J. F. Rodrigues de Miranda

The Pharmacon-Receptor-Effector Concept

The biological effect of chemical agents can only be the result of an interaction of the molecules of the active agent and particular molecules or molecular complexes present in the biological object. The same holds true for endogenous bioactive agents such as hormones, neurotransmitters, etc. Often these messengers have their molecular sites of action, referred to as their specific receptors, located on the outer cell membrane (Lefkowitz et al., 1976). They then transmit information from remote or nearby cells to the surface of the target cells where they trigger these cells, thus inducing certain changes in cell function. The interaction between the bioactive agent and its specific receptors initiates the transduction of information received from outside to the cell interior. An example is the generation of intracellular mediators such as cyclic AMP. Similarly, the hormone-receptor complex, in the case of the interaction of steroid hormones and their specific proteins in the cytoplasm, is involved in the transduction of information to the cell nucleus. The presence of specific receptor for the bioactive agents either on the outer cell membrane or in the cytoplasm of only particular tissue cells accounts for a selectivity in action on these tissues.

Amiloride is a cholinergic antagonist in the rabbit pancreas

The effect of amiloride on fluid and protein secretion in the isolated rabbit pancreas and on amylase secretion in rabbit pancreatic acini has been studied. Amiloride (1 mM) has no effect on the pancreatic fluid secretion either in a normal incubation medium (143 mM Na+), or in a medium containing only 25 mM Na+. The carbachol-induced enzyme secretion is inhibited by amiloride in both systems, whereas the enzyme secretion induced by the C-terminal octapeptide of cholecystokinin ( PzO ) is not affected. Amiloride also inhibits the carbachol-induced 45Ca efflux from rabbit pancreatic acini, but again not that induced by PzO . The amiloride concentrations for half-maximal inhibition of carbachol-induced amylase secretion and 45Ca efflux are 40 and 80 microM, respectively. Amiloride also competitively inhibits the specific binding of [3H]quinuclidinyl benzylate ( [3H]QNB) to rabbit pancreatic acini, suggesting that the amiloride effect is due to competition on the level of the muscarinic acetylcholine receptor.

Cholinergic muscarinic receptors in rat cochlea.

Specific 3H-1-quinuclidinylbenzilate (3H-1-QNB) binding to rat cochlea homogenates occurs to a homogeneous class of binding sites with Kd = 0.13 +/- 0.01 nM and Bmax = 0.57 +/- 0.07 fmol per cochlea. Binding is stereoselectively inhibited by benzetimide enantiomers. Dexetimide was more effective than levetimide in displacing 3H-1-QNB from its binding sites (Ki = 4 x 10(-10) M and 6.5 x 10(-6) M, respectively). Pirenzepine inhibits 3H-1-QNB binding with low affinity (Ki = 2 x 10(-6) M), classifying the binding sites as muscarinic M2 receptors.

Ligand binding to dopamine-receptors: Analysis & interpretation

Abstract Dopamine receptor interaction was studied by concentration dependent inhibition of 3 H-Spiperone binding to calf caudate nucleus homogenates with (+)-butaclamol, haloperidol, ergometrine, apomorphine and dopamine. The results were analyzed in terms of a one and a two site receptor model using computerized curve fitting procedures. Evidence is given in favour of a two site receptor model on basis of statistical criteria. The presence of two independent non-interconverting receptor sites for dopamine in calf caudate nucleus is consistent with binding data from others and with in vivo experiments.

Muscarinic receptor binding in central airway musculature in chronic airflow obstruction

The hypothesis of an increased muscarinic receptor sensitivity in airway musculature of patients with chronic airflow obstruction (CAO) has been investigated by in vitro radioligand binding studies. The receptor binding profiles were determined in membrane homogenates of tracheal and main bronchial smooth muscle, as well as in segmental bronchial tissue preparations. The number of receptors was measured by binding of the radioactive muscarinic antagonist [3H]-(-)-quinuclidinyl benzilate ([3H]-(-)-QNB). The affinity for agonists was investigated by studying the inhibition of [3H]-(-)-QNB binding by the full muscarinic agonist methylfurtrethonium. Binding characteristics were determined (1) in tracheal smooth muscle from 7 patients with chronic airflow obstruction (2) in main bronchial smooth muscle of 5 patients with CAO and (3) in segmental bronchial tissue of 10 patients with CAO. The receptor binding properties were compared with values obtained on airway tissue from 31 subjects without CAO. Muscarinic receptors in smooth muscle preparations of trachea and main bronchus were present in normal density, and showed normal [3H]-(-)-QNB binding affinity and methylfurtrethonium binding properties. In segmental bronchial tissue preparations which contain smooth muscle and glandular tissue, also normal receptor numbers and [3H]-(-)-QNB affinity values were found. Morphometric examination of these preparations revealed normal amounts of smooth muscle and submucosal glands. This suggests that the excessive mucus production of CAO is not accompanied by an increased muscarinic receptor density in submucosal glands. These observations suggest unaltered muscarinic receptor characteristics in central airway smooth muscle of patients with chronic airflow obstruction.

Pancuronium masks the prejunctional muscarinic autoreceptor in guinea pig tracheal smooth muscle.

The effect of pancuronium pretreatment on the function of the prejunctional muscarinic receptor in guinea-pig trachea was studied by using electrical field stimulation (EFS). The effects of cumulative doses of the muscarinic M2 receptor antagonist gallamine were investigated in tracheal smooth muscle strips from guinea-pigs after addition of pancuronium in vitro and in strips from guinea-pigs which had been pretreated with doses of pancuronium that caused 100% neuromuscular blockade. The results of both types of experiments were compared to those of control groups of the same size. In all strips a dose response curve with cumulative doses of methacholine was made before EFS was switched on. No differences were found between the mean pD2 value and slope of the concentration-response curves of untreated guinea-pigs and animals treated with anaesthetics and pancuronium. The animals showed variable responses to pancuronium. The bath concentration of pancuronium which decreased the EFS-induced contraction to half the original value varied between 14-61 microM. The intravenous dose necessary to paralyze the muscles, varied among the different guinea-pigs from 0.017-0.085 mg.kg-1. The EFS-induced contraction for the concentration range of gallamine 0.32 microM-0.32 mM was found to differ significantly between the strips treated with pancuronium in the organ bath and their control group. For the guinea-pigs anaesthetized and pretreated with pancuronium a significant difference with control was observed at gallamine concentrations ranging from 0.032-0.32 mM. These results show that pancuronium, added to the organ bath as well as administered intravenously to the guinea-pig, masked the inhibitory muscarinic receptor.

Folates: epileptogenic effects and enhancing effects on [3H]TBOB binding to the GABAA-receptor complex

The biochemical mechanism responsible for the convulsive effects of folates was investigated. The epileptogenic effects of folates were determined in vivo by quantification of the seizures following intracortical application in rats. The rank order of epileptogenic effects is: folic acid greater than or equal to 5-HCO-H4 folate greater than H2 folate greater than 5-CH3-H4 folate. This sequence of epileptogenicity in vivo is compared to the rank order of the effects of folates on radioligand binding to the GABAA-receptor complex in vitro. The inhibitory potencies of folates on [3H]muscimol and [3H]diazepam bindings did not correlate with their epileptogenic effects. However, folates reverse the inhibiting effect of GABA on the binding of the cage convulsant [3H]TBOB [( 3H]t-butylbicycloorthobenzoate). The rank order of this in vitro effect (folic acid greater than 5-HCO-H4 folate greater than H2 folate = 5-CH3-H4 folate) resembles the rank order of epileptogenicity determined in vivo. A relationship between the in vivo and in vitro effects is therefore suggested.

Kinetics of (-)125Iodocyanopindolol Binding to Intact Human Mononuclear Cells

In association experiments of (-)125Iodocyanopindolol (125ICYP) with human mononuclear cells (MNC) at 70 pM and a temperature of 37 degrees C equilibrium was reached within 30 min. However, when the same experiments were performed at a concentration of 4 pM 125ICYP, equilibrium was only reached after 3 hours. The consequences of incomplete equilibrium for the interpretation of binding experiments under the incorrect assumption that equilibrium has been reached, was investigated at equilibration times of one, two and three hours. The dissociation constant, Kd, decreased from 7.4 +/- 0.2 pM after one hour to 2.5 +/- 0.4 pM after three hours of incubation while the receptor density, RO, decreased from 970 +/- 170 to 713 +/- 58 sites/cell. Analysis of computer simulated binding curves confirmed the decrease in Kd and RO at prolonged incubations. We conclude that in 125ICYP binding in intact MNC one hour of incubation is not sufficient to obtain equilibrium at the lower concentrations. This leads to an overestimation of Kd- and to a lesser extent of RO-values. Extending the incubation time to three hours on the other hand may lead to a loss of cells and therefore to an underestimation of RO.

Nonexponential relaxation of the methyl protons of acetrizoate in solution

Abstract The time dependence of the magnetization of the methyl group of acetrizoate dissolved in D 2 O is found to be strongly nonexponential. This effect is attributed to the influence of cross-correlations and is in nice agreement with the theory of Hubbard and its extended form presented by Werbelow and Marshall. The rotational diffusion of the methyl group around its axis is estimated to be 30 times as fast as the overall rotation of the whole molecule. In the presence of BSA the nonexponential relaxation effect disappears. Inaddition it is shown that, for a symmetric top molecule with the axis of the methyl group along the molecular symmetry axis and the methyl group rotation in the extreme narrowing limit, analytical expressions can be derived for the time decay of the magnetization, when the rotation of the molecule perpendicular to this axis is not in the limit of extreme narrowing. There is, however, a lower limit to the rate of this motion.

Binding of RO 5–4864 To Brain Membranes of Rats with and Without Absence-Like Phenomena

Spontaneously occurring spike-wave discharges (SWD) in rats are used as a model for absence epilepsy in humans. In vitro, the binding parameters of 3H-Ro 5-4864, a ligand labelling the peripheral benzodiazepine receptor, were determined for brain membranes of WAG/Rij rats, an inbred strain showing SWD, and for ACI rats, an inbred strain showing no SWD. No difference in the Kd but a small difference in the Bmax values between the strains were found. Recently, other investigators reported a correlation between a decrease in affinity for 3H-Ro 5-4864 and the occurrence of SWD. Our results suggest however that it is doubtful that a change in Kd of the peripheral benzodiazepine receptor is causal in the etio-pathology of the spontaneous absence like phenomena in rats.