1-trifluoromethoxyphenyl-3(1-propionylpiperidin-4-yl) urea (TPPU), a soluble epoxide hydrolase inhibitor attenuates high fat diet-induced cardiovascular and metabolic disorders in rats.

OBJECTIVE: Obesity was induced in rats by feeding on a high fat diet (HFD), 60% w/w cholesterol, 20% w/w carbohydrates, and 20% w/w proteins for two months. MATERIALS AND METHODS: Animals were fed on a HFD and treated concurrently with a single daily dose of vehicle or TPPU (2 mg/kg p.o) for two months. Body weights, blood pressure, and biochemical investigations of all animals were registered at 0, 1, and 2 months of the experimental period. RESULTS: Vehicle-treated rats fed on a HFD had a considerable increase in body weight compared to age-matched control animals fed on a regular diet (regular diet; 311.40 ±9.60 vs. HFD; 446 ± 12.67). The body weight of rats fed on a HFD and concurrently treated with 1-trifluoromethoxyphenyl-3-(1-propionylpiperidin-4yl) urea (TPPU; 2 mg/kg p.o) daily for two months was significantly decreased (p<0.01). A significant (p<0.01) increase in the systolic blood pressure of animals and vascular dysfunction with blunted relaxant response to acetylcholine and sodium nitroprusside was evident in vehicle-treated animals fed on a HFD compared to control rats fed on a regular diet. These HFD-induced disorders were markedly attenuated in animals fed on a HFD and treated concurrently with a single daily dose of TPPU (2 mg/kg p.o). HFD diet-induced deleterious metabolic changes were prevented with concurrent administration of TPPU (2 mg/kg p.o). TPPU treatment decreased the HDF-induced increase in plasma creatinine levels (p<0.001) in rats. The adiponectin levels were decreased (p<0.001) in vehicle-treated rats fed on HFD for two months compared to control rats fed on a normal diet (p<0.001). Adiponectin levels were significantly (p<0.001) increased in rats fed on HFD and treated concurrently with TPPU (2 mg/kg p.o). HFD diet caused a marked increase in plasma leptin levels of animals which were significantly decreased in animals fed on a HFD and treated concurrently with TPPU for two months. Obese animals exhibited increased levels of plasma insulin compared to control animals fed on a regular diet which were significantly suppressed (p<0.001) by TPPU treatment. In the current investigation, TPPU treatment had a favorable impact on the levels of other metabolic parameters such as plasma cholesterol, triglycerides (TGs), low density lipoproteins (LDLs), and high density lipoproteins (HDL). HFD caused a profound increase in the serum liver enzymes, the effect was reversed by treatment of animals with TPPU (2 mg/kg p.o). CONCLUSIONS: The findings of our current study indicate the promising therapeutic potential of TPPU as a new drug candidate to manage obesity-induced cardiovascular and metabolic disorders. Soluble epoxide hydrolase inhibitors such as TPPU could prevent HFD-induced obesity and related cardiovascular and metabolic complications.

Effect of piperine, a major component of black pepper, on the pharmacokinetics of domperidone in rats.

The present study was aimed to investigate the effect of piperine, a major active ingredient of black pepper, on the pharmacokinetics of domperidone in rats. Animals were given oral (p.o.) or intraperitoneal (i.p.) domperidone (20 mg/kg) alone or together with piperine (20 mg/kg, p.o.). Plasma samples were collected at 0.25, 0.5, 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0 and 12 hours after drug administration. The concentration of domperidone in the plasma was measured using a HPLC method. The concomitant administration of piperine with oral or intraperitoneal domperidone resulted in a significant (P<0.05) increase in the maximum plasma concentration (Cmax), the mean area under the plasma concentration-time curve (AUC), and the elimination half-life (t1/2) of domperidone as compared to those obtained for domperidone alone. These results suggest that an important pharmacokinetic interaction may occur if piperine is administered concurrently with domperidone.

Evidence for the binding of beta-adrenoceptor blockers to microsomal Na+/K+-ATPase in guinea pig heart preparations.

We reported in a previous study that beta-adrenoceptor blockers inhibit the Mg2+-dependent ATP-hydrolytic function of Na+/K+-ATPase. To determine if this action is a result of binding of beta-blockers to the receptor sites that bind the digitalis glycosides, we performed displacement binding assays of eight beta-blockers with [3H]-ouabain (OUA) in guinea pig myocardial microsomal preparations. In the first series of experiments, 10-200 microM of the beta-blockers were displaced with 250 nM OUA. In the second set of experiments, 10-500 nM of OUA was displaced using 200 microM of the beta-blockers. The drugs showed concentration-dependent receptor occupancy at the different OUA levels. Propranolol (PPN), metoprolol (MTP), and sotalol (STL) showed the strongest binding; nadolol (NDL), indenolol (IDN), and atenolol (ATN) had intermediate binding; carazolol (CRZ) and celiprolol (CLP) had the weakest binding properties. The results suggest that beta-blockers may compete for the same binding sites with ouabain in their inhibition of the Na+/K+-ATPase. These actions may contribute to the mechanism for some of their cardiac effects, especially their proarrhythmic and arrhythmogenic actions.

Class I antiarrhythmic drug effects on ouabain binding to guinea pig cardiac Na+ -K+ ATPase.

The notion that the inhibition of the Mg2+ -dependent ATP-hydrolytic function of the myocardial Na+ -K+ ATPase by class I antiarrhythmic agents occurs as a result of their binding to the same receptor sites as the digitalis glycosides was tested by performing competitive binding assays of [3H]ouabain (OUA) with eight drugs: disopyramide, encainide, lidocaine, lorcainide, phenytoin, procainamide, quinidine, and tocainide in guinea pig heart microsomal preparations. In the first set of experiments, 10-200 microM concentrations of the drugs were preincubated with the enzyme and displacement assays performed with 250 nM OUA. The drugs showed receptor occupancy of 19-32% at 50 microM, 25-44% at 100 microM, and 37-56% at 200 microM. Then, 10-500 nM concentrations of OUA were preincubated with the enzyme, and competitive assays were performed using 200 microM concentrations of the drugs. OUA occupied 39-51% of the receptor sites at 100 nM, 44-67% at 250 nM, and 62-82% at 500 nM, displacing the drugs in a concentration-dependent fashion. The results show that antiarrhythmic drugs interact with the same or similar receptor sites as ouabain on the Na+ -K+ ATPase, pointing to a possible contribution of these interactions to the mechanism for their inhibitory actions on the enzyme, and perhaps their arrhythmogenic effects.

Effects of amiodarone on ouabain binding to microsomal Na+, K(+)-ATPase in guinea-pig heart preparations.

1. The influence of amiodarone on [3H]ouabain (OUA) binding to myocardial Na+, K(+)-ATPase was studied at various KCl concentrations in guinea pig heart microsomal preparations to test the hypothesis that the drug acts on the same receptor sites as cardiac glycosides. 2. First, a series of assays for OUA binding to Na+, K(+)-ATPase were performed in the range of 64-800 nM at 2.5, 5.0 and 10.0 mM K+ concentration. The drug exhibited increasing binding tendency with increasing concentrations and elevation in K+ levels. 3. Competitive binding assays were then performed at 256, 512 and 800 nM OUA in the presence of 50, 100 and 200 microM amiodarone at 5.0 mM KCl, respectively. At each OUA concentration, a concentration-dependent left-to-right shift was observed in the binding affinity with increasing amiodarone concentration. similar assays at 2.5 and 10.0 mM K+ showed the same trends. These effects were significant for 200 mM amiodarone at all K+ levels and OUA concentrations. 4. Furthermore, different OUA concentrations were also shown to displace amiodarone in a concentration-dependent fashion. 5. These results indicate that amiodarone competes with OUA for specific binding sites on myocardial microsomal Na+, K(+)-ATPase. They lead to the conclusion that myocardial Na+, K(+)-ATPase is a possible receptor for some of the cardiac actions of amiodarone, such as its proarrhythmic effects.

Sympathetic activity in children undergoing balloon valvuloplasty of pulmonary stenosis.

We studied the influence of balloon valvuloplasty on alpha- and beta-adrenoceptor densities, plasma catecholamine, and cAMP levels in children and infants with pulmonary stenosis before and 10 min after balloon dilatation, employing as controls children undergoing transcatheter occlusion of patent ductus arteriosus (PDA) with Qp/Qs ratio < 1.5. In the PDA group, the alpha-adrenoceptor density (Bmax) was 3.75 +/- 0.72 fmol/10(7) cells (n = 15) before occlusion and remained unchanged at 3.35 +/- 0.47 fmol 10 min thereafter. In the pulmonary stenosis patients (n = 31), the receptor density was 59% higher (p < 0.05) before, and decreased to PDA levels 10 min after, the procedure. The control beta-adrenoceptor density was 64.8 +/- 11.0 fmol/10(6) cells before, and 71.2 +/- 13.2 fmol 10 min after, occlusion. In the study group, the density was 23% lower (p < 0.07) and increased to the PDA levels 10 min after the dilatation. Compared with the PDA, pre- and postdilatation plasma norepinephrine levels were not significantly changed; epinephrine was slightly elevated before, but increased by 73% after, dilatation; dopamine was 80% (p < 0.05); and cAMP was 37% higher before, and remained elevated at 70 and 23% above the PDA values after, the procedure. Accordingly, alpha-adrenoceptor density is significantly elevated in children with pulmonary stenosis and decreases significantly immediately after balloon valvuloplasty. On the other hand, beta-adrenoceptor density is attenuated and increases toward normal levels after the procedure. The immediate reversal of the receptor levels after balloon valvuloplasty suggests that this procedure exerts acute effects on the sympathetic functional level in this disease.

Effect of left ventricular pressure and volume overload on alpha-adrenoceptor activity in patients with rheumatic heart valvular disease.

We have investigated the possibility that the various left ventricular load conditions may exert different effects on the sympathetic function by comparing the influence of volume (VOL) and pressure (POL) overload on platelet alpha-adrenoceptor activity, plasma catecholamines and cAMP in 44 patients with rheumatic heart valvular disease. Receptor activity was determined by radioligand binding methods, catecholamines by HPLC using an electrochemical detector, and cAMP by radioimmunoassay. The mean alpha-adrenoceptor density (Bmax) of the control group (n = 29) was 4.71 +/- 0.41 fmol per 10(7) platelets and the corresponding dissociation constant (Kd) was 2.47 +/- 0.15 nM. In VOL patients, the density was elevated by 70% (P < 0.0001), but it remained unchanged in the POL patients. In contrast to the Bmax, the Kd of the VOL group was not changed, and it increased by 34% (P < 0.01) in the POL group. Norepinephrine was elevated by 91% (P < 0.05) in POL, and epinephrine increased by 65% (P < 0.05) in POL and 71% (P < 0.05) in VOL. These results suggest that the sympathetic nervous system responds to left ventricular volume overload by increasing alpha-adrenoceptor density with no apparent change in receptor affinity toward [3H]-yohimbine binding, and to left ventricular pressure overload by decreasing their binding affinity without a parallel decrease in receptor density. The increase in receptor density in VOL is accompanied by an increase in plasma epinephrine, and the decrease in binding affinity in POL is associated with increased plasma norepinephrine and epinephrine levels.

Regulation of sympathetic activity in children with various congenital heart diseases.

We studied the alpha- and beta-adrenoceptor activity and catecholamine and cAMP levels in 112 children and infants admitted to the hospital for diagnostic or interventional catheterization of tetralogy of Fallot, ventricular septal defects with or without hypertension, pulmonary stenosis, coarctation of the aorta, and various complex cyanotic congenital cardiac diseases and compared them with 14 children undergoing transcatheter occlusion of patent ductus arteriosus with insignificant left-to-right-shunts. The mean total platelet alpha-adrenoceptor density of the study population was elevated by 73%. Both the increases in acyanotic (p < 0.05) and cyanotic (p < 0.005) patients as well as the difference between the two groups (p < 0.01) were significant. Based on the congenital disease classification, the elevation in receptor density was also significant in all groups of patients, except coarctation of the aorta. On the other hand, the mean lymphocyte beta-adrenoceptor density was attenuated by 27%, showing significant difference between the acyanotic and the patent ductus arteriosus groups, but none between acyanotic and cyanotic or cyanotic and the patent ductus arteriosus groups. Among the congenital groups, only the left-to-right shunts and the pulmonary stenosis group showed significant (p < 0.05) decrease in beta-adrenoceptor density, whereas the affinity of all the groups toward [125I]iodocyanopindolol was hardly influenced. The plasma levels of all three catecholamines, norepinephrine, epinephrine, and dopamine, were elevated, but cAMP remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of lymphocyte beta-adrenoceptor activity and Gs-protein in patients with rheumatic heart valvular disease.

In order to test whether the beta-adrenoceptor activity in rheumatic heart valvular disease depends on the ventricular load conditions, we determined their density and binding affinity to [125I]-iodocyanopindolol in lymphocytes, as well as plasma catecholamine and cAMP levels in 69 patients with regurgitant and stenotic lesions of the aortic and mitral valves. The patients were classified as having left ventricular pressure overload (LVP), left ventricular volume overload (LVV), mixed lesions (MOL) or right ventricular pressure overload (RVP). The beta-adrenoceptor activity was determined by radioligand binding methods, catecholamines by high performance liquid chromatography using an electrochemical detector and cAMP by radioimmunoassay. The mean beta-adrenoceptor density (Bmax) of the control group was 60.1 +/- 9.5 fmol (n = 29) per 10(6) lymphocytes. In the study population, the density was decreased by 83% in LVP, 78% in LVV, 87% in MOL and 86% in RVP. Plasma norepinephrine was elevated by 89% in LVP and 60% in MOL, epinephrine by 43% in LVP, 50% in VOL, 115% in MOL and 20% in RVP, while dopamine was not significantly changed, and cAMP was slightly elevated in all four groups. Screening for activating mutational changes in the Gs alpha-protein gave negative results, possibly dissociating the elevation in plasma cAMP from stimulatory effects of such abnormalities in the Gs-protein signaling.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of amiodarone on mitochondrial ATPase and lactate dehydrogenase activities in guinea pig heart preparations.

The effects of amiodarone on mitochondrial ATPase (EC 3.6.1.3) and lactate dehydrogenase (LDH: EC 1.1.1.27) activities were studied in guinea pig mitochondrial preparations in order to test the hypothesis that amiodarone exerts some of its effects as a result of multiple actions on membrane-bound enzymes and receptors. Amiodarone inhibited the ATPase activity in the range of 10 pM to 10 mM (n = 10) with IC50 values of 56.4 +/- 7.2 microM. However, although the inhibitory action was very significant (P < 0.0001, compared to the control) in the concentration range of 100 pM to 10 microM, the differences in individual enzyme responses showed very weak correlation with drug concentration. In this region, the inhibitory effects were almost constant at approximately 37%. Below 100 pM and above this range however, the concentration-response relationships were steep, reaching total inhibition at approximately 2.5 mM. Amiodarone also exerted concentration-dependent inhibitory effects on lactate dehydrogenase activity. However, over the effective inhibitory concentration range (5-95%) of 7.5 microM to 2.5 mM (n = 8) and IC50 value of 108 +/- 6 microM, its inhibitory potency was twofold weaker than that of its ATPase inhibition. We propose that these actions contribute, at least in part, to the mechanism(s) of some of the pharmacological actions of amiodarone.

Synthesis and pharmacological evaluation of the antifibrillatory effect of fluorinated derivatives of carazolol and celiprolol: comparison with propranolol.

1. The effects of carazolol, celiprolol and their respective fluorinated derivatives (FD) on electrical stimulation threshold (EST) and ventricular fibrillation threshold (VFT) were compared with those of propranolol and propranolol-FD in Langendorff-perfused rabbit hearts. 2. Carazolol, carazolol-FD, propranolol and propranolol-FD produced significant dose-dependent elevation of both EST and VFT at all tested concentration levels. In contrast, celiprolol and celiprolol-FD did not produce a significant change in either threshold. 3. On a dosage basis, the order of antifibrillatory potency of these compounds is: carazolol-FD > propranolol-FD > carazolol > propranolol > celiprolol-FD > celiprolol. 4. The results of this study seem to indicate the importance of membrane stabilizing effect for a potent antifibrillatory action of beta-adrenoceptor blocking agents. Furthermore, fluorination is demonstrated to produce more potent antifibrillatory activity than that of the parent drugs.

Investigation of class I anti-arrhythmic drug actions on guinea-pig cardiac mitochondrial lactate dehydrogenase activity.

1. The effects of the Class I anti-arrhythmic drugs quinidine, procainamide, lidocaine, phenytoin and tocainide on mitochondrial lactate dehydrogenase activity were compared in guinea-pig heart preparations. 2. All the tested drugs inhibited the enzyme activity in a concentration-dependent fashion, exhibiting varying profiles in their actions. Lidocaine exhibited inhibitory concentration 20% (IC20) and IC50 values of 0.52 +/- 0.02 mmol/L and 25.6 +/- 0.5 mmol/L, procainamide 6.0 +/- 0.2 mmol/L and 108 +/- 7.2 mmol/L, phenytoin 3.4 +/- 0.06 mumol/L and 0.34 +/- 0.02 mmol/L, quinidine 39.2 +/- 1.2 mumol/L and 9.8 +/- 0.8 mmol/L and tocainide 2.7 +/- 0.3 mmol/L and 44.6 +/- 2.5 mmol/L. 3. According to the IC50 values, this is the order of their inhibitory potencies: phenytoin > quinidine > lidocaine > tocainide > procainamide. This trend is in general agreement with the lipophilicity rank of the drugs. 4. It is concluded, therefore, that inhibition of mitochondrial lactate dehydrogenase is a property shared by most Class I anti-arrhythmic drugs which may depend on their lipophilicity and possibly their membrane stabilizing effects.

Effects of class I antiarrhythmic drugs on mitochondrial ATPase activity in guinea pig heart preparations.

1. The effects of three class I antiarrhythmic drugs quinidine, lidocaine and lorcainide on undamaged myocardial mitochondrial ATPase [ATP: phosphohydrolase, EC 3.6.1.3] activity were evaluated in guinea pig heart preparations. 2. All three drugs inhibited the enzyme activity in a concentration-dependent fashion. 3. Lorcainide was the most potent, exerting inhibitory effects in the range of less than 1.0 nM-2.0 mM, with IC20 and IC50 values of 9.4 +/- 0.6 nM and 87.2 +/- 5.5 microM. However, in the range of approx. 10 nM-10 microM, the enzyme response decreased only slightly with increasing lorcainide concentrations. 4. Quinidine and lidocaine, on the other hand, inhibited the enzyme activity in the range of 1.0 microM-100 mM. 5. The IC20 and IC50 values for quinidine were 0.92 +/- 0.04 mM and 4.8 +/- 0.6 mM and for lidocaine were 115 +/- 6 microM and 2.3 +/- 0.3 mM. 6. The results show that all three drugs inhibit mitochondrial ATPase activity and that lorcainide is the most potent. 7. These inhibitory effects may be related to the lipophilicity and membrane stabilizing activity of this class of antiarrhythmic drugs.

Inhibition of myocardial Na(+)-K(+)-ATPase activity by bretylium: role of potassium.

The effects of bretylium on the Mg(2+)-dependent ATP-hydrolytic action of myocardial Na(+)-K(+)-ATPase (EC 3.6.1.3) were studied in guinea-pig heart preparations in media containing various K+ concentrations. Under "standard" conditions (5 mM K+), bretylium inhibited the Na(+)-K(+)-ATPase activity in a concentration-dependent fashion in the range of 0.001-80 mM, with an IC20 value of 0.19 +/- 0.05 mM and an IC50 value of 1.38 +/- 0.11 mM. Reducing the K+ concentration from 5 to 2.5 mM enhanced the inhibitory action, shifting the effective (5-95% inhibition) concentration range to 0.0003-9.6 mM, with IC20 and IC50 values of 0.036 +/- 0.004 mM and 0.92 +/- 0.30 mM, respectively. On the other hand, increasing the K+ concentration to 10 mM shifted the range to 10-90 mM. The corresponding IC20 value was 0.48 +/- 0.02 mM and the IC50 was 2.24 +/- 0.36 mM. The results show that K+ plays an important role in the inhibition of myocardial Na(+)-K(+)-ATPase activity by bretylium. This is probably pertinent to the mode by which the drug may interfere with the electrogenic Na+/K+ pump activity of the enzyme, and, consequently, cause or aggravate cardiac arrhythmias.

Effects of beta-adrenoceptor blockers on mitochondrial ATPase activity in guinea pig heart preparations.

The effects of three beta-adrenoceptor blockers atenolol, indenolol and nadolol on myocardial mitochondrial ATPase (ATP: phosphohydrolase EC 3.6.1.3) activity were evaluated and compared with that of propranolol in guinea pig heart preparations. Propranolol and indenolol inhibited ATPase activity with IC50 values of 4.4 +/- 0.5 and 5.3 +/- 0.4 mM, respectively. In contrast, however, nadolol and atenolol markedly enhanced mitochondrial ATPase activity. Atenolol increased the enzyme activity by approximately 5, 240 and 950%, while nadolol enhanced it by 13, 280 and 2800% at 100 microM, 1.0 mM and 10.0 mM, respectively. The results indicate that these drugs exhibit two modes of interaction with the mitochondrial ATPase: inhibition by propranolol and indenolol and stimulation by atenolol and nadolol. The inhibitory actions are probably related to the membrane-stabilizing effects and therefore antiarrhythmic actions of the two drugs, while the stimulatory effects of atenolol and nadolol are probably a result of interactions with some component of oxidative phosphorylation or the respiratory chain.

Variations in potassium concentration modify the inhibitory effect of lorcainide on myocardial Na(+)-K(+)-ATPase activity.

1. The inhibitory action of lorcainide on the myocardial Na(+)-K(+)-ATPase (EC 3.6.1.3) activity was studied in guinea-pig heart preparations at medium K+ concentrations of 2.5, 5.0 and 10 mM. 2. Lorcainide exhibited characteristically similar concentration-dependent inhibitory effects at all three K+ concentrations tested. However, the inhibitory potencies were significantly increased at decreased K+ concentrations. 3. The IC50 values were 10.4 +/- 3.2 microM at 2.5 mM, 28.3 +/- 7.9 microM at 5.0 mM and 40.7 +/- 9.2 microM at 10.0 mM K+ respectively. Thus, reduction in the K+ concentration from the 'standard' 5.0 to 2.5 mM enhanced the inhibitory potency of lorcainide, the effective concentrations being shifted towards much lower ranges, while increasing it to 10 mM on the other hand produced opposite but less marked effects. 4. These results show that the inhibition of myocardial Na(+)-K(+)-ATPase activity by lorcainide depends on the K+ concentration of the incubation medium. These effects are probably related to the mechanism by which lorcainide interferes with the electrogenic Na+/K+ pump activity of the enzyme, and hence may contribute to some of its cardiac actions.

Interaction between potassium concentration and inhibition of myocardial Na(+)-K(+)-ATPase by two class 1A antiarrhythmic drugs: quinidine and procainamide.

The inhibitory actions of quinidine and procainamide on the Mg(2+)-dependent ATP hydrolytic action of myocardial Na(+)-K(+)-ATPase (EC 3.6.1.3) were studied in guinea-pig heart preparations incubated in media containing 2.5, 5 and 10 mM of K+. The IC50 values for quinidine were 0.23 +/- 0.02 mM at 2.5 mM K+, 0.56 +/- 0.18 mM at 5 mM K+ and 0.82 +/- 0.05 mM at 10 mM K+, and those of procainamide were 7.2 +/- 1.8 mM at 2.5 mM K+, 13.7 +/- 2.3 mM at 5 mM K+ and 35.5 +/- 3.3 mM at 10 mM K+. Thus, reducing the K+ concentration from the "standard" 5 mM to 2.5 mM showed a significant right-to-left shift in the inhibitory potencies of the drugs, while increasing it to 10 mM resulted in opposite effects. The results show that the inhibitory actions of both drugs on the ATP-hydrolytic action of myocardial Na(+)-K(+)-ATPase depend on the K+ concentration of the incubation medium. These effects seem to be related to the mode by which antiarrhythmic drugs may induce or aggravate cardiac arrhythmias. In addition, the present results suggest that hypokalemia may exacerbate arrhythmias during treatment with these drugs.

The influence of potassium concentration on the inhibitory effect of amiodarone on guinea-pig microsomal Na(+)-K(+)-ATPase activity.

The effects of varying incubation K+ concentration on the inhibitory action of amiodarone on the Mg(2+)-dependent ATP hydrolysis by myocardial Na(+)-K(+)-ATPase (EC 3.6.1.3) were studied in guinea pig heart preparations. In the first part of the study, it was established that the activity of the enzyme increased with growing concentrations up to approximately 20 mM K+. The concentration-response relationships for amiodarone were investigated in incubation media containing 2.5, 5.0 and 10 mM K+ respectively. Amiodarone exhibited similar concentration-dependent inhibitory effects in the range of 0.01 nM-80 microM at 2.5 mM, 0.13-150 microM at 5.0 mM and 0.3-700 microM at 10.0 mM K+. The corresponding IC50 values were 10.4 +/- 3.2 microM, 28.3 +/- 7.6 microM at 5.0 mM and 33.3 +/- 9.2 microM at 10.0 mM K+, respectively. Thus, reduction in the K+ concentration from the "standard" 5.0 to 2.5 mM was accompanied by a significant right-to-left shift in the inhibitory potency of amiodarone, the effective concentrations being shifted from microM into nM ranges. Increasing K+ concentration to 10 mM on the other hand attained opposite but less remarkable effects. The results show that the in vitro inhibition of myocardial Na(+)-K(+)-ATPase activity by amiodarone is related to the K+ concentration of the incubation medium. These effects may be pertinent to the mechanism by which this drug interferes with the electrogenic Na+/K+ pump activity of the enzyme, thereby probably contributing to the mechanism(s) responsible for some of its cardiac actions.

Effects of beta-adrenergic blockers on Na(+)-K(+)-ATPase activity in guinea-pig heart preparations: comparison with ouabain.

The effects of the beta-adrenergic receptor antagonists carazolol, celiprolol, indenolol and propranolol on myocardial Mg(2+)-dependent, Na(+)- and K(+)-activated adenosine triphosphatase (Na(+)-K(+)-ATPase, EC 3.6.1.3) were compared with those of ouabain in guinea-pig heart preparations. All four drugs inhibited the enzyme activity in a concentration-dependent fashion similar to that shown by ouabain. Celiprolol exerted inhibitory actions (5-95%) in the concentration range of 0.5-3000 microM, while propranolol showed similar activity in the range of 100-4000 microM. Carazolol and indenolol exhibited intermediary effects. Their inhibitory effective ranges were 95-4000 and 10-5000 microM, respectively. The concentrations for half maximal inhibition of the enzyme activity were 0.74 +/- 0.12 mM for carazolol, 0.62 +/- 0.09 mM for celiprolol, 0.80 +/- 0.11 mM for indenolol and 1.03 +/- 0.19 mM for propranolol. Accordingly, the rank order of their potencies exhibited the following trend: celiprolol greater than carazolol greater than indenolol greater than propranolol. The present results show that these drugs exhibit inhibitory actions on the Na(+)-K(+)-ATPase activity, which may be suggested as a possible biochemical basis for their proarrhythmic effects.

Influence of lorcainide on microsomal Na+, K(+)-ATPase in guinea-pig isolated heart preparations.

1. The effects of lorcainide on the myocardial Mg2(+)-dependent, Na+ and K(+)-activated adenosine triphosphatase (Na+, K(+)-ATPase) were compared in guinea-pig heart preparations with those of ouabain, a specific inhibitor of the enzyme activity. 2. Both ouabain and lorcainide inhibited the microsomal Na+, K(+)-ATPase activity in a concentration-dependent fashion. Their inhibitory effective ranges were 0.05-100 microM and 0.15-125 microM, respectively, and the concentrations for half maximal inhibition (IC50 values) were 2.1 +/- 0.3 and 33.5 +/- 7.3 microM, respectively. 3. In a second series of experiments, the combined effects of the two drugs on the enzyme activity were studied. In these experiments, lorcainide produced a concentration-dependent potentiation of the inhibitory effects of ouabain on Na+, K(+)-ATPase activity. 4. The present study demonstrates that lorcainide is a potent inhibitor of myocardial Na+, K(+)-ATPase.