Desensitization, two-state receptors and pharmacological parameters

Excitation and adaptation in bacteria-a model signal transduction system that controls taxis and spatial pattern formation

The machinery for transduction of chemotactic stimuli in the bacterium E. coli is one of the most completely characterized signal transduction systems, and because of its relative simplicity, quantitative analysis of this system is possible. Here we discuss models which reproduce many of the important behaviors of the system. The important characteristics of the signal transduction system are excitation and adaptation, and the latter implies that the transduction system can function as a "derivative sensor" with respect to the ligand concentration in that the DC component of a signal is ultimately ignored if it is not too large. This temporal sensing mechanism provides the bacterium with a memory of its passage through spatially- or temporally-varying signal fields, and adaptation is essential for successful chemotaxis. We also discuss some of the spatial patterns observed in populations and indicate how cell-level behavior can be embedded in population-level descriptions.

A family of autocrine growth factors in Mycobacterium tuberculosis

Mycobacterium tuberculosis and its close relative, Mycobacterium bovis (BCG) contain five genes whose predicted products resemble Rpf from Micrococcus luteus. Rpf is a secreted growth factor, active at picomolar concentrations, which is required for the growth of vegetative cells in minimal media at very low inoculum densities, as well as the resuscitation of dormant cells. We show here that the five cognate proteins from M. tuberculosis have very similar characteristics and properties to those of Rpf. They too stimulate bacterial growth at picomolar (and in some cases, subpicomolar) concentrations. Several lines of evidence indicate that they exert their activity from an extra-cytoplasmic location, suggesting that they are also involved in intercellular signalling. The five M. tuberculosis proteins show cross-species activity against M. luteus, Mycobacterium smegmatis and M. bovis (BCG). Actively growing cells of M. bovis (BCG) do not respond to these proteins, whereas bacteria exposed to a prolonged stationary phase do. Affinity-purified antibodies inhibit bacterial growth in vitro, suggesting that sequestration of these proteins at the cell surface might provide a means to limit or even prevent bacterial multiplication in vivo. The Rpf family of bacterial growth factors may therefore provide novel opportunities for preventing and controlling mycobacterial infections.

New insights in insurmountable antagonism

Antagonists that produce parallel rightward shifts of agonist dose-response curves with no alteration of the maximal response are traditionally classified as surmountable, while insurmountable antagonists also depress the maximal response. Although the longevity of the antagonist-receptor complex is quoted in many studies to explain insurmountable antagonism, slowly interconverting receptor conformations, allosteric binding sites, and receptor internalization have been evoked as alternative explanations. To complicate matters even further, insurmountable antagonism is not only drug-related; it may also depend on the tissue, species and experimental design. For the sake of drug development, it is important to elucidate the molecular mechanisms of insurmountable antagonism. New experimental approaches, such as intact cell studies and the use of computer-assisted simulations based on dynamic receptor models, herald the advent of better insight in the future.

Angiotensin II type 1 receptor antagonists. Why do some of them produce insurmountable inhibition?

Chinese hamster ovary (CHO) cells expressing human recombinant angiotensin II type 1 (AT(1)) receptors offer a useful experimental system in which antagonist binding and inhibition of AT-induced inositol mono-, bis-, and trisphosphate accumulation can be measured under identical experimental conditions. The major conclusions of the current work are: All investigated AT(1) antagonists are competitive with respect to AT. They bind to a common or overlapping binding site on the receptor in a mutually exclusive way. Reduction of the maximal angiotensin II response, i.e. insurmountable inhibition, is observed only when the cells are preincubated with candesartan, EXP3174, or irbesartan and is strictly related to the dissociation rate of the antagonist-receptor complex. On the other hand, inhibition by losartan is fully surmountable by AT, and its dissociation is very rapid. With respect to the binding kinetics, the antagonist-receptor complex can adopt a fast and a slow reversible state. The equilibrium between both states, which is dependent upon the nature of the antagonists, determines the extent of insurmountable inhibition. Consequently, the dissociation rate of the different antagonists correlates with the amount of insurmountable inhibition. In addition to the relatively slow dissociation of candesartan, reassociation to the receptor, which is measurable in CHO-AT(1) cells, likely contributes to its long-lasting blood pressure lowering effect in vivo.

Dynamic mechanisms of non-classical antagonism by competitive AT(1) receptor antagonists

Selective competitive angiotensin AT(1) receptor antagonists exhibit diverse patterns of antagonism of angiotensin-II-mediated responses in functional assays. These range from the classical parallel rightward shift of agonist concentration-response curves with no depression of the maximum response to an apparently straightforward insurmountable antagonism with complete depression of the maximum response and no rightward shift. This article reviews some earlier equilibrium-based models that have been used to explain the insurmountable antagonism, and suggests that a kinetic model might provide a more satisfactory account of the observations. Such a model might provide deeper insights into the pharmacology of G-protein-coupled receptors than the more popular equilibrium models.

Action of AT1 receptor antagonists on angiotensin II-induced tone in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were studied under isometric conditions in a myograph. 2. Addition of angiotensin II (AII) induced a concentration-dependent increase in tone in isolated arteries. The active metabolite of candesartan (CV 11974), losartan and the active metabolite of losartan, E-3174 antagonized AII-induced tone in a non-competitive manner, but the AT2 selective antagonist, PD123319, was without effect on responses to AII. The effects of candesartan, losartan and E-3174 were analysed using a classical model of non-competitive antagonism and a two-state receptor model. 3. Mechanical removal of the endothelium; pre-incubation with Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME); pre-incubation with indomethacin, a cyclo-oxygenase inhibitor; or pre-incubation with BQ 485, an endothelin antagonist; had no significant effect on contractions induced by AII. 4. Our results suggest AII contracts human isolated resistance arteries by an action on AT1 receptors and does not involve release of endothelial factors. Use of a two-state receptor model successfully described the action of the AT1 antagonists without sacrificing assumptions regarding the competitive nature of binding of these antagonists.

Insurmountable angiotensin AT1 receptor antagonists: the role of tight antagonist binding

Angiotensin II increased the inositol phosphates production (EC50 = 3.4+/-0.7 nM) in Chinese hamster ovary (CHO) cells expressing the cloned human angiotensin AT1 receptor (CHO-AT1 cells). Coincubation with angiotensin AT1 receptor antagonists produced parallel rightward shifts of the concentration-response curve without affecting the maximal response. The potency order is 2-ethoxy-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]-1H-benz imidazoline-7-carboxylic acid (candesartan) > 2-n-butyl-4-chloro-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]i midazole-5-carboxylic acid (EXP3174) > 2-n-butyl-4-spirocyclopentane-1-[(2'-(1H-tetrazol-5-yl)biphe nyl-4-yl)methyl]2-imidazolin-5-one (irbesartan)> of 2-n-butyl-4-chloro-5-hydroxymethyl-1-(2'-(1H-tetrazol-5-yl)bipheny l-4-yl)methyl]imidazole (losartan). Additionally, preincubation with these antagonists depressed the maximal response, i.e., 95%, 70%, 30% of the control response for candesartan, EXP3174 and irbesartan and not detectable for losartan. Increasing the antagonist concentration or prolonging the preincubation time did not affect this depression. Furthermore, these values remained constant for candesartan and EXP3174, when the angiotensin II incubation time varied between 1 and 5 min. Our data indicate that antagonist-receptor complexes are divided into a fast reversible/surmountable population and a tight binding/insurmountable population at the very onset of the incubation with angiotensin II.

Binding of the antagonist [3H]candesartan to angiotensin II AT1 receptor-transfected [correction of tranfected] Chinese hamster ovary cells

Binding of the non-peptide angiotensin II AT1 antagonist [3H](2-ethoxy-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]- H-benzimidazoline-7-carboxylic acid ([3H]candesartan) to human angiotensin II AT1 receptor-transfected Chinese hamster ovary (CHO-AT1) cells was inhibited to the same extent by angiotensin II and non-peptide angiotensin II AT1 antagonists. No binding was observed in control CHO-K1 cells. Dissociation was slow (k(-1) = 0.0010+/-0.0001 min(-1)) after removal of the free [3H]candesartan but increased 5-fold upon addition of supramaximal concentrations of angiotensin II AT1 antagonists. Angiotensin II responses recovered equally slow from candesartan-pretreatment. When washed and further incubated, these angiotensin II responses also recovered more rapidly in the presence of 2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)biphen yl-4-yl)methyl]imidazole (losartan), indicating that unlabelled ligands prevented reassociation. [3 H]candesartan saturation binding experiments required a long time to reach equilibrium. Therefore, the equilibrium dissociation constant (Kd = 51+/-8 pM) was calculated from the association and dissociation rate constants. Our findings indicate that the insurmountable nature of candesartan in functional studies is related to its slow dissociation from the receptor.

Distinction between surmountable and insurmountable selective AT1 receptor antagonists by use of CHO-K1 cells expressing human angiotensin II AT1 receptors

1. CHO-K1 cells that were stably transfected with the gene for the human AT1 receptor (CHO-AT1 cells) were used for pharmacological studies of non-peptide AT1 receptor antagonists. 2. In the presence of 10 mM LiCl, angiotensin II caused a concentration-dependent and long-lasting increase of inositol phosphates accumulation with an EC50 of 3.4 nM. No angiotensin II responses are seen in wild-type CHO-K1 cells. 3. [3H]-Angiotensin II bound to cell surface AT1 receptors (dissociates under mild acidic conditions) and is subject to rapid internalization. 4. Non-peptide selective AT1 antagonists inhibited the angiotensin II (0.1 microM) induced IP accumulation and the binding of [3H]-angiotensin II (1 nM) with the potency order: candesartan > EXP3174 > irbesartan > losartan. Their potencies are lower in the presence of bovine serum albumin. 5. Preincubation with the insurmountable antagonist candesartan decreased the maximal angiotensin II induced inositol phosphate accumulation up to 94% and, concomitantly, decreased the maximal binding capacity of the cell surface receptors. These inhibitory effects were half-maximal for 0.6 nM candesartan and were attenuated by simultaneous preincubation with 1 microM losartan indicating a syntopic action of both antagonists. 6. Losartan caused a parallel rightward shift of the angiotensin II concentration-response curves and did not affect the maximal binding capacity. EXP3174 (the active metabolite of losartan) and irbesartan showed a mixed-type behavior in both functional and binding studies. 7. Reversal of the inhibitory effect was slower for candesartan as compared with EXP3174 and irbesartan and it was almost instantaneous for losartan, suggesting that the insurmountable nature of selective AT1 receptor antagonists in functional studies was related to their long-lasting inhibition.

Non-competitive binding of the nonpeptide antagonist BIBP3226 to rat forebrain neuropeptide Y1 receptors

[3H]Neuropeptide Y labelled neuropeptide Y receptors in rat forebrain membranes as a homogenous class of high-affinity sites. Between 80 and 85% of these receptors showed high affinity for Y1-selective antagonists such as (R)-N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-D-arginine amide (BIBP3226). While competitive in functional studies, BIBP3226 produced parallel shifts of the Scatchard plots of [3H]neuropeptide Y saturation binding in rat forebrain membranes. Mechanisms which are routinely invoked to explain non-competitive binding do not apply to BIBP3226. Wash-out experiments, involving successive treatment of the membranes with BIBP3226, buffer (wash-out step) and [3H]neuropeptide Y, argue against irreversible or a pseudo-irreversible binding of the antagonist. Allosteric inhibition is also unlikely since BIBP3226 did not affect the rate of dissociation of [3H]neuropeptide Y in isotope dilution experiments. The non-hydrolyzable guanine nucleotide, 5'-guanylylimidodiphosphate (Gpp(NH)p), abolished the binding of [3H]neuropeptide Y and increased its rate of dissociation in isotope dilution experiments. This suggests that the initial [3H]neuropeptide Y-receptor association is a low affinity process and that the observed binding of [3H]neuropeptide Y is related to the formation of a ternary [3H]neuropeptide Y-receptor-G protein complex. Two- or even multistate models (in which BIBP3226 could potentially behave as an inverse agonist) could therefore be needed to explain the non-competitive antagonism of BIBP3226 in broken cell preparations.

Molecular pharmacology of LR-B/081, a new non-peptide angiotensin AT1 receptor antagonist

This report describes the molecular pharmacological properties of LR-B/081 (methyl 2-[[4-butyl-2-methyl-6-oxo-5-[[2'-(1H-tetrazol-5- yl) [1,1'-biphenyl]-4-yl]methyl]-1 (6H)-pyrimidinyl]methyl]- 3-thiophenecarboxilate), a novel non-peptide angiotensin II receptor antagonist. This compound potently displaced [3H]angiotensin II from angiotensin AT1 (Ki = 1.4 nM, rat adrenal cortex), but not from angiotensin AT2 (Ki > 1 microM, bovine cerebellar cortex) receptors and did not show affinity for other receptor systems (Ki > 10 microM). In saturation studies, LR-B/081 both increased KD and decreased Bmax values in a dose-dependent fashion. The rate of dissociation of [3H]angiotenin II from angiotensin AT1 receptors was not affected by the presence of 1 microM LR-B/081 and the association rate of [3H]angiotensin II was not decreased by the presence of 1 or 30 nM LR-B/081, indicating that the Bmax reduction was not due to an allosteric interaction or to a delay in reaching the steady-state conditions. These data underline the complexity of the antagonistic nature of LR-B/081, presenting features of both competitive and noncompetitive antagonism.

Models to explain dose-response relationships that exhibit a downturn phase

Over a broad dose (or concentration) range, dose-response relationships frequently display a bell-shaped form. Mechanisms leading to this phenomenon may be manifold, but so far they have not been adequately identified. In this article, Vladimir Pliska discusses two models that are based on multiple-state cell-signaling pathways and enable an estimate of generally applicable descriptors. These can be used for expression of drug potency and for similar purposes.

Potential errors in agonist dissociation constant estimation caused by desensitization

A simple model of receptor desensitization is developed and analysed to predict the consequences of acute receptor loss on the pharmacological quantification of agonist action. The model incorporates the desensitization scheme of Katz and co-workers (1957) into the operational model of agonism (Black & Leff, 1983) and, therefore, it assumes the occupancy theory of agonist action. Pharmacological effect-time profiles are simulated which illustrate (i) the extent to which overt fade may be detectable under different conditions and (ii) the extent to which measured pharmacological effects deviate from those which would be measured in the absence of desensitization. It is shown that the resulting agonist concentration-effect curves may be displaced rightwards from their equilibrium positions and that the agonist dissociation constants estimated from them may be overestimated. Such errors are predicted to occur regardless of whether or not fade is detectable in the effect-time profiles. Estimates of agonist efficacy appears to be unaffected by desensitization. The results of this analysis are discussed with respect to their implications for receptor classification using agonist dissociation constant estimates and for the development of agonist drugs.

Desensitization of the acetylcholine-induced increase of potassium conductance in rabbit cardiac Purkinje fibres

In rabbit cardiac Purkinje fibres, acetylcholine (ACh) changes membrane electrophysiological properties in a biphasic time course. On wash-out of ACh a rebound phenomenon is observed (Mubagwa & Carmeliet, 1983). The underlying mechanisms have been studied by the voltage-clamp technique. The ACh-induced increase in K+ current follows a biphasic time course during exposure to ACh. This time course is not due to intercellular accumulation or depletion of K+, but results from a desensitization process. On wash-out a rebound is obtained, i.e. the membrane K+ conductance transiently decreases below the control value. In contrast, the inhibition of ACh of the catecholamine-induced increase of slow inward current follows a monophasic time course. The desensitization process or secondary decrease of ACh-induced change in K+ current follows a mono- or a biexponential time course. The extent and rate of desensitization depend on ACh concentration. The rate of desensitization is not influenced by membrane potential but its extent seems to be increased by depolarization. Recovery from desensitization is relatively rapid and has a half-time of about 2 min. Different existing models for desensitization are discussed, no one of which accounts for all results in rabbit Purkinje fibres. Therefore, a three-state receptor model is proposed to explain the results. The model assumes that the K+ channel is directly associated with the muscarinic receptor and that the channel-receptor complex may be in closed, open or desensitized state, in the presence as well as in the absence of agonist.

Desensitization and coupled receptors: a model of drug dependence

It is assumed that certain drug receptors are so coupled with certain physiological receptors that stimulation of either receptor increases the sensitivity of the other. If the drug receptor suffers tolerance (i.e. slow desensitization) and if insensitivity of the drug receptor also makes the physiological receptor insensitive, then tolerance must be responsible for a physiological deficiency. This may be remedied by increased drug administration which will raise the sensitivity of the remaining physiological receptors so that a normal or near-normal physiological situation is achieved. Thus the organism is not only tolerant to the drug but also dependent on it. If such theoretical considerations apply to opiate receptors (as drug receptors) and to catecholamine receptors (as physiological receptors), then the theory predicts that acute morphine administration increases the sensitivity of dopamine receptors, that sympathetic stimulation decreases pain sensitivity, that opioid tolerance provokes increased catecholamine activity, that alpha-receptor stimulants attenuate and alpha-receptor antagonists exacerbate morphine abstinence, and that catecholaminergic inhibition results in increased morphine toxicity. All of these predictions have been verified experimentally.