Dopamine-induced diuresis in the cat without changes in renal hemodynamics

Summary The effects of intravenous (i.v.) and intraarterial (i.a.) injection and infusion of dopamine (DA) on renal hemodynamics, regional sympathetic activity and kidney function were investigated in anaesthetized cats. In response to the i.v. bolus injection of DA (25 g/kg), mean arterial blood pressure (MABP) was increased by 19.7%, renal blood flow (RBF) by 16.6%, and regional sympathetic discharges were inhibited. The principal effect of i.a. bolus injection of DA into the renal artery was vasoconstriction. Vasodilation was observed neither after lower doses of DA nor after pretreatment with phenoxybenzamine.During continuous i.v. infusion of 10 g DA kg^–1 min^–1 MABP, RBF, renal sympathetic discharges and glomerular filtration rate (GFR) did not change, whereas urine volume was increased by 120.5%, sodium excretion by 99.7%, chloride excretion by 143.2%, and potassium excretion by 31.9%. Urine osmolality was decreased and osmolal clearance increased. Raising the DA dose to 25 g kg^–1 min^–1 resulted in a fall of GFR, but the diuretic response was not significantly different from that of the low dose. Bulbocapnine (6 mg/kg i.v.) antagonized the DA-induced diuresis.In conclusion, the diuretic effect of DA in the cat is not dependent on a change in RBF, GFR or renal sympathetic activity. This suggests that a tubular site of action is primarily responsible for DA diuresis.     

Adenosine and the endothelium-dependent modulation of 3H-noradrenaline release in the canine pulmonary artery

This study aimed at characterizing the influence of endothelium on noradrenaline release from the canine pulmonary artery. Tritium overflow from intact or endothelium-free vessels preloaded with 0.2 mol.1^{–1 3}H-noradrenaline was evoked by electrical stimulation (1 Hz, during 5 min) or potassium (25–100 mmol. 1^–1).The fractional release of tritium evoked by electrical stimulation was increased by removing the endothelium [from 1.7 (1.2; 2.4) to 2.7(2.3; 3.2) × 10^–5. pulse^–1, n = 10; P < 0.05]. Neither N^G-nitro-l-arginine methyl ester (l-NAME) (up to 300 mol.l^–1) nor indomethacin (up to 30 ml.l^–1), nor endothelin-1 (up to 30 nmol.l^–1), nor suramin (up to 300 mol.l^–1) changed tritium release evolved by electrical stimulation. In contrast, the selective A₁-adenosine antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (3.3-33 nmol.l^–1) concentration-dependently increased, and the selective A₁-adenosine agonist N⁶-cyclopentyladenosine (CPA) (3.3–100 nmol. l^–1) concentration-dependently decreased the evoked release of noradrenaline. Since the effects of DPCPX were observed in endothelium-intact tissues only, it may be concluded that adenosine secreted by the endothelium activates prejunctional release-inhibiting A₁-receptors. Tetraethylammonium (TEA) (3.3–33 mmol. l^–1) enhanced tritium overflow evoked by electrical stimulation more in endothelium-free than in endothelium-intact vessels, indicating that some K⁺-channel opener is involved in the inhibitory role of endothelium on noradrenaline release. Since it had been previously shown that A₁-adenosine receptors are coupled to K⁺-channels, it is suggested that adenosine may inhibit noradrenaline release through the opening of K⁺-channels.In conclusion, the results show that in the canine pulmonary artery, adenosine is a good candidate for the endothelium-dependent inhibitory factor which is responsible for the reduction of noradrenaline release evoked by electrical stimulation.     

Facilitatory and inhibitory modulation by endogenous adenosine of noradrenaline release in the epididymal portion of rat vas deferens

Summary The present study aimed at determining the modulation by adenosine of the release of noradrenaline in the epididymal portion of the rat vas deferens. The tissues were treated with pargyline and perifused in the presence of desipramine and yohimbine. Up to four periods of electrical stimulation were applied (5 Hz, 9 min).The A₁-adenosine receptor selective agonist R-N⁶-phenylisopropyladenosine (R-PIA; 100–900 nmol·l^–1) reduced, whereas the A_2A-receptor selective agonist 2-p-(2-carboxyethyl)phenethylamino-5-N-ethylcarboxamidoadenosine (CGS21680; 3–30nmol·l^–1) increased the electrically-evoked noradrenaline overflow in a concentration-dependent manner. The nonselective agonist 5-N-ethy1carboxamidoadenosine (NECA; 30–300 nmol·l^–1) reduced noradrenaline overflow, but the effect did not depend on the concentration. Adenosine deaminase at the concentration of 0.5 ·ml^–1 decreased but at that of 2.0 ·ml^–1 increased noradrenaline overflow. The inhibitors of adenosine uptake, S-(4-nitrobenzyl)-6-thioinosine (NBTI; 50 nmol·l^–1) and dipyridamole (3 mol·l^–1), increased the electrically-evoked noradrenaline overflow. The A₁-adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 20 nmol·l^–1) caused an increase whereas the A₂-adenosine receptor antagonist 3,7-dimethyl-1-(2-propynyl)xanthine (DMPX; 0.1 mol·l^–1) caused a decrease. NBTI (50 nmol·l^–1), partially antagonized the effect of both DPCPX (20 nmol·l^–1) and DMPX (0.1 mol·l^–1).It is concluded that, in the epididymal portion of the rat vas deferens, endogenous adenosine tonically modulates the release of noradrenaline evoked by electrical stimulation, through activation of both inhibitory (A₁) and facilitatory (A_2A) adenosine receptors.Abbreviations CGS 21680 2-p-(2-carboxyethyl)phenethylamino-5-N-ethylcarboxamidoadenosine - DMPX 3,7-dimethyl-l-(2-propynyl)xanthine - DPCPX 1,3-dipropyl-8-cyclopentylxanthine - NBTI S-(4-nitrobenzyl)-6-thioinosine - NECA 5-N-ethylcarboxamidoadenosine - R-PIA R-N⁶-phenylisopropyladenosine Correspondence to J. Gongalves at the above address     

Binding of adenosine receptor ligands to brain of adenosine receptor knock-out mice: evidence that CGS 21680 binds to A1 receptors in hippocampus

The adenosine receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5-N-ethylcarboxamidoadenosine (CGS 21680) is generally considered to be a selective adenosine A_2A receptor ligand. However, the compound has previously been shown to exhibit binding characteristics that are not compatible with adenosine A_2A receptor binding, at least in brain regions other than the striatum. We have examined binding of [³H]CGS 21680 and of antagonist radioligands with high selectivity for adenosine A₁ or A_2A receptors to hippocampus and striatum of mice lacking either adenosine A₁ (A1R^(–/–)) or A_2A (A2AR^(–/–)) receptors. Both receptor autoradiography and membrane binding techniques were used for this purpose and gave similar results. There were no significant changes in the binding of the A₁ receptor antagonist [³H]DPCPX in mice lacking A_2A receptors, or in the binding of the A_2A receptor antagonists [³H]SCH 58261 and [³H]ZM 241385 in mice lacking A₁ receptors. Furthermore, [³H]CGS 21680 binding in striatum was abolished in the A2AR^(–/–), and essentially unaffected in striatum from mice lacking A₁ receptors. In hippocampus, however, binding of [³H]CGS 21680 remained in the A2AR^(–/–), whereas binding was virtually abolished in the A1R^(–/–). There were no adaptive alterations in A_2A receptor expression in this region in A1R^(–/–) mice. Thus, most of the [³H]CGS 21680 binding in hippocampus is dependent on the presence of adenosine A₁ receptors, but not on A_2A receptors, indicating a novel binding site or novel binding mode.     

The effect of renal function on the pharmacokinetics of ranitidine

This open study evaluated the influence of renal function on the pharmacokinetics of ranitidine (50 mg iv infusion given over 6 min). Five groups, each of 8 subjects, 1 with normal renal function and 4 with different degrees of renal impairment were studied.Renal function was assessed in each patient by ⁵¹Cr-EDTA (glomerular filtration rate, GFR), creatinine clearance (GFR) and N-methylnicotinamide clearance (reflecting glomerular and tubular function). Sixteen blood samples (5 ml) taken up to 48 h post dose from each subject were analysed for plasma ranitidine concentrations by reversed phase HPLC.Patient groups with renal impairment had significantly increased AUC and t_1/2 with corresponding decreases in CL_p and _z when compared with normal subjects. There was also a significant increase in t_max but not in C_max. There was a high linear correlation between the degree of renal impairment and ranitidine clearance.In patients with GFR 20 ml min^–1, the AUC mean ratio (compared with normal subjects) was up to 4.6 while for patients with GFR 20–50 ml min^–1, the average AUC ratio was 2.6. It is recommended that the dose of ranitidine is halved in patients with GFR 20 ml min^–1.     

Effect of epinine on tension of human renal arteries

Background: The present study aimed to characterize the effects of epinine, the active metabolite of ibopamine on tension development in human renal arteries.Methods and results: Experiments were performed on isolated human renal arteries rings obtained during surgery due to kidney tumors (n = 12). Epinine concentration-dependently relaxed isolated precontracted (PGF₂) human renal artery rings (P < 0.05) in the presence of phentolamine, as effectively (epinine – 30 +/– 4 mN, dopamine – 31 +/– 5 mN) and with the same potency as dopamine (epinine EC₅₀ 0.7 mol/l (0.4–1.2 mol/l), dopamine 0.5 mol/l (0.2–1.7 mol/l)). This effect was antagonized by the specific D₁-receptor-antagonist SCH 23390. Effective -adrenoceptor antagonistic concentrations of propranolol did not affect epinine-induced vasorelaxation. In the absence of -and -adrenoceptor-antagonists the potency of epinine to contract renal artery rings was significantly higher compared to dopamine indicating a higher affinity of epinine to -adrenoceptors.Conclusion: The present study provides evidence for direct vasodilatory effects of epinine via activation of D₁-receptors on human renal arteries.     

Labelling of Ri adenosine receptors in rat fat cell membranes with (−)-[125iodo]N6-hydroxyphenylisopropyladenosine

Summary A new adenosine analogue, (–)-iodo-N⁶-p-hydroxyphenylisopropyladenosine [(–)-IHPIA], has been developed for radioligand binding studies of R_i adenosine receptors. In addition, the effects of (–)IHPIA on adenosine-mediated responses of rat fat cells have been characterized. (–)IHPIA is slightly less potent at R_i adenosine receptors than (–)N⁶-phenylisopropyladenosine [(–)PIA] as assessed by adenylate cyclase and lipolysis studies. (–)IHPIA inhibited basal adenylate cyclase activity with an IC₅₀ of 60 nmol/l compared to an IC₅₀ of 16.3 nmol/l for (–)PIA. (–)PIA and (–)IHPIA inhibited adenosine deaminase-stimulated lipolysis of intact rat fat cells with an IC₅₀ of 0.55 and 3.6 nmol/l. The potency of (–)N⁶-p-hydroxyphenylisopropyladenosine [(–)HPIA] was intermediate. (–)HPIA has been labelled with carrier-free Na[¹²⁵I] to very high specific activity (2,175 Ci/mmol) and used as agonist radioligand in binding studies of R_i adenosine receptors. The binding of (–)[¹²⁵I]HPIA was saturable, reversible and stereospecific. Saturation analysis revealed two affinity states with dissociation constants (K _D) of 0.7 and 7.6 nmol/l and maximal number of binding sites (B _max) of 0.94 and 0.95 pmol/mg protein. The rate constant of association, k ₁, was 3.7×10⁸ l×mol^–1×min^–1. Binding was slowly reversible with a t_1/2 of 88 min. In competition experiments specific binding was most potently inhibited by (–)PIA, N⁶-cyclohexyladenosine (CHA), (–)HPIA and (–)IHPIA, followed by 5-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine. 1,3-Diethyl-8-phenylxanthine (DPX) and 8-phenyltheophylline were the most potent adenosine antagonists with K _i-values of 67 and 83 nmol/l, whereas the methylxanthines 3-isobutyl-1-methylxanthine, theophylline and caffeine had K _i-values between 1 and 21 mol/l. Binding is highly stereospecific, as indicated by an approximately 20-fold higher K _i-value of the (+)isomer of PIA in comparison to the (–)isomer. The pharmacological profile of (–)[¹²⁵I]HPIA binding sites is consistent with an interaction at R _i adenosine receptors. (–)[¹²⁵I]HPIA appears to be a suitable agonist for radioligand binding studies at R _i adenosine receptors.     

Endogenous adenosine as a modulator of hippocampal acetylcholine release

Summary Modulation of acetylcholine release via adenosine receptors was studied in rabbit hippocampal slices, which were preincubated with ³H-choline and then continuously superfused. Electrical field stimulation of the slices elicited a release of acetylcholine, which was inhibited in a concentration-dependent manner by various adenosine receptor agonists. The effects of the agonists were antagonized by the methylxanthines. From the order of potency: cyclohexyladenosine > (–)phenylisopropyl-adenosine ((–)PIA) > 5-N-ethylcarboxamideadenosine (NECA) > 2-chloradenosine > (+)phenylisopropyladenosine > adenosine, the inhibitory adenosine receptor may be classified as A₁-(R₁-)receptor. In experiments on rabbit caudate nucleus slices, adenosine receptor agonists only slightly decreased the evoked acetylcholine release.The presence of an inhibitory tone of endogenous adenosine on hippocampal acetylcholine release is supported by the following findings: 1) the methylxanthines theophylline, 8-phenyltheophylline and 3-isobutylmethyl-xanthine (IBMX) increased the evoked acetylcholine release in concentrations below those required for phosphodiesterase inhibition. 2) Adenosine uptake inhibitors, in contrast, decreased the evoked transmitter release. 3) Deamination of endogenous adenosine by addition of adenosine deaminase to the medium enhanced the acetylcholine release.In conclusion, acetylcholine release in the hippocampus is depressed at the level of the cholinergic nerve terminals by endogenous adenosine via A₁-(R_i-)receptors.     

Ra Adenosine receptors in human platelets

Summary Adenosine receptors in human platelet membranes have been characterized by radioligand binding and measurement of adenylate cyclase activity. Binding of 5-N-ethylcarboxamido[³H]adenosine ([³H]NECA) was rapid, reversible and dependent on protein concentration, pH and temperature. Due to a rapid rate of dissociation (t _1/2 approximately 20 s) binding was highest at 0° C. Adenosine deaminase and GTP alone did not influence [³H]NECA binding, whereas several divalent cations decreased binding. Saturation experiments revealed two different binding sites for [³H]NECA, with K _d values of 0.16 and 2.9 mol/l and B _max values of 8.4 and 33.4 pmol/mg of protein. In competition experiments NECA was the most potent adenosine agonist (IC₅₀ 0.5 mol/l), followed by 2-chloroadenosine (IC₅₀ 6.3 mol/l) and adenosine (IC₅₀ 12mol/l). A similar rank order of potencies was observed for the stimulatory effect of adenosine analogues on platelet adenylate cyclase. NECA stimulated adenylate cyclase activity with an EC₅₀ value of 0.5 mol/l and was approximately 4-fold more potent than (–)N⁶-phenylisopropyladenosine [(–)PIA]. However, (–)PIA and N⁶-cyclohexyladenosine did not significantly affect [³H]NECA binding, an observation not consistent with the stimulatory effect on adenylate cyclase. The adenosine antagonists 3-isobutyl-1-methylxanthine, theophylline and caffeine showed IC₅₀ values between 98 and 5,600 mol/l. [³H]PIA bound to platelet membranes with very low affinity and was not displaced by NECA. The [³H]NECA binding to human platelet membranes satisfies essential criteria for R _a adenosine receptors and, with some limitations, should be of value for the characterization of adenosine receptors in R _a subtype selective cells.     

Pharmacokinetic–Pharmacodynamic Modeling of Doxacurium: Effect of Input Rate

One of the basic assumptions in pharmacokinetic–pharmacodynamic modeling (PK–PD) is that drug equilibration rate constant between plasma concentration and effect (K_e0 ) is not changed by input rate. To test this assumption in a clinical setting, a 25 g/kg iv dose of doxacurium was administered either by bolus injection or 10-min infusion to 15 anesthetized patients. Neuromuscular function was monitored using train-of-four stimulation of the ulnar nerve. For the short infusion dose, arterial concentrations were measured at 1-min intervals during infusion and at frequent intervals thereafter. Following the iv bolus dose, the early PK profile of doxacurium was investigated by measuring doxacurium arterial concentrations every 10 sec during the first 2 min and at frequent intervals thereafter. PK–PD modeling was performed using nonparametric approach with and without including a finite receptor concentration (R_tot ) in the effect compartment. Kinetic parameters were unchanged. For the bolus and the infusion, K_e0 values were 0.053±0.006 and 0.056±0.009 min ^–1 , respectively. Using the R_tot model, corresponding K_e0 values were 0.148±0.016 and 0.150±0.024, respectively. The relatively faster K_e0 obtained with the R_tot model is compatible with the high potency of doxacurium. Our results show that PK–PD parameters derived with either a bolus or an infusion mode of administration are equally reliable.     

Glomeruli and microvessels of the rabbit kidney contain both A1- and A2-adenosine receptors

Summary Rabbit renal cortices were fractionated by collagenase dispersion and glomeruli, microvessels and tubuli purified on a discontinuous sucrose gradient. Binding experiments with (–)[¹²⁵I]N⁶-(4-hydroxyphenylisopropyl)-adenosine ([¹²⁵I]HPIA) provided evidence for the presence of A₁-adenosine receptors in the glomerular and microvascular fraction. With glomeruli, saturation isotherms for specific [¹²⁵I]HPIA binding were mono-phasic with a K _D of 1.3 nmol/l and a B _maxof 7.7 fmol/mg protein. In kinetic experiments, an association rate constant of 4.9 × 105 (mol/ 1)^–1 s^–1 and a dissociation rate constant of 4.3 × 10^–4 s^–1 were obtained, yielding a K _D of 0.9 nmol/l. Adenosine analogs displaced [¹²⁵I]HPIA binding with a rank order of potency typical of A₁-adenosine receptors; furthermore, binding was inhibited by methylxanthines and modulated by GTP. Saturation experiments with the microvessels revealed a K _D of 1.9 nmol/l and a B _max of 13.4 fmol/mg protein. However, no inhibition of glomerular and microvascular adenylate cyclase activity could be demonstrated, but instead both 5-N-ethylcarboxamido-adenosine (NECA) and N⁶-(R-phenylisopropyl)-adenosine (R-PIA) stimulated enzyme activity, with EC₅₀ values of 0.14 mol/l and 1.5 mol/l, respectively. The concentration-response curve for NECA was shifted to the right (factor 9) by 10 mol/l 8-phenyltheophylline. On the other hand, computer simulation of biphasic curves (adenylate cyclase inhibition in the presence of activation via a stimulatory receptor) indicates that the failure to observe an A₁-adenosine receptor-mediated inhibition of adenylate cyclase activity in the presence of stimulatory adenosine receptors may be attributable to methodological constraints. The results demonstrate that both A₁- and A₂-adenosine receptors are present in rabbit glomeruli and microvessels. It is suggested that both receptors are involved in the control of renin secretion.Abbreviations R-PIA (–)N⁶(R-phenylisopropyl)-adenosine - NECA 5-(N-ethyl-carboxamido)-adenosine - S-PIA (+)N⁶-(S-phenylisopropyl)-adenosine - I-HPIA (–)N⁶-(3-iodo-4-hydroxy-phenylisopropyl)-adenosine - HPIA (–)N⁶-(4-hydroxyphenylisopropyl)-adenosine - [¹²⁵I]HPIA (–)N6-(3-[¹²⁵I]iodo-4-hydroxy-phenylisopropyl)-adenosine - ATP adenosine-5-triphosphate - cAMP cyclic 3,5-adenosine-monophosphate - GTP guanosine-5-triphosphate - HEPES 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid - EDTA (ethylenedinitrilo)-tetraacetic acid Send offprint requests to W. Schütz at the above addressThis study was supported by the Fonds zur Förderung der wissenachiftlichen Forschung in Österreich (Project 5712)     

Evidence for an A2 adenosine receptor in guinea pig lung

Summary Adenosine receptors in guinea pig lung were characterized by measurement of cyclic AMP formation and radioligand binding. 5-N-Ethylcarboxamidoadenosine (NECA) increased cyclic AMP levels in lung slices about 4-fold over basal values with an EC₅₀ of 0.32 mol/l. N⁶-R-(–)-Phenylisopropyladenosine (R-PIA) was 5-fold less potent than NECA. 5-N-Methylcarboxamidoadenosine (MECA) and 2-chloroadenosine had EC₅₀-values of 0.29 and 2.6 mol/l, whereas adenosine and inosine had no effect. The adenosine receptors in guinea pig lung can therefore be classified as A₂ receptors. Several xanthine derivatives antagonized the NECA-induced increase in cyclic AMP levels. 1,3-Diethyl-8-phenylxanthine (DPX; K _i 0.14 mol/l) was the most potent analogue, followed by 8-phenyltheophylline (K _i 0.55 mol/l), 3-isobutyl-1-methylxanthine (IBMX; K _i 2.9 mol/l) and theophylline (K _i 8.1 mol/l). In contrast, enprofylline (1 mmol/l) enhanced basal and NECA-stimulated cyclic AMP formation. In addition, we attempted to characterize these receptors in binding studies with [³H]NECA. The K _D for [³H]NECA was 0.25 mol/l and the maximal number of binding sites was 12 pmol/mg protein. In competition experiments MECA (K _i 0.14 mol/l) was the most potent inhibitor of [³H]NECA binding, followed by NECA (K _i 0.19 mol/l) and 2-chloroadenosine (K _i 1.4 mol/l). These results correlate well with the EC₅₀-values for cyclic AMP formation in lung slices. However, the K _i-values of R-PIA and theophylline were 240 and 270 mol/l, and DPX and 8-phenyltheophylline did not compete for [³H]NECA binding sites. Therefore, a complete characterization of A₂ adenosine receptors by [³H]NECA binding was not achieved. In conclusion, our results show the presence of adenylate cyclase-coupled A₂ adenosine receptors in lung tissue which are antagonized by several xanthines.     

Investigations on GABAB receptor-mediated autoinhibition of GABA release

Summary In this study, we have investigated the effects of phaclofen on the [³H] overflow from [³H]GABA prelabelled rat cortical slices and its interaction with the effects of (–)-baclofen in dependence of the stimulation frequency. (–)-Baclofen strongly depressed the [³H] overflow in the frequency range of 0.125 to 4 Hz to a constant residual level (ICIn50 = 0.37 mol/l at 0.125 Hz), but became inactive above. The potency of the (+)-enantiomer was considerably weaker by a factor of nearly 1000. The GABAB antagonist, phaclofen, increased [³H] overflow at 300 mol/l and, moremarkedly, at 3 and 1 mmol/l, respectively. However, the increase was virtually independent of the frequency between 0.125 and 16 Hz. If the compound interacted only with the putative GABA_B autoreceptor involved in the regulation of GABA release, the extent of the enhancing effect should increase with increasing frequency because of the concomitant rise in synaptic GABA concentration. In order to further investigate this phenomenon, the IC₅₀ of (–)-baclofen and antagonism of phaclofen against (–)-baclofen were determined at 0.125 Hz and 2 Hz, respectively. Whereas the IC₅₀ of (–)-baclofen was 0.63 ± 0.04 mol/l at 0.125 Hz, it increased to 4.88 + 0.45 mol/l at 2 Hz. The pA₁₀-values of phaclofen were about the same at both frequencies, whereas the pA₂-values differed by a factor of 2.3. Therefore, the possibility should be considered that (–)-baclofen does not only interact with presynaptic GABA autoreceptors, but also may interact with other - presumably somatodendritic- GABA_B-receptors whose pharmacology is not identical with that of the receptors by which (–)-baclofen exerts its effects on GABA release. Send offprint requests to P. A. Baumann at the above address     

Pharmacokinetics of cephacetrile in patients undergoing haemodialysis

Summary The pharmacokinetics of cephacetrile were studied after its administration as a single i.v. bolus injection of 15 mg/kg body weight to 11 patients with terminal renal inpairment undergoing haemodialysis for 6 h. A two-compartment kinetic model was used to describe the biphasic decrease in plasma concentration. The quantities of antibiotic in the central and peripheral compartments, and the amounts eliminated, were calculated for different times. During haemodialysis sessions, the average pharmacokinetic parameters of cephacetrile determined at the dialyser input were:=5.03 h^–1,=0.458 h^–1, K₁₂=2.337 h^–1, K₂₁=1.996 h^–1 K₁₃=1.154 h^–1, V_c=5.508 l, V_p=6.448 l, Vd_ss=11.956 l.As a function of the pharmacokinetic parameters of cephacetrile, a regimen of multiple doses was established for patients with terminal renal impairment, which will guarantee safe and effective concentrations of the antibiotic.     

Renal action of adenosine: Effect on renin secretion in the rat

Summary The present study was undertaken to further evaluate the interaction of adenosine and the renin angiotensin system. In 12 sodium-restricted (SR) Sprague-Dawley rats plasma renin activity in the arterial carotid (PRA_a) and the renal venous (PRA_v) blood was measured using a radioimmunoassay for angiotensin I (AI). Renin secretion rate (RS) was calculated from the arteriovenous difference of PRA and renal plasma flow obtained from the reading of an electromagnetic flowmeter and hematocrit. Adenosine infused into the thoracic aorta at a rate of 33 nmol/min·100g body weight, decreased RS from 250±29 ng AI/min in the control periods to 115±17 ng AI/min (P<0.001, n=9). After cessation of the adenosine infusion RS recovered incompletely to 167±21 ng/AI/min. Adenosine reduced glomerular filtration rate (GFR) by 56% (P<0.001) but renal blood flow (RBF) by only 20%. The effects of adenosine were also studied in a group of sodium loaded (SL) rats. In 6 non-laparotomized SL rats (PRA_a=16±5 ng AI/ml·h) GFR was not significantly changed following adenosine infusion (1.54±0.16 vs 1.42±0.09 ml/min). In additional 7 SL rats (PRA_a=39±4 ng AI/ml·h) in which RBF was measured with an electromagnetic flow probe, adenosine reduced GFR by 20% (P<0.01), whereas RBF increased from 13.7±1.2 to 16.5±2.2 ml/min (P<0.05). RS in SL rats was not affected by adenosine infusion (59±16 vs 62±26 ng AI/min). There was a close correlation between the adenosine induced decrease of GFR and PRA_a (r=0.91, n=25) suggesting a higher sensitivity of the renal vasculature to adenosine when the renin-angiotensin system was stimulated. Micropuncture experiments in 9 Sprague-Dawley rats on a normal sodium diet demonstrated that adenosine infused in the low dose of 5 nmol/min·100 g of body weight markedly decreased the superficial nephron GFR from 38.6±2.65 to 20.5±1.67 nl/min (P<0.001) whereas whole kidney GFR was not changed significantly. This disproportionate response of superficial nephron compared to whole kidney GFR indicates that adenosine affects preferentially the superficial nephrons. It is concluded that adenosine depresses the renin secretion in SR rats. This depression was accompanied by a marked fall of the GFR. The renal response to adenosine was nearly abolished in SL rats suggesting that the renal action of adenosine is functionally related to the reninangiotensin system.Supported by Deutsche Forschungsgemeinschaft (Os 42/2)     

Renal response to amino acid infusion in rats: effect of dopamine receptor antagonists and benserazide

Previously, we have found that feeding is a dominant factor controlling urinary dopamine excretion (U_DA) in conscious rats (Mühlbauer and Osswald 1992). Since the renal response to feeding is also characterized by an increase in glomerular filtration rate (GFR), we wanted to investigate in a first step whether the feeding-induced elevations of GFR and U_DA could be causally related phenomena. Therefore, we studied the influence of dopamine synthesis and dopamine receptor blockade on the renal response to amino acid infusion (AA) in thiopental anesthetized rats. AA infusion (n = 7) increased GFR by 33±7% (P<0.001) and U_DA by 87±19% (P<0.001). In the presence of benserazide (BZD, n = 5), an inhibitor of dopamine synthesis, infused i.v. at a dose of 30 g/min/kg, U_DA was suppressed to values below detection limit and the AA-induced GFR increase was abolished. Continuous intravenous infusion of the DA₁ receptor antagonist SCH 23390 (SCH, n = 7) in a dose of 4.0 g/kg/min did not prevent the AA-induced increase in GFR (33±3%, P<0.001) and U_DA (97±12%, P< 0.001). In contrast, S-sulpiride (SUL), a specific DA₂ receptor antagonist, infused continuously i.v. in a dose of 5 g/kg/min, completely abolished the AA-induced GFR increase, while U_DA was increased 1.6-fold (P<0.01). Like BZD, both dopamine receptor antagonists did not affect renal sodium excretion substantially.Our results suggest, that endogenous dopamine could act as a mediator in the renal response to amino acid infusion in the rat, most likely by activation of DA₂ receptors. Correspondence to:B. Mühlbauer at the above address     

Endogenous adenosine inhibits lipolysis induced by nerve stimulation without inhibiting noradrenaline release in canine subcutaneous adipose tissue in vivo

Summary The influence of adenosine on blood flow, lipolysis and adrenergic neurotransmission was studied in canine subcutaneous adipose tissue in situ. Tissue concentrations of endogenous adenosine were elevated in 2 ways: (1) by inhibition of adenosine elimination by i. a. infusion of the uptake blocker dipyridamole (0.5–1.5 M in arterial plasma) + the deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA; 3–5M) or (2) by prolonged nerve stimulation (4 Hz for 30 min). Furthermore, effects of exogenous adenosine and the adenosine antagonist theophylline were studied. Dipyridamole+EHNA increased resting vascular conductance from 0.055 to 0.096 ml×min^–1×100 g^–1×mm Hg^–1, whereas lipolysis induced by sympathetic nerve stimulation (2,4 and 8 Hz during 2 min) was inhibited (by 91, 71 and 34%, respectively). Therapeutic concentrations of theophylline (5–15 g/ml plasma) completely reversed the effects of dipyridamole + EHNA on blood flow and significantly reversed the antilipolytic effect of these drugs. In untreated adipose tissue theophylline reduced resting vascular conductance by 40% and enhanced the lipolytic response to prolonged nerve stimulation but not that to brief nerve stimulation. Transmitter release, evaluated by the outflow of endogenous noradrenaline and/or by the outflow of radioactivity after ³H-noradrenaline labelling, was linearly related to the stimulation frequency (2,4 and 8 Hz). Dipyridamole + EHNA increased plasma noradrenaline by inhibiting uptake or binding of noradrenaline to blood cells. However, noradrenaline release induced by nerve stimulation was not significantly affected by dipyridamole + EHNA or by theophylline. It is concluded that a rise in endogenous adenosine concentration caused by drugs or by prolonged sympathetic nerve stimulation inhibits the lipolytic response to nerve stimulation without any effect on noradrenaline release. The results also show that theophylline in a therapeutic dose range is capable of enhancing lipolysis only if endogenous adenosine concentrations are raised.     

Purinoceptor-mediated modulation by endogenous and exogenous agonists of stimulation-evoked [3H]noradrenaline release on rat iris

Summary To investigate whether endogenous purinoceptor agonists affect the sympathetic neurotransmission in the rat isolated iris, and to classify the purinoceptors modulating exocytotic [³H]-noradrenaline release, we have determined the effect of adenosine receptor antagonists on, and the relative potency of selected agonists in modulating, the field stimulation-evoked (3 Hz, 2 min) [³H]-noradrenaline overflow. In addition, the apparent affinity constants of 8-phenyltheophylline (8-PT) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) in antagonizing the prejunctional effects of purinoceptor agonists were estimated.The relatively A₁-selective DPCPX 10 and 100 nmol/l increased the evoked [³H]-noradrenaline overflow by about 25%–35%a indicating a minor inhibition of evoked release by endogenous purinoceptor agonists probably via an A₁ adenosine receptor. Whereas the A₁/A₂-antagonist 8-PT failed to increase the evoked [³H]-noradrenaline overflow in the absence of exogenous agonists (without or with dipyridamole 1 pmol/l present), the relatively A₂-selective antagonist CP-66,713 (4-amino-8-chloro -1-phenyl(1,2,4)triazolo(4,3-a)quinoxaline) 100 nmol/l decreased it by 20%–30% in the absence and continuous presence of DPCPX. This may be compatible with a minor A₂-mediated facilitation by an endogenous purinoceptor agonist.All exogenous agonists tested (except UTP 100 mol/l) inhibited the evoked [³H]-noradrenaline overflow. The relative order of agonist potency (IC4o, concentration in mol/l for inhibition of evoked release by 40%) was CPA (N⁶-(cyclopentyl)adenosine, 0.004) > R-PIA (R(–)N6-(2phenylisopropyl)adenosine, 0.066) = CHA (N⁶-(cyclohexyl)adenosine, 0.082) > NECA (N⁵-(ethyl-carboxamido)adenosine 0.44) > ADO (adenosine, 4.1). ATP was n early equipotent with ADO. Maximum inhibition was 70%–80% and similar for all agonists. Adenosine deaminase 1 u/ml failed to affect the ATP-induced, but abolished the adenosine-induced prejunctional inhibition. The adenosine uptake inhibitor S-p-nitrobenzyl-6-thioguanosine (NBTG) failed to enhance the potency of ADO and ATP. The A₁-selective antagonist DPCPX 10 nmol/l did not reduce the ATP potency indicating an effect of ATP per se not mediated via an A₁ purinoceptor.Prejunctional affinity constants of 8-PT were 6.07 when tested against adenosine (in the presence of dipyridamole), and 6.60 against CHA. The apparent -log K_B of DPCPX tested against CPA was 9.71. The high DPCPX affinity is compatible with an A₁ adenosine receptor mediating inhibition of sympathetic neurotransmission in rat iris. This receptor may not be the only prejunctional purinoceptor on rat iris sympathetic nerves. The receptor by which ATP acts prejunctionally in this tissue remains to be determined.This study was supported by the Deutsche Forschungsgemeinschaft (Fu 163/2 and 163/3) Send offprint requests to H. Fuder at the above address     

Lack of presynaptic modulation by isoprenaline of 3H-noradrenaline release from rabbit isolated ear artery

Summary The aim of the present investigation was to examine whether or not presynaptic facilitatory -adrenoceptors are detectable on the postganglionic nerves in the rabbit isolated ear artery. Strips of rabbit central ear artery were incubated with ³H-noradrenaline (10^–7 mol/l; 30 min or 10^–6 mol/l; 60 min). Subsequently, they were washed repeatedly with physiological salt solution. The strips were subjected to electrical-field stimulation (S₁–S₈) and the resultant ³H-overflow was determined.When the ear artery was stimulated with 150 pulses (0.5 ms; 3 Hz; 225 mA), isoprenaline (10^–9–10^–6 mol/l) either alone or in the presence of either rauwolscine (10^–6 mol/l) or phentolamine (10^–6 mol/l) did not alter the stimulation-evoked ³H-overflow. This was also the case in the presence of rauwolscine (10^–6 mol/l) plus either the selective phosphodiesterase inhibitor ICI 63 197 (3 × 10^–5 mol/l) or forskolin (10^–6 mol/l). When the ear artery was stimulated with 300 pulses (1 ms; 5 Hz; 225 mA), isoprenaline had no effect on the stimulation-evoked ³H-overflow. This was also the case when phentolamine (10^–6 mol/l) was present. Propranolol (10^–7–10^–5 mol/l) did not alter the stimulation-evoked ³H-overflow. In some experiments, the stimulation current was reduced to 175 mA in order to obtain similar reference release (S₃) values despite the presence of rauwolscine (150 pulses; 0.5 ms; 3 Hz). Even then, isoprenaline (10^–9–10^–6 mol/l) did not change stimulation-evoked ³H-overflow. The results suggest that postganglionic sympathetic nerves in rabbit central ear artery do not possess presynaptic facilitatory -adrenoceptors. Send offprint requests to J. Abrahamsen at the above address     

Effects of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a highly selective adenosine receptor antagonist,on force of contraction in guinea-pig atrial and ventricular cardiac preparations

Summary The effects of the A₁ adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on force of contraction were examined in isolated electrically driven auricles and papillary muscles from guinea-pigs in the absence and presence of (–)-N⁶-phenylisopropyladenosine (PIA) and 5-N-ethylcarboxamidadenosine (NECA).In auricles DPCPX (30–1000 nmol/l) alone increased force of contraction. DPCPX produced only a minor inhibition of phosphodiesterase I–III activity. PIA and NECA alone exerted concentration-dependent negative inotropic effects and the concentration-response curves for PIA and NECA were shifted competitively to the right by the adenosine receptor antagonist DPCPX with similar potency and efficacy. The pA₂-value for the inhibition of the effects of PIA and NECA were 9.1 and 8.8, respectively.In papillary muscles DPCPX alone had no inotropic effect. In the presence of isoprenaline PIA and NECA alone exerted concentration-dependent negative inotropic effects and again DPCPX shifted the concentration-response curves for PIA and NECA competitively to the right with similar potency and efficacy. The pA₂-value for the inhibition of the effects of PIA and NECA were 9.3 and 9.0, respectively.It is concluded that DPCPX is a potent competitive A₁ adenosine receptor antagonist in guinea-pig atrial and ventricular cardiac preparations. Since PIA and NECA were equally potent the cardiac adenosine receptor may constitute a subtype of A₁ adenosine receptors differing from the receptor in other tissues such as fat cells. Furthermore, DPCPX has a positive inotropic effect in atrial tissue which cannot be attributed to the A₁ receptor antagonism.Abbreviations cAMP cyclic adenosine-3,5-monophosphate - DPCPX 1,3-dipropyl-8-cyclopentylxanthine - IBMX 3-isobutyl-lmethylxanthine - NECA 5-N-ethylcarboxamidadenosine - PDE phosphodiesterase - PIA (–)-N⁶-phenylisopropyladenosine Send offprint requests to J. Scholz at the present addressParts of the results have been presented at the 54th Annual Meeting of the German Society for Heart and Circulation Research (Scholz et al. 1988)