The Effect of Lipopolysaccharide on the Disposition of Xanthines in Rats

Abstract— The effect of lipopolysaccharide (LPS) isolated from Klebsiella pneumoniae O3 on the pharmacokinetic behaviour and metabolism of the xanthines, theophylline and 1-methyl-3-propylxanthine (MPX), which are mainly metabolized by the liver, was investigated in rats. LPS was infused at 0·25 mg kg^?1 over a period of 20–30 min, 2 h before the administration of theophylline (10 mg kg^?1) or MPX (2·5 mg kg^?1). Concentrations of both xanthines in plasma and concentrations of the parent drug and metabolites in urine were measured by HPLC. Model-independent methods were applied to estimate the pharmacokinetic parameters for both xanthines. No significant changes in the pharmacokinetic parameters or metabolism of theophylline were observed in rats pretreated with LPS. However, the total body clearance and volume of distribution of MPX were significantly increased by pretreatments with LPS. Significant decreases in the binding capacity and number of binding sites on the albumin molecule were observed in the presence of LPS. Changes occurring in the protein binding behaviour as a result of the introduction of LPS is a primary factor which not only increases the volume of distribution but also increases total body clearance. These results indicate that LPS has no effect on the pharmacokinetics and metabolic pathway of theophylline although it changes the disposition of MPX due to decreases in the extent of the protein binding of MPX which is highly bound to protein.     

Cardiovascular effects of two different xanthines in healthy subjects. Studies at rest,during exercise and in combination with a beta-agonist,terbutaline

Summary The haemodynamic response to two xanthines, enprofylline and theophylline, was studied in 6 healthy male volunteers at rest, during exercise and in combination with the beta₂-agonist, terbutaline. At rest the haemodynamic effects of both xanthines were small and were qualitatively different from each other. While theophylline exerted a pressor response, enprofylline seemed to have arterial dilating ability. During exercise both xanthines as compared to placebo were associated with a higher heart rate and in general with increased systolic blood pressure. In combination with terbutaline enprofylline and theophylline both increased systolic blood pressure more than placebo, i. e. they augmented the positive inotropic effect of terbutaline. The systolic blood pressure was higher after theophylline than enprofylline despite their equipotent bronchodilator activity. This may reflect different inotropic effects of the xanthines as well as a difference in their influence on the response to adenosine.     

Enprofylline: Pharmacokinetics and comparison with theophylline of acute effects on bronchial reactivity in normal subjects

Summary In a double blind, placebo controlled, crossover study the pharmacokinetics and acute effects of enprofylline and theophylline on airway reactivity during histamine challenge were investigated in 10 healthy volunteers. The pharmacokinetic parameters of enprofylline were (mean): elimination half-life 1.9 h, total body clearance 191.1 ml · kg^–1 · h^–1, volume of distribution 0.48 l · kg^–1, and protein binding 49%. Bronchial reactivity in the histamine inhalation test was expressed as the concentration causing a 20% fall in FEV_1.0 (PC₂₀). Mean PC₂₀ values were lowest after placebo and highest after theophylline with the enprofylline values in between. Only the difference in PC₂₀ ^Safter placebo and theophylline was statistically significant (p<0.05). At the time of determination of the PC₂₀, the serum concentration of enprofylline was between 16.5 and 11.8 µmol/l, and that of theophylline was between 78.3 and 61.1 µmol/l. Adverse actions of enprofylline were nausea (3/10) and cardiovascular reactions (2/10), whereas theophylline mainly caused restlessness (3/10) and tremor (2/10). Thus enprofylline, in one-fifth of the serum concentration of theophylline cannot be regarded as equipotent in terms of bronchoprotection.In fulfillment of his thesis (Dr. med.)     

The Actions of Some β-Receptor Agonists and Xanthines on Isolated Muscle Strips from the Human Oesophago-Gastric Junction

Isolated preparations from the circular muscle layer of the human oesophago-gastric junction were mounted in organ baths and isometric tension recorded. During an equilibration period, active resting tension developed suggesting that the preparations were representing the lower oesophageal sphincter. Active tension was abolished by exposing the preparations to Ca⁺⁺-free medium. The two xanthines theophylline and enprofylline almost equipotently relaxed the preparations in a concentration-dependent manner (10^-7-10^-3M). Within therapeutic concentrations, theophylline inhibited active resting tension by 30–60%, while enprofylline lowered tension by less than 20%. Inhibitory actions of adenosine were demonstrated, and this suggests that adenosine antagonism is not the mechanism of action for xanthines in the oesophagus. Non-selective β-receptor stimulation with isoprenaline inhibited active tension by 70% (10^-7M), while β₂-receptor stimulation with terbutaline inhibited tension by 47% (10^-5M). Dobutamine, believed to preferentially stimulate β₁-receptors, inhibited active tension in a concentration-dependent manner (10^-7-10^-4M). Metoprolol (10^-6M), a selective β₁-receptor antagonist, shifted the concentration-response curve for isoprenaline to the right, but left the maximal response unchanged. It is concluded that xanthines and β-receptor agonists have inhibitory actions on circular muscle from the human oesophagogastric junction. The experimental data suggest the presence of β₁- as well as β₂-receptors, both mediating inhibition of active resting tension.     

The possible mechanism of interaction between xanthines and quinolone

To clarify the mechanism of interaction between theophylline and enoxacin, the effects of enoxacin and its metabolite, 4-oxo-enoxacin, on the disposition of new xanthine derivatives, 1-methyl-3-propylxanthine (MPX) and 3-propylxanthine (enprofylline), as models of theophylline have been investigated in rats. Pretreatment with enoxacin significantly delayed the elimination of MPX from plasma. No significant change in the volume of distribution of MPX was observed in the presence of enoxacin, but the total body clearance of MPX was significantly decreased by approximately 60 and 80% after pretreatment with 25 and 100 mg kg-1 of enoxacin, respectively. The amount of the decrease in total body clearance depended on the dose of enoxacin. 4-Oxo-enoxacin had little or no effect on MPX disposition. A newly developed quinolone, NY-198, which does not affect the disposition of theophylline, also did not affect the disposition of MPX. Enoxacin also had no effect on the disposition of enprofylline. These results indicate that the mechanism for decrease in theophylline clearance induced by enoxacin may not be due to its metabolite, 4-oxo-enoxacin, but to enoxacin itself, and that enoxacin does not inhibit solely the elimination process depending on cytochrome P450 isoenzyme for N-demethylation. It is likely that enoxacin has no influence on the renal excretion of xanthines.     

Imidazodiazepinediones: a new class of adenosine receptor antagonists

A series of imidazo[4,5-e][1-4]diazepine-5,8-diones were synthesized from hypoxanthines. Certain of these cyclic homologues of caffeine, theophylline, theobromine, 3-isobutyl-1-methylxanthine, and enprofylline were inhibitors of binding of adenosine analogues to rat brain A1 and A2 adenosine receptors and were antagonists of A2 adenosine receptors stimulatory to adenylate cyclase in rat PC12 cell membranes. Activity at adenosine receptors was lower than the corresponding xanthines, perhaps because imidazodiazepinediones contain a boat-shaped seven-membered ring rather than the planar heteroaryl ring system of the xanthines. The imidazodiazepinediones had low affinity for brain benzodiazepine sites.     

Actions and interactions of adenosine, theophylline and enprofylline on the guinea-pig spirally cut trachea

Adenosine, theophylline and enprofylline induced concentration-dependent relaxations of guinea-pig isolated tracheal spirals whether they had intrinsic tone or were precontracted with carbachol or histamine. The potency order was enprofylline greater than theophylline greater than adenosine and the maximum relaxation in absolute terms was generally less for adenosine. The maximum relaxation (measured in absolute terms of change in tension) induced by the three spasmolytics in preparations with intrinsic tone was generally less than in precontracted tissues. This was attributed to the higher resting tone of precontracted tissues than the intrinsic tone. The adenosine transport inhibitor dipyridamole potentiated adenosine but not theophylline or enprofylline so that the potency order became adenosine greater than enprofylline greater than theophylline. Without dipyridamole, theophylline in a concentration producing 10-30% relaxation of the trachea, failed to antagonize adenosine. However, in the presence of dipyridamole, adenosine was antagonized, indicating that the relaxation by adenosine was mediated via an extracellular P1 receptor. Enprofylline, in a concentration producing equivalent direct effects, failed to antagonize adenosine. It is concluded that the tracheal relaxation by xanthines is independent of adenosine antagonism.     

The pharmacokinetics of theophylline and enprofylline in patients with liver cirrhosis and in patients with chronic renal disease

Summary We have studied the pharmacokinetics of theophylline and enprofylline in patients with liver cirrhosis, patients with chronic renal failure, and healthy subjects, and have assessed the predictive value of routine tests of liver function and renal function (creatinine clearance) for theophylline and enprofylline total body clearances.Theophylline clearance was significantly decreased in the patients with liver cirrhosis compared with both the patients with renal failure and the healthy subjects (the mean values in the three groups were 24, 47, and 46 ml·h^–1·kg^–1 respectively.Enprofylline clearance was significantly decreased in the patients with chronic renal failure, compared with both the patients with liver cirrhosis and the healthy subjects (the values in the three groups were 64, 250, and 289 ml·h^–1·kg^–1 respectively.There was a strong correlation between creatinine clearance and enprofylline clearance, while there was only a poor correlation between the liver function tests and theophylline clearance.It appears that in various clinical situations enprofylline elimination can be predicted more precisely than theophylline elimination, which may make the drug safer in clinical practice.     

BRONCHODILATORY ACTIVITY AND PHARMACOKINETICS OF NEW XANTHINES IN GUINEA-PIGS

1. The in vitro biological activities and the effect of protein binding on the relaxant effects in vivo of N-3-alkylxanthine and N-3-alkyl-N-1-methylxanthine derivative were investigated in guinea-pigs. 2. A significantly positive correlation was observed among the in vitro muscle relaxant activity, the cyclic adenosine monophosphate (cAMP) phosphodiesterase (PDE) inhibitory activity and the protein-binding potency of xanthine derivatives. However, there was a weak relationship between these activities and affinity for adenosine receptors. 3. When theophylline, enprofylline and 1-methyl-3-propylxanthine (MPX) were injected intravenously in guinea-pigs, their ED50 values were 6.1, 3.3 and 1.0 mg/kg, respectively. Plasma concentrations of these drugs obtained following the intravenous injection of the ED50 approximated the theoretically effective concentration (EC50) predicted from both the relaxant effects in vitro and the protein binding parameters. A good linear correlation was observed between bodyweight in four species (rats, guinea-pigs, rabbits and humans) and certain pharmacokinetic parameters of enprofylline and theophylline. 4. The present study indicates that differences in the relaxant effects of these drugs in vitro and in vivo can be explained in part by protein binding, and that the protein binding of these xanthine bronchodilators is an important determinant for their pharmacological activity. Guinea-pigs provide a useful model for studying pharmacodynamic-pharmacokinetic relationships of new bronchodilators.     

Activities of caffeine, theophylline, and enprofylline analogs as tracheal relaxants

A variety of xanthines cause tracheal relaxation, an activity predictive of antiasthmatic potential. Structural analogs of caffeine, theophylline, and enprofylline were examined for their abilities to relax carbamylcholine-stimulated guinea pig trachea in vitro. All caffeine analogs tested were more potent than caffeine (EC50 = 551 +/- 81 microM) except the 8-p-sulfophenyl analog. 1,3,7-Tripropylxanthine and 1,3,7-tripropargylxanthine were among the more potent analogs with EC50 values of 12 +/- 1.3 and 65 +/- 11 microM respectively. Increasing the polarity at the 1- or 3-position by substituting a propargyl group for an n-propyl group decreased relaxant activity, an effect not observed at the 7-position. The 8-cyclohexyl-, 8-cyclopentyl- and 8-phenyl-derivatives of caffeine were relatively potent (EC 50 = approximately 75 microM). The theophylline analog 1,3-di-n-propylxanthine was approximately two times more active than theophylline (EC50 = 162 +/- 17 microM). 3-Isobutyl-1-methylxanthine (EC50 = 7.1 +/- 1.8 microM) and 1-isoamyl-3-isobutylxanthine (EC50 = 37 +/- 4.2 microM) were among the most potent tracheal relaxants. The 8-substituted theophylline analogs were weak or inactive relaxants except for 8-cyclopentyl- and 8-cyclohexyltheophylline, which were more potent or as potent as theophylline. In contrast to enprofylline (EC50 = 56 +/- 9 microM), 8-substituted enprofylline analogs were weak or inactive as relaxants with the exception of the 8-cyclohexyl analog. The potency of xanthines as tracheal relaxants was unrelated to potency as adenosine receptor antagonists and may reflect activity as phosphodiesterase inhibitors.     

Enprofylline and theophylline on small human placental arteries: studies of in vitro effects and mode of action

Vascular effects of theophylline and enprofylline, a novel xanthine derivative lacking adenosine receptor antagonism, were studied comparatively in tubular preparations of small human placental arteries mounted in an isometric myograph. Both xanthines produced concentration-dependent (10(-7)-3 X 10(-3) M) relaxation of arteries contracted by PGF2 alpha or PGE2 in both normal Ca2+-medium and in Ca2+-depleted medium. Enprofylline was about three times more potent than theophylline. Also in vasopressin-contracted arteries enprofylline was a more potent vasodilator in both media. In contrast the xanthines were equally potent in relaxation of the tonic as well as the phasic part of a K+-induced contraction, but less potent than in relaxation of PG-induced contractions. Propranolol, phentolamine, atropine, indomethacin or tetrodotoxin did not influence the xanthine relaxations. It is concluded that the theophylline-induced relaxation of small human placental arteries is not due to adenosine receptor antagonism. A common important mechanism of action, in which enprofylline is more potent than theophylline, seems to be interference with intracellular Ca2+-binding/mobilisation processes. Some decrease in cellmembrane Ca2+-permeability produced by the xanthines seems to take part in the mechanism of relaxation.     

Positive inotropic and chronotropic effects and coronary vasodilation in vitro by two antiasthmatic xanthines with different abilities to antagonize adenosine

The direct actions and the ability to antagonize adenosine of theophylline (1,3-dimethylxanthine) and enprofylline (3-propylxanthine) were examined in isolated guinea pig heart preparations. Enprofylline was approximately six times as potent as theophylline in reducing coronary perfusion pressure, and between three and five times more potent in increasing rate and force of contraction in isolated perfused hearts, spontaneously beating right atrias, and electrically stimulated left atrias. Adenosine reduced the coronary perfusion pressure and had negative inotropic and chronotropic actions. Theophylline (5-75 microM) concentration-dependently antagonized the coronary vasodilation and the negative chronotropic and inotropic actions of adenosine, whereas enprofylline (2.5-75 microM) did not antagonize adenosine. Since adenosine may be a protective autacoid in the heart under select stress conditions, the possibility that adenosine nonblocking xanthines, like enprofylline, may offer therapeutic advantages over theophylline should be investigated.     

Influence of Age on the Disposition and Renal Handling of Enprofylline in Rats

Abstract— The effects of ageing on the pharmacokinetics, renal handling and protein binding of enprofylline were investigated in 6-, 13- and 18-month-old male Fischer 344 rats. Concentrations of enprofylline in plasma and urine were determined by HPLC, and pharmacokinetic parameters were estimated by model-independent methods. No significant differences in the volume of distribution, systemic clearance of enprofylline or urinary recovery of unchanged enprofylline (> 85%) were observed among any of the groups of rats. The dissociation constant and free fatty acid concentration in plasma increased with age. Age-dependent decreases in the systemic clearance for unbound drug were observed, and the volume of distribution for unbound drug tended to decrease with age. The ratio of systemic clearance for unbound drug to the glomerular filtration rate (GFR) decreased with ageing. Ageing was associated with decreases in the apparent maximum capacity of transport (V_max) (223·33,160·24 and 142·98 μg min^?1 kg^?1 for 6-, 13- and 18-month-old rats, respectively) and in the tubular secretory intrinsic clearance (V_max/K_m) of enprofylline (75·45, 51·03 and 44·13 mL min^?1 kg^?1, respectively), while a slight change in the Michaelis-Menten constant (K_m) was observed. These results indicate that the mechanism responsible for age-related changes in the disposition and renal handling of enprofylline may be responsible for a decrease in the ability of the tubular anion transport system.     

Adenosine A1 receptors in mammalian brain: Species differences in their interactions with agonists and antagonists

The binding of the selective ligand [³H]cyclohexyladenosine (CHA) to adenosine A1 receptors was studied in brain membrane from six species; cow, rabbit, rat mouse, human, and guinea pig. Saturation analysis gave evidence for a single binding site in each species. The number of binding sites (2.6–4.7 pmol/mg protein), based on saturation analysis, was virtually identical in each species. The Kd values were, however, different; whereas rat, rabbit, and mouse A1 receptors had similar affinities (Kd=1–2 nM), the binding site in bovine tissue had three to four times greater affinity (Kd=0.59 nM). Adenosine binding sites in human and guinea pig brain had two to three times less affinity, with Kd values 3.7 and 6.6 nM, respectively, than those in rat, rabbit, and mouse brain. Examination of the binding of five agonists, the R and S diastereomers of N⁶-phenylisopropropyladenosine (PIA), CHA, 5′N-ethylcarboxamido adenosine (NECA), and 2-chloroadenosine, and of five xanthine antagonists, PACPX [1,3,dipropyl-8-(2-amino-4-chloro)phenylxanthine], 1,3 diethyl-8-phenylxanthine (DPX), 8-phenyltheophylline (8PT), 8-parasulfophenyl-theophylline (8PST), and theophylline, showed that across the species N⁶-substituted adenosine analogs showed consistent differences in rank order activity. R-PIA ≥ CHA >S-PIA, with the compounds being most active in bovine tissue and least active in guinea pig, showing an 18-fold difference. In contrast, the 2-substituted analog, 2-chloroadenosine was most active in guinea pig and human brain membranes, and least active in rat and bovine brain. The 5′-substituted analog NECA had similar activity in all species studied. PACPX was the most potent of the xanthines studied, being most active in bovine brain tissue and least active in guinea pig; the difference in activities was 393-fold. The interaction of the remainder of the xanthines studied was complex. In general, the rank order of potency was PACPX > DPX=8 phenyltheophylline > 8PST >> theophylline. However, in human and rabbit brain 8PT was more active than DPX, and in guinea pig 8PT was twofold less active than DPX. In addition, in human and guinea pig brain, 8PST had comparable activities. These differences in pharmacological profiles may be attributable to differences in the adenosine systems studied as previously noted by Murphy and Snyder [Mol. Pharmacol. 22:250–257, 1982], which may in turn reflect distinct differences in A1 receptors or the presence of different subtypes of adenosine receptor in the various species. While these interspecies studies need to be extended to both A2 receptors and to coupled, i.e., adenylate cyclase, systems, they underline the need for care in choosing the mammalian species in which structure activity relationships are generated. Some caution is necessary in the use of bovine brain tissue in development programs targeted towards therapeutic entities in the adenosine A1 antagonist area.     

Pharmacodynamics of ZM 241385, a Potent A2a Adenosine Receptor Antagonist,after Enteric Administration in Rat,Cat and Dog

4-(2-[7-Amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) is currently the most selective for the A_2a adenosine receptor antagonist. This paper describes the in-vivo activity of ZM 241385 after administration by both oral and intraduodenal routes. In conscious spontaneously hypertensive rats, ZM 241385 (1–10 mg kg^?1) selectively attenuated the mean arterial blood pressure response produced by exogenous adenosine (1 mg kg^?1 min^?1, i.v.) by up to 45% after oral administration. Activity of ZM 241385 was maintained for at least 6 h after 3 and 10 mg kg^?1 (p.o.). In conscious normotensive cats, ZM 241385 attenuated the blood pressure responses to adenosine (0.6–10 mg kg^?1, i.v.) by 94% after 10 mg kg^?1 (p.o.) and by up to 74% after 0.3 mg kg^?1 (i.v.). Duration of action of ZM 241385 up to 12 h (36% inhibition) was observed after 3 mg kg^?1 (p.o.). In anaesthetized dogs and cats, ZM 241385, after intraduodenal administration (1–10 mg kg^?1), produced a rapid (dose ratio 100-fold 15 min after administration of 10 mg kg^?1 in the cat) and prolonged (dose ratio of 14 at 6 h after administration of 10 mg kg^?1) attenuation of the vasodilatation responses to adenosine receptor stimulation. When administered by this route ZM 241385 was six times more potent than theophylline in the cat and at least twice as potent as theophylline in the dog. In conclusion, ZM 241385 is a potent, selective A_2a adenosine receptor antagonist which is orally active, with a good duration of action by the enteric route in cat, rat and dog. It could therefore be used to evaluate the role of adenosine A_2a receptors in the action of adenosine in-vivo.     

XANTHINES INHIBIT HUMAN PLATELET AGGREGATION INDUCED BY PLATELET-ACTIVATING FACTOR

1. Platelet-activating factor (PAF) may be involved in the pathogenesis of asthma, and therefore the effects of the anti-asthma drugs theophylline and enprofylline on human platelet aggregation and adenosine triphosphate (ATP) release induced by PAF and adenosine diphosphate (ADP) were studied. 2. Enprofylline (50% inhibitory concentration [IC50] = 94.8 +/- 13.2 mumol/L) was more potent than theophylline (IC50 = 934.1 +/- 40.1 mumol/L) as an inhibitor of PAF-induced aggregation, and the xanthines were twice as potent as inhibitors of PAF-induced aggregation when compared with ADP-induced aggregation. ATP release was 1.4 times more sensitive to inhibition by the xanthines than aggregation. 3. Although high concentrations of xanthines inhibited platelet aggregation and ATP release induced by PAF, therapeutic concentrations are unlikely to inhibit PAF-induced effects.     

Alterations in Renal Uptake Kinetics of the Xanthine Derivative Enprofylline in Endotoxaemic Mice

The pharmacokinetics and renal uptake of enprofylline, which is primarily excreted into the urine by an active tubular secretion mechanism, were investigated in endotoxaemic mice by lipopolysaccharide isolated from Klebsiella pneumoniae. Lipopolysaccharide (1 mg kg^?1) was infused 2 h before starting the examination, thereby inducing a decrease in the systemic clearance and an increase in the steady-state volume of distribution of enprofylline while inducing no changes in the urinary recovery (> 90%). The protein binding of enprofylline significantly decreased in the presence of lipopolysaccharide. Both the systemic clearance for unbound enprofylline and glomerular filtration rate decreased in the treated mice. A nonlinear relationship was found in both groups between the steady-state unbound plasma concentration and renal uptake of enprofylline after constant infusion for 1 h. The renal uptake rate of enprofylline decreased in the treated mice. Lipopolysaccaharide caused increases in the apparent maximum capacity for renal uptake (V_max) from 17.3 to 32.2 μg h^?1 g^?1 of kidney and in the Michaelis–-Menten constant (K_m) from 2.7 to 21.7 μg mL^?1 and decrease in the nonsaturable uptake rate constant (K_d) from 0.87 to 0.43 mL h^?1 g^?1 of kidney. These results indicate that lipopolysaccharide decreases the renal tubular secretion of enprofylline by inducing a decrease in the renal uptake ability.     

Effects of enprofylline on A1 and A2 adenosine receptors

The effects of enprofylline were studied on A₁ adenosine receptors of rat fat cells and on A₂ adenosine receptors of human platelets and of guinea-pig lung. Enprofylline antagonized the 5′-N-ethylcarboxamidoadenosine (NECA)-induced stimulation of platelet adenylate cyclase activity with a K_B of 130 μM. In human platelets, enprofylline did not antagonize but potentiated the NECA-induced inhibition of aggregation. This potentiation was abolished in the presence of the phosphodiesterase inhibitor papaverine. An adenosine antagonistic effect of enprofylline could not be evaluated on A₂ receptors of guinea-pig lung because the xanthine enhanced basal and NECA-stimulated cyclic AMP accumulation. Enprofylline antagonized the N⁶-R-(−)-phenylisopropyladenosine (R-PIA)-induced inhibition of rat fat cell adenylate cyclase with a K_B of 32 μM. The K_i value for inhibition of [³H]PIA binding to fat cell membranes was 45 μM. Enprofylline inhibited cyclic AMP phosphodiesterase activity of human platelets, guinea-pig lung and rat fat cells with K_i values of 15, 130 and 110 μM, respectively. The results show that enprofylline was nearly equipotent as antagonist at A₁ and A₂ adenosine receptors. Mechanisms other than adenosine antagonism or phosphodiesterase inhibition may be involved in the pharmacological effects of enprofylline.     

On the High Affinity of 8-Cyclohexylcaffeine for the Presynaptic Inhibitory Adenosine Receptor Present in Rat Motor Nerve Terminals

Abstract: Rat neuromuscular junction was used to study the characteristics of presynaptic A₁ adenosine receptors. We investigated the ability of the 8-substituted caffeine, 8-cyclohexylcaffeine (CHC), as well as of 1,3,8-substituted xanthines, 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX) and 8-p-sulfophenyl-1-isoamyl-3-isobutylxanthine (SPIIBX) to antagonize the inhibitory effect of 2-chloroadenosine on the amplitude of nerve-evoked twitches of the rat phrenic-hemidiaphragm, and we compared the affinity of these xanthines with that of 1,3-dipropyl-8-cyclopenthylxanthine (DPCPX). CHC, DPSPX and SPIIBX in a near parallel manner shifted to the right the log concentration-response curve for the inhibitory effect of 2-chloroadenosine on nerve-evoked twitch amplitude. Linear Schild plots with slopes near to unity were obtained for all these xanthines. The order of potency of the xanthines was DPCPX (K_i=0.53 nM) >DPSPX (38 nM) ≈CHC (41 nM) >SPIIBX (404 nM). The affinities of DPSPX and SPIIBX for the A₁ receptor at the rat neuromuscular junction are in agreement with the affinities described for A₁ receptors at brain membranes. The now reported affinity of CHC for the presynaptic A₁ receptor is 683 times higher than that obtained in binding studies in rat brain membranes, and is only 49 times higher than that obtained in functional assays (adenylate cyclase activity) in non-neuronal preparations (rat fat cells).     

Pharmacodynamics and pharmacokinetics of enprofylline and theophylline in the isolated rabbit heart

The direct effects of theophylline and enprofylline, a new anti-asthma xanthine derivative without adenosine receptor blocking action, were studied in the isolated, spontaneously beating rabbit heart. At increasing concentrations from 2 x 10(-5)-5 x 10(-4) M both drugs produced increases in heart rate up to 143% and 162% and in contractility up to 135% and 147% from control values (100%), respectively. At concentrations higher than 10(-3) M contractures and heart block occured. Enprofylline showed a potency about 2.3 times higher than theophylline. Coronary flowrate did not increase. Myocardial oxygen consumption was moderately increased by the drugs. The myocardial pharmacokinetics showed two-compartment characteristics for both drugs. Half-times of the two phases of accumulation were for theophylline 0.28 and 0.98 min. and for enprofylline 0.31 and 0.78 min. The terminal disposition half-time for enprofylline was, however, 2.7 times higher than that of 0.91 min. for theophylline, apparently due to a stronger binding of the former drug intracellularly. Both xanthines accumulated against a concentration gradient showing myocardial tissue-perfusion liquid ratios of about 2.9 for theophylline and 3.7 for enprofylline. The drugs seem to exert their primary action on sarcolemmal binding sites which probably are unrelated to adenosine receptors.