Hemodynamic differences between carvedilol and labetalol in the cutaneous circulation

The effects of labetalol and carvedilol on local cutaneous microvascular perfusion and calculated local cutaneous microvascular resistance were investigated in anesthetized rats at submaximal doses that produced equivalent reductions in blood pressure and heart rate. Labetalol decreased cutaneous perfusion (– 25% ± 3%) without significantly affecting cutaneous vascular resistance ( – 6% ± 3%). In marked contrast, carvedilol dramatically increased cutaneous perfusion ( + 64% ± 9%) and significantly reduced cutaneous vascular resistance ( – 57% ± 3%). These results suggest that carvedilol and labetalol possess differences in the mechanisms by which they produce vasodilation in vivo.     

Seroquel (ICI 204 636), a putative “atypical” antipsychotic,in schizophrenia with positive symptomatology: results of an open clinical trial and changes of neuroendocrinological and EEG parameters

Preclinical data indicated that seroquel (ICI 204 636), a dibenzothiazepine with 5-HT₂ and D₂-like receptor antagonistic properties, might be an effective antipsychotic agent, causing fewer extrapyramidal side effects than typical neuroleptics. In the present study, 12 patients suffering from schizophrenia or schizophreniform disorder with predominantly positive symptomatology were treated in an open clinical trial for 4 weeks with seroquel at a maximum dosage of 750 mg/day. The drug was generally well tolerated, and virtually no adverse extrapyramidal side effects such as acute dystonia, parkinsonism or akathisia were observed. Total scores for BPRS (item score 0–6; baseline: 42.0±2.3; mean±SeM), SAPS (64.5±4.8) and SANS (55.0±4.3) showed a moderate decrease at the end of treatment (BPRS: 30.0±3.5; SAPS: 36.1±6.7; SANS: 42.5±5.9), when intention-to-treat analysis was applied. There were considerable interindividual differences in treatment response, with some subjects showing almost full remission of positive symptoms, in contrast to about half of the patients who showed no satisfactory clinical improvement. Interestingly, patients showing good antipsychotic response reported slight initial side effects like mild sedation. Prolactin and TSH levels were not altered during seroquel administration. As to pharmaco-EEG investigations, seroquel caused a moderate increase of the absolute power in the alpha, theta, and beta frequency bands, paralleled by a decrease of delta activity. There were no signs of paroxysmal EEG activity under seroquel. Our results suggest that seroquel may be a well tolerated drug with some antipsychotic properties, exhibiting no extrapyramidal side effects that could be of use in the treatment of schizophrenic patients with positive symptomatology. Further double-blind studies with higher doses, in order to test presumably better efficacy, and with monitoring of plasma levels, are needed to extend the present results.     

Effects of α- and β-receptor blockade on systolic time intervals

Summary The effects of single oral doses of oxprenolol (20 and 80 mg), phentolamine (20 and 40 mg), and combinations of oxprenolol and phentolamine (80:20 and 20:40 mg, respectively) on blood pressure, heart rate and the systolic time intervals (Q—S_2c, PEP_c, LVET_c and PEP/LVET) have been studied under double-blind conditions in seven healthy volunteers during recumbency and passive tilting. Heart rate was slowed by oxprenolol and slightly increased by phentolamine, but was not affected by either of the combinations. There were only minimal non-significant changes in blood pressure in the subjects, all of whom were normotensive. Of the systolic time intervals, PEP_c was most affected: it was prolonged by oxprenolol because of reduction in myocardial contractility, and shortened by phentolamine as a result of the cardiostimulant effect of this compound. When the two drugs were administered in combination, the negative inotropic effects of the -blocker always predominated. The changes induced by the drugs were more pronounced during upright titling than in the recumbent position. Measurement of PEP_c provides a suitable means of assessing the influence of cardiovascular drugs on left ventricular function in pharmacological studies in man.     

Rolipram forms a potent discriminative stimulus in drug discrimination experiments in rats

Long Evans rats were trained to discriminate 0.2 mg/kg IP (±)-rolipram from vehicle injection in a food-motivated two-lever operant task. Eight out of nine rats acquired the discrimination after an average of 91 sessions (min 65, max 137). The ED₅₀ of (±)-rolipram was 0.06 mg/kg IP. Generalization tests with (–)- and (+)-rolipram showed that the (–)-isomer was 8 times more active than (+)-rolipram with an ED₅₀ of 0.06 and 0.4 mg/kg IP respectively. The phosphodiesterase inhibitor RO 20-1724 partially (83%) generalized to (±)-rolipram in doses of 0.6 and 1.0 mg/kg IP. IBMX 5 mg/kg IP showed 63% generalization. Tests with imipramine and the (+)- and (–)-isomer of the noradrenaline uptake inhibitor oxaprotiline suggest that NA-uptake inhibiting drugs do not form an interoceptive cue which is (±)-rolipram-like. dbcAMP 12.5 mg/kg SC and 100 mg/kg SC dbcGMP did not generalize to the training drug. The nature of the discriminative stimulus produced by this dose of (±)-rolipram in rats remains to be elucidated. Offprint requests to: R. Ortman     

Coronary vascular responses to nicotine in the anaesthetized dog

Summary The effect of the intra-coronary (i.c.) injection of nicotine on large coronary artery diameter and coronary blood flow was examined in anaesthetized dogs. In sixteen untreated dogs nicotine (20 g i.c.) had a biphasic effect on arterial pressure (initial increase, 7 ± 2 mmHg; secondary decrease, –8 ± 3 mmHg) which was accompanied by small and variable effects on heart rate and an increase in LV dP/dt. Nicotine increased large coronary artery diameter by 5.8 ± 0.8% but had a biphasic effect on coronary blood flow (initial increase, 41 ± 7 ml/min; secondary decrease, –10 ± 2 ml/min). Bilateral vagotomy or muscarinic receptor blockade with atropine (0.1 mg/kg i. v.) did not significantly affect the nicotine-induced changes in coronary artery diameter or coronary blood flow. The additional antagonism of -adrenoceptors with propranolol (1 mg/kg i. v.) abolished the effect of nicotine in coronary artery diameter ( CD = 0.2 ± 0.2%) and the initial increase in coronary blood flow ( CBF = 1 ± 1 ml/min) but enhanced the secondary decrease in flow ( CBF = –25 ± 3 ml/min). The nicotine-induced decrease in coronary blood flow observed after muscarinic and -adrenoceptor blockade was attenuated by antagonism of ₁-adrenoceptors with prazosin (10 g/kg i. c., CBF = –15 ± 3 ml/min) and abolished after additional antagonism of ₂-adrenoceptors with idazoxan (50 g/kg i. c., CBF = –2 ± 1 ml/min). These results indicate that in the anaesthetized dog intra-coronary injection of nicotine results in -adrenoceptor mediated dilatation of both large and small coronary arteries. In the coronary resistance vessels, but not in the large coronary artery, the dilatation is opposed by ₁-and ₂-adrenoceptor mediated vasoconstriction. Send offprint requests to O. L. Woodman at the above address     

The β-adrenergic receptor-adenyl-cyclase system of rat reticulocytes

Summary Non-nucleated red blood cells from rats contain adenyl cyclase, the activity of which is predominantly localized in the reticulocytes. Basal enzyme activities in membrane preparations from reticulocyte-rich blood (pretreatment of rats with acetyl-phenylhydrazide: about 60% reticulocytes) are about 5 times higher than in preparations from reticulocyte-poor blood (untreated animals: 2–3% reticulocytes). The enzyme activities are stimulated 10-fold by sodium fluoride (10^–2 M) and 6 to 8-fold by isoprenaline (10^–4 M).Adenyl cyclase activities in membrane preparations from reticulocyte-rich and reticulocyte-poor blood can be ascribed to identical enzymes since identical apparent K _m (ATP; 3×10^–4 M), K _a (isoprenaline; 3×10^–6 M) and K _i (propranolol vs. isoprenaline; 3×10^–7 M) values were obtained in both preparations.Besides NaF, only phenylethanolamine derivatives with -adrenergic receptor stimulant properties were effective as stimulators of adenyl cyclase activity. The affinities (apparent K _a values) of the investigated compounds decreased in the order isoprenaline—hexoprenaline—fenoterol—salbutamol—adrenaline—terbutaline—noradrenaline—phenylephrine. For maximal intrinsic activity, the catechol structure was essential; the relative intrinsic activities of resorcinol derivatives did not exceed 0.6.The isoprenaline-stimulated adenyl cyclase activities in erythrocyte membrane preparations were competitively inhibited by -adrenergic blocking drugs, the affinities (apparent K _i values) decreasing in the order prindolol—penbutolol—propranolol—practolol. The dextrorotatory enantiomers of penbutolol and propranolol were 1/100 to 1/200 as active as the resp. levorotatory enantiomers.From experiments with -adrenergic agonists (e.g. phenylephrine) and antagonists (e.g. phentolamine), it is concluded that -adrenergic receptors do not interfere with the -adrenergically-mediated cAMP formation in these particular membranes.A variety of hormones and drugs known to stimulate adenyl cyclase activities in various tissues, e.g. ACTH, glucagon, STH, erythropoietin, prostaglandin E ₁ etc. did not affect adenyl cyclase activity in reticulocyte-rich erythrocyte membrane preparations.In contrast to adenyl cyclase activity, phosphodiesterase activities in erythrocyte membrane and cytoplasmic fractions were only twice as high in reticulocyterich as in reticulocyte-poor preparations.From the experiments described, it is obvious that the adenyl cyclase of the rat reticulocyte is subject to monovalent-hormonal, i.e. -sympathomimetic stimulation. Moreover, the premature red blood cell provides a useful model for quantitative studies of the interaction of drugs with the -adrenergic receptor.This study was supported by the Deutsche ForschungsgemeinschaftPreliminary accounts were presented at meetings of the Deutsche Pharmakologische Gesellschaft (Gauger et al., 1972; Gauger and Palm, 1973; Quiring et al., 1974a).     

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.     

Effects of small doses of bisoprolol on blood pressure and lipoprotein concentrations in hypertensive patients

Summary The effects of bisoprolol 2.5 and 5 mg per day on blood pressure, and lipoprotein and apolipoprotein concentrations were compared in 18 newly detected hypertensives in a double-blind, crossover study. All treatment results were related to the values at the end of a four-week placebo run-in period. Each of the two following treatment periods lasted for 3 months.The systolic and diastolic pressures in the supine position were reduced by 19.5/11.7 mm Hg and 14.6/10.4 mm Hg by 2.5 and 5 mg bisoprolol per day, respectively, with no significant difference in effect. Supine heart rate was reduced by 4.7 and 8.2 beats · min^–1, respectively, (P=0.0517 for different effects).The cholesterol concentration in low-density (LDL) and high-density (HDL) lipoproteins was reduced during both regimens, by about 0.3 and 0.1 mmol·l^–1, respectively, difference not significant. Triglyceride concentrations were not significantly affected during either regimen.We conclude that, in this study population, treatment with bisoprolol 2.5 mg per day was equally effective as 5.0 mg per day in reducing blood pressure. The effects on lipoprotein concentrations were small and included an unexpected reduction in LDL-cholesterol concentration. A low dose of a highly selective -adrenoceptor blocker like bisoprolol appears to retain the blood pressure reducing capacity and has lost most of the unfavourable effects on lipoproteins characteristic of higher doses.     

In vivo studies on alpha-adrenergic receptor subtypes in human veins

Summary We studied in vivo responsiveness of venous ₁ and ₂-adrenoceptors, measuring the diameter changes in superficial veins in response to -adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations.Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% ± 10%), methox amine (97% ± 5%), phenylephrine (95% ± 6%), clonidine (54% ± 12%), and azepexole (68% ± 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% ± 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbineantagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60–180 min.Results show that the in vivo effects on veins of -adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with ₁- and ₂-adrenoceptor subtype selectivity cause venoconstriction in vivo, but ₂-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic ₂-receptors. At high doses, ₂-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo. Send offprint requests to H. G. Eichler at the above address     

Efflux of Zidovudine and 2′,3′-Dideoxyinosine Out of the Cerebrospinal Fluid When Administered Alone and in Combination to Macaca nemestrina

To determine if there is active efflux of zidovudine (ZDV) and 2,3-dideoxyinosine (ddl) out of the cerebrospinal fluid (CSF), and if this efflux is saturable, we investigated the steady-state CSF/plasma concentration ratio of the two drugs when administered alone or in combination. Constant-rate infusions of ZDV, ddl or both were administered to seven macaques (Macaca nemestrina) through a chronic venous catheter for a minimum of 28 hr. Antipyrine, a marker of passive diffusion, was coinfused in all experiments. Blood (5 mL) and CSF samples (0.5–1 mL) were collected by venous and lumbar/thoracic punctures, respectively, at 24 and 28 hr after beginning the infusion. When ZDV and ddl were administered alone, the steady-state CSF/plasma concentration ratios were significantly different from unity (ZDV, 0.20 ± 0.08; ddI, 0.09 ± 0.04) and were independent of the plasma concentration (P > 0.05). In contrast, the CSF/plasma concentration ratio of antipyrine (0.82 ± 0.19) was close but significantly smaller than unity (P > 0.05). The CSF/ plasma concentration ratios after simultaneous administration of ZDV and ddI were not significantly different (P > 0.05) from those obtained after administration of the drugs alone. These results suggest that ZDV and ddI are actively transported out of the CSF; however, within the concentration range studied, this efflux is neither saturable nor mutually competitive. Concomitant administration of ZDV and ddI did not produce a systemic interaction in the animals, indicating that the pharmacokinetics of either drug is unaffected by the presence of the other.     

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.     

Influence of the renin-angiotensin system on sympathetic neurotransmission in canine skeletal muscle in vivo

Summary The physiological importance of interactions between angiotensin II and sympathetic neurotransmission was studied in an in vivo model with constant flow blood perfused gracilis muscle in situ in dogs pretreated with desipramine and atropine. Sympathetic nerve stimulation- (2 and 8 Hz, 480 pulses) evoked overflow of endogenous noradrenaline and vasoconstriction, and vasoconstrictor responses to exogenous noradrenaline (0.5 nmol, locally i. a.) were evaluated.Angiotensin converting enzyme inhibition by benazeprilat (10 mg i. v.; n = 8) reduced arterial angiotensin II levels from 26 ± 8 to 2 +- 1 pM and reduced mean arterial and basal muscle perfusion pressures. Subsequent resubstitution of angiotensin II (3, 30 and 90 ng kg^–1 min^–1 i.v.) elevated arterial angiotensin II dose-dependently (to 67 ± 14, 622 ± 63 and 1940 ± 251 pM, respectively), as well as mean arterial and muscle perfusion pressures. Nerve stimulation-evoked noradrenaline overflow was unchanged following benazeprilat (–4 ± 4 and + 1 ± 8% at 2 and 8 Hz, respectively) and during subsequent infusions of angiotensin II. Vasoconstrictor responses to nerve stimulation and exogenous noradrenaline were also uninfluenced by these treatments. Thus, angiotensin II did not enhance sympathetic neurotransmission at the postjunctional level.Another group of animals was pretreated with noncompetitive -adrenoceptor blockade locally by phenoxybenzamine and benextramine (0.5 mg kg^–1 i. a. of each; n = 7), which abolished vasoconstrictor responses to nerve stimulation. The effects of benazeprilat and subsequent angiotensin II infusions (3 and 30 ng kg^–1 min^–1 i.v.) on circulating angiotensin II levels, mean arterial and muscle perfusion pressures were similar in this group. Following -adrenoceptor blockade, however, inhibition of angiotensin converting enzyme reduced sympathetic nerve stimulation-evoked noradrenaline overflow by 23 ± 4 and 21 ± 5% at 2 and 8 Hz, respectively (P < 0.01 for both). Angiotensin II infusions failed to enhance evoked noradrenaline overflow (–5 ± 10 and –18 ± 10% at 2 Hz; +6 ± 13% and –3 ± 14% at 8 Hz) also under these conditions.It is concluded that circulating angiotensin II does not influence sympathetic vascular control in canine skeletal muscle even at very high levels in arterial plasma. Angiotensin converting enzyme inhibition reduces nerve stimulation-evoked noradrenaline overflow only in the presence of -adrenoceptor blockade, suggesting that prejunctional -adrenoceptors have an overriding importance over prejunctional angiotensin II-receptors in the modulation of noradrenaline release in this model. The effect of converting enzyme inhibition may be related to merely local changes in angiotensin II concentration or — unrelated to the renin-angiotensin system — to other consequences of the blockade of this unspecific enzyme. Send offprint requests to J. Schwieler at the above address     

Vasopressin stimulates release of β-lipotropin and β-endorphin in conscious rats as measured by radioimmunoassay of unextracted plasma

Summary Using a newly developed radioimmunoassay to determine the -endorphin-like immunoreactivity (-EI) in unextracted plasma, the effect of vasopressin injections on plasma -EI was investigated in conscious rats. Arginine vasopressin caused a dose-dependent increase of plasma -EI from 34.5±7.8 fmol ml^–1 (n=6) in vehicle-treated animals to 205.0±36.1 fmol ml^–1 (n=7) after injection of the highest vasopressin dose employed (486 ng/100 g b.w.). In view of the appreciable cross-reactivity of -lipotropin (-LPH) in the radioimmunoassay used, plasma was extracted and subjected to gel chromatography on a Sephadex G-50 column. On average, about 70% of the -EI co-eluted with human -LPH and about 30% with human -endorphin in plasma extracts obtained from both control and vasopressin-treated rats. No peripheral conversion of human -LPH occurred under the experimental conditions, since after i.v. bolus injection of human -LPH 97% of the -EI comigrated with human -LPH during gel filtration. A similar blood pressure increase to that induced by the vasopressin injections, when elicited by noradrenaline or angiotensin II i.v., was not followed by an elevation of plasma -EI.These data indicate that vasopressin stimulates -lipotropin and -endorphin release into the systemic circulation in vivo.     

Acebutolol in hypertension: Relationships between drug concentration and effects

Summary Relationships between plasma concentrations of acebutolol (Ac) and its N-acetyl metabolite (Am) and pharmacological effects were studied in patients with essential hypertension. In an acute study (N=7) 400 mg oral acebutolol produced similar peak plasma levels of Ac and Am but Am had a longer half life. Group mean pulse and blood pressure fell significantly, although the size of individual blood pressure fall varied five-fold. There was no significant change in plasma renin activity. Correlations were found between drug concentration and -blockade, assessed as % reduction in exercise tachycardia, and between both of these and the fall in post exercise systolic blood pressure. Similar comparisons were made in a chronic optimum dose study (N=11) both pre-dose and three hours post-dose. Am concentration was consistently higher than Ac at both times. Blood pressure was lowered equally pre-and post-dose when compared to placebo and plasma renin activity (PRA) was reduced on active treatment. Correlations were again shown between drug concentrations and -blockade but none were found with changes in blood pressure or PRA. Between subject differences in blood pressure response could not be fully explained in terms of the other measurements but it is unlikely that pharmacokinetic differences are the major source of this variability.     

Venoconstrictor effect of dihydroergotamine in superficial hand veins

Summary The effect of dihydroergotamine on superficial veins of the hand has been investigated in healthy volunteers. The compliance of the veins was assessed by measurement of their diameter at a standard congestion pressure. Direct local infusion into the vein under study of 0.4 µg and 2.0 µg dihydroergotamine reduced venous compliance by 15±5% and 30±3%, respectively. Local infusion of phentolamine 20 µg, an -adrenoceptor blocking drug, did not affect venous compliance, but markedly inhibited the venoconstrictor effect of dihydroergotamine 2 µg. It was concluded that the venoconstrictor effect of dihydroergotamine demonstrated in this experiment was mainly due to -adrenoceptor stimulation. Venous compliance also decreased after oral administration of dihydroergotamine 10 mg; after 150 min, venous diameter was reduced by 30±5%.     

Depression of exploratory activity by clonidine in rats as a model for the detection of relative pre- and postsynaptic central noradrenergic receptor selectivity of α-adrenolytic drugs

Summary The effects of various -adrenoceptorblocking drugs on the depression of exploratory activity (ambulation and rearing) induced by 0.1 mg/kg i.p. clonidine were investigated in the rat. In parallel experiments, the effects of the same drugs on pre- and postsynaptic -receptors were determined in vitro (field-stimulated cortex slices and isolated vas deferens of the rat, respectively). Tolazoline, esproquine, yohimbine and piperoxan distinctly antagonized the inhibition of exploration produced by clonidine. All these drugs were found to possess relatively higher selectivity for the presynaptic -receptors, as judged by the ratios of the concentrations inducing a 50% increase in field-stimulated ³H-noradrenaline-overflow and the concentrations required to shift the EC₅₀ for the antagonism of noradrenaline-induced contractions of the vas deferens to the right by a factor of 2 (pA₂, ratio <1): In contrast, phentolamine and phenoxybenzamine which showed preferential postsynaptic -receptor blocking activity (ratio>1), potentiated rather than antagonized the effects of clonidine. Mianserin, although preferentially blocking the postsynaptic receptors, had no effect on clonidine-induced hypoactivity up to the high dose of 100 mg/kg i.p., probably because of its additional NA-uptake-inhibiting properties. The antagonism of clonidine by the selective presynaptic -receptor blockers was observed within a limited dose-range. Increasing the doses above an optimal level, which varied from one compound to another, resulted in a decrease in the effect. It is suggested that this phenomenon reflects the counter-balancing postsynaptic -adrenoceptor blockade occuring at higher concentrations of these drugs. In general, the results show a fairly good correlation between antagonism of clonidine in vivo and preferential blockade of presynaptic -receptors in vitro. Clonidine-induced suppression of exploration therefore seems to be a valuable model for the investigation of drug interactions with -adrenergic receptors in the central nervous system.These results were presented in part at the Spring Meeting of German Pharmacological Society, Mainz, 16–18 March, 1977     

Absorption of 2′, 3′-Dideoxyinosine from Lower Gastrointestinal Tract in Rats and Kinetic Evidence of Different Absorption Rates in Colon and Rectum

This study explored the rectal route of administration for 2,3-dideoxyinosine (ddI). Rats were given a rectal infusion of nonradiolabeled ddI (200 mg/kg in 0.7 mL saline) over 35 min along with an intravenous (iv) bolus injection of [8-³H]ddI (20 µCi, equivalent to 2.1 µg), which was used to calculate the absolute rectal bioavailability of ddI. Maximal plasma concentrations of rectally administered unlabeled ddI were 5.4 ± 2.2 µg/mL and were reached at the end of the infusion. The rectal bioavailability averaged 15.6 ± 4.4% (n = 9). The second aim of this study was to examine the kinetics of ddI absorption from the colorectal region. Analyses of the absorption rate–time profiles by the Loo–Riegelman and deconvolution methods showed biphasic absorption: a rapid phase during infusion and a slow phase postinfusion. These profiles were inconsistent with a mammillary model with absorption from a single site with one apparent rate constant. The model which gave the best fit for infusion and postinfusion data consisted of two different sites (colon and rectum) with different apparent absorption rate constants. The two sites were connected by a first-order transfer of drug solution from rectum to colon. The apparent absorption rate constant in the rectum was 39-fold higher than that in the colon. In conclusion, these results show absorption of ddI from the colorectal region and suggest the rectal route as an alternative to the oral route. The data further suggest different absorption sites in the colorectal region, with a more rapid absorption in the rectum than in the colon.     

The role of α1- and α2-adrenoceptors in the coronary vasoconstrictor responses to neuronally released and exogenous noradrenaline in the dog

Summary 1. Coronary vasoconstriction was examined in response to the neuronal release of noradrenaline produced by bilateral carotid occlusion and the infusion of tyramine (5 – 50 Erg/kg/min i. v.) in anaesthetized dogs which had been vagotomized and treated with the -adrenoceptor antagonist propranolol (1.0 mg/kg i. v.). These responses were compared to those produced by the infusion of noradrenaline (0.1 – 0.5 g/kg/min i. v.). 2. Similar increases in late diastolic coronary resistance were produced by bilateral carotid occlusion (0.70 ± 0.25 mmHg min/ml), and intravenous infusions of tyramine, 20 g/kg/min (0.70 ± 0.12 mm Hg min/ml) and noradrenaline, 0.5 gg/kg/min (0.59 ± 0.11 mm Hg min/ml). 3. Selective antagonism at ₁-adrenoceptors with prazosin (0.5 mg/kg i. v.) attenuated the coronary constrictor response to bilateral carotid occlusion (0.36 ± 0.09 mm Hg min/ml), tyramine (0.12 ± 0.06 mm Hg min/ml) and noradrenaline (0.18 ± 0.07 mm Hg min/ml). Antagonism at ₂-adrenoceptors with idazoxan (1 mg/kg i. v.) attenuated the coronary vasoconstriction produced by bilateral carotid occlusion (0.30 ± 0.06 mmHg min/ml), tyramine (0.17 ± 0.08 mmHg min/ml) and noradrenaline (0.12 ± 0.03 mm Hg min/ml). Combined antagonism at both ₁- and ₂-adrenoceptors with prazosin and idazoxan abolished the responses to bilateral carotid occlusion, tyramine and noradrenaline. 4. These results show that coronary vasoconstriction produced by either neuronally released or exogenous noradrenaline is mediated by both ₁ and ₁-adrenoceptors. It appears that in the coronary resistance vessels of the dog postjunctional ₁- and ₂-adrenoceptors are both innervated by sympathetic nerves.     

Bis(carbamoyloxymethyl) Esters of 2′,3′-Dideoxyuridine 5′-monophosphate (ddUMP) as Potential ddUMP Prodrugs

No HeadingPurpose. We previously reported the synthesis of bis(pivaloyloxymethyl) 2,3-dideoxyuridine 5-monophosphate (POM₂-ddUMP) (1a) as a membrane-transport prodrug formulation of the free parent nucleotide, ddUMP. Although successful at delivering ddUMP into cells in culture, POM₂-ddUMP was rapidly degraded by plasma carboxylate esterases after intravenous administration to experimental animals, and therefore has limited therapeutic potential as a systemically administered prodrug. We now report the synthesis of bis(N,N-dimethylcarbamoyloxymethyl)- and bis(N-piperidinocarbamoyloxymethyl) 2,3-dideoxyuridine 5-m onophosphate [DM₂-ddUMP (1b) and DP₂-ddUMP (1c), respectively], analogues of POM₂-ddUMP that were designed to be more resistant to degradation by plasma esterases..Methods. After entering cell by passive diffusion, it was anticipated that loss of one of the carbamoyloxymethyl groups of 1b and 1c would occur by spontaneous chemical hydrolysis to give the intermediate phosphodiesters, 2b and 2c. Cleavage of the remaining carbamoyloxymethyl groups by cellular phosphodiesterase I would generate ddUMP. 1b and 1c were prepared by condensation of 2,3-dideoxyuridine (ddU) with the appropriate bis(N-alkylcarbamoyloxymethyl) phosphate in DMA in the presence of triphenylphosphine and diethyl azodicarboxylate (the Mitsunobo reagent).Results. The half-lives of 1b and 1c when incubated at a concentration of 10^–4 M in human plasma at 37°C were 3.5 h and 3.7 h, respectively, similar to the half-lives observed under the same temperature conditions in 0.05 M aqueous phosphate buffer, pH 7.4. By contrast, the half-life of the POM₂ prodrug, 1a, in plasma was only 5 min. The initial products of degradation of 1b and 1c were the phosphodiesters 2b and 2c. The latter compounds gave rise to ddUMP when incubated with snake venom phosphodiesterase I.Conclusions. These findings support the premise inherent in the design of 1b and 1c, namely that the carbamate prodrugs are far more resistant to hydrolysis by plasma carboxylate esterases than their POM counterparts and can revert to the free parent 5-mononucletides by successive chemical and enzymatic hydrolysis. Further studies of 1b and 1c as membrane-permeable prodrugs of ddUMP are in progress.     

Minimal α1- and α2-adrenoceptor-mediated coronary vasoconstriction in the anaesthetized swine

Summary -Adrenoceptor-mediated coronary vasoconstriction contributes to the initiation and aggravation of experimental and clinical myocardial ischaemia. However, the extent of ₁- and ₂-adrenoceptor-mediated constriction has not been characterized in the porcine coronary circulation despite the frequent use of this experimental model.Fifteen swine were anaesthetized with either -chloralose, enflurane or isoflurane to determine the amount of -adrenoceptor-mediated coronary constriction elicited by either the selective ₁-adrenoceptor agonist methoxamine or the selective ₂-adrenoceptor agonist azepexole. The left anterior descending coronary artery was cannulated and perfused by an external pump delivering constant blood flow from the carotid artery. Following bilateral cervical vagotomy and ß-adrenoceptor blockade with propranolol (2 mg kg^–1), graded dosages of either one of the -adrenoceptor agonists (9–45 g kg^–1 min^–1) were infused into the coronary perfusion line while coronary arterial pressure (CAP) was measured through a distal side arm of the cannula to detect changes in coronary vascular resistance. Infusion of the -adrenoceptor agonists was terminated when systemic arterial pressure increased. Sonomicrometers were used to measure anterior left ventricular wall thickening for the assessment of regional contractile function. During methoxamine infusion, no increase in vascular resistance was observed during -chloralose, enflurane or isoflurane anaesthesia, whereas the infusion of azepexole increased CAP from 103 ± 31 mmHg to 120 ± 35 mmHg (-chloralose), from 101 ± 16 mmHg to 122 ± 11 mmHg (enflurane) and from 84 ± 20 mmHg to 94 ± 19 mmHg (isoflurane), respectively. In four additional swine anaesthetized with enflurane, the intracoronary infusion of the full catecholamine agonist noradrenaline in the presence of propranolol (6 mg kg^–1) increased CAP from 98 ± 10 to 105 ± 10 mmHg prior to an increase in regional left ventricular function or systemic arterial pressure.These results indicate that there are no ₁- and relatively little ₂-adrenoceptor-mediated coronary constrictive effects in swine. Furthermore, neither -adrenoceptor agonist produced any detectable change in regional myocardial contractile function, regardless of the anaesthesia used.Supported by the German Research Foundation (He 1320/3-2). Dr. Guth is the recipient of a scholarship from the Alexander von Humboldt-Foundation. Send offprint requests to G. Heusch at the above address