Norbert Kolassa

Vasodilatation elicited by 5-HT1A receptor agonists in constant-pressore-perfused rat kidney is mediated by blockade of α1A-adrenoceptors

The vasodilator mechanism of the putative serotonin1A (5-HT) receptor agonists, urapidil, 5-methyl-urapidil, ipsapirone, flesinoxan and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was investigated in constant-pressure perfused rat kidneys. The compounds (10(-12)-10(-7) mol bolus injection) neither enhanced basal flow nor evoked vasodilatation in kidneys preconstricted by 27 mM KCl, 1.5 mM BaCl2 or 10(-6) M prostaglandin (PG)F2 alpha, but evoked a dose-dependent, reversible and spiroxatrine-resistant increase in vasodilatation of organs preconstricted by 6 x 10(-7) M noradrenaline. 5-Carboxamidotryptamine and sumatriptan did not reverse the vasoconstriction induced by all stimuli or that induced by noradrenaline in the presence of 5-HT2 plus 5-HT3 receptor blockade. No correlation for the vasorelaxant drugs was found between their -log ED50 in rat kidney and pKi values at 5-HT1A binding sites in pig cortex as determined in radioligand experiments. The relaxation in rat kidney induced by 5-HT1A receptor agonists and alpha 1A-adrenoceptor-selective antagonists (WB 4101 and (+)-niguldipine) was significantly correlated with pKi values at alpha 1A binding sites in rat cortex and the pA2 values derived from contraction studies for competitive antagonism at alpha 1-adrenoceptors in prostatic portions of the rat vas deferens, but differed from pKi values for alpha 1B binding sites in rat cortex. Thus, the vasodilator effect of the 5-HT1A receptor agonists urapidil, 5-methyl-urapidil, ipsapirone, flesinoxan and 8-OH-DPAT in the noradrenaline-perfused rat kidney appears to be mediated by their concomitant alpha 1A-adrenoceptor blockade. No evidence for a vasodilator effect mediated through 5-HT1A receptors was found under our experimental conditions.

Mechanism of calcium‐independent phosphorylation of sarcoplasmic reticulum ATPase by orthophosphate

Magnesium affects several reaction steps of the calcium transport cycle in sarcoplasmic reticulum membranes [ 1,2]. Recent studies on the effect of magnesium on sarcoplasmic reticulum function [3] indicate a dual role of magnesium on the phosphorylation of sarcoplasmic reticulum transport ATPase from ATP, as evaluated from analysis of the exchange rate of y-phosphate between ATP and ADP [4] in the presence of saturating calcium concentrations: (i) That magnesium activates the enzyme directly; (ii) That it represents part of the substrate MgATP for the phosphorylation reaction [3]. From studies on the role of magnesium in calciumindependent and calciumdependent phosphorylation of sarcoplasmic reticulum transport ATPase from orthophosphate [5-191, the formation of a magnesium-phosphoprotein in calcium-independent phosphorylation was suggested [ 181. Based on the good fit of a reaction scheme to the data the following features were proposed: (i) The phosphoprotein (Mge E-P,), in which the phosphate is covalently bound to the enzyme, is in equilibrium with the Michaelis complex (Mg . E . Pi), the concentration of which is determined by the concentration of free orthophosphate and free magnesium; (ii) The binding of orthophosphate and magnesium appears to be interdependent, both ligands apparently bind randomly [ 181.

pH and temperature dependence of adenosine uptake in human erythrocytes

Kinetic analysis of the saturable adenosine uptake in human erythrocytes suggests the existence of two saturable components, distinguished by different Km values (1.4 and 260 micron, respectively, at pH 7.4 and 25 degrees C). Both components were abolished by p-nitrobenzylthioguanosine or dipyridamole. Total uptake was significantly higher at pH 8 than at pH 7 at adenosine concentrations above 2 micron. The increase in uptake at the higher pH was brought about mainly by an increase in the maximum rate of transport of the low-affinity uptake system. With rising temperature the Km and the V of both uptake components increased. No transition temperature was observed between 12 and 37 degrees C.

Relaxation of coronary artery strips by adenosine and acidosis

Cumulative dose-response curves of Ca2+-induced tension increments were studied in K+-depolarized helical strips of dog coronary arteries. Adenosine 10(-4) M reduced the Ca2+ sensitivity of the strips without altering the maximal tension with full Ca2+ activation. In contrast, acidosis of pH 7.05 significantly diminished the maximal tension with full Ca2+ activation. The relaxing effect of acidosis was almost completely abolished by 10(-4) M adenosine. It is concluded that adenosine inhibits Ca2+ influx, whereas acidosis depresses the contractile process of vascular smooth muscle directly.

Intestinal transfer of the quaternary ammonium compound N-methyl-scopolamine by two transport mechanisms in series

Abstract In order to localize the previously demonstrated intestinal transport mechanism for quaternary ammonium compounds, counter-transport and efflux experiments were performed with N -methyl-scopolamine (NMScop) on isolated jejunal epithelia of guinea pigs. (1) Both [ 3 H]NMScop uptake from and [ 3 H]NMScop release to the blood side of jejunal epithelia were stimulated by the presence of high concentrations of unlabelled NMScop in the compartment, into which flux was occurring. The phenomenon of counter-transport was not apparent at the luminal cell membrane. (2) From preloaded epithelia [ 3 H]NMScop efflux to the lumen side was markedly reduced during NaCN-intoxication as compared to controls, whereas that to the blood side increased. These effects were brought about mainly by a decrease in the luminal transfer coefficient. (3) At the luminal membrane the efflux coefficient far exceeded that for influx, in spite of the negative potential of the cell interior. It is concluded that NMScop is transferred across the jejunal epithelium by two transport mechanisms in series, one located in the basolateral and one in the luminal cell membrane.

Effects of the carboxylesterase inhibitor bis-(p-nitrophenyl)-phosphate on disposition and metabolism of hexobendine

Abstract The effect of pretreatment with the carboxylesterase inhibitor bis-( p -nitrophenyl)-phosphate (BNPP) on the disposition and metabolism of intravenously-administered [ 14 C]hexobendine was studied in rats. Inhibition of hexobendine ester cleavage by BNPP produced a marked diminution in the concentrations of the hexobendine hydrolysis products, 3,4,5-trimethoxybenzoic acid and N,N′-dimethyl-N-[3-(3′,4′,5′-trimethoxybenzoxy)-propyl]-N′-3-hydroxypropyl-ethylendiamine in plasma and in the liver and kidneys. A simultaneous elevation occurred in the levels of demethylated metabolites following BNPP pretreatment. The concentration-time relationship of non-metabolized [ 14 C]hexobendine was not significantly altered by BNPP pretreatment. The results suggest that BNPP pretreatment produced a shift towards the dealkylating pathway in hexobendine metabolism. Consequently, esterase inhibition did not significantly affect the hexobendine plasma and tissue levels.

Myocardial glucose uptake and breakdown during adenosine-induced vasodilation.

SummaryIn isolated K+ (16.2 mM)-arrested cat hearts perfused at constant pressure adenosine infusions (0.8 μmoles · min−1 · 100 g−1 for 10 min) caused an increase in myocardial14C-glucose uptake and release of14CO2+H14CO−3 and14C-lactate simultaneously with a rise in coronary flow. The ratio of the release of14CO2+H14CO−3 to that of14C-lactate and the specific activity of lactate in the effluate were not altered. In K+-arrested hearts perfused with constant volume neither glucose uptake nor glucose breakdown were influenced by 0.8 or 100 μmoles · min−1 · 100 g−1 adenosine with 0.1–5 mM glucose in the perfusion medium. It is concluded that adenosine does not affect directly the myocardial glucose carrier system, aerobic or anaerobic glucose breakdown or glycogenolysis, but enhances glucose uptake secondarily by increasing coronary flow. This interpretation is substantiated by the finding that mechanically produced increases in perfusion volume caused similar increases in myocardial glucose uptake as were observed with comparable adenosine-induced coronary flow increments.

Induction of drug metabolism in the rat by taglutimide, a sedative-hypnotic glutarimide derivative

SummaryPretreatment with taglutimide significantly decreased the plasma dicoumarol level and shortened the duration of hexobarbital-induced narcosis in rats. Furthermore, taglutimide pretreatment accelerated thein vitro metabolism of dicoumarol, hexobarbital, o-nitrophenyl acetate and procaine, but not of 3,4-benzpyrene, as assayed in the 10,000xg supernatant fraction of rat liver homogenate. No definite increase was observed in liver wet weight, nor in the amount of microsomal and total liver protein in comparison with the control values. No marked differences were found between the effects of short-(4-day) and long-term (17-day) pretreatment on any of the studied parameters. The changes in drug metabolism and liver protein observed after taglutimide pretreatment differed from those observed after pretreatment with either phenobarbital or 3,4-benzpyrene. Taglutimide, like other inducing agents, is lipophilic, but differs from them in not being a substrate of monooxygenases.

Effects of the carboxylesterase inhibitor bis-(p-nitrophenyl)-phosphate on elimination of hexobendine.

Abstract A study was conducted of the effect of pretreatment of rats with the carboxylesterase inhibitor, bis-( p -nitrophenyl)-phosphate (BNPP) on the biliary and urinary excretion of intravenously-administered 14 C-hexobendine (HB) and its metabolites. Inhibition of HB ester cleavage by BNPP produced a marked decrease in the urinary excretion of HB hydrolysis products: this effect was accompanied by a definite increase in the biliary excretion of conjugates of O -demethylated HB metabolites. The results indicate the participation of complementary metabolic and excretory pathways in the elimination of HB; this hypothesis is further supported by the finding that BNPP did not alter the half-life of the plasma 14 C level.

Weak electrolyte transfer in the guinea pig jejunum: secretion of trimethoxybenzoic acid.

SummaryIn isolated epithelia of guinea pig jejunum the transcellular permeation of 10−4 M (carboxyl-14C)-3,4,5-trimethoxybenzoic acid (TMBA) in the direction blood-lumen was more than 10 times greater the transcellular permeation of 10−4 M (carboxyl-was reduced to less than 2 by anaerobiosis or by increasing TMBA concentrations of up to 10−2 M.Under aerobic conditions the cellular uptake of TMBA (10−4 M) from the blood side was twice as high as that from the lumen side. In anaerobiosis the percentage of TMBA taken up into the epithelium was enhanced, when TMBA was administered on the lumen side, while the percentage was unchanged after administration on the blood side; thereby the difference in cellular TMBA concentrations was abolished. Similar results were obtained under aerobic conditions, if the TMBA concentration was increased up to 10−2 M.The results are consistent with a three-compartment model with an intermediate compartment distinguished by a high pH as compared to that of the outer compartments and by a luminal boundary highly permeable for the ionized form of the substrate in contrast to the contraluminal boundary.