorelaxant effects) was compared to that of previously reported K(ATP) channel openers belonging to 3-isopropylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides. The present study aimed at evaluating the impact on biological activity of the isosteric replacement of the NH group of 3-alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides by a O, S, S(═O), or CH(2) group. By comparing compounds bearing identical substituents, the following rank order of potency on pancreatic β-cells was observed: 3-isopropylamino > 3-isobutyl > 3-isopropoxy > 3-isopropylsulfanyl > 3-isopropylsulfinyl-substituted 4H-1,2,4-benzothiadiazine 1,1-dioxides (NH > CH(2) > O > S > S(═O)). A molecular modeling study revealed that 3-isopropoxy-, 3-isopropylsulfanyl-, and 3-isopropylamino-substituted compounds adopted a similar low-energy conformation (preferred orientation of the isopropyl chain). Moreover, no direct relationship was detected between the conformational freedom of the different classes of benzothiadiazines (from the most to the lowest conformationally constrained compounds: NH > O > S > CH(2)) and their biological activity on insulin-secreting cells. Therefore, the present study confirmed the critical role of the NH group at the 3-position for the establishment of a strong hydrogen bond responsible for optimal activity expressed by 3-alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides on insulin-secreting cells. Radioisotopic and fluorimetric experiments conducted with 7-chloro-3-isopropoxy-4H-1,2,4-benzothiadiazine 1,1-dioxide 10c demonstrated that such a compound, bearing a short branched O-alkyl group instead of the NH-alkyl group at the 3-position, also behaved as a specific K(ATP) channel opener. Lastly, the present work further identified 3-(alkyl/aralkyl)sulfanyl-substituted 7-chloro-4H-1,2,4-benzothiadiazine 1,1-dioxides as a class of promising myorelaxant drugs acting on uterine smooth muscles, at least in part, through the activation of K(ATP) channels.

Antidiabetic activity and toxicity of Zizyphus spina-christi leaves

The effect of the butanol extract of Zizyphus spina-christi (L.), Willd (Rhamnaceae) leaves and its major saponin glycoside, christinin-A, on the serum glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment either with 100 mg/kg butanol extract or christinin-A potentiated glucose-induced insulin release in non-diabetic control rats. In type-II but not in type-I diabetic rats pretreatment with the butanol extract or christinin-A improved the oral glucose tolerance and potentiated glucose-induced insulin release. Treatment either with 100 mg/kg butanol extract or christinin-A reduced the serum glucose level and increased the serum insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of the butanol extract and christinin-A were similar. Pretreatment of non-diabetic control and type-II diabetic rats either with 100 mg/kg butanol extract or christinin-A enhanced the glucose lowering and insulinotropic effects of 5 g/kg glibenclamide. The hyperglycemic and hypoinsulinemic effects of 30 mg/kg diazoxide in non-diabetic control and type-II diabetic rats were inhibited and antagonized, respectively by pretreatment with the butanol extract or christinin-A. The relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips were inhibited by 100 micromol/l christinin-A or 10 micromol/l glibenclamide. The combination of glibenclamide and christinin-A led to complete inhibition of the relaxant effects of different concentrations of diazoxide. At a dose level much higher than that required to produce satisfactory insulinotropic and hypoglycemic effects, the butanol extract of Zizyphus spina-christi leaves produced a depressant effect on the central nervous system in rats. Treatment of rats with 100mg/kg butanol extract for 3 months produced no functional or structural disturbances in liver and kidney and no haematological changes. In addition, the oral LD50 of the butanol extract in mice was 3820 mg/kg, while that of glibenclamide was 3160 mg/kg. Thus, Zizyphusspina-christi leaves appears to be a safe alternative to lower blood glucose. The safe insulinotropic and subsequent hypoglycemic effects of Zizyphus spina-christi leaves may be due to a sulfonylurea-like activity.

Blockade of calcium entry in smooth muscle cells by the antidepressant imipramine

The present study was designed to evaluate the effects of antidepressants on smooth muscle contractile activity. In rat aortic rings, the antidepressants imipramine, mianserin and sertraline provoked concentration-dependent inhibitions of the mechanical responses evoked by K+ (30 mM) depolarization. These myorelaxant effects were not modified by the presence of glibenclamide or 80 mM K+ in the bathing medium. Moreover, the vasodilator properties of imipramine were not affected by atropine, phentolamine and pyrilamine. Radioisotopic experiments indicated that imipramine failed to enhance 86Rb outflow from prelabelled and perifused aortic rings whilst counteracting the increase in 45Ca outflow provoked by a rise in the extracellular K+ concentration. Simultaneous measurements of contractile activity and fura-2 fluorescence revealed that, in aortic rings, imipramine reduced the mechanical and fluorimetric response to K+ challenge. In A7r5 smooth muscle cells, whole cell recordings further demonstrated that imipramine inhibited the inward Ca2+ current. Under different experimental conditions, the ionic and relaxation responses to the antidepressants were reminiscent of those mediated by the Ca2+ entry blocker verapamil. Lastly, it should be pointed out that imipramine exhibited a myorelaxant effect of similar amplitude on rat aorta and on rat distal colon. All together, these findings suggest that the myorelaxant properties of imipramine, and probably also setraline and mianserin, could result from their capacity to inhibit the voltage-sensitive Ca2+ channels.

KATP channel openers: structure-activity relationships and therapeutic potential

ATP-sensitive potassium channels (K(ATP) channels) are heteromeric complexes of pore-forming inwardly rectifying potassium channel subunits and regulatory sulfonylurea receptor subunits. K(ATP) channels were identified in a variety of tissues including muscle cells, pancreatic beta-cells, and various neurons. They are regulated by the intracellular ATP/ADP ratio; ATP induces channel inhibition and MgADP induces channel opening. Functionally, K(ATP) channels provide a means of linking the electrical activity of a cell to its metabolic state. Shortening of the cardiac action potential, smooth muscle relaxation, inhibition of both insulin secretion, and neurotransmitter release are mediated via K(ATP) channels. Given their many physiological functions, K(ATP) channels represent promising drug targets. Sulfonylureas like glibenclamide block K(ATP) channels; they are used in the therapy of type 2 diabetes. Openers of K(ATP) channels (KCOs), for example, relax smooth muscle and induce hypotension. KCOs are chemically heterogeneous and include as different classes as the benzopyrans, cyanoguanidines, thioformamides, thiadiazines, and pyridyl nitrates. Examples for new chemical entities more recently developed as KCOs include cyclobutenediones, dihydropyridine related structures, and tertiary carbinols.

The reactivity of serotonin, acetylcholine and kcl-induced contractions to relaxant agents in the rat gastric fundus

The effects of nifedipine, cromakalim, diazoxide, caffeine and sodium nitroprusside (SNP) on acetylcholine, serotonin and KCl-induced contractions were studied in rat stomach fundus. Thus, we aimed to demonstrate how these contractions are modified by the substances acting on Ca (2+)influx and intracellular Ca (2+)stores. Serotonin (10(-9) - 10(-5) M) and KCl (20-80 mM) showed a similar contraction profile, which was slightly different from that of acetylcholine (10(-8)- 3 x 10(-3) M). In the experiments with the incubation of calcium-free/EGTA (0.5 mM) Krebs solution for 20 min, serotonin (3 x 10(-7)M) and KCl (40 mM) did not produce any contraction whereas, 10% of contraction to acetylcholine (3 x 10(-5) M) was still intact. Serotonin-induced contractions were readily reversed by nifedipine (10(-10) - 10(-4) M), cromakalim (10(-9) - 10(-4) M), diazoxide (10(-9) - 10(-4) M), caffeine (10(-5) - 10(-2) M) and SNP (10(-4) M) whereas, acetylcholine-induced contractions showed relative refractoriness to the above relaxant agents. 1 mM caffeine nearly fully inhibited serotonin-induced contraction, but not acetylcholine and high K-induced contractions whereas, 10 mM caffeine completely inhibited all the contractions. The relaxation pattern of nifedipine on serotonin and high K (+)-induced contractions was quite similar. Moreover, nifedipine and cromakalim showed equal dose effectiveness in relaxing acetylcholine and serotonin. The maximum relaxations induced by nifedipine and cromakalim in acetylcholine contractions were 61.51 +/- 6.92 % and 58.97 +/- 7.55 %, respectively. However their maximum relaxations in serotonin and high K (+)-induced contractions were almost 100%. The similarity in myorelaxants properties of cromakalim and nifedipine may relate to the similarity of their effects on calcium influx by a different mechanism of action in rat stomach fundus. In conclusion, acetylcholine-induced contraction is partially mediated both by calcium release from the intracellular Ca (2+) pool and calcium influx via L-type Ca (2+) channels. However, serotonin-induced contraction is possibly triggered by Ca (2+) release from sarcoplasmic reticulum and basically mediated by Ca (2+) influx via L-type Ca (2+)channels.

The potential antidiabetic activity of some alpha-2 adrenoceptor antagonists

The effect of alpha-2 adrenoceptor antagonists, yohimbine and efaroxan, on the plasma glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment with either yohimbine or efaroxan potentiated glucose-induced insulin release in non-diabetic control rats and produced an improvement of the oral glucose tolerance and potentiated glucose-induced insulin release in type-II but not in type-I diabetic rats. Treatment with either yohimbine or efaroxan reduced the plasma glucose level and increased the plasma insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of efaroxan were more marked. Pretreatment of non-diabetic control and type-II diabetic rats with either yohimbine or efaroxan inhibited clonidine-induced hyperglycaemia and suppressed or reversed clonidine-induced hypoinsulinaemia. Also, pretreatment of these animals with either yohimbine or efaroxan enhanced the hypoglycaemic and insulinotropic effects of glibenclamide. The combination of glibenclamide and efaroxan led to a synergistic increase in insulin secretion, while that of glibenclamide and yohimbine led to an additive increase. The hyperglycaemic effect of diazoxide in non-diabetic control and type-II diabetic rats was inhibited by pretreatment with either yohimbine or efaroxan. The hypoinsulinaemic effect of diazoxide in these animals was antagonized and reversed by pretreatment with yohimbine and efaroxan, respectively. In type-I diabetic rats, there was no change in the plasma glucose and insulin levels induced by the treatment of animals with each of clonidine or diazoxide alone or in combination with either yohimbine or efaroxan. Glibenclamide produced a slight decrease in the plasma glucose level of type-I diabetic rats, at the end of the 120 min period of investigation but there was no change in the plasma insulin level. Pretreatment of these animals with either yohimbine or efaroxan produced no change in glibenclamide effects. Additionally, bath application of efaroxan or glibenclamide inhibited the relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips, while the application of yohimbine produced insignificant changes. The combination of glibenclamide and efaroxan led to complete inhibition of the relaxant effects of different concentrations of diazoxide, while that of glibenclamide and yohimbine did not produce such an effect. It is concluded that yohimbine, via blockade of postsynaptic alpha-2 adrenoceptors, and efaroxan, via blockade of postsynaptic alpha-2 adrenoceptors and adenosine triphosphate-sensitive potassium channels in the pancreatic beta-cell membrane, produce insulinotropic and subsequent hypoglycaemic effects.

Synthesis and characterization of a quinolinonic compound activating ATP-sensitive K(+) channels in endocrine and smooth muscle tissues

Original quinolinone derivatives structurally related to diazoxide were synthesized and their effects on insulin secretion from rat pancreatic islets and the contractile activity of rat aortic rings determined. A concentration-dependent decrease of insulin release was induced by 6-chloro-2-methylquinolin-4(1H)-one (HEI 713). The average IC(50) values were 16.9+/-0.8 microM for HEI 713 and 18.4+/-2.2 microM for diazoxide. HEI 713 increased the rate of (86)Rb outflow from perifused pancreatic islets. This effect persisted in the absence of external Ca(2+) but was inhibited by glibenclamide, a K(ATP) channel blocker. Inside-out patch-clamp experiments revealed that HEI 713 increased K(ATP) channel openings. HEI 713 decreased (45)Ca outflow, insulin output and cytosolic free Ca(2+) concentration in pancreatic islets and islet cells incubated in the presence of 16.7 or 20 mM glucose and extracellular Ca(2+). The drug did not affect the K(+)(50 mM)-induced increase in (45)Ca outflow. In aortic rings, the vasorelaxant effects of HEI 713, less potent than diazoxide, were sensitive to glibenclamide and to the extracellular K(+) concentration. The drug elicited a glibenclamide-sensitive increase in (86)Rb outflow from perifused rat aortic rings. Our data describe an original compound which inhibits insulin release with a similar potency to diazoxide but which has fewer vasorelaxant effects. Our results suggest that, in both aortic rings and islet tissue, the biological effects of HEI 713 mainly result from activation of K(ATP) channels ultimately leading to a decrease in Ca(2+) inflow.

The effect of L-type calcium channel modulators on the mobilization of intracellular calcium stores in guinea-pig intestinal smooth muscle

1. The action of Ca2+ channel modulators has been examined on the intracellular Ca2+ signal in the longitudinal smooth muscle cells of the guinea-pig intestine after exposure to histamine and to agents known to affect intracellular Ca2+ stores. Isometric contraction has been measured simultaneously with front-surface fluorometry of fura 2-loaded preparations. 2. Histamine (10 microM) evoked a phasic and tonic increase in [Ca2+]i and contraction which were both sensitive to the Ca2+ channel blockers, nimodipine and D600. 3. Caffeine (10 mM) evoked in rapid increase in [Ca2+]i which was sustained as long as the preparation was exposed to the drug, whereas the contractile response was only phasic. In the presence of nimodipine 1 microM, the phasic contraction was absent although the fura 2-Ca2+ signal amounted to 32% of the control. 4. Ryanodine (10 microM) evoked a slow increase in [Ca2+]i and a contraction, both of which were reversed after exposure to nimodipine (1 microM) or D600 (10 microM). In the presence of diazoxide (500 microM), a hyperpolarizing agent, the ryanodine-evoked increase in [Ca2+]i and in muscle tone were inhibited. 5. Thapsigargin (1 microM) also produced an increase in [Ca2+]i and a contraction both of which were blocked by nimodipine (1 microM). 6. In Ca2+-free solution, histamine 10 microM evoked non-reproducible phasic Ca2+ signal and contraction. This response was recovered after refilling in Ca2+ containing solution. The recovery was blocked by nimodipine, D600 or diazoxide and was facilitated by the Ca2+ channel activator, Bay K 8644. When the refilling medium was supplemented with thapsigargin, the recovered response was significantly reduced, but Bay K 8644 still had some action. 7. The present results show that blockage of L-type Ca2+ channels inhibited changes in [Ca2+]i evoked by histamine, caffeine and ryanodine which are generally attributed to Ca2+ mobilization from intracellular stores. They also show that when the tissue was exposed to nimodipine, D600 and diazoxide during the procedure of refilling after depletion of intracellular stores, the action of histamine on [Ca2+]i and contraction was blocked. Bay K 8644 had an opposite effect even when the Ca2+ pumping activity of the sarcoplasmic reticulum was reduced by thapsigargin. This indicates that refilling of intracellular Ca2+ stores depleted by histamine in guinea-pig intestine mainly occurred through L-type Ca2+ channels.

Effects of diazoxide on KCl- and norepinephrine-induced contraction in isolated rat aorta

Diazoxide produced endothelium-independent relaxations of the rat aorta contracted by either low concentrations of KCl or norepinephrine (NE). However, contraction with a high concentration of KCl was not altered by diazoxide. Hydrochlorothiazide did not affect the contraction induced by either KCI or NE. Tetraethylammonium and glibenclamide inhibited the relaxations of the aorta induced by diazoxide. Diazoxide significantly reduced the phasic and tonic contractions under Ca2+-free conditions induced by NE and CaCl2 following NE addition, respectively. These results suggest that diazoxide dilates vascular smooth muscle by opening the ATP-sensitive K+ channels and inhibiting receptor-operated Ca2+ movements from either the intracellular store sites or extracellular space to the cytoplasm, but not antagonizing voltage-dependent Ca2+ channels directly.

Effects of glibenclamide on systemic and splanchnic haemodynamics in conscious rats

1. The effects of the sulphonylurea, glibenclamide (20 mg kg-1, i.v.), at a dose that blocks vascular potassium channels, on systemic and splanchnic haemodynamics (radioactive microspheres) were studied in conscious rats. 2. Glibenclamide significantly decreased cardiac index and hepatic artery blood flow while it significantly increased vascular resistance in systemic, portal and hepatic arterial territories. 3. In rats with suppressed cardiovascular reflexes, glibenclamide induced vasoconstriction in systemic, portal and hepatic arterial territories. 4. Intracerebroventricular administration of glibenclamide did not alter systemic or regional vascular tone. 5. Glibenclamide blunted the vasodilator effect of the potassium channel opener, diazoxide but not that of the L-type calcium channel blocker, nicardipine. 6. Another sulphonylurea, glipizide (20 mg kg-1, i.v.), induced significant systemic and splanchnic vasoconstriction. 7. Thus, the glibenclamide-induced blockade of vascular potassium channels caused a vasoconstriction in the systemic and splanchnic vascular beds. In these territories, therefore, the opening of glibenclamide-sensitive potassium channels might be responsible for a basal vasodilator tone.

Effect of diazoxide on serum and tissue electrolyte levels in rats with deoxycorticosterone acetate-induced hypertension

Effects of diazoxide, a benzothiadiazine derivative, on the blood pressure, excretion of Na+, K+, and Ca2+, and the levels of these electrolytes in serum, kidney, and aorta were studied in rats with deoxycorticosterone acetate (DOCA)-induced hypertension in comparison with rats treated with hydrochlorothiazide. After a lateral nephrectomy, 5 mg kg-1 of DOCA was injected subcutaneously three times a week. Drinking water for the rats injected with DOCA contained 1% NaCl. Systolic blood pressure in the DOCA-induced hypertensive rats became about 170 mmHg, with associated elevation of serum level of Na+ 7 weeks after the treatment, whereas that in the normotensive rats was about 100 mmHg. In the DOCA-hypertensive rats, urinary output, and renal excretion of Na+ and Ca2+ increased, and aortic tissue level of Ca2+ decreased as compared with those in the normotensive rats. After 7 weeks of the treatment, diazoxide (10 mg kg-1) or hydrochlorothiazide (10 mg kg-1) was injected intravenously. Diazoxide decreased the blood pressure in both DOCA-hypertensive and normotensive rats, whereas hydrochlorothiazide decreased it only in the DOCA-hypertensive rats. Diazoxide recovered the tissue aortic Ca2+ level that had been decreased by hypertension to that in the normotensive rats. Diazoxide may improve Ca2+ perturbation in the vascular smooth muscle due to DOCA-induced hypertension.