Drug-Receptor Interaction: Interaction of One or More Drugs with One Receptor System

Aerobic fitness is associated with cardiomyocyte contractile capacity and endothelial function in exercise training and detraining

BACKGROUND

Physical fitness and level of regular exercise are closely related to cardiovascular health. A regimen of regular intensity-controlled treadmill exercise was implemented and withdrawn to identify cellular mechanisms associated with exercise capacity and maximal oxygen uptake (VO2max).

METHODS AND RESULTS

Time-dependent associations between cardiomyocyte dimensions, contractile capacity, and VO2max were assessed in adult rats after high-level intensity-controlled treadmill running for 2, 4, 8, and 13 weeks and detraining for 2 and 4 weeks. With training, cardiomyocyte length, relaxation, shortening, Ca2+ decay, and estimated cell volume correlated with increased VO2max (r=0.92, -0.92, 0.88, -0.84, 0.73; P<0.01). Multiple regression analysis identified cell length, relaxation, and Ca2+ decay as the main explanatory variables for VO2max (R2=0.87, P<0.02). When training stopped, exercise-gained VO2max decreased 50% within 2 weeks and stabilized at 5% above sedentary controls after 4 weeks. Cardiomyocyte size regressed in parallel with VO2max and remained (9%) above sedentary after 4 weeks, whereas cardiomyocyte shortening, contraction/relaxation- and Ca2+-transient time courses, and endothelium-dependent vasorelaxation regressed completely within 2 to 4 weeks of detraining. Cardiomyocyte length, estimated cell volume, width, shortening, and Ca2+ decay and endothelium-dependent arterial relaxation all correlated with VO2max (r=0.85, 0.84, 0.75, 0.63, -0.54, -0.37; P<0.01). Multiple regression identified cardiomyocyte length and vasorelaxation as the main determinants for regressed VO2max during detraining (R2=0.76, P=0.02).

CONCLUSIONS

Cardiovascular adaptation to regular exercise is highly dynamic. On detraining, most of the exercise-gained aerobic fitness acquired over 2 to 3 months is lost within 2 to 4 weeks. The close association between cardiomyocyte dimensions, contractile capacity, arterial relaxation, and aerobic fitness suggests cellular mechanisms underlying these changes.

Desensitization of AMPA receptors on horizontal cells isolated from crucian carp retina

In horizontal cells freshly dissociated from crucian carp (Carassius auratus) retina, we recorded the whole-cell responses to rapid application of glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate. Currents induced by glutamate and AMPA, but not by kainate, usually showed extremely rapid desensitization. 1-(4-aminophenyl)-3-methylcarbamyl- 4-methyl-7,8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (GYKI 53655), a selective AMPA receptor antagonist, was found to completely block glutamate- and kainate-induced currents, which were supposed to be mediated by activation of AMPA receptors. We further extensively studied the kinetics of desensitization of glutamate- and AMPA-induced currents in horizontal cells. The time constants for decay of whole-cell currents induced by glutamate and AMPA were 1.9 and 1.4 ms, respectively, and the equilibrium responses to glutamate and AMPA at concentrations over 1 mM were invariably less than 10% of the corresponding peak responses. We have determined the values of EC50 for glutamate and AMPA as 1.08 and 1.05 mM, respectively, which are nearly 100-fold higher than that reported previously. Dose dependence of desensitization was also investigated and the glutamate concentration for a half desensitization was 26 microM, much lower than the EC50. Furthermore, kainate and AMPA interacted at AMPA receptors of horizontal cells in a dual competitive manner: the response to kainate of low concentration (10 microM) was potentiated by the addition of 300 microM AMPA, while the responses induced by kainate of relatively higher doses (300 microM or more) were reduced. We conclude that crucian carp horizontal cells may exclusively express the AMPA subtype of glutamate receptors, which is characterized by extremely rapid desensitization.

A new method for determining affinity constants on isolated organs when a threshold value and spare receptors are present

It was shown in previous studies that fixed ratio combinations of agonists with competitive antagonists emulate partial agonists and, therefore, mediate submaximal concentration-effect (E/[A]) curves. A linear plot was obtained by plotting the relative height (H) of these submaximal curves against phiH, where phi=[antagonist]/[agonist]. In this study, it was shown that in an agonist-effector system comprising a possible threshold value, the use of fixed ratio agonist-competitive antagonist concentrations (artificial partial agonists) induces E/[A] curves of different heights which allows one to quantify the threshold value. A nonlinear parabolic-like plot may be obtained when a threshold value is present in an agonist-effector system. By quantifying the threshold value, the nonlinear plot was converted to a linear plot from which the apparent affinity (KA) and an efficacy related parameter (eES) of an agonist could be estimated. The practical applicability of the method was shown by applying the method to E/[A] curves obtained for agonists (noradrenaline and acetylcholine) obtained in the presence and absence of increasing concentrations of their respective competitive antagonists on different effectors.

Anti-M2 muscarinic receptor antibodies inhibit beta-adrenoceptor-mediated inotropic response in rat myocardium

The modulation of the inotropic effect by affinity-purified antibodies against a synthetic peptide corresponding to the second extracellular loop of the human muscarinic M2 receptors was studied in adult rat ventricular myocardium. These anti-muscarinic M2 receptor antibodies shifted the dose-response relationship of the beta-adrenoceptor agonist isoproterenol to higher concentrations whereas preimmune rabbit immunoglobulin G (IgG) or antibodies against the N-terminus of the beta 1-adrenoceptor had no effect. This effect of anti-muscarinic M2 receptor antibodies was fully blocked after preincubation with the antigenic peptide. No significant change of maximal inotropic response to isoproterenol was observed in the presence of anti-muscarinic M2 receptor antibodies. The anti-muscarinic M2 receptor antibodies did apparently not hamper the access of the muscarinic receptor agonist carbachol. The muscarinic receptor antagonist atropine attenuated the effect of the anti-muscarinic M2 receptor antibodies. The present study demonstrates for the first time in intact adult ventricular myocardium a specific stimulatory muscarinic activity of antibodies raised against a part of the muscarinic M2 receptor protein.

Alpha 1-adrenoceptor subtypes involved in increased 86Rb+ influx rate and inositol 1,4,5-trisphosphate mass in adult rat cardiomyocytes

The aim of the present study was to identify the receptor subtypes involved in the alpha 1-adrenoceptor-mediated increase in 86Rb+ influx rate and in inositol 1,4,5-trisphosphate (IP3) accumulation in isolated ventricular cardiomyocytes from adult rat heart, in order to identify a possible response pattern compatible with a causative relationship. Subtype-selective receptor antagonists used were: 5-methylurapidil (alpha 1A), WB 4101 [2([2,6-dimethoxyphenoxy-ethyl]aminomethyl)-1,4-benzodioxane] (alpha 1A), chloroethylclonidine (alpha 1B) and BMY 7378 [8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]dec ane-7,9-dione] (alpha 1D). The basal 86Rb+ influx rate was 0.22 +/- 0.01 ml/g protein x min. At 15 min, 5 x 10(-5) mol/l noradrenaline in the presence of 3 x 10(-5) mol/l timolol increased the 86Rb+ influx rate by 33 +/- 1%. This response was not affected by either chloroethylclonidine or BMY 7378 at concentrations up to 10(-5) mol/l. 5-Methylurapidil dose dependently inhibited the response to 5 x 10(-5) mol/l noradrenaline with a -logIC50 value of 5.27 +/- 0.12 and 5.61 +/- 0.27 in the presence and absence of 10(-5) mol/l chloroethylclonidine, respectively. WB 4101 in the presence of 10(-5) mol/l chloroethylclonidine dose dependently inhibited the response to 5 x 10(-5) mol/l noradrenaline with a -logIC50 value of 6.10 +/- 0.14. Noradrenaline in the presence of 10(-5) mol/l chloroethylclonidine dose dependently increased the 86Rb+ uptake rate with a -logEC50 value of 6.19 +/- 0.35. The basal IP3 level was 2.15 +/- 0.19 pmol/mg protein. Incubation with 10(-5) mol/l noradrenaline for 2 min increased this by 65 +/- 7% of control levels. 10(-5) mol/l chloroethylclonidine and 10(-4) mol/l 5-methylurapidil reduced the response to 27 +/- 6% and 18 +/- 9% of control level, respectively. BMY 7378 dose dependently inhibited the IP3 response at relatively high concentrations, and it was completely eliminated at 10(-5) mol/l BMY 7378. The combination of chloroethylclonidine and 5-methylurapidil or 3 x 10(-6) mol/l prazosin alone completely abolished the hormone-induced effect. We conclude that whereas the alpha 1-adrenoceptor-stimulated increase in 86Rb+ influx rate is mediated via the alpha 1A-adrenoceptor subtype only, both alpha 1A- and alpha 1B-adrenoceptor subtypes are involved in the increase in IP3 mass. Furthermore, a contribution from the alpha 1D-adrenoceptor in the IP3 response cannot be excluded. Thus there does not appear to be a simple causative relationship between an increase in 86Rb+ influx rate and an increase in IP3.

Efficacy. II. Estimation of a newly defined efficacy related parameter

A new method for estimation of agonist-affinity (KA) and relative efficacy was introduced. This method afforded a procedure by which relative efficacy may be estimated while the actual KA values of agonist-receptor complexes are unknown. The relative efficacy may be estimated by employing a newly defined drug parameter, namely the eES value. The eES value is related to drug efficacy and is defined in such a manner that an isolated eES is a meaningful quantity which may indicate whether or not spare receptors are present in an agonist-effector system. The estimation of eES was based on the fact that fixed agonist-competitive antagonist combinations mimic partial agonists and mediate submaximal concentration-effect curves. However, for the practical estimation of eES one may employ data acquired from agonistic concentration-effect curves determined in the absence and presence of increasing concentrations of a competitive antagonist. This procedure was illustrated by utilizing theoretical concentration-effect curves and applied practically by estimating eES and KA values acquired from sets of carbachol and salbutamol curves. The sets of carbachol and salbutamol concentration-effect curves were determined in the absence and presence of their respective competitive antagonists, namely tripitramine and pindolol.

Efficacy. I: A new method for estimating relative efficacy of full agonists via a newly defined efficacy related parameter

A new method for estimating relative efficacies and relative intrinsic efficacies of agonists is described. Relative efficacy is estimated by employing a newly defined efficacy related parameter (eES) and it may be estimated without prior knowledge of efficacy values or the value of the equilibrium dissociation constants, KA, of agonist-receptor complexes. The parameter eES is directly related to efficacy (e) and is defined as the ratio of maximal stimulus to maximal effect of an agonist. The value of eES indicates whether or not spare receptors are present for a particular agonist-effector system. The eES values of agonists are estimated by utilizing submaximal concentration-effect curves determined with fixed agonist-competitive antagonist concentration combinations and choosing a suitable reference (height of an agonistic concentration-effect curve) to which the height of the stimulus concentration-effect curves of the agonist may be compared. In addition to eES, other new agonist-effector parameters, namely SEm/Sm and phi min, were also defined.