The nitric oxide donors, SNAP and DEA/NO, exert a negative inotropic effect in rat cardiomyocytes which is independent of cyclic GMP elevation

The role of guanosine 3',5'-cyclic monophosphate (cGMP) in the regulation of cardiac contractility remains controversial. The present study has examined the effects of high concentrations of the nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine (SNAP) and 1,1-diethyl-2-hydroxy-2-nitroso-hydrazine (DEA/NO), on cGMP levels and isoproterenol-induced increases in contractility in rat cardiomyocytes before and after selective inhibition of soluble guanylyl cyclase with 1 H -[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). In control myocytes, 100 microm SNAP or 100 microm DEA/NO increased cGMP levels by more than 15-fold at 2 and 6 min and produced marked attenuations of isoproterenol-mediated increases in maximal cell shortening over the same time period. The NO donors had no significant effect on basal cell shortening (in the absence of isoproterenol). Pretreatment of myocytes with 25 microm ODQ for 30 min resulted in a complete blockade of the SNAP- or DEA/NO-induced increases in cGMP with no reversal of negative inotropy. ODQ did not affect basal contractility, basal cGMP levels or isoproterenol-induced increases in cell shortening. Furthermore, myocytes exposed to the cGMP analog, 8-bromo-cGMP (100 microm), did not exhibit significant differences in basal contractility or isoproterenol-induced increases in cell shortening. These results suggest that attenuation of cardiac contractility by NO donors in rat cardiomyocytes occurs by a mechanism independent of increases in cGMP levels.

Effect of itraconazole on the pharmacokinetics of digoxin in guinea pigs

The effect of itraconazole (ITZ) on the pharmacokinetics of digoxin (DGX) was investigated in guinea pigs. The plasma concentrations of DGX in guinea pigs following treatment with ITZ (20 mg/kg intraperitoneally (i.p.)) after intravenous (i.v.) administration of DGX (0.125 mg/kg) were significantly higher than in the controls. The percentage cumulative excretion (6.9%) of DGX in bile of the ITZ-treated group up to 6 h after administration of DGX was significantly reduced (p < 0.05) compared with 10.5% in the control. The percentage cumulative excretion (3.1%) of DGX in urine of the ITZ-treated group up to 6 h after administration was also one third that in the controls. The total, biliary, renal and metabolic clearances for DGX in the ITZ-treated group were significantly reduced, while the V(dss) was unaffected. The plasma unbound fraction of DGX (ranged 47-58%) in the ITZ-treated group was generally similar to that in the controls (50-57%). The blood-to-plasma concentration ratio of DGX (range 1.28-1.42) in the absence of ITZ did not change in the presence of ITZ. Based on these results, the pharmacokinetic interaction between DGX and ITZ may be due not only to a reduction in the renal clearance but also to a reduction in the metabolic clearance of DGX by ITZ.

Characterisation of the binding of digitoxin and acetyldigitoxin to human serum albumin by high-performance affinity chromatography

Zonal elution and high-performance affinity chromatography were used to examine interactions of the drugs digitoxin and acetyldigitoxin with the protein human serum albumin (HSA). This was done by injecting small amounts of digitoxin and acetyldigitoxin onto an immobilized HSA column in the presence of mobile phases that contained various concentrations of digitoxin, acetyldigitoxin or other solutes as competing agents. A fixed concentration of beta-cyclodextrin was also present in the mobile phase as a solubilising agent. It was found that digitoxin and acetyldigitoxin each had strong interactions at a single common binding site on HSA, but with slightly different equilibrium constants for this region. Neither compound showed any competition with warfarin or L-tryptophan, which were used as probes for binding at the warfarin-azapropazone and indole-benzodiazepine sites of HSA. These results confirmed the presence of a separate binding region on HSA for digitoxin-related compounds.

[Prolonged half-life of digitoxin in the elderly]

Digitoxin is frequently used in Norway in the treatment of cardiac failure. Digitalis glycosides may give rise to a number of side effects difficult to separate from disease in the elderly. Six patients aged 77-93 years, treated with digitoxin 0.05 mg/day, were hospitalized due to digitalis intoxication. Mean digitoxin half-life was 25.2 days. This is significantly more than reported in other studies on younger patients. The symptoms of digitoxin intoxication disappeared on discontinuation of medication. The slow elimination of digitoxin may be related to reduced serum albumin concentration. When digitoxin is used in the treatment of heart failure in the very elderly patients, one should be aware of the possibility of digitoxin intoxication, even at a low dose.

Clozapine-induced dopamine levels in the rat striatum and nucleus accumbens are not affected by muscarinic antagonism

The effect of the muscarinic antagonist, scopolamine, was examined for a change in the increase in extracellular dopamine, dihydroxyphenyl acetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA), induced by haloperidol or clozapine in the striatum and nucleus accumbens of anaesthetised and awake rats, monitored using in vivo cerebral microdialysis. Rats received scopolamine (1 mg kg(-1); s.c.) or vehicle followed by haloperidol (1 mg kg(-1); s.c.) or clozapine (20 mg kg(-1); s.c.). Dopamine, DOPAC, HVA and 5-HIAA overflow into striatal or accumbens perfusates was determined using high performance liquid chromatography with electrochemical detection (HPLC-ECD). Scopolamine failed to modify the clozapine- or haloperidol-induced efflux of dopamine or its metabolites in either the striatum or nucleus accumbens following systemic administration in anaesthetised or awake rats. Although pretreatment with scopolamine tended to produce a smaller increase in the clozapine-induced efflux of DOPAC in striatal perfusates than following clozapine treatment alone, this was not statistically significant. Furthermore, local infusion of scopolamine (100 microM) with clozapine (1 mM) via the microdialysis probe did not attenuate the elevated efflux of dopamine observed following clozapine alone, in either the striatum or nucleus accumbens, in anaesthetised rats. This treatment did prevent the clozapine-induced increase in DOPAC and HVA in the striatum but not the nucleus accumbens. Carbachol (50 microM) infused into the dorsolateral striatum or nucleus accumbens raised extracellular dopamine levels 200% and 150%, respectively above baseline. Our data suggest that the increased efflux of dopamine and its metabolites in the rat basal ganglia following clozapine administration is not significantly dependent upon the interaction of clozapine with muscarinic receptors.

Effects of nicotine on hydroxyl free radical formation in vitro and on MPTP-induced neurotoxicity in vivo

Parkinson's disease (PD) is one of the most frequent disorders of the basal ganglia. From epidemiological studies there is a controversial discussion on the question whether tobacco smoking is correlated with a decreased incidence of PD. The present study aimed to elucidate the role of nicotine and its potential neuroprotective effects in a rodent model of PD. These effects may be related to an altered hydroxyl radical formation; this possibility was studied in vitro. Nicotine and alpha-phenyl-N-tert-butyl nitrone (PBN) were examined in a cell-free in vitro Fenton system (Fe3+/EDTA + H2O2) for their radical scavenging properties using the salicylate trapping method. Salicylic acid (0.5 mM) was incubated in the presence and absence of nicotine or PBN and the main products of the reaction of hydroxyl radicals with salicylic acid, namely 2,3- and 2,5-dihydroxybenzoic acid, were immediately determined using HPLC in combination with electrochemical detection. Nicotine and PBN were both able to significantly reduce hydroxyl radical levels at concentrations of 1, 2.5 and 5 mM. Interestingly, at 5 mM nicotine was able to reduce hydroxyl radical levels significantly more than the radical scavenger PBN (5 mM). To investigate the in vivo effects of nicotine, male C57BL/6 mice were used in the MPTP mouse model of PD. Nicotine (0.1 or 0.4 mg/kg s.c.) was administered twice daily for a period of 14 days. On day 8 a single injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 30 mg/kg s.c.) was given as well as an enhanced protocol of nicotine treatment (0.1 or 0.4 mg/kg s.c., 30 min before MPTP and 30, 90, 210, 330, 450, 570 min after MPTP) for a total of seven injections of nicotine. High dosage nicotine treatment significantly increased the MPTP-induced loss of body weight and resulted in a significantly decreased striatal dopamine content and an increased dopamine turnover in comparison with the MPTP-treated controls at day 15. However, the lower dosage of nicotine did not significantly alleviate the MPTP-induced effects, although some parameters showed a slight tendency in this direction. These results demonstrate that in vitro nicotine has radical scavenging properties which might suggest neuroprotective effects. In vivo experiments with nicotine, however, showed that a low dosage of nicotine did not alleviate the MPTP-induced dopamine depletion, but a large dosage even enhanced it.

N6-2-(4-aminophenyl)ethyladenosine (APNEA), a putative adenosine A3 receptor agonist, enhances methamphetamine-induced dopamine outflow in rat striatum

In the present study, the effect of adenosine A3 receptor agonist, N6-2-(4-aminophenyl)ethyladenosine (APNEA), on methamphetamine (MTH)-induced dopamine (DA) release in rat striatum was evaluated using microdialysis in freely moving animals. MTH at a dose of 5 mg/kg injected 3-times every two hours produced massive overflow of DA and decline in the level of DOPAC (3,4-dihydroxyphenylacetic acid) and HVA (homovanillic acid). APNEA perfused locally to the striatum via microdialysis probe triggered opposite effects, at 75 microM it diminished MTH-induced DA overflow during first 2 h of the experiment (p<0.05), but potently enhanced it at higher 100 microM concentration for entire period of treatment (p<0.001). Concomitant release of glutamate in striatum was slightly decreased by MTH alone, and significantly diminished by coadministration of 100 M APNEA (p<0.001). The data indicate that activation of adenosine A3 receptor exerts rather toxic effect on DA neurons and exacerbates neurotoxicity of MTH. In addition, MTH-induced DA overflow does not seem to result from the increased release of striatal glutamate level.

PST2238: a new antihypertensive compound that antagonizes the long-term pressor effect of ouabain

The inhibition of the long-term pressor effect of ouabain may be useful for the therapy of essential hypertension. Here, for the first time, a selective inhibitor of the ouabain pressor effect is described. In vitro, 17beta-(3-furyl)-5beta-androstane-3beta, 14beta, 17alpha-triol (PST 2238) displaced ouabain from its binding sites on purified sodium, potassium ATPase enzyme (Na-K ATPase) (IC50 1.7 x 10(-6) M) without interacting with other receptors involved in blood pressure regulation or hormonal control. In cultured renal cells, incubation with ouabain (10(-10) to 10(-8) M) for 5 days stimulated the Na-K pump at Vmax, whereas PST 2238 showed the same effect at micromolar concentration. The ouabain-dependent increase in the Na-K pump rate was abolished by PST 2238 at concentrations from 10(-14) to 10(-9) M. In rats made hypertensive by chronic infusion of 50 microg/kg/day of ouabain, PST 2238 given p.o at very low doses (0.1-1 microg/kg/day for 4 weeks) abolished the increase in blood pressure and renal Na-K ATPase activity caused by ouabain. PST 2238 did not affect either blood pressure or renal Na-K ATPase activity in normotensive rats. In conclusion, PST 2238 is a very potent compound that normalizes both blood pressure and alterations in the Na-K pump caused by ouabain. Thus it represents the prototype of a new class of antihypertensive drugs that could be effective in forms of hypertension sustained by the concomitant increase of endogenous ouabain levels and alterations in the Na-K pump.

Bovine adrenals and hypothalamus are a major source of proscillaridin A- and ouabain-immunoreactivities

Besides an isomer of the cardenolide ouabain, a material with a similar HPLC retention time as ouabain but cross-reactivating with antibodies against the bufadienolide proscillaridin A and inhibiting the sodium pump is known to circulate in human blood plasma (B. SICH et al., Hypertension 27, 1073-1078 (1996).). The concentrations of both substances are known to correlate with the blood pressure. It was the intention of this work to localize tissues that contain the highest concentrations of the proscillaridin A immunoreactive material, to correlate its concentration with that of ouabain and to get information whether the concentration of this material simply reflects the number of sodium pumps of the tissue extracted. Specific antibodies for each cardiotonic steroid were used to test the tissue concentration. This report shows that in bovine tissues the distribution pattern of proscillaridin A and ouabain immunoreactivities are similar and that hypothalamus and adrenals show the highest concentrations. The cross-reactive material did not reflect the number of sodium pumps per g of wet weight tissue as measured by [3H]ouabain binding. Therefore, it is unlikely that the tissue concentrations in both immunoreactivities reflects the tissue capacity of sodium pumps labeled with cardiotonic steroids via the blood plasma. The study rather favors the concept that two different types of inhibitors of the sodium pump exist within both tissues.

Investigation of the potential interaction between terfenadine and tedisamil in human liver microsomes

1. The potential drug-drug interaction of terfenadine and tedisamil has been investigated. Terfenadine is a widely used antihistamine drug with the potential for QTC prolongation. Tedisamil is a potassium channel blocking agent known to produce bradycardia and prolong the effective refractory period in man. 2. Tedisamil and terfenadine were incubated with human liver microsomes for 30 min at 37 degrees C. No significant inhibition of terfenadine biotransformation was seen with 0.1 or 10 microM tedisamil as the formation of the terfenadine alcohol and acid metabolites were unaffected. 3. Based on the in vitro results it is suggested that tedisamil will not interact pharmacokinetically with terfenadine as it does not impair metabolism of terfenadine.

The stereoisomers quinine and quinidine exhibit a marked stereoselectivity in the inhibition of hepatobiliary transport of cardiac glycosides

BACKGROUND/AIMS

Certain basic (cationic) drugs are known to interact with the hepatic transport, and renal and/or biliary clearance of cardiac glycosides. The mechanisms behind these interactions are not fully understood. In the present study our aim was to investigate the effects of the two diastereomers, quinidine and quinine, as well as the calcium antagonist verapamil, on the hepatobiliary elimination of digoxin and ouabain in the isolated perfused rat liver.

METHODS

Livers from male, fasting Wistar rats were perfused by recirculation of Krebs-Henseleit bicarbonate buffer supplemented with 1% BSA. Disposition of digoxin or ouabain was studied at an initial perfusion medium concentration (Ci) of 100 or 10 nmol/l for digoxin and a Ci of 30 micromol/l for ouabain. The Ci of quinine, quinidine or verapamil was 50 micromol/l. Concentrations of the drugs in perfusion medium and bile were followed up to 2 h.

RESULTS

A marked reduction in the initial medium disappearance rate of digoxin and ouabain by quinine was found, whereas quinidine did not affect the hepatic disposition of the cardiac glycosides. The stereoselective inhibition of digoxin and ouabain clearance by quinine, and not by quinidine, was shown to be due to an effect on the hepatic uptake level rather than on the metabolic conversion and/or the biliary excretion steps. An allosteric type of inhibition by the basic drugs, exerted from the inside of the cells, is inferred. This interaction may occur at the sinusoidal plasma membrane on the level of multi-specific carrier proteins for cardiac glycosides and cationic drugs, as cloned recently by various groups.

CONCLUSIONS

A marked stereoselective difference was found in the effect of the stereoisomers quinidine and quinine on the hepatic uptake of digoxin and ouabain, quinine being the potent inhibitor.

Biotransformation of the novel inotropic agent toborinone (OPC-18790) in rats and dogs. Evidence for the formation of novel glutathione and two cysteine conjugates

The metabolism of toborinone, (+/-)-6-[3-(3,4-dimethoxybenzylamino)-2-hydroxypropoxy]-2(1H)-quin - olinone, a novel inotropic agent, was studied in rats and dogs after intravenous administration. Chemical structures of the 13 metabolites were characterized by direct-probe FAB/MS and field desorption/MS, LC/FAB/MS, and various NMR measurements. After intravenous dosing of 10 mg/kg [14C]toborinone, fecal and urinary recoveries of the 14C dose were approximately 70% and 26-30%, respectively, in both rats and dogs. The predominant component of radioactivity was the unchanged toborinone in every biological specimen in rats and dogs. Although unchanged toborinone was predominantly observed, toborinone underwent extensive conjugations with glucuronic acid, sulfate, and glutathione, either directly or following phase I reaction. Metabolites resulting from oxidative N-C cleavage were minor both in number and in quantity in every biological specimen in rats and dogs. In rats, toborinone underwent O-demethylation to form M-7 and successive phase it reaction to yield the glucuronide M-1 and the sulfoconjugate M-2, and deconjugation to yield M-7, which was a primary metabolite accounted for 35.67% of the radioactivity excreted in the feces by 48 hr. Conjugates M-1 and M-2 were the major metabolites in rat plasma. In dogs, toborinone was metabolized via mercapturic acid pathway to yield the primary metabolites, cysteine conjugates M-10 and M-11 that accounted for 19.10% and 6.70% of the radioactivity excreted in the feces by 48 hr and that were detected species specifically in dogs. The glutathione conjugate M-13, which was isolated from in vitro incubations using dog liver, led us to consider a possible mercapturic acid pathway from the parent compound to M-10. Metabolites in dog plasma and those in urine in both rats and dogs were minor in quantity. The metabolic pathways of toborinone in rats and dogs are proposed herein.

Transport and epithelial secretion of the cardiac glycoside, digoxin, by human intestinal epithelial (Caco-2) cells

1. Human intestinal epithelial Caco-2 cells have been used to investigate the transepithelial permeation of the cardiac glycoside, digoxin. 2. Transepithelial basal to apical [3H]-digoxin flux exceeds apical to basal flux, a net secretion of [3H]-digoxin being observed. At 200 microM digoxin, net secretory flux (Jnet) was 10.8 +/- 0.6 nmol cm-2 h-1. Maximal secretory flux (Jmax) of vinblastine was 1.3 +/- 0.1 nmol cm-2 h-1. Cellular uptake of digoxin was different across apical and basal cell boundaries. It was greatest across the basal surface at 1 microM, whereas at 200 microM, apical uptake exceeded basal uptake. 3. Net secretion of [3H]-digoxin was subject to inhibition by digitoxin and bufalin but was not inhibited by ouabain, convallatoxin, and strophanthidin (all 100 microM). Inhibition was due to both a decrease in Jb-a and an increase in Ja-b. Uptake of [3H]-digoxin at the apical surface was increased by digitoxin and bufalin. All cardiac glycosides decreased [3H]-digoxin uptake at the basal cell surface (except for 100 microM digitoxin). 4. The competitive P-glycoprotein inhibitors, verapamil (100 microM), nifedipine (50 microM) and vinblastine (50 microM) all abolished net secretion of [3H]-digoxin due to both a decrease in Jb-a and an increase in Ja-b. Cellular accumulation of [3H]-digoxin was also increased across both the apical and basal cell surfaces. I-Chloro-2,4,-dinitrobenzene (10 microM), a substrate for glutathione-S-transferase and subsequent ATP-dependent glutathione-S-conjugate secretion, failed to inhibit net secretion of [3H]-digoxin. The increase in absorptive permeability Pa-b (= Ja-b/Ca) and cellular [3H]-digoxin uptake upon P-glycoprotein inhibition, showed that the intestinal epithelium was rendered effectively impermeable by ATP-dependent extrusion at the apical surface. 5. A model for [3H]-digoxin secretion by the intestinal epithelium is likely to involve both diffusional uptake and Na(+)-K+ pump-mediated endocytosis, followed by active extrusion at the apical membrane.

Interaction of cardiotonic thiadiazinone derivatives with cardiac troponin C

The cardiotonic effects of thiadiazinone derivative EMD 57033 are mediated by direct actions on myofilaments (Lues, I., Beier, N., Jonas, R., Klockow, M., and Haeusler, G. J. (1993) Cardiovasc. Pharmacol. 21, 883-892). Cardiac troponin C has been postulated to be a potential target of the drug (White, J., Lee, J. A., Shah, N., and Orchard, C. H. (1993) Circ. Res. 73, 61-70). This study tested whether EMD 57033 interacts directly with recombinant human cardiac TnC (hcTnC). EMD 57033 caused concentration-dependent quenching of tyrosine (Tyr) fluorescence of hcTnC in the presence of Ca2+ (100 microM) and little change of the fluorescence in the presence of Mg2+ (2 mM). Kd for the drug-hcTnC interaction in the presence of Ca2+, determined by Tyr fluorescence titrations, was approximately 40 microM. The binding of EMD 57033 was stereo-selective: the optical isomer of EMD 57033 bound hcTnC much more weakly. The Ca2+ dependence and stereo-selectivity of EMD 57033 binding were substantiated by a dialysis-based direct binding assay. EMD 57033 was found to interfere with Ca(2+)-dependent binding of hydrophobic probe 1,1'-bi-(4-anili-no)naphthalene-5,5'-disulfonate (bis-ANS) to hcTnC. The relationships between [Ca2+] and Tyr fluorescence of hcTnC and between [Ca2+] and bis-ANS fluorescence in the presence of hcTnC were substantially altered by EMD 57033 in the range of [Ca2+] where Ca2+/Mg2+ sites of hcTnC were titrated by Ca2+. EMD 57033 was found to bind as tightly to 2 Ca2+.hcTnC as to 3 Ca(2+).hcTnC. These observations were interpreted as indicating that a EMD 57033-binding site is induced by Ca2+ binding, but not Mg2+ binding, to the Ca2+/Mg2+ sites of hcTnC. The drug-binding site most likely resides in the carboxyl domain of hcTnC.

Metabolism and disposition of kyushin, a drug containing senso (ch'an su)

The method of enzyme immunoassay (EIA) was developed for measurement of anti-resibufogenin IgG reactive substance (RRS) in plasma and urine of healthy volunteers receiving a Senso containing drug, Kyushin (KY). Unchanged bufalin (BF), cinobufagin (CB), and resibufogenin (RB) and their metabolites in plasma and urine were also measured by EIA after the separation of these compounds by high performance liquid chromatography. In a single dosing study, the concentration of BF, CB and RB in plasma reached maximum at 1-2 hr after drug administration, and the half-lives (T1/2 of these were 15.8, 8.94 and 11.5 hr, respectively. In a multiple dosing study, the concentration of BF, CB and RB measured fitted on the estimated curve calculated from data of the single dosing study. These results suggest that BF, CB and RB, important components of Senso, do not have a cumulative nature.

Assessment of the immunoreactivity of digoxin metabolites and the cross-reactivity with digoxin-like immunoreactive factors in the Roche-TDM ONLINE digoxin assay

Immunoassays for digoxin measurement have long had the problem of low specificity. Antisera used in these assays may not only measure digoxin and the active metabolites but also cross-react with the noncardioactive metabolites and digoxin-like immunoreactive factors (DLIFs). In this study, we describe the analytical performance of the newly introduced Roche-TDM ONLINE digoxin assay on the COBAS FARA II centrifugal analyzer. The assay possessed linearity up to 5.0 ng/ml, sensitivity of 0.19 ng/ml, average recovery of 100.4%, and day-to-day variability of < 6%. The assay demonstrated no cross-reactivity with DLIFs or spironolactone and its metabolites and negligible reactivity with the digoxin noncardioactive metabolites. In addition, the immunoreactivity of the digoxin active metabolites reflected their cardioactivity. We conclude that the Roche-TDM ONLINE digoxin assay is highly specific and precise and suitable for the therapeutic monitoring of this drug.

Metabolism of pimobendan in long-term human hepatocyte culture: in vivo-in vitro comparison

The aim of this study was to investigate further the potential of a new hepatocyte culture based on the hypothesis that liver cells in an appropriate in vitro environment (immobilizing gel technique) maintain high metabolic activity comparable with that in vivo. Pimobendan (UD-CG 115), a pyridazinone derivative, is a cardiotonic vasodilator that increases myocardial contractility through calcium sensitization and relaxation of vascular smooth muscle, probably due to phosphodiesterase inhibition. In man, pimobendan is O-demethylated to UD-CG 212. This latter is metabolized to O- and N-glucuronides. Pimobendan itself is also glucuronidated to a N-glucuronide. Human hepatocytes immobilized in collagen gel were incubated with pimobendan to investigate their metabolic activity in the long-term and to compare the results to the data from clinical trials. 14C-labelled pimobendan was incubated at two concentrations (10 and 100 microM) at day 3, 11 and 22 of culture, and samples were analysed after 4, 24 and 48-h incubation. Metabolic patterns were evaluated by hplc with radioactivity-, diode array-, and mass spectral-detection. In vitro, pimobendan was O-demethylated and subsequently O-glucuronidated. The rate of metabolism of pimobendan could be maintained in this culture system for > 3 weeks. However, the relative amount of a putative N-glucuronide under in vitro conditions was lower than in vivo.

Propionyl-L-carnitine: labelling in the N-methyl position with carbon-11 and pharmacokinetic studies in rats

The prospective therapeutic, propionyl-L-carnitine, was labelled in the N-methyl position with the positron-emitter, carbon-11 (t1/2 = 20.4 min), with a view to studying its pharmacokinetics in humans using PET. Labelling was achieved by methylating nor-propionyl-L-carnitine hydrochloride with no-carrier-added [11C]iodomethane (produced from cyclotron-produced [11C]carbon dioxide) in ethanol in the presence of 1,2,2,6,6-pentamethylpiperidine. HPLC of the reaction mixture on a strong cation exchange column provided high purity [N-methyl-11C]propionyl-L-carnitine in 62% radiochemical yield (decay-corrected from [11C]iodomethane), ready for intravenous administration within 35 min from the end of radionuclide production. [N-methyl-11C]Propionyl-L-carnitine, given intravenously to rats, cleared rapidly from plasma. A slow uptake of radioactivity into myocardium and striated muscle was observed. In plasma, unchanged tracer represented 84% of the radioactivity at 2.5 min and 2.5% of the radioactivity at 60 min. In heart, unchanged tracer represented 18% of radioactivity at 2.5 min and 2.4% at 15 min. The remainder of radioactivity detected in plasma and heart was identified as [N-methyl-11C]L-carnitine and [N-methyl-11C]acetyl-L-carnitine.

Comparison of the stereoselective effects of a thiadiazinone derivative on contractile parameters and protein phosphorylation in the mammalian ventricle

In papillary muscles from reserpinized guinea pigs, EMD 57033, the most potent Ca(2+)-sensitizer known, effectively increased force of contraction (FOC) and concomitantly increased duration of contraction. In isolated 32P-labeled guinea pig ventricular cardiomyocytes, EMD 57033 increased the phosphorylation state of phospholamban as well as the inhibitory subunit of troponin, which are usually linked to reductions in contraction time. Therefore, EMD 57033 dissociates phospholamban-phosphorylation and effects on contractile parameters, probably owing to its strong Ca(2+)-sensitizing properties.

Metabolism and pharmacokinetics of the cardiotonic agent piroximone and of its major metabolite in dog

1. Piroximone was administered orally (p.o.) and intravenously (i.v.) to male Beagle dog. In vitro, piroximone was incubated with dog liver microsomes. 2. Piroximone was metabolized in vivo to five metabolites (1-5) representing approximately 20% of the total administered dose. 3. The parent drug and its metabolites were totally eliminated in urine. 4. Reduced piroximone (piroximole), representing approximately 10% of the administered dose, was identified as the major metabolic product in vivo. 5. In vitro, piroximone was metabolized by dog liver microsomes to isonicotinic acid (1) and piroximole (4), with the same ratio as in vivo (1:4 = 0.2). The Michaelis-Menten parameters were determined for piroximole formation and were: Kmapp = 733 microM and Vmax app = 232 pmol/mg protein/min. 6. Comparison of the pharmacokinetics of piroximone and piroximole revealed that both compounds were very well absorbed (F = 93 +/- 7 and 89 +/- 8% respectively), slightly distributed (Vd app = 0.78 +/- 0.04 and 1.02 +/- 0.09 l/kg p.o., and 0.95 +/- 0.05 and 0.76 +/- 0.13 1/kg i.v. respectively) and excreted into urine to the same extent (UEx = 54.7 +/- 1.2 and 53.2 +/- 12.6% p.o., and 59.1 +/- 5.3 and 51.2 +/- 5.7% i.v. respectively), except that the clearance of piroximone was two-fold higher than that observed for piroximole (ClT = 7.77 +/- 1.35 and 4.12 +/- 0.44 ml/min/kg p.o., and 7.68 +/- 1.25 and 4.06 +/- 0.51 ml/min/kg i.v. respectively).

Binding of a new Ca2+ sensitizer, levosimendan, to recombinant human cardiac troponin C. A molecular modelling, fluorescence probe, and proton nuclear magnetic resonance study

The binding of a new calcium sensitizer, levosimendan, to human cardiac troponin C (cTnC) is described. Fluorescence studies done on dansylated recombinant human cTnC and a site-directed mutant showed that levosimendan modulated the calcium-induced conformational change in cTnC, and revealed the role of Asp-88 in the binding of the drug to the NH2-terminal domain of cTnC. Furthermore, NMR studies performed on the NH2-terminal fragment of cTnC showed a spatial proximity between levosimendan and Met81, Met85, and Phe77 in the drug-protein complex. These data were used to build an optimized model of the drug-protein complex, in which levosimendan binds cTnC at the hydrophobic pocket of the NH2-terminal domain. The role of the binding of levosimendan to cTnC in the pharmacological action of this drug in vivo is discussed.

Metabolism of vesnarinone by activated neutrophils: implications for vesnarinone-induced agranulocytosis

Vesnarinone is an important new drug that significantly decreases mortality rates in severe congestive heart failure; however, its use is associated with a relatively high incidence (approximately 1%) of agranulocytosis. The authors studied its metabolism by activated neutrophils, the target for this toxicity, and evidence pointed to a pathway that involved a reactive iminium ion. Hydrolysis of the iminium ion led to a reactive quinone imine. The same pathway was observed with a combination of myeloperoxidase/hydrogen peroxide/chloride, the major oxidizing system of neutrophils, or hypochlorous acid, which is generated by this system. Activation of the neutrophils could be achieved by phorbol ester or by influenza vaccine and there is evidence to suggest that the administration of influenza vaccine during vesnarinone therapy may increase the risk of agranulocytosis. Incubation of radiolabeled vesnarinone with activated neutrophils led to covalent binding of almost 5% of the drug to the cells. Both the iminium ion and quinone imine generated by hypochlorous acid could be trapped with glutathione. It was proposed that these reactive metabolites, generated by neutrophils or neutrophil precursors in the bone marrow, may be responsible for the vesnarinone-induced agranulocytosis. Factors such as infection or vaccination that activate neutrophils may increase the risk of agranulocytosis.

Tubulin binding agent CI-980 has positive inotropic and local anesthetic actions

Tubulin binding agents inhibit tubulin polymerization by actions at specific binding sites. CI-980 acts at the colchicine-binding site, which is distinct from the vinca-alkaloid binding site. We studied the actions of CI-980 in two models: neonatal rat myocytes in tissue culture and adult canine Purkinje fibers. In the first model, experiments on cell shortening and calcium signaling (using fluo3) showed that CI-980 increased the amplitude of both cell shortening and the Ca signal. The comparison drug, vinblastine, shared the effect on Ca signaling, but not that on cell shortening. In addition, high concentrations of CI-980 decreased the beating rate of spontaneously firing cell cultures. In canine Purkinje fibers, CI-980 decreased action potential amplitude (APA), Vmax, and conduction velocity and prolonged repolarization. It also decreased automaticity and suppressed delayed afterdepolarizations (DAD). These studies suggest that CI-980 is a novel compound in that it exerts antiarrhythmic effects on the AP but is positively inotropic. Whether all these actions derive from a primary effect on tubulin or whether they reflect action on both tubulin and transsarcolemmal ion channels remains to be determined.

Metabolic effects of propionate, hexanoate and propionylcarnitine in normoxia, ischaemia and reperfusion. Does an anaplerotic substrate protect the ischaemic myocardium?

It has been suggested that propionyl-L-carnitine administration to ischaemic hearts facilitates the restoration of cardiac function upon reperfusion, but it is still a matter of dispute whether its effect is conveyed via the metabolic effect of the propionyl moiety, the carnitine moiety or other mechanisms involving membrane receptor interactions. The metabolism of propionylcarnitine involves the formation of succinyl-CoA, which causes an increase in the total amount of tricarboxylic acid cycle intermediates. According to the current paradigm, anaplerosis ensures rapid restoration of tricarboxylic acid cycle activity during reperfusion. To evaluate the contribution of anaplerosis to the protective effect of propionylcarnitine during ischaemia and reperfusion, isolated rat hearts were perfused with Krebs-Henseleit bicarbonate buffer containing 5 mM glucose+insulin (12 IU per litre), to which 1 mM propionate, 0.8 mM hexanoate or 1 mM propionylcarnitine were added. Global 20 or 24 min no-flow ischaemia was followed by 10 min reperfusion. The flavoprotein redox state, myoglobin oxygenation, oxygen consumption and mechanical functioning of the heart were recorded and metabolites determined in freeze-trapped tissue. In parallel experiments, the cellular energy state was studied with phosphorus nuclear magnetic resonance spectrometry. The addition of 1 mM propionylcarnitine failed to cause an anaplerotic effect, but did bring about an oxidation of flavins, probably due to citrate synthase inhibition. Propionate showed similar but stronger effects and a marked anaplerosis, but still failed to improve the recovery of the heart upon reperfusion. The addition of hexanoate caused marked anaplerosis upon reperfusion and flavin reduction. The results failed to demonstrate that propionylcarnitine had any beneficial effect on the ischaemic myocardium.

Simultaneous determination of the enantiomers of pimobendan and its main metabolite in rat plasma by high-performance liquid chromatography

A high-performance liquid chromatographic method for the simultaneous analysis of the enantiomers of pimobendan and its main metabolite, with an n-hexane-ethanol-acetic acid solvent system, has been developed. After solid-phase extraction from plasma, the enantiomers were separated from each other using a Sumichiral OA-4400 column, which is commercially available and contains a chiral stationary phase composed of (S)-proline and (S)-1-(alpha-naphthyl)ethylamine coated on silica. The enantiomers were detected with a fluorescence detector (excitation at 330 nm, emission at 415 nm). The intra- and inter-day precision studies showed good reproducibilities: the coefficients of variation were less than 10.3% for pimobendan enantiomers and 13.0% for metabolite enantiomers. The calibration curves were linear (r2 > 0.996) in the concentration range 1.25-200 ng/ml. The minimum measurable level was 125 pg per 100 microliters of plasma. The method was used in a preliminary pharmacokinetic study in three male rats after intravenous administration of racemic pimobendan (2 mg/kg).

Management of congestive heart failure: neuroendocrine approach

The term heart failure is a complex of clinical syndromes caused by "neuroendocrine" compensatory responses that are renal, neural, hormonal and hemodynamic in nature. Activation of these multiple systems--as a result of renin, angiotensin II and the action of circulatory catecholamines--causes the release of norepinephrine vasopressin and aldosterone. These responses contribute to the arteriolar constriction and salt and water retention that promote central pooling with an increase in pre- and afterload of the already failing heart, causing deterioration of the congestive heart failure (CHF). Therapy for this clinical syndrome is aimed at reducing both pre- and afterload with vasodilators, which produce a profoundly favorable effect on left ventricular performances. Beta blocking agents, which inhibit the increased sympathetic mediated vasoconstriction, up-regulate beta receptors and thus restore responsiveness to the failing heart. Inotropic agents, such as digitalis and amrinone/milrinone are reserved for patients with dilated failing heart and impaired systolic function; recently a synthetic atrial natriuretic factor has been developed for potential use in CHF. Ultrafiltration is also used in refractory preoperative CHF with cardiac abnormalities. Neuroendocrine responses to CHF are treated today directly by improving central hemodynamic imbalance in these patients.

Cordil reversibly inhibits the Na,K-ATPase from outside of the cell membrane. Role of K-dependent dephosphorylation

Cordil-LND796 is a new cardiotonic glycoside under development. In rat brain microsomes where three isoforms of the Na,K-ATPase with differential affinities for cardiac glycosides have been identified, Cordil had higher affinity for the alpha 3 (IC50 = 0.02 microM) than for the alpha 2 (IC50 = 0.6 microM) and the alpha 1 (IC50 = 30 microM) isozymes. Cordil is potentially a selective inhibitor for both alpha 2 and alpha 3 Na,K-ATPase isoforms. Using inside out vesicles we have shown that Cordil binds to and inhibits Na,K-ATPase at an extracellular site. The dissociation kinetic rates (k-1) from the ATPase and the phosphatase activity (K-dependent dephosphorylation) of the Na,K-ATPase were similar for Cordil. Despite these similarities to ouabain comparison of the kinetics of the Na,K-ATPase inhibition by ouabain and Cordil revealed marked differences in their association rates (k+1 = 0.7 l mol-1 min-1 and k+1 = 6 x 10(-3) l mol-1 min-1 respectively) and their dissociation rates (k-1 = 1.3 +/- 0.2 x 10(-4) s-1 and k-1 = 69 +/- 7 x 10(-4) s-1 respectively). Both binding association and dissociation rates were enhanced for Cordil. These data are compatible with a stabilizing effect of Cordil on the E2P conformational state of Na,K-ATPase.

Linear and cyclic peptide analogues of the polypeptide cardiac stimulant, anthopleurin-A. 1H-NMR and biological activity studies

A loop corresponding to residues 8-17 in the polypeptide cardiac stimulant anthopleurin-A is known to be important for the cardiostimulant activity of this molecule. To investigate the activity and possible conformations of this loop in isolation, two synthetic peptides have been studied. The first corresponds to residues 6-20 of anthopleurin-A with Cys6 replaced by Thr, and the second to residues 6-21 of anthopleurin-A, with Thr21 replaced by Cys. The introduction of an additional cysteine in the latter peptide enabled an intramolecular disulfide to be formed between the N- and C-terminal residues. Both linear peptides and the disulfide-containing analogue lack the cardiostimulant and Na(+-)-channel binding activity in the parent molecule, anthopleurin-A, indicating that although the loop is important for the function of anthopleurin-A, other regions of the molecule must also be involved in activity. Assignments of the 1H-NMR spectra of both peptides are presented, and their pH and temperature dependences investigated. The results show that the amide protons of Gly5 and Asn11 (corresponding to Gly10 and Asn16 in anthopleurin-A) sample hydrogen-bonded conformations in solution. Based on these NMR data, two regions of non-random structure, encompassing residues 2-5 and 8-11, respectively, are proposed, and the possible involvement of such structures in the activity of anthopleurin-A is discussed.

Isomazole disposition in man as a function of dose and route of administration

The disposition of a new cardiotonic agent (isomazole (ISO] was evaluated in healthy volunteers after various single oral (p.o.), intravenous (i.v.), and multiple p.o. doses. Blood samples were collected after dosing in all studies, with urine collected in the single i.v. and multiple dose studies. All biological samples were measured for ISO. In the multiple dose study, samples were collected for analysis after the first and last doses administered. In addition to ISO, several known metabolites (hydroxyisomazole (OHISO), sulfone (SULF), and hydroxysulfone (OHSULF) analogs) were measured after the first and last doses given in the multiple dose study. Pharmacokinetic values compared between doses suggested no saturable processes existed over the entire dose range. The single i.v. dose data showed ISO experienced some extravascular distribution (mean V beta = 1.82 l kg-1), with a high clearance (mean Cls = 18.8 ml min-1 kg-1) and a short half-life (mean t 1/2 = 1.1 h). Elimination was primarily nonrenal (Clr = 3.5 ml min-1 kg-1). Single p.o. data supported these findings and further suggested rapid absorption. ISO data from the first dose of the multiple dose study was in agreement with these data; however, the last dose showed a higher Cls (33.0 ml min-1 kg-1) (p = 0.055). Although not statistically significant, metabolite plasma data and and urinary excretion patterns changed. An increase was observed in plasma AUC and metabolite excretion of SULF and OHSULF, while a decrease was observed in the same parameters for OHISO. These results suggest that multiple dosing of ISO produces autoinduction of ISO metabolism through selective metabolic routes.

Synthesis and structural characterization of a cardioactive biotinylated digoxin analogue

We have biotinylated the terminal glycose of digoxin by reaction of the periodate-oxidized steroid with biotin hydrazide. A biotinylated product (BD-1) was formed which retained significant digoxin receptor (Na+/K+ ATPase) binding activity. Sustained reaction resulted in a second biotinylated product (BD-2) which showed reduced receptor binding activity. The products were characterized by FAB mass spectroscopy and shown to be the mono- and di-biotinylated digoxin conjugates of the oxidized glycose moiety. These analogues may prove useful in determining the subcellular site of digoxin binding.

The metabolic disposition of an orally active pyridyl thiadiazinone cardiotonic agent (MPTD) in rat and baboon

1. Oral absorption and bioavailability of the orally active cardiotonic agent, (6RS)-6-methyl-5-(pyrid-4-yl)-3H,6H-1,3,4-[6-14C]thiadiaz in-2-one (MPTD) (5 mg/kg), in rat and baboon were high. Peak blood concentrations of MPTD and total radioactivity were reached by 1.5-4 h when MPTD accounted for 60-70% of total radioactivity. In both species, elimination of MPTD from blood was rapid (t 1/2 = 3-4 h), although total nonspecific radioactivity was eliminated more slowly. 2. Radioactivity was rapidly eliminated by both species mainly into urine. In rat, about 3% dose was collected as 14CO2 and 2% remained in the carcass after 4 days. Recovery from baboon was incomplete (78-86%). 3. Examination of urine indicated extensive metabolism of MPTD showing a marked species difference. In baboon, MPTD was metabolized largely by glucuronidation at the pyridyl nitrogen to yield a quaternary ammonium conjugate and only about 1% of the dose was excreted unchanged. In rat, the major urinary component was unchanged MPTD and no glucuronide conjugate was found. Both species formed the pyridine N-oxide of MPTD as well as a number of unidentified minor components. 4. Distribution of radioactivity in rat was rapid and extensive. In general, elimination from tissues was also rapid, although radioactivity was eliminated much more slowly from the nasal and bronchiolar epithelium and from the preputial gland.

[Determination of enoximone and its principle metabolite in serum and urine using high pressure liquid chromatography]

We developed a high performance liquid chromatography method for the monitoring of enoximone and its main metabolite in serum and urine. Samples handling involves a unique chemical extraction step by ethylacetate. Serum needs at first to be deproteinized by acetonitril. The chromatographic separation is realized on a reversed phase analytical column by gradient elution with acetonitrile. Quantification is by U.V. absorbance at 365 nm. We compared our new method with the method so far considered as reference one. When specificity, accuracy and linearity of both procedures are similar, we greatly enhanced the detection limit [(5 ng/ml for (E) and (SE)] and the practicability: ease of use, rapidity and lower cost.

Toxicological evaluation of the cardiotonic isomazole in the dog

Acute, subchronic, and chronic toxicity studies were conducted in dogs with the new vasodilator/cardiotonic drug isomazole (IMZ) to support, in part, clinical investigations of this agent in humans. Single oral doses of IMZ of 25, 50, or 100 mg/kg given to English pointer dogs (2/dose) caused a marked drop in systemic blood pressure and reflex-induced increases in heart rate to values well over 200 beats per minute. These responses were maintained for 12 to 22 hr depending on the dose given. One of the dogs receiving 100 mg/kg died at 4.5 hr postdose. Results of subchronic (3 months) and chronic (1 year) studies in beagle dogs (4/sex/dose group), in which measurable plasma levels of the drug and its metabolites were found, indicated that IMZ did not produce any discernible adverse findings when given in doses up to 16 mg/kg, other than expected cardiotoxic effects. The plasma t1/2 of IMZ at 16 mg/kg increased to between 4 and 8 hr from 2 hr noted at lower doses. In the 1-year study, at all doses and in both sexes, plasma levels of IMZ declined over the first month, stabilizing (at the 2 and 6 mg/kg doses) thereafter for the duration of the study. At the high dose of 16 mg/kg, after 1 year plasma levels of IMZ exceeded (females) or equaled (males) the 1-month values. At peak plasma levels of IMZ (2 hr postdose), plasma levels of parent drug increased linearly with the dose. The cardiotoxic effects consisted of substantial postdose increases in heart rate throughout the course of treatment (5 mg/kg and above), significant increases in heart weight (6 mg/kg and above), and multifocal myocardial fibrosis (6 mg/kg and above). There was a decline in basal heart rate at doses of 12.5 mg/kg and higher. The results of these studies demonstrated that repeated IMZ administration, as expected, was cardiotoxic to the dog, a species relatively sensitive to the pharmacological activity and hemodynamic changes induced by vasodilator/cardiotonic drugs. The no-effect dose level for cardiotoxicity in the repeated dose studies was considered to be 2 mg/kg, the lowest dose tested.

Cardiac glycoside-like structure and function of 5 beta,14 beta-pregnanes

5 beta-Reduction and 14 beta-substitution convert the planar progesterone molecule to the cardiac glycoside configuration--A and D rings of the steroid moiety are bent toward the alpha-face relative to the B and C rings. Potency of the 5 beta,14 beta-derivative in a [3H]ouabain binding assay or its ability to inhibit the sodium pump in red blood cells is enhanced by 3 beta-hydroxylation, 20 beta-hydroxylation, and 3 beta-glycosidation. Synthesis of 14,20 beta-dihydroxy-3 beta-(beta-D-glucopyranosyloxy)- 5 beta,14 beta-pregnane from digitoxin is described. The glucoside is 1/20 as potent as ouabain and elicits prominent, sustained, positive inotropy in isolated cardiac muscle.

Analysis of the binding sites for the cardiotonic phosphodiesterase inhibitor [3H]LY186126 in ventricular myocardium

The positive inotropic action of the newer cardiotonic phosphodiesterase inhibitors such as indolidan, milrinone, and imazodan has been previously attributed to selective inhibition of cGMP-inhibitable Type IV (high affinity) cAMP phosphodiesterase activity. However, the subcellular binding site(s) for this class of compounds has not been defined. We have characterized the binding of [3H]LY186126, an analogue of indolidan, in subcellular fractions prepared from rabbit and sheep ventricular myocardium. Binding required magnesium ion and exhibited rapid association and dissociation kinetics. Specific binding (defined by ligand displacement with 5 microM indolidan) to enriched rabbit sarcoplasmic reticulum (SR) membrane vesicles was saturable (Bmax = 714 +/- 77 fmol/mg of protein) and of high affinity (Kd = 6.2 +/- 1.4 nM). Linear and nonlinear analyses of the binding isotherms fit a single-site model. Mixed SR preparations from sheep myocardium exhibited binding characteristics (Bmax = 944 +/- 115 fmol/mg; Kd = 8.5 +/- 2.3 nM) comparable to those of rabbit cardiac SR. Further subfractionation of sheep SR indicated that the binding sites were equally distributed between free (Bmax = 630 fmol/mg; Kd = 4.4 nM) and junctional SR (Bmax = 569 fmol/mg; Kd = 10.9 nM). Specific binding of [3H]LY186126 was also demonstrated in the cytosolic subfraction of rabbit myocardium that contained Type IV phosphodiesterase activity (Peak III from anion exchange chromatography). Competition for [3H] LY186126 binding studied in rabbit SR showed that, of the compounds tested, lixazinone (RS 82856) competed most effectively (IC50 = 0.030 +/- 0.008 nM), followed by indolidan (0.14 +/- 0.05 nM), cGMP (17.8 +/- 2.6 nM), milrinone (39.3 +/- 13.2 nM), and imazodan (192 +/- 73 nM). In contrast, rolipram, which does not inhibit SR-associated Type IV phosphodiesterase activity, was not effective at competing for [3H]LY186126 binding (IC50 greater than 30 microM). These results indicate that [3H]LY186126 has specific binding sites in myocardial subcellular fractions that contain cGMP-inhibitable Type IV (high affinity) cAMP phosphodiesterase activity.

Immunocytochemical analysis of potential neurotransmitters present in the myenteric plexus and muscular layers of the corpus of the guinea pig stomach

Recent electrophysiological studies of neurons of the myenteric plexus of the corpus of the guinea pig stomach have revealed that slow synaptic events are extremely rare. In contrast, they are commonly encountered in similar investigations of myenteric ganglia of the guinea pig small intestine. The current immunocytochemical analysis of the myenteric plexus and innervation of the muscularis externa of the corpus of the guinea pig stomach was undertaken in order to determine whether putative neurotransmitters capable of mediating slow synaptic events are present in gastric ganglia. A major difference between the small intestine and the stomach was found in the innervation of the musculature. Whereas the longitudinal muscle layer of the small intestine contains very few nerve fibers and is innervated mainly at its interface with the myenteric plexus, the longitudinal muscle of the corpus of the stomach contained as many varicose substance P (SP)-, vasocative intestinal polypeptide (VIP)-, and neuropeptide Y (NPY)-immunoreactive axons as the circular muscle layer. These putative neurotransmitters were also present in the ganglia of the myenteric plexus, where varicose SP-, VIP-, and NPY-immunoreactive fibers encircled nonimmunoreactive neurons. Varicose 5-hydroxytryptamine (5-HT)-immunoreactive terminal axons were essentially limited to the myenteric plexus and were found both in ganglia and in interganglionic connectives, where they were particularly numerous; 5-HT-immunoreactive neurons appeared to be more abundant in the stomach than in the small intestine. Tyrosine hydroxylase (TH)- and calcitonin-gene-related-peptide (CGRP)-immunoreactive axons were also more common in the myenteric plexus than in the musculature, but of these, only the TH-immunoreactive neurites tended, like those of the other putative transmitters, to encircle neurons in myenteric ganglia. Evidence was obtained that, as in the small intestine, at least some of the SP-, VIP-, NPY-, and 5-HT-immunoreactive fibers in the stomach are derived from intrinsic gastric myenteric neurons. In contrast, unlike the small intestine, gastric myenteric ganglia appeared to lack intrinsic CGRP-immunoreactive neurons; therefore, the CGRP-immunoreactive gastric axons are probably of extrinsic origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization and pharmacological relevance of high affinity binding sites for [3H]LY186126, a cardiotonic phosphodiesterase inhibitor, in canine cardiac membranes

[3H]LY186126, an analogue of the cardiotonic agent indolidan, was shown to bind reversibly and with high affinity (Kd = 4 nM) to a single class of binding sites within canine myocardial vesicles. Binding site density measured in various cardiac membrane fractions correlated well with Ca2+-ATPase activity (r = 0.94; p less than 0.01), but not with Na+,K+-ATPase or azide sensitive ATPase, indicating a localization of these sites within sarcoplasmic reticulum membranes. Divalent cations were required for binding and displayed the following order of activation: Zn2+ greater than Mn2+ greater than Mg2+ greater than Ca2+. Differential activation of [3H]LY186126 binding by various divalent cations was due to alterations in binding site density, rather than affinity. cGMP and selective inhibitors of type IV membrane-bound phosphodiesterase (SR-PDE), for example, indolidan, milrinone, imazodan, and enoximone, selectively displaced bound [3H]LY186126 caffeine, theophylline, and rolipram were relatively impotent as inhibitors of radiolabel binding. Kd values from displacement curves were highly correlated with IC50 values for inhibition of SR-PDE (r = 0.92; p less than 0.001). In addition, Kd values correlated well with published ED50 values for increases in cardiac contractility in pentobarbital-anesthetized dogs (r = 0.94; p less than 0.001). The results support the hypothesis that [3H]LY186126 labels the pharmacological receptor for the class of positive inotropic agents characterized as isozyme-selective phosphodiesterase inhibitors. Furthermore, the data suggest that the identity of the site labeled by [3H]LY186126 is SR-PDE, the type IV isozyme of cardiac phosphodiesterase located in the sarcoplasmic reticulum.

Specific binding of [3H]LY186126, an analogue of indolidan (LY195115), to cardiac membranes enriched in sarcoplasmic reticulum vesicles

LY186126 was found to be a potent inhibitor of type IV cyclic AMP phosphodiesterase located in the sarcoplasmic reticulum of canine cardiac muscle. This compound, a close structural analogue of indolidan (LY195115), was prepared in high specific activity, tritiated form to study the positive inotropic receptor(s) for cardiotonic phosphodiesterase inhibitors such as indolidan and milrinone. A high-affinity binding site for [3H]LY186126 was observed (Kd = 4 nM) in purified preparations of canine cardiac sarcoplasmic reticulum vesicles. Binding was proportional to vesicle protein, was inactivated by subjecting membranes to proteolysis or boiling, and was dependent on added Mg2+. Scatchard analysis suggested the presence of a single class of binding sites in the membrane preparation. Indolidan, milrinone, and LY186126 (all at 1 microM) produced essentially complete displacement of bound [3H]LY186126, while nifedipine, propranolol, and prazosin had little or no effect at this concentration. This represents the first reported use of a radioactive analogue to label the inotropic receptor for cardiotonic phosphodiesterase inhibitors. The results suggest that [3H]LY186126 is a useful radioligand for examining the subcellular site(s) responsible for positive inotropic effects of these drugs.

The metabolism and disposition of 14C-indolidan, a potent and orally active cardiotonic agent, in four species of laboratory animals

1. The metabolism and disposition of 14C-indolidan, a potent, orally-active positive inotrope with vasodilator properties, has been studied after single dose oral administration to rats, mice, dogs and monkeys. 2. Excretion of 14C in all 4 species was mostly via the urine, largely as parent drug together with two other major metabolites. 3. The two metabolites have been isolated and identified, by mass spectroscopy and 1H-n.m.r., as a dehydro-compound, with a double bond in the pyridazanone ring, and a hydroxylated derivative of the parent drug. 4. Plasma t 1/2 values, based on 14C, were 14 h in dog, 5 h in mouse and 8 h in monkey. Plasma t 1/2 of parent drug, by h.p.l.c. was 10 h in dog, approx. 5 h in rodents, and 8 h in monkeys. 5. Tissue distribution in rats showed no accumulation in any tissue; 14C concn. in all tissues were indistinguishable from background 48 h after dosage. 14C peaked at 6-8 h for most tissues but in blood and plasma, 14C was maximal 1 h after dosing.

Details of mode and mechanism of action of denopamine, a new orally active cardiotonic agent with affinity for beta 1-receptors

We investigated the detailed mode and mechanism of action of denopamine in canine right ventricular muscle. When administered in single doses, denopamine produced a positive inotropic effect (PIE) and increased cyclic AMP levels. Concentration--response curves for both variables were sigmoid. The maximum PIE of denopamine was almost the same as that produced by isoproterenol. However, the maximum increase in cyclic AMP caused by denopamine was approximately 65% of that attained with isoproterenol. When administered cumulatively, concentration--PIE curves for denopamine were bell-shaped, ascending at 10(-7) to 10(-6) M, reaching a maximum at approximately 3 X 10(-6) M which was approximately 75% of that attained with single administrations, and descending at higher concentrations. The bell-shaped curve, which was computer-fitted, suggested that denopamine behaves as an agonist with pD2 of 6.12 and as an antagonist with pD2 of 4.50. The PIE of denopamine was antagonized by atenolol (pA2 = 7.66) but not by ICI 118,551 (less than 10(-7) M), indicating that denopamine is a selective beta 1-receptor agonist. The PIE of denopamine was augmented by 3-isobutyl-1-methyl-xanthine. The PIE and increase in cyclic AMP level caused by 3 X 10(-6) M denopamine were abolished by 10(-6) M atenolol or 3 X 10(-6) M carbachol. From these results we concluded that the PIE of denopamine is derived from the mechanism of action as a selective beta 1-receptor partial agonist. We suggested the close coupling of the beta 1-receptor-adenylate cyclase-cyclic AMP system to positive inotropy.

Calcitonin gene-related peptide: release by capsaicin and prolongation of the action potential in the guinea-pig heart

The mechanisms underlying the stimulatory effects of capsaicin on the contractility of the guinea-pig heart were studied in vitro. Capsaicin (10(-7) to 10(-5) M) caused an increased overflow of immunoreactive material, suggesting release of calcitonin gene-related peptide (CGRP)-and neurokinin A (NKA)-like immunoreactivity (-LI), but not of neuropeptide Y (NPY)-LI from the isolated Langendorff-perfused whole heart. The capsaicin-induced release was calcium-dependent. During exposure to capsaicin, the heart rate was increased, while the contractile force was reduced. In addition to releasing CGRP and NKA-LI, potassium (60 mM) also increased the overflow of NPY-LI. The potassium-induced release of peptides was less calcium-dependent than the response to capsaicin. Considerably higher tissue levels of CGRP-LI were found in the atria (about 30 pmol g-1) than in the ventricles (about 10 pmol g-1). In experiments on the right atria using transmembrane action-potential recordings of myocytes, CGRP induced a prolongation of the action potential concomitantly with an increase in rate and contractile force, which was similar to the effect of noradrenaline. Furthermore, CGRP increased the contractile force and relaxation velocity of the electrically stimulated atria. Capsaicin (10(-7) M) also increased the duration of the atrial action potential. In conclusion, CGRP-like material is released by capsaicin from the isolated guinea-pig heart. Both CGRP and capsaicin prolong the plateau phase of the action potential of atrial myocytes. Therefore, the present data give further evidence that CGRP release from sensory nerves within the heart underlies the cardiostimulatory actions of capsaicin.

Lymphocyte-induced stimulation of the contractile response of the heart

In previous reports we have shown that phytohemagglutinin (PHA)-activated human lymphocytes had positive inotropic effects on spontaneously beating isolated rat atria. In this study, we demonstrated that the stimulatory effect on heart contractility induced by lymphocytes was linked to early events of lymphocyte activation by lectins. Active soluble factors were gradually released to the fluid phase. Similar results were obtained with both mitogenic (PHA) and nonmitogenic (WGA) lectins indicating that the stimulatory action of activated lymphocytes did not require cell division. Absence of Ca2+ inhibited both the generation of the stimulatory activity and lymphocyte proliferation. In contrast, verapamil, dexamethasone and low concentrations of cycloheximide eliminated only the appearance of the stimulatory effect.

Competition of milrinone, a non-iodinated cardiac inotropic agent, with thyroid hormone for binding sites on human serum prealbumin (TBPA)

Milrinone [2-methyl-5-cyano-(3,4'-bipyridin)-6(1H)-one] is a positive cardiac inotropic agent recently shown to have thyromimetic activity in vitro in a rabbit myocardial membrane Ca2+-ATPase system [K. M. Mylotte et al., Proc. natn. Acad. Sci. U.S.A. 82, 7974 (1985)]. In the present studies, milrinone was examined for activity as an inhibitor of iodothyronine binding by human serum thyroid hormone transport proteins, thyroxine-binding globulin (TBG), prealbumin (TBPA) and albumin. Polyacrylamide gel electrophoresis at pH 9.0 of sera equilibrated with [125I]thyroxine showed that milrinone competed with L-thyroxine (T4) for binding sites on TBPA (10 and 100 microM milrinone caused 61 and 73% reductions, respectively, in T4 binding to TBPA, P less than 0.01); T4 displaced from TBPA was bound by TBG and albumin. Comparable reductions in T4 binding to TBPA were observed in electrophoretic studies conducted at pH 7.4. Binding of triiodo-L-thyronine (T3) to TBPA was electrophoretically confirmed and shown to be decreased in the presence of milrinone. Electrophoresis of purified TBPA also demonstrated that [14C]milrinone co-migrated with this transport protein and that milrinone displaced tracer T4 from TBPA. Amrinone, the 2-H-5-NH2 analog of milrinone, had less than 5% of the activity of milrinone as an inhibitor of T4 binding in electrophoretic studies. Scatchard analysis of T4 and milrinone binding to purified TBPA, measured by equilibrium dialysis, showed two classes of binding sites, with association constants, respectively, of 6.1 X 10(7) M-1 and 1.6 X 10(6) M-1 for T4, and 1.7 X 10(6) M-1 and 8.9 X 10(2) M-1 for milrinone. Computer graphic modeling of the binding of milrinone to the T4 site in the crystal structure of TBPA showed that milrinone best occupied this site when the substituted bipyridine ring overlapped the phenolic ring of T4. In this orientation the 5-cyano group, which has an electronegativity similar to that of iodine, occupied the same volume as the 5'-iodine of T4. The 5-amino group of amrinone lacks these characteristics. In this orientation, the keto function of milrinone overlapped the T4 4'-hydroxyl and could participate in similar intermolecular interactions. Thus, milrinone, a non-iodinated bipyridine, and thyroid hormone share structural and biochemical homologies and compete for the same binding site on TBPA.

Biotransformation and pharmacokinetic overview of enoximone and its sulfoxide metabolite

Enoximone possesses both positive inotropic and vasodilatory activities and may be useful in the treatment of patients with congestive heart failure (CHF). In all animal species investigated (rat, dog, monkey and man), the major urinary metabolite is the sulfide oxidation product (sulfoxide); very little unchanged drug appears in urine. Both in vitro and in vivo animal studies indicate reversibility of the sulfoxidation reaction; therefore, it is presumed that sulfoxidation is reversible in man. In normal healthy subjects, no difference in extent of absorption due to dietary state is observed. In patients with New York Heart Association class III to IV CHF, median terminal disposition half-lives for enoximone and its sulfoxide metabolite are 6.2 to 7.6 hours, respectively. Enoximone and sulfoxide plasma concentrations from high dose intravenous infusion studies in patients with class III to IV CHF were also investigated. The collective data suggest nonlinearity in one or more pharmacokinetic processes, of which one may be saturation of sulfoxidation. No direct relation between enoximone and/or the sulfoxide metabolite plasma concentration and pharmacologic effect has been established.

The pharmacokinetics and pharmacodynamics of newer inotropic agents

In the past few years an intense effort has been directed toward the development of new inotropic agents for the treatment of chronic cardiac failure. Traditionally, therapy of this disease has included treatment with digitalis glycosides, diuretics, sodium restriction and vasodilators. While digitalis has proven to be an effective inotropic agent, it possesses a low therapeutic index and many patients remain symptomatic or 'refractory' despite its inotropic effects. This review focuses on the pharmacokinetics and pharmacodynamics of newer inotropic agents that have been developed or which are currently undergoing investigation. Amrinone and milrinone are two bipyridine derivatives which have been shown to be effective in the short term treatment of cardiac failure. Milrinone is currently being evaluated for its long term efficacy. The mechanism of action of amrinone and milrinone appears to be unrelated to the cardiac glycosides and sympathomimetic agents, and they are rapidly and well absorbed following oral administration. The bioavailability of milrinone appears to be somewhat reduced in patients with chronic cardiac failure. The distribution of these drugs to extravascular tissues is very rapid; the volume of distribution suggests that they are not extensively bound to tissues. While the volume of distribution of amrinone appears to be unaffected by the presence of heart failure, that of milrinone appears to be somewhat enhanced. The major route of elimination of both drugs appears to be excretion into urine as unchanged drug. A substantial fraction of the amrinone dose, however, undergoes hepatic metabolism to many metabolites, including an N-acetyl derivative. Clearance of amrinone and milrinone is dramatically reduced in patients with chronic cardiac failure compared with normal volunteers, resulting in proportionate increases in the serum half-lives of these drugs. Studies examining the acute and chronic disposition of these agents in cardiac failure patients have not demonstrated changes in their pharmacokinetics secondary to improvements in cardiocirculatory function. Both drugs show strong correlations between mean improvements in haemodynamics and drug serum concentrations, although considerable intrapatient variability may exist. It is currently unclear as to whether the site for the pharmacological action of amrinone is pharmacokinetically distinguishable from plasma. Enoximone and its sulphoxide metabolite, piroximone, are two compounds currently undergoing investigation for the treatment of chronic cardiac failure.(ABSTRACT TRUNCATED AT 400 WORDS)