Horst Lemoine

Direct labelling of ? 2-adrenoceptors: Comparison of binding potency of 3H-ICI 118,551 and blocking potency of ICI 118,551

A radioligand that selectively labels beta 2-adrenoceptors, 3H-ICI 118,551 (3H-ICI), is introduced. Experiments were performed on guinea-pig tissues. The binding characteristics of 3H-ICI on lung membrane particles are compared with the blocking characteristics of ICI 118,551 against the tracheo-relaxant effects of (-)-noradrenaline, (-)-adrenaline and (+/-)-fenoterol. Binding to both beta 1- and beta 2-adrenoceptors were also performed with 3H-(-)-bupranolol on lung and ventricular myocardium. The binding inhibition characteristics of unlabelled ICI 118,551 on ventricle were compared with its characteristics as antagonist of the positive chronotropic effects of (-)-noradrenaline, (-)-adrenaline and (+/-)-fenoterol in spontaneously beating right atria. 1. ICI 118,551 blocked more the relaxant effects of (+/-)-fenoterol and (-)-adrenaline than those of (-)-noradrenaline on trachea. The positive chronotropic effects of (+/-)-fenoterol on sinoatrial node were blocked more than those of both (-)-adrenaline and (-)-noradrenaline. A non-linear regression analysis of blocking data with ICI 118,551 according to the model of Lemoine and Kaumann (1983) revelas that both beta 1- and beta 2-adrenoceptors contribute to the tracheo-relaxant and positive chronotropic effects of agonists. The estimated equilibrium dissociation constants pKB (-log KB = pKB; mol/l) were 7.1 and 9.6 for beta 1- and beta 2-adrenoceptors, respectively. Tracheal beta 2-adrenoceptors contribute 99%, 97% and 7%, sinoatrial beta 2-adrenoceptors contribute 76%, 3% and 0% to the fractional stimuli induced by (+/-)-fenoterol, (-)-adrenaline and (-)-noradrenaline, respectively. 2. 3H-ICI associated to beta 2-adrenoceptors of lung membranes with a kon of 0.521 X nmol-1 X min-1 and dissociated with a koff of 0.19 min-1. 3H-ICI bound to lung beta 2-adrenoceptors with an equilibrium dissociation constant pKL* of 9.2. Unlabelled ICI 118,551, (-)-bupranolol, (+)-bupranolol, (-)-adrenaline, (-)-noradrenaline and (+/-)-fenoterol competed with 3H-ICI for lung beta 2-adrenoceptors with pKL-values of 9.0, 9.4, 8.1, 5.9, 4.9 and 6.4, respectively. 3. 3H-(-)-bupranolol associated to beta-adrenoceptors of lung membranes with a kon 1.21 X nmol-1 X min-1 and dissociated with a koff of 0.26 min-1. 3H-(-)-bupranolol bound to lung beta 2-adrenoceptors and to heart beta 1-adrenoceptors with a pKL of 9.6 and a pKL of 8.8, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulant and blocking effects of optical isomers of pindolol on the sinoatrial node and trachea of guinea pig. Role of β-adrenoceptor subtypes in the dissociation between blockade and stimulation

SummaryThe blocking and stimulant potencies of (−)-pindolol and (+)-pindolol were estimated on right atria and tracheae of guinea pig. Blocking affinities were estimated for β-adrenoceptor subtypes by using several agonists. Binding affinities of (−)-pindolol and (+)-pindolol were also estimated for β-adrenoceptors labelled with 3H-(−)-bupranolol in membranes of ventricular myocardium and lung of guinea pig.1.Both (−)-pindolol and (+)-pindolol caused tracheal relaxation with intrinsic activities of 0.3. The concentration-effect curve for (−)-pindolol exhibits a high-sensitivity and a low-sensitivity relaxant component; the curve for (+)-pindolol was nearly monophasic. The EC50s were (−log mol/l) 9.2 and 6.1 for (−)-pindolol and 7.6 for (+)-pindolol. Using subtype-selective blockers it was found that the relaxant effects of (+)-pindolol and those of the high-sensitivity component of (−)-pindolol are mediated through β2-adrenoceptors. The low-sensitivity component of relaxation of (−)-pindolol was antagonized by β-blockers less than expected from their affinities for β-adrenoceptors.2.Both (−)-pindolol and (+)-pindolol caused an increase of atrial beating rate with an intrinsic activity of 0.2. The concentration-effect curve of (−)-pindolol was biphasic; the curve of (+)-pindolol was monophasic. The EC50s were (−log mol/l) 9.1 and 7.0 for (−)-pindolol and 7.5 for (+)-pindolol. From the use of subtype-selective antagonists we conclude that the positive chronotropic effects of (+)-pindolol are mediated predominantly by β2-adrenoceptors. On the other hand, the high-sensitivity component of the positive chronotropic effects of (−)-pindolol appears to be mediated predominantly through β1-adrenoceptors, although β2-adrenoceptors may also participate. The low-sensitivity component of the positive chronotropic effects of (−)-pindolol is resistant to blockade by subtype-selective antagonists at concentrations causing at least 98% β-adrenoceptor occupancy. Only high but nondepressant concentrations of non-selective (−)-bupranolol antagonized the low-sensitivity component of (−)-pindolol.3.(−)-Pindolol antagonized the effects of several agonists to similar extent in both trachea and right atrium. (+)-Pindolol was less potent as antagonist of the relaxant effects of (−)-noradrenaline on trachea than against those of (−)-adrenalinc, (−)-isoprenaline and (±)-salbutamol. On right atrium (+)-pindolol antagonized non-selectively the positive chronotropic effects of (−)-isoprenaline, (−)-adrenaline and (−)-noradrenaline. The estimated blocking constant KB was (−log mol/l) 9.5–9.6 for (−)-pindolol; the KBs of (+)-pindolol were 6.7–7.2 for β1- and 7.8 for β2-adrenoceptors. The evidence is consistent with different steric characteristics of β1- and β2-adrenoceptors.4.(−)-Pindolol and (+)-pindolol competed in a stereoselective manner with 3H-(−)-bupranolol for β-adrenoceptors of ventricle and lung. The affinity ratio of (−)-pindolol/(+)-pindolol was 80 in lung and 200 in ventricle, supporting the concept of different steric properties of β1- and β2-adrenoceptors.5.3H-(−)-bupranolol labelled a ventricular site that was recognized with low affinity (−log mol/l KD=4.9) by (−)-pindolol. This low-affinity site may be involved in causing the low-sensitivity stimulant effects of (−)-pindolol.6.The dissociation between blockade (high potency) and stimulation (low potency) observed with (−)-pindolol and racemic pindolol can be accounted by different receptor populations mediating blockade and stimulation of (−)-pindolol.

A model for the interaction of competitive antagonists with two receptor-subtypes characterized by a schild-plot with apparent slope unity

Summary1.A model is presented that predicts the double log relationship between the agonist concentration ratio \t-1 against antagonist concentration for the case of two receptor subtypes mediating the same qualitative effect of an agonist and interacting with both agonist and antagonist. The relationship is a nonlinear function which is approximated for small and high antagonist concentrations by two lines of slope 1.0 united by an inflection. The function depends on the concentration of the antagonist, on the two equilibrium dissociation constants KBQ and KBR for the corresponding complexes of agonist-receptor-subtype and on the fractional stimuli σQ = 1 − σR generated by the agonist via the receptor-subtypes Q and R.2.To test the model, the antagonism of the effects of 4 agonists was investigated with 9 antagonist concentrations in a single guinea-pig trachea. Optically pure (+)-bupranolol was used as an antagonist selective for \gb-adrenoceptor subtypes. (+)-Bupranolol blocked the relaxant effects of (\+-)-salbutamol, (\t-)-adrenaline and (\t-)-isoprenaline to the same extent whereas the effects of (\t-)-noradrenaline were blocked less effectively.Experimental points of the double-log plots were well approximated by the non-linear functions based upon the two-receptor model. 77% of the relaxant effects of (−)-noradrenaline (σQ = 0.77) appear to be mediated through a low-affinity (KBQ=45 nmol/l) β-adrenoceptor subtype; 23% (σQ = 0.23) through a high-affinity (KBR=3.1 nmol/l) β-adrenoceptor subtype for (+)-bupranolol. (−)-Adrenaline, (−)-isoprenaline and (±)-salbutamol caused most of their effects (93%, 92% and 99%, respectively) through the high-affinity R-subtype.3.In contrast to (+)-bupranolol, the antagonism of the relaxant effects of agonists by (\t-)-bupranolol were nearly independent of the agonist used. KB-values between 0.2 and 0.4 nmol/l were estimated for the (\t-)-bupranolol-\gb-adrenoceptor complex of the trachea. The enantiomeric affinity ratio (i.e. KBQ(+)-bupranolol/KB(\t-)-bupranolol) was 102 for \gb-adrenoceptors (Q-subtype) activated predominantly by (\t-)-noradrenaline. The enantiomeric affinity ratio was 14 for \gb-adrenoceptors (R-subtype) activated mostly by (\t-)-adrenaline, (\t-)-isoprenaline and (\+-)-salbutamol.4.In guinea-pig right atria, the blocking potencies of both (\t-)-bupranolol and (+)-bupranolol were nearly independent of the used agonist. The enantiomeric affinity ratio of bupranolol was 89 for sinoatrial \gb-adrenoceptors. The agreement of this ratio with that for Q-receptors of tracheae suggests that most sinoatrial \gb-adrenoceptors resemble the tracheal Q-receptors.5.Tissue selectivities (i.e. tracheal/sinoatrial affinity ratios) were agonist-dependent. Against (\t-)-noradrenaline the ratio was less than 2 for both (+)-bupranolol and (\t-)-bupranolol. Against (\t-)-adrenaline, (\t-)-isoprenaline and (\+-)-salbutamol the ratio was 26 for (+)-bupranolol and 4 for (\t-)-bupranolol.

Regional differences of β1- and β2-adrenoceptor-mediated functions in feline heart

The effects of (-)-adrenaline and (-)-noradrenaline were studied on isolated preparations of kitten heart. To define the contribution of beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2AR) we used as tools the highly beta 1AR-selective antagonist CGP 20,712 A and non-linear analysis of antagonism. The beta 2AR-mediated responses to the catecholamines, disclosed by CGP 20,712 A, were verified by blockade with the beta 2AR-selective ICI 118,551. The relative density and contribution of beta 1AR and beta 2AR to (-)-adrenaline- and (-)-noradrenaline-induced adenylyl cyclase stimulation was also estimated in right ventricular membranes. 1. In the sinoatrial pacemaker (-)-adrenaline caused positive chronotropic effects through both beta 1AR and beta 2AR while (-)-noradrenaline does so predominantly through beta 1AR. During beta 1AR blockade (-)-adrenaline did cause the same maximum effects through beta 2AR as (-)-noradrenaline did through beta 1AR. 2. In left atria (-)-adrenaline caused positive inotropic effects predominantly through beta 1AR. CGP 20,712 A also uncovered a beta 2AR component at high (-)-adrenaline concentrations comprising one third of the maximum beta 1AR-mediated response. 3. Receptor binding assays revealed that 80% of right ventricular beta AR were beta 1AR and 20% beta 2AR. Consistent with this finding, around 80% of the adenylyl cyclase stimulation by both (-)-noradrenaline and (-)-adrenaline was mediated through beta 1AR, around 20% through beta 2AR. The positive inotropic effects of (-)-noradrenaline appeared to be nearly exclusively mediated through beta 1AR in right ventricular papillary muscles. 4. The positive inotropic effects of (-)-adrenaline were quite variable with regard to beta 1AR and beta 2AR in right ventricular papillary muscles. Although beta 1AR-mediated effects are predominant in many muscles with only a small contribution of beta 2AR, in some muscles beta 2AR mediated around 50% of the maximum effect elicited through beta 1AR. In 3 out of 17 muscles beta 2AR mediated the same maximum effect of (-)-adrenaline as beta 1AR. 5. On occasion, we found marked beta AR heterogeneity amongst two muscles from the same right ventricle. One muscle only exhibited beta 1AR-mediated effects of (-)-adrenaline whereas in the other muscle maximal effects could be elicited through beta 2AR.(ABSTRACT TRUNCATED AT 400 WORDS)SummaryThe effects of (−)-adrenaline and (−)-noradrenaline were studied on isolated preparations of kitten heart. To define the contribution of β1-adrenoceptors (β1AR) and β2-adrenoceptors (β2AR) we used as tools the highly β1AR-selective antagonist CGP 20,712 A and non-linear analysis of antagonism. The β2AR-mediated responses to the catecholamines, disclosed by CGP 20,712 A, were verified by blockade with the β2AR-selective ICI 118,551. The relative density and contribution of β1AR and β2AR to (−)-adrenaline- and (−)-noradrenaline-induced adenylyl cyclase stimulation was also estimated in right ventricular membranes. 1.In the sinoatrial pacemaker (−)-adrenaline caused positive chronotropic effects through both β1AR and β2AR while (−)-noradrenaline does so predominantly through β1AR. During β1AR blockade (−)-adrenaline did cause the same maximum effects through β2AR as (−)-noradrenaline did through β1AR.2.In left atria (−)-adrenaline caused positive inotropic effects predominantly through β1AR. CGP 20,712A also uncovered a β2AR component at high (−)-adrenaline concentrations comprising one third of the maximum β1AR-mediated response.3.Receptor binding assays revealed that 80% of right ventricular βAR wereβ1AR and 20% β2AR. Consistent with this finding, around 80% of the adenylyl cyclase stimulation by both (−)-noradrenaline and (−)-adrenaline was mediated through β1AR, around 20% through β2AR. The positive inotropic effects of (−)-noradrenaline appeared to be nearly exclusively mediated through β1AR in right ventricular papillary muscles. were quite variable with regard to β1AR and β2AR in right ventricular papillary muscles. Although β1AR-mediated effects are predominant in many muscles with only a small contribution of β2AR, in some muscles β2AR mediated around 50% of the maximum effect elicited through β1AR. In 3 out of 17 muscles β2AR mediated the same maximum effect of (−)-adrenaline as β1AR.5.On occasion, we found marked βAR heterogeneity amongst two muscles from the same right ventricle. One muscle only exhibited β1AR-mediated effects of (−)-adrenaline whereas in the other muscle maximal effects could be elicited through β2AR.6.CGP 20,712 A had the same affinity for β1AR (pKB = 9.6) and β2AR (pKB = 5.4) in all 3 heart regions studied, suggesting identical affinities of the corresponding receptors. The pronounced tissue differences in the function of β1AR and β2AR appear therefore due to marked differences in receptor density or receptor coupling.7.Surprisingly, pronounced β2AR-mediated inotropic effects of (−)-adrenaline in right ventricular papillary muscles showed prolonged relaxation times; furthermore, the time to peak force was not shortened. These observations are inconsistent with an involvement of cyclic AMP. In contrast, β1AR-mediated effects of both (−)-noradrenaline and (−)-adrenaline shortened time to peak force, as expected from an involvement of cyclic AMP.8.We suggest that feline ventricular β2AR mediate an inotropic effect of (−)-adrenaline that does not involve cyclic AMP. This effect may be due to the direct opening of calcium channels by Gs protein reported by Yatani et al. (1987).

An initial characterization of human heart β-adrenoceptors and their mediation of the positive inotropic effects of catecholamines

SummaryThe positive inotropic effects of catecholamines were studied on samples of ventricular myocardium taken from patients undergoing open heart surgery. The adenylyl cyclase and binding of 3H-(−)-bupranolol were examined in membrane particles prepared from similarly obtained samples.The equilibrium dissociation constant (KD) for (−)-bupranolol was estimated in 4 ways: blockade of the positive inotropic effects of catecholamines, blockade of the stimulation of the adenylyl cyclase by catecholamines, saturation binding of 3H-(−)-bupranolol, inhibition of the binding of 3H-(−)-bupranolol by its unlabeled stereoisomers. The estimates of KD fall in the range 0.5–1.4 nmol/l. The stereo-selectivity ratio (KD (+)-isomer/KD (−)-isomer) is 73. Both values for bupranolol are very similar in cat and man.The inotropic potency of (−)-noradrenaline is nearly 2 orders of magnitude higher in cat heart tissues than in tissues from human hearts. The difference in inotropic potencies between species is only partially accounted for by the five-fold lower potency of (−)-noradrenaline for the human heart adenylyl cyclase as compared to the cat enzyme.

Allosteric properties of 5-HT2 receptors in tracheal smooth muscle

Summary5-Hydroxytryptamine (5-HT)-induced contractions were investigated in isolated tracheal smooth muscle of guinea pig and calf. In guinea-pig tracheae, ketanserin reduced to 60% the maximum response to 5-HT, but also shifted the concentration-effect curve for 5-HT to higher 5-HT concentrations, as expected from its affinity for 5-HT2 receptors [pKB=−log KB=9.6, KB in mol/l]. The concentration effect-data for the depressant effect of ketanserin are closely associated with the curve for occupancy of 5-HT2-receptors by ketanserin. In calf tracheae, ketanserin caused surmountable antagonism of the effects of 5-HT with a pKB of 9.5. Methysergide reduced to 25% the maximum response to 5-HT in guinea-pig tracheae and to 20% in calf tracheae. The methysergide-depressed response to 5-HT was restored by ketanserin to 60% of maximum in guinea-pig tracheae, and to 100% in calf tracheae. The results support for tracheal smooth muscle a model of an allosteric regulation of 5-HT2-receptors which was proposed for arterial smooth muscle by Kaumann and Frenken (this journal 328:295–300, 1985). The model requires that: 1) the 5-HT2 receptor exists in two interconvertible states R⇌R′;2) 5-HT causes its effects through R; 3) methysergide, by acting on an allosteric site near or on the 5-HT2 receptor, shifts the equilibrium into the inactive state R′;4) ketanserin competes with 5-HT for the 5-HT2 receptor and with methysergide for the allosteric site, thereby restoring the active state R of the 5-HT2-receptor. All four requirements were experimentally verified in calf trachea. In guinea-pig trachea ketanscrin also appears to possess a partial ability to shift the 5-HT2 receptor into the inactive R′ state. Thus, ketanserin is both a competitive antagonist at the 5-HT2 receptor and also appears to be an allosteric modulator at the allosteric site.

Improved evaluation of binding of ligands to membranes containing several receptor-subtypes

SummaryIn order to evaluate accurately affinity characteristics and relative size of populations of receptor-subtypes in one system we analysed three relevant problems encountered in binding assays. 1.Binding to receptors caused a decrease in the free ligand concentration (i.e. “depletion”). The neglect of depletion may lead to significant distortions of the estimates of affinity and size of receptor-subtype population when the concentrations of both receptor and ligand are of similar magnitude. The distortion is particularly marked when the affinity of a competing ligand is higher than the affinity of the radioligand. We present a formula that describes binding inhibition in a system with receptor-subtypes under conditions of depletion.2.Binding data usually exhibit heteroscedasticity (i.e. heterogeneous variance), which can not be neglected especially in a system with receptor heterogeneity. Assuming a log normal distribution of experimental errors and a Poisson distribution for errors due to radioactivity counting we derived a function for the transformation of binding data. Transformed data show homoscedasticity, as illustrated with experiments on membranes of guinea-pig lung using ICI 118,551 as inhibitor of 3H-(−)-bupranolol binding to β1- and β2-adrenoceptors.3.The hypothesis that affinity characteristics of receptor subtypes are independent of the tissue class can not be tested accurately by the use of standard methods because of interferences of errors between experiments. We propose a method to account for differences between experiments. Assuming invariance of affinity characteristics one is able to perform common fits of data from different tissue classes. This procedure provides more certain estimates of affinities and subtype fractions, as documented with binding inhibition experiments on β1- and β2-adrenoceptors of guinea pig heart and lung.

Regulation of endothelial permeability by β-adrenoceptor agonists : contribution of β1- and β2-adrenoceptors

The barrier function of cultured, macrovascular endothelial cells derived from bovine aorta was analyzed using confluent monolayers of cells and measuring the exchange of fluorescein dextrans of different molecular masses. The effects of beta-adrenoceptor agonists with different selectivity for beta 1- and beta 2-adrenoceptors (AR) were investigated. Formoterol, a novel high-affinity agonist for beta 2-AR recently introduced in the treatment of bronchial asthma, showed a significant reduction of cell permeability with subnanomolar concentrations, whereas the catecholamines (-)-isoproterenol and (-)-norepinephrine only showed significant effects with micromolar concentrations. In order to elucidate if this difference in potential to regulate cell permeability is related to appropriate changes in the selectivity and affinity of the agonists for beta 2 AR, we investigated the beta AR-coupled adenylate cyclase (AC) in membranes from endothelial cells and compared AC stimulation with the binding of agonists to the receptors using [125I](-)-iodopindolol as radioligand. beta-Adrenoceptors revealed to be closely coupled to AC as assessed by a similar magnitude of effects by receptor agonists in comparison to GTP analogues and direct stimulants of AC activity. AC activity was increased by formoterol in parallel to its receptor occupancy of beta 2AR with nanomolar concentrations which were 50-fold higher than those used for the regulation of cell permeability indicating the existence of spare receptors. In contrast to formoterol, the catecholamines (-)-isoproterenol and (-)-norepinephrine stimulated AC activity through both beta 1AR and beta 2AR. From the overproportional high contribution of beta 1AR to AC stimulation (42%) in comparison to its low fraction (13%) in receptor binding we calculated that beta 1AR is 3-4-fold more effectively coupled to AC than beta 2 AR.

Neuronally released (−)-noradrenaline relaxes smooth muscle of calf trachea mainly through β1-adrenoceptors: comparison with (−)-adrenaline and relation to adenylate cyclase stimulation

SummaryThe nature of the receptors that mediate the relaxation of smooth muscle by field stimulation, (−)-noradrenaline and (−)-adrenaline was investigated in calf tracheal smooth muscle. The relation between relaxation, stimulation of the adenylate cyclase and density of β-adrenoceptor subtypes was studied with the help of antagonists of β1- and β2-adrenoceptors. The question of the existence of catecholamine-containing nerves was also investigated. (1) Nerves with varicosities exhibiting catecholaminergic fluorescence were observed between bundles of smooth muscle cells. (2) Consistent with the existence of adrenergic nerves (−)-noradrenaline was also found. The content of (−)-noradrenaline (1 μg · g−1 w.w.) was the same in smooth muscle strips from the sublaryngeal region and from the region close to the bifurcation of the calf trachea. (−)-Adrenaline was not detected. (3) Smooth muscle relaxation by low (−)-noradrenaline concentration (0.6–2 nmol/l) was mediated through β1-adrenoceptors. Low concentrations of (−)-adrenaline (0.06–1 nmol/l) relaxed through β2-adrenoceptors. High concentrations of (−)-noradrenaline and (−)adrenaline also caused relaxation through β2- and β1-adrenoceptors respectively. (4) Field stimulation caused relaxation which was half maximal at 0.2–0.8 Hz. Blockade of β1-adrenoceptors strongly attenuated the relaxant response to field stimulation and shifted the frequency-relaxation curves to 4 times higher frequencies. These results are consistent with a β1-adrenoceptor-mediated relaxation caused by (−)-noradrenaline released from sympathetic nerve endings at low stimulation frequencies. (5) Blockade of β2-adrenoceptors failed to reduce smooth muscle relaxation caused by field stimulation at low stimulation frequencies (0.1–1 Hz). However, after β1-adrenoceptor blockade, additional blockade of β2-adrenoceptors reduced the relaxant effects observed at high frequencies (2–400 Hz). The results suggest that high concentrations of endogenous (−)-noradrenaline cause relaxation through β2-adrenoceptors. (6) Binding experiments with 3H-(−)-bupranolol and 3H-ICI 118,551 revealed between 10,000 and 20,000 β-adrenoceptors per smooth muscle cell of which 3/4 were β2 and 1/4 β1. The equilibrium dissociation constant of (−)-adrenaline for both β1- and β2-adrenoceptors and of (−)-noradrenaline for β1-adrenoceptors was 1 μmol/l. The affinity of (−)noradrenaline for β2-adrenoceptors was 10 to 20 times lower than for β1-adrenoceptors. (7) The adenylate cyclase of smooth muscle cells was stimulated more through β2-adrenoceptors by both (−)-noradrenaline (77%) and adrenaline (83%) than through β1-adrenoceptors. Half maximum stimulation of the cyclase was observed at 2 to 4 times lower catecholamine concentrations than the concentrations that half saturate β1- or β2-adrenoceptors. (8) Both the low affinity and low cyclase stimulating potency of the catecholamines suggest considerable amplification of the receptor signals. Half maximum relaxation of tracheal muscle by (−)-noradrenaline is associated with the activation of less than 50 β1-adrenoceptors per cell producing less than 250 molecules cyclic AMP per second.

Relations between β-adrenoceptor occupancy and increases of contractile force and adenylate cyclase activity induced by catecholamines in human ventricular myocardium

SummaryThe function of β-adrenoceptors was investigated in ventricular myocardium obtained from patients undergoing open heart surgery.1.Dopamine increased contractile force up to 1/2 and 1/4 of the maximum increase caused by (−)-noradrenaline or (−)-adrenaline in right and left ventricular preparations, respectively.2.β-adrenoceptors were labelled with 3H-(−)-bupranolol. For 3/4 of the receptors (β1) the affinity of (−)-noradrenaline was 20 times higher than for the remaining 1/4 (β2). (−)-Adrenaline and dopamine appeared to be nonselective for β1 and β2.3.Dopamine was able to stimulate the adenylate cyclase only up to 1/3 of the maximum stimulation caused by (−)noradrenaline and (−)-adrenaline.4.Increases in contractile force by (−)-noradrenaline were closely associated with small increases of cyclase activity through β1-adrenoceptors, consistent with a common link.5.The experiments on human myocardium were compared with similar experiments on feline myocardium. Feline ventricle exhibited a 20- to 30-fold higher sensitivity to catecholamines as activators of contractile force than did human ventricle. However, the binding affinities for catecholamines were similar in cat and man.6.A 3 h exposure of human and feline ventricular myocardium to (−)-isoprenaline caused desensitization by uncoupling β-adrenoceptors from the adenylate cyclase. Desensitization reduced the maximum contractile response to (−)-isoprenaline in human but not in feline ventricle.7.The more efficient activation of contractile force by (−)-noradrenaline in cat, compared to man, appears to be related to a 2-fold higher density of β1-adrenoceptors, a 6-fold higher production of cyclic AMP per β1-adrenoceptor and possibly to a more effective use of cyclic AMP for contraction.