Effect of lead exposure on some biological indices related to porphyrin metabolism and the activity of erythrocyte pyrimidine 5′-nucleotidase in the mice

The urinary excretion of -aminolevulinic acid (ALA) and coproporphyrin (CP) isomers (I and III) was investigated in mice exposed to lead in drinking water (200 and 500 ppm) for 14 or 30 days. Furthermore, the inhibition of pyrimidine 5-nucleotidase (P5N) and -aminolevulinic acid dehydratase (ALAD) was studied in the same mice. In the lead-exposed mice, urinary excretion of ALA was significantly elevated (12-times control), while that of CP remained unchanged. This discrepancy on the urinary excretion may be of interest. The P5N activity in both erythrocytes and bone marrow cells was significantly inhibited by exposure to lead. However, the degree of inhibition was greater in the erythrocytes (45% inhibition) than in the bone marrow cells (25% inhibition). The inhibition of ALAD was much higher in erythrocytes (90% inhibition) than in liver (20–40% inhibition). The increase in erythrocyte protoporphyrin and the decrease in both hemoglobin and hematocrit were not observed in mice exposed to lead under the above conditions.     

2,2′,3′,4,4′,5,5′-Heptachlorobiphenyl (PCB 180) — on its toxicokinetics,biotransformation and porphyrinogenic action in female rats

The toxicokinetics and biotransformation of 2,2,3,4,4,5,5-heptachlorobiphenyl, as well as its influence on the activity of microsomal and cytosolic enzymes and on the porphyrin pathway in the liver were studied in female rats following oral treatment with 7 mg/kg every other day for 3 months. One day after cessation of treatment the concentration of the compound in liver, spleen, CNS and blood was 100–500 times and in the trachea it was only 5 times less than in the adipose tissue. The daily excretion with the feces and urine amounted to 35 and 1.5 g, respectively. In both excreta, heptachlorobiphenylol was identified as a metabolite. The biotransformation rate was estimated to be about 5%. Investigations of the liver revealed increases in the relative liver weight, total cytochrome P-450 content, O-deethylation of 7-ethoxycoumarin and in the activity of glutathione S-transferases. Disturbances of the hepatic porphyrin pathway were not detected. Only at the end of a post-dosing period of 12 months did the hepatic uroporphyrinogen decarboxylase show diminished activity. Only one of these animals with diminished enzyme activity showed drastically elevated porphyrins. In these animals, the fecal and urinary porphyrins did not differ from controls. At no time did heptachlorobiphenyl influence the urinary excretion of delta-aminolevulinic acid and porphobilinogen. The results indicate 1) that this congener shows expected toxicokinetics with the exception of being accumulated in the trachea and 2) that this congener induces disturbances of the hepatic porphyrin pathway several months after cessation of treatment.     

A simple method for sampling murine pulmonary and cardiac tissues for analysis of cyclic nucleotide levels

Summary A simple method for the rapid removal and freezing of mouse cardiac and pulmonary tissues is described. Samples thus obtained were judged to be suitable for valid estimation of in vivo levels of cyclic AMP and cyclic GMP based on the following findings: (a) the samples could be obtained and frozen in the very short time period of a few seconds; (b) no indication of adverse effects of the collection procedure was found upon examination of chemical indicators of energy metabolism; (c) the apparent rates of change of cyclic AMP and cyclic GMP levels during the first seconds after tissue isolation could produce small, but acceptable errors; and (d) dose-dependent elevations of pulmonary cAMP levels consistent with known effects in vitro were found after in vivo administration of isoproterenol.Abbreviations cAMP adenosine-3,5-cyclic monophosphate - cGMP guanosine-3,5-cyclic monophosphate     

A role of red cell pyrimidine 5′-nucleotidase in experimental lead poisoning

Intravenous administration of lead acetate to rabbits for 10 weeks at 2 week intervals resulted in significantly elevated blood lead levels, slight anemia with marked microspherocytosis and moderate basophilic stippling, and marked depression of red cell-aminolevulinic acid (ALA) dehydratase activity. However the decrease in red cell pyrimidine 5-nucleotidase (P5N) activity was slight when compared to the red cell P5N activity of comparable reticulocyte-rich blood, and intracellular accumulation of pyrimidine nucleotides could not be demonstrated. In the in vitro inhibition test the same degree of inhibition of red cell P5N activity seen in hereditary red cell P5N deficiency was obtained by using a lead concentration 200–400 times higher than the lead levels detected in human plumbism. Most importantly, there were no differences in the lead-induced inhibition of human and rabbit red cell P5N.From the results of the in vitro inhibition test, lead-induced red cell P5N deficiency appears to be one of several pathogenic mechanisms in chronic lead exposure associated with the accumulation of lead in bone marrow. A decrease in red cell P5N activity could not be demonstrated despite the marked depression in red cell ALA dehydratase activity, and slight anemia with marked microspherocytosis and moderate basophilic stippling in this experiment. These results suggest that lead affects red cells at multiple metabolic loci.     

Effects of indomethacin on changes in renal blood flow induced by adenosine and its analogues in conscious dogs

Summary The effects of indomethacin on changes in renal blood flow induced by adenosine, NECA (adenosine-5-N-ethyl-carboxamide) and 2,3-dinitro-NECA were investigated in 6 chronically instrumented conscious dogs. Adenosine (187.5, 375 and 750 nmol/kg, i.v.) induced a dose-dependent initial decrease, followed by a reactive increase in renal blood flow. NECA (1.5 nmol/kg, i.v.) also induced an initial decrease, which was, however, followed by a prolonged reactive increase in renal blood fow. 2,3-dinitro-NECA (50 nmol/kg, orally) induced only an increase in renal blood flow. Indomethacin (27.9 mol/kg, i.v.) caused no relevant change of the initial decrease and a significant attenuation of the reactive increase in renal blood flow induced by adenosine. NECA-induced changes in blood flow were affected by indomethacin in the same direction but to a greater extent than were adenosine-induced changes in blood flow. Indomethacin reversed the increase to a decrease in renal blood flow induced by 2,3-dinitro-NECA. Thus, prostaglandins seem to be involved in mediating the response of renal blood flow to adenosine, NECA and 2,3-dinitro-NECA.Part of this study was presented at the fall meeting of the German Pharmacological Society, September 1982 in Vienna, Austria     

Pharmacokinetic Interaction Between Intravenous 2′,3′-Dideoxyinosine and Pentamidine in Rats

Purpose. This study examined the pharmacokinetic interaction between 2',3'-dideoxyinosine (ddl) and pentamidine. Background, ddl and pentamidine are often coadministered to patients with acquired immunodeficiency syndrome, and are both associated with pancreatic toxicity. Information on potential interaction would be useful to assess the need for dose modification and the basis of the higher incidence of pancreatic toxicity associated with coadministration of the two drugs. Methods. ddl (200 mg/kg) and pentamidine (10 mg/kg) were administered by continuous infusion to rats over 3 hr, either alone or concomitantly. Drug analysis was by high pressure liquid chromatography with UV or fluorescence detection, or by radioimmunoassay. Results. Pentamidine coadministration significantly increased the apparent volume of distribution at steady state of ddl from 1.4 to 3.4 l/kg (p = 0.004), and increased the mean residence time from 36.3 to 50.0 min (p = 0.015). Pentamidine enhanced the distribution of ddl from plasma into pancreas (p = 0.001) and muscle (p = 0.026). ddl distribution into spleen and liver was also increased, with differences approaching statistical significance (p = 0.08 and 0.06, respectively). In contrast, ddl coadministration did not affect the total body clearance but increased the urinary excretion and the renal clearance of pentamidine by about 5-fold (p = 0.0003). Conclusions. These data indicate that pentamidine increased the distribution of ddl into pancreas and muscle, whereas ddl increased the renal elimination of pentamidine.     

Evidence that the aversive effects of opioid antagonists and κ-agonists are centrally mediated

The role of central versus peripheral opioid receptors in mediating the aversive effects of opioids was examined by use of an unbiased place preference conditioning procedure in rats. The non-selective opioid antagonist naloxone (NLX) produced conditioned aversions for the drug-associated place after subcutaneous (SC) as well as intracerebroventricular (ICV) administration. Place aversions were also observed in response to the ICV administration of the selective -antagonist CTOP. In contrast, the selective -antagonist ICI 174,864 and the selective -antagonist norbinaltorphimine (nor-BNI) (ICV) were without effect. Place aversions were also produced by central applications of the selective -agonist U50,488H and the dynorphin derivative E-2078. For those opioid ligands tested, the doses required to produce place aversions were substantially lower following ICV as compared to SC administration. These data confirm that -agonists and opioid antagonists produce aversive states in the drug-naive animal and demonstrate that this effect is centrally mediated. Furthermore, the ability of NLX and CTOP, in contrast to both ICI 174,864 and nor-BNI, to produce place aversions suggests that the aversive effects of opioid antagonists result from the blockade of -receptors.     

CNS stimulant properties and physiologic disposition of Β,Β-difluorophenylethylamine in mice

,-Difluorophenylethylamine, because of the strong electrophilic nature of the fluorines, is a much weaker base than is phenylethylamine. The pK for the difluoro compound is 6.75 compared to 9.55 for phenylethylamine. At physiologic pH, difluorophenylethylamine exists primarily as a neutral molecule, whereas phenylethylamine is almost entirely cationic. Difluorophenylethylamine caused increased locomotor activity in mice, and its effects were enhanced by a monoamine oxidase inhibitor (tranylcypromine) as were the effects of phenylethylamine. Comparable levels of difluorophenylethylamine and phenylethylamine in brain appeared to lead to equivalent increases in locomotor activity. At doses above those giving the maximum increase in locomotor activity, behavioral differences between phenylethylamine and difluorophenylethylamine were observed. Difluorophenylethylamine localized in adipose tissue to a greater degree than in other organs; it disappeared from adipose tissue and from brain with a physiologic half-life of 8 min in mice.     

Pharmacokinetics of 2′,3′-dideoxyadenosine in dogs

The pharmacokinetics of 2,3-dideoxyadenosine (ddAdo) and 2-3-dideoxyinosine (ddIno) were determined after intravenous bolus administration and long-term intravenous infusion of ddAdo in dogs. ddAdo was rapidly deaminated to ddIno and ddAdo plasma concentrations were only a fraction of ddIno concentrations. The total body clearance of ddAdo exceeded the literature value for the cardiac output of the dog, indicating an extremely rapid metabolism, and the existence of extrahepatic metabolism. Urinary excretion of unchanged ddAdo was a minor route of elimination ( 1%). The pharmacokinetics of ddIno was determined assuming complete conversions of ddAdo to ddIno. ddIno elimination was dose-dependent with total body clearance ranging from 4 to 55 ml/min/kg in individual animals. The plasma half-life was approximately 30 min after most routes of administration, but increased to approximately 60 min in two animals receiving a large intravenous dose of 500 mg/kg. ddIno penetrated into the cerebrospinal fluid to a limited extent, reaching concentrations of 3–11% of those in plasma. Urinary excretion of unchanged ddIno accounted for approximately 20% of the administered dose of ddAdo, while uric acid and hypoxanthine were minor urinary metabolites.Concentrations exceeding the in vitro minimal viral inhibitory concentration (2.4 g/mL) could be safely maintained in plasma for a 10-day period. Infusions which gave cerebrospinal fluid concentrations of 12 to 17 Lg/mL resulted in dose limiting myelosuppression and intestinal toxicity, after less than 10 days of infusion. Orally administered ddAdo was absorbed as ddIno, with bioavailabilities ranging from 28 to 93% in experiments where no emesis occurred. These studies indicate the rapid in vivo conversion of ddAdo to ddIno, and support the selection of ddIno over ddAdo for further drug development.     

Disposition of 2′, 3′-dideoxyadenosine and 2′, 3′-dideoxyinosine in mice

Summary Mice were dosed with [³H]2,3-dideoxyadenosine ([³H]ddA) in three procedures: intravenously, intraperitoneally, and interperitoneally following a dose of 2 -deoxycoformycin (dCF). For mice dosed intravenously, the content of radioactivity in plasma and tissue samples were essentially constant after 30 min. Of the radioactivity in plasma and brain samples collected between 30 min and 24 hr, more than 94% was present as ³H₂O, indicating that most of the tritium from [³H]ddA had exchanged with water. No intact ddA was detected, and the deamination product, 2,3 -dideoxyinosine (ddI), was present only transiently. In the urine, the major radioactive material was [³H]ddI. Also detected were ³H₂O and small amounts of [³H]hypoxanthine and [³H]ddA. Following intraperitoneal doses to mice, levels of radioactivity in plasma, liver, and kidney increased to a maximum by 15–30 min after dosing but dropped to essentially constant levels thereafter, again indicating that the tritium had exchanged with water. At 5, 15, and 30 min after dosing, ddI was the major radioactive component in plasma. Only small amounts of ddA were present. When dCF was administered 24 hr prior to intraperitoneal [³H]ddA, levels of radioactivity in plasma, liver, and kidney reached a maximum at 30 to 60 min after dosing and decreased to essentially constant levels thereafter. The dCF transiently inhibited the deamination of ddA to ddI, since, in plasma, [³H]ddA was the main radioactive component at 5 and 15 min after dosing. Comparison of HPLC assays based on radioactivity detection and UV absorbance showed that they were equivalent for measuring ddA and ddI in samples derived from dosed mice. Therefore, exchange of tritium must have occurred at a metabolic step beyond ddI.For mice dosed intravenously and orally with unlabeled ddI, there was evidence of a saturated process. Nevertheless, for the high and low intravenous doses of ddI, the percent of dose excreted in the urine as unchanged drug was the same.     

Effect of six virustatic nucleoside analogues on the development of fetal rat thymus in organ culture

The effects of the virustatic agents zidovudine (azidothymidine, AZT) 23-dideoxycytidine (ddC), 23-dideoxyinosine (ddI), acyclovir (ACV), ganciclovir (GCV), and vidarabine phosphate (VP) on the in vitro development of thymic lobes of 17-day-old rat fetuses were tested in an organ culture system. The virustatics were added to the medium for a culture period of 7 days. All nucleoside analogues inhibited the proliferation and differentiation of lymphatic cells. However, differences were observable with respect to the potency of the six drugs to interfere with thymic development. Compared to untreated controls, reduction in the number of thymocytes was significant at concentrations of 30 M AZT and ddI. In the case of ACV, GCV, VP, and ddC concentrations as low as 10 M were sufficient to cause a significant reduction, ddC being the most potent derivate. Increasing concentrations of the nucleoside analogues led to a dose-dependent further inhibition of cell proliferation. At a concentration of 30 M flow cytometry revealed a decrease in the relative number of double positive CD4⁺ CD8⁺ and single positive CD4⁺ CD8^– cells but an increase in the relative number of CD4-CD8⁺ cells. At the same concentration the expression of the CD5 antigen was reduced by the antimetabolites, indicating that maturation of the thymocytes was inhibited. Distribution of the forward light scatter, a cell size-related parameter, showed that the formation of small thymocytes was reduced by the nucleoside analogues. Light and electron microscopic investigations indicated cytotoxic effects of the drugs on the thymocytes, whereas the epithelium was only slightly affected.     

Inhibition by papaverine and eupaverin of 3′,5′-cyclic AMP phosphodiesterase from rabbit skeletal muscle

Summary Papaverine and eupaverin are potent inhibitors of 3,5-cyclic AMP phosphodiesterase from rabbit white skeletal muscle. These drugs inhibit more the activity associated with the particulate fractions than that associated with the soluble fraction.     

Hemmung von Adenyl-Cyclase-Präparationen aus der Rattenniere durch Calciumionen und verschiedene Diuretica

Summary Antidiuretic hormone (ADH) increases the permeability to water of certain epithelial membranes. This effect, found in the urinary bladder of the toad and in the distal tubules and the collecting ducts of kidney, is mediated intracellularly by adenosine 35-monophosphate (Ado-35-P). Calcium ions and the diuretic ethacrynic acid are known to inhibit the ADH-induced increase in water permeability of the toad bladder. In adenyl cyclase preparations from rat renal cortex and medulla, the influence of these substances as well as of other diuretics added in vitro has been studied. Adenyl cyclase activity has been determined, excepted as noted, by measuring Ado-35-P formed from 1 mM ¹⁴C-ATP in the presence of 10 mM Mg⁺⁺, an ATP regenerating system, and 5 mM unlabeled Ado-35-P to reduce the enzymatic degradation of the labeled Ado-35-P.Calcium ions reduced the rate of Ado-35-P formation by particles from renal cortex and medulla when the activity was measured in the presence of either Mg⁺⁺ or Mn⁺⁺. With 10 mM Mg⁺⁺, 1 mM Ca⁺⁺ decreased adenyl cylase activity by about 50%. Activities of cortical adenyl cyclase stimulated by parathyroid hormone, thyrocalcitonin or ADH and of medullary adenyl cyclase stimulated by ADH were also reduced by about 50% in the presence of 1 mM Ca⁺⁺. The inhibition was independent of the ATP concentration, but was influenced by the Mg⁺⁺ content of the incubation medium.Adenyl cyclase activities of cortical and medullary membrane preparations were reduced by about 50% by 0.2 mM ethacrynic acid. The extent of this inhibition was essentially the same whether the enzymatic activity was determined in the absence or presence of stimulating hormones. The inhibitory action of ethacrynic acid was partially prevented by simultaneous addition of dithioerythritol (DTE). A derivative of ethacrynic acid, L 589420-0-2, also inhibited renal adenyl cyclase, but its action was not influenced by the addition of DTE. Adenyl cyclase from both parts of the kidney was inhibited by about 90% by 0.2 mM mersalyl. This action was almost completely prevented by the addition of 1 mM DTE. The pharmacological significance of adenyl cyclase inhibition by these diuretics is still uncertain since the role of Ado-35-P in the regulation of sodium transport is as yet unclear.Other diuretics, hydrochlorothiazide, furosemide, mefruside, amiloride, and the non-diuretic benzothiadiazine, diazoxide, had essentially no effect on cortical and medullary adenyl cyclase preparations when they were added in 0.1–0.5 mM concentration.The methylxanthines, theophylline and caffeine, which are known to inhibit nucleoside 35-monophosphate phosphodiesterase, reduced the rate of Ado-35-P formation. The unstimulated and the hormone-stimulated adenyl cyclases were inhibited to the same extent by theophylline. When adenyl cyclases was stimulated by fluoride, however, we found only a very small inhibition by theophylline. Inhibition of the medullary adenyl cyclase was greater than that of the enzyme prepared from renal cortex. At a concentration of 1 mM these methylxanthines significantly inhibited the medullary enzyme, but the inhibition became asymptotic at about 50% when concentrations up to 20 mM were used. Therefore, it is likely that inhibition by these substances varies in different cell types and tissues.Instead of phosphodiesterase inhibitors, unlabeled Ado-35-P can be used in the assay of adenyl cyclase activity to reduce the degradation of enzymatically formed labeled Ado-35-P. This addition, though, can also influence adenyl cyclase activity. In a medullary enzyme preparation 0.2 mM Ado-35-P reduced the adenyl cyclase activity by 13%, 5 mM Ado-35-P by 35%.

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Abkürzungen Ado-35-P Adenosin-35-monophosphat - Guo-35-P Guanosin-35-monophosphat - ADH antidiuretisches Hormon, Vasopressin - PTH Parathormon - TCT Thyreocalcitonin - DTE Dithioerythrit - EDTA Äthylendiamintetraessigsäure Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.Über einen Teil der Ergebnisse wurde auf der 11. Frühjahrstagung der Deutschen Pharmakologischen Gesellschaft berichtet (Jakobs et al., 1970). Einige der vorliegenden Ergebnisse sind der Inauguraldissertation von K. H. J. (Medizinische Fakultät der Universität Heidelberg, 1971) entnommen.     

The antinociception induced by \-endorphin administered intrathecally is mediated by the activation of μ and κ-opioid receptors in the rat

The antinociception induced by -endorphin given supraspinally has been previously demonstrated to be mediated by the stimulation of -, but not -, - or -opioid receptors in rats and mice. The present study was designed to determine what types of opioid receptors in the spinal cord are involved in the antinociception induced by intrathecally (i.t.) administered -endorphin. Antinociception was assessed by the tailflick test in male Sprague-Dawley rats. CTOP (0.9–6.6 nmol), a selective -opioid receptor antagonist, or nor-BNI(13.6–95.3 nmol), a selective -opioid receptor antagonist, given i.t. dose-dependently reversed i.t. administered -endorphin-induced inhibition of the tail-flick response. On the other hand, naltrindole (6.6–44.4 nmol), a selective -opioid receptor antagonist, or -endorphin (1–27) (1–6.7 nmol), a selective -opioid receptor antagonist given i.t., did not antagonize the inhibition of the tail-flick response induced by i.t. administered -endorphin. The results are consistent with the previous study in mice [Tseng LF and Collins KA (1992) Eur J Pharmacol 214: 59–65] that the antinociception induced by -endorphin given i.t. is mediated by the stimulation of - and -, but not - and -opioid receptors.Abbreviations i.t. Intrathecal - CTOP D-Phe-Cys-Tyr-D-Try-Orn-Thr-Pen-Thr-NH₂ - nor-BNI Norbinaltorphimine     

The effects of ethanol,estrogen, and hexachlorobenzene on the activities of hepatic δ-aminolevulinate synthetase, δ-aminolevulinate dehydratase,and uroporphyrinogen decarboxylase in male rats

To determine if clinically observed disorders in heme biosynthetic enzymes, known as sporadic porphyria cutanea tarda (PCT), could be reproduced in experimental animals, male Fischer rats were treated with ethanol, estrogen and hexachlorobenzene (HCB). A series of heme biosynthetic enzymes were assayed. In the rats given free access to 8% ethanol-drinking water for 15 weeks, -aminolevulinate (ALA) dehydratase was significantly reduced in erythrocytes. In the liver, ALA synthetase and uroporphyrinogen (UROgen) decarboxylase activities remained unchanged. In bone marrow cells, these activities did not change markedly. In the rats treated with estrogen (1 mg estrioltripropionate /rat/week, IM), no body weight gain was observed during the treatment for 15 weeks and urinary ALA excretion increased to 1.7 fold over normal level. In the liver, a significant increase was observed in the activity of ALA dehydratase, but other enzymes remained within the normal level. In bone marrow cells and erythrocytes, ALA dehydratase was also increased. ALA synthetase increased only in bone marrow cells to 2.1 times higher than the control level. In rats fed 0.3% HCB-diet for 8 weeks, urinary excretion of ALA, coproporphyrin and uroporphyrin increased to 2.4, 3.3 and 3.8 times higher than the controls, respectively. In the liver, an increase was observed in ALA synthetase, while a decrease was observed in ALA dehydratase and UROgen decarboxylase. In bone marrow cells and erythrocytes, ALA dehydratase was reduced and activities of other enzymes did not show any changes.These results indicate that alcohol, estrogen and HCB do not produce phenomena similar to those observed clinically in PCT.     

Enhancement by α-fluoromethylhistidine of the thiopental sleep-prolonging action of Δ9-tetrahydrocannabinol

The effect of -fluoromethylhistidine (-FMH), a specific inhibitor of histidine decarboxylase, on the potentiation of thiopental-induced sleep by ⁹-tetrahydrocannabinol (THC), which inhibits the histamine turnover in the brain, was examined in mice and rats. The sleeping time after injection of thiopental sodium (40 mg/kg, IV) was prolonged by THC (10 mg/kg, IP, 1 h before) to approximately twice the control value. -FMH (50 mg/kg, IP) administered alone had no significant influence on the thiopental sleeping time. However, -FMH given 1 or 3 h before THC treatment markedly enhanced the THC potentiation of thiopental-induced sleep. Such an enhancement by -FMH was not observed when -FMH was administered 15 h before THC treatment. The brain histamine level decreased by 60% during the first 4 h after -FMH injection and remained low until 15 h after the treatment. The thiopental sleep-potentiating action of morphine, chlorpromazine and diazepam was not affected by pretreatment with -FMH. The transient enhancing effect of -FMH on the THC potentiation of thiopental-induced sleep suggests that the histaminergic system is one of the activating transmitter systems in the brain.     

Differences between full and partial α-adrenoceptor agonists in eliciting phasic and tonic types of responses in the longitudinal smooth muscle of the rat portal vein

Summary The aim of the present investigation was to study, taking into account both quantitative and qualitative differences, the influence of full and partial -adrenoceptor agonists on spontaneous myogenic activity in the rat portal vein.We found that the -adrenoceptor agonists cirazoline, adrenaline, noradrenaline, phenylephrine, St 587, Sgd 101/75, B-HT 920 and UK-14,304 could increase the amplitude of the phasic myogenic contractions in the rat portal vein with apparent differences in EC₅₀ and E_max values. In addition to an increase in phasic myogenic activity, the -adrenoceptor agonists cirazoline, adrenaline, noradrenaline, and phenylephrine were also able (in higher concentrations) to increase the basal tone of the rat portal vein preparation, again with apparent differences in EC₅₀ and E_max values. Changing the extracellular Ca²⁺ concentration from 0.9 mmol/l to 2.5 mmol/1 had no influence on the phasic character and the concentration range in which St 587 and UK-14,304 increased spontaneous myogenic activity, although changes in amplitude and frequency of the spontaneous myogenic contractions were less pronounced at a higher extracellular Ca²⁺ concentration (2.5 mmol/1). By the use of Schild analysis with the competitive a-adrenoceptor antagonists prazosin (pA₂ = 8.74) and 5-methyl-urapidil (pA₂ = 8.37), it was established that the contractile responses to St 587 were mediated by the same ₁-adrenoceptor subtype as the phasic and tonic type of contraction elicited by phenylephrine as described in a previous study. The concentration-response curve of UK-14,304 was significantly shifted to the right by low concentrations of prazosin (3 nmol/1–30 nmol/1), indicating stimulation of ₁-adrenoceptors by UK-14,304 in the rat portal vein. The -adrenoceptor antagonists phenoxybenzamine and chloroethylclonidine irreversibly blocked the contractile responses to St 587. Based on the method of receptor alkylation with phenoxybenzamine an affinity constant was calculated for St 587 (pK_a = 5.91). Phenoxybenzamine was approximately 1000-fold more potent in inactivating ₁-adrenoceptors than chloroethylclonidine.In conclusion there appeared to be a divergence in the excitation-contraction coupling of ₁-adrenoceptors in the rat portal vein, which is reflected by two types of contraction (phasic versus tonic). The extent to which both the phasic and tonic types of contraction are stimulated by agonists depends on the affinity and intrinsic efficacy for each of the receptor-coupled effector pathways. Thus, partial and full agonism can only meaningfully be discussed if confined to one particular effector pathway. Send offprint requests to H. R. Schwietert at the above address     

Synthesis,Physicochemical Properties,and Cytotoxicity of a Series of 5′-Ester Prodrugs of 5-Iodo-2′-deoxyuridine

Five aliphatic 5-esters of 5-iodo-2deoxyuridine (IDU) were synthesized via an acid chloride alcoholysis reaction. The solubility in pH 7.4 phosphate buffer, lipophilicity as determined by partition experiments in octanol/pH 7.4 buffer, and cytotoxicity of these potential prodrugs were evaluated. The esters showed a 43- to 250-fold increase in lipophilicity and a 1.6- to 14-fold decrease in aqueous solubility relative to IDU. At a concentration of 50 µM, all esters showed reduced cytotoxicity toward uninfected Vero cells relative to IDU.     

Use of the Peptide Carrier System to Improve the Intestinal Absorption of L-α-Methyldopa: Carrier Kinetics,Intestinal Permeabilities,and In Vitro Hydrolysis of Dipeptidyl Derivatives of L-α-Methyldopa

Intestinal permeabilities of five dipeptidyl derivatives of L--methyldopa (I) were studied by an in situ intestinal perfusion method. The dipeptides displayed a significant increase in their permeabilities compared to L--methyldopa. The increases ranged from 4 to 20 times. These results suggest that the peptide transport system is less structurally specific than the amino acid transport systems and can be used as an absorption pathway for peptide analogues. The kinetic advantage demonstrated by the dipeptide, L--methyldopa-L-phenylalanine, over the amino acid analogue, L--methyldopa, suggests that the peptide carrier would be a possible route for improving the intestinal absorption of pharmacologically active amino acid analogues. Furthermore, the preliminary results of in vitro hydrolysis studies of selected dipeptidyl derivatives indicate that the peptide carrier system could be used as a base for a prodrug strategy.