Gen. Pharmac. Vol. 22, No. 6, pp. 1169-1177, 1991 Printed in Great Britain. All dghts reserved
0306-3623/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press plc
EFFECTS OF (+_)- (+)- AND (-)-METOPROLOL, (_)- (+)- AND (-)-PINDOLOL, (_)-MEPINDOLOL AND (+_)-BOPINDOLOL ON THE RAT LEFT ATRIA A N D PORTAL VEIN SHEILAA. DOGGRELL Department of Pharmacology, School of Medicine, University of Auckland, Private Bag, Auckland, New Zealand [Fax 649-770-956] (Received 22 February 1991)
Abstract--1. The effects of ( + )- (+)- and (-)-metoprolol, ( + )- ( +)- and ( -)-pindolol, ( d-)-mepindolol and (+)-bopindolol on the/J~-adrenoceptor mediated responses of the rat left atria and the fl2-adrenoceptot mediated responses of the rat portal vein to isoprenaline have been determined. 2. Racemic and (-)-metoprolol were selective /]l-adrenoceptor antagonists. (+)-Metoproiol was devoid of/Ladrenoceptor antagonistic activity. 3. Racemic and (-)-pindolol were potent and (+)-pindoioi was a modest fl-adrenoceptor antagonist. 4. (+)-Mepindolol and (+)-bopindolol were apparently competitive antagonists of the isoprenaline flradrenoceptor mediated responses of the rat left atria but non-competitive antagonists of the isoprenaline fl2-adrenoceptor mediated responses of the rat portal vein. 5. It is suggested that (±)-mepindolol and (±)-bopindoloi are slowly dissociating fl-adrenoceptor antagonists and the non-competitive antagonism can only be detected on tissues with modest receptor reserves for maximum responses to isoprenaline.
INTRODUCTION
lung (Walter et ai., 1984). (-)-Pindolol has a pA L value of 9.6, 9.2 and 8.6 at the fl2-adrenoceptors of the Racemic B-adrenoceptor antagonists (/Lblockers) guinea-pig trachea and soleus muscle (Jeppsson et al., are commonly used in the treatment of hypertension, 1984) and the rat aorta (Doggrell, 1990a), respectively. angina and cardiac arrhythmias. These drugs act as pA2 values for (+)- and (-)-pindolol at fll-adrenoantagonists at/~t" and ~2-adrenoceptors and may also ceptors have not been determined. have ancillary properties. The properties of racemic Mepindolol and bopindolol are r-blockers that are drugs are not always shared equally amongst the structurally related to pindolol and have asymmetric stereoisomers. There have been many studies centres (Fig. 1). (_+)-Mepindolol is a nonselective of racemic fl-blockers but only a few studies of the /~-blocker having a pA L value of 9.1, 8.8 and 8.3 at fl-blocker stereoisomers. the/]radrenoceptors of the rat atria (Beckmann et al., Metroprolol and pindolol (Fig. 1) are both fl-block- 1986) and guinea-pig atria and ventricles (Amerini et al., ers with asymmetric centres and ( + ) - and (-)-stereo1986), respectively and of 8.3 at the fl2-adrenoceptors isomers. (+)-Metropolol is a/]l-adrenoceptor selective of guinea-pig trachea (Amerini et al., 1986). (+_)antagonist having a pA2 value of 7.9 at the fl~-adreno- Bopindolol was a more potent flradrenoceptor ceptors of the rat right ventricle (Doggrell, 1989a) antagonist than propranolol on the guinea-pig atria and of 6.1 at the/]2-adrenoceptors of the guinea-pig (Berthold et al., 1981). However pA2 values for trachea (Fitzgerald, 1984). The effects of (+)- and bopindolol have not been reported. (-)-metoprolol at B-adrenoceptors are unknown. (_+)-Metoprolol and (+)-bopindolol have not been (+)-Pindolol is a nonselective/~-blocker with a pA L reported to have ancillary properties to/~-blockade. value of 8.7 at the fl~-adrenoceptors of the guinea-pig In contrast (+)-pindolol (Walter et al., 1984) and atria (Fitzgerald, 1984) and of 8.8 and 8.7 at the f12" (+)-mepindolol (Beckmann et al., 1986) have been adrenoceptors of the guinea-pig trachea (Fitzgerald, reported to be partial agouists at fl-adrenoceptors. 1984) and rat aorta (Doggrell, 1990a), respectively. The aim of the present study was to determine the Binding studies have demonstrated that (+)-pindolol pharmacodynamic profiles of ( + ) - and ( + ) - and is quite a potent ligand at ~-adrenoceptors with (-)-metoprolol, ( + ) - ( + ) - and (-)-pindolol, ( + ) selectivity for the fl2-adrenoceptors of the guinea-pig mepindolol and (_+)-bopindolol on isolated cardiolung over the Bm-adrenoceptors of the guinea-pig vascular preparations with special attention being ventricle (Walter et al., 1984). Contractility studies given to the effects of the drugs alone. Thus I have have produced pA2 values for (+)-pindolol at l]2- studied the effects of the drugs alone on the electrically adrenoceptors of 7.0 and 7.4 on the guinea-pig soleus driven rat left atria and on the spontaneous contractile muscle and trachea, respectively (Jeppsson et al., 1984). activity of the rat portal vein. Also I describe the effects (-)-Pindolol is a more potent figand than (+)-pindolol of (:t:)- (+)- and (-)-metoprolol, (+_)- ( + ) - and at ~-adrenoceptors but does not select between the/~m (-)-pindolol, (+)-mepindoloi and (+)-bopindolol and B2-adrenoceptors of the guinea-pig ventricles and on the ~radrenoceptor mediated contractions of the 1169
1170
SHEILA A. DOGGRELL
OH
H
X
'
,
H
CH s
H
I
I
O-CH,-C*-CH~-NHC-CH a
O-CH 2-C-CH 2-NHC-CHo ~
OH "
I
1
CH a
(-*)-METOPROLOL
( -+) - PINDOLOL !
CH2"CH 2"O'CH,~
OH I,
H
H
O
I
I
O-CH2-C-CH2-NHC-CH3 CH 3
H
CHs
( -+ ) - MEPINDOLOL
CH s I
O-CH2-C*-CH 2- NHC - CH3 H
H
CH s
H3
( +- ) - BOPINDOLOL
Fig. 1. Structure of (__+)-metoprolol, (_)-pindolol, (+)-mepindolol and (+)-bopindolol. *Asymmetric centre. electrically-driven rat atria a n d the fl2-adrenoceptor m e d i a t e d a t t e n u a t i o n s o f the rat portal vein to isoprenaline. MATERIALS AND M E T H O D S
General Male Wistar rats (250-350g) were stunned and exsangninated. The heart or portal vein was rapidly removed and placed in Krebs solution saturated with 5% CO2 in oxygen. All experiments were performed in the presence of a modified Krebs solution [composition (mM); NaC1, 116; KC1, 5.4; CaCI~, 2.5; MgCI 2, 1.2; NaH2PO 4, 1.2; NaHCO 3, 22.0; Dglucose, 11.2; Na2EDTA, 0.04] at 37°C which was bubbled with 5% CO2 in oxygen. Contractile responses were measured isometrically with force--displacement transducers (Grass model FTO3.C) and displayed on a polygraph (Grass model 79B). In each series of experiments, the individual values obtained were subject to Student's t-test. Differences were considered significant when P < 0.05. Mean values+ SEM were also obtained.
Contractile responses of the electrically driven rat left atria (method described by Doggrell, 1988) Left atria were removed from the heart and halved. Each atria half was mounted longitudinally between two platinum electrodes (approx. 3 era apart, above and below the tissue) under 1 g tension in 5 ml organ baths containing Krebs solution (with 10-5 M guanethidine to prevent the release of noradrenaline from nerve endings, and atropine at 10 -6 M) and allowed to equilibrate for 60 min. During the equilibration period, the tissues were washed by overflow. Tissues were electrically stimulated at 4Hz (5 msec, 10 V). After 9 rain of stimulation, a cumulative challenge with isoprenaline was initiated. Thus atria were challenged with isoprenaline at 3 rain intervals to give a final concentration of isoprenaline in the organ bath successfully of I 0 - " , 10 -~°, 10 -9 M, etc. This cycle was continued until a maximum response was obtained. One of the tissues was treated with a drug for 75 rain while the other tissue of the pair remained untreated throughout. During this 75 min about 1 litre of drug-free or -containing Krebs overflowed the tissue. The tissues were electrically
stimulated and cumulatively challenged with isoprenaline until an isoprenaline maximal response was obtained. This procedure was repeated with one tissue remaining untreated throughout and the paired tissue being treated with a higher concentration of the same drug. The contractile responses to electrical stimulation just prior to the second and third challenges with isoprenaline were calculated as a percentage of the response to stimulation prior to the first challenge with isoprenaline. For each challenge with isoprenaline, the response to electrical stimulation just prior to challenge with isoprenaline was subtracted from the combined response to electrical stimulation and isoprenaline. The maximal combined responses to electrical stimulation and isoprenaline were calculated as a percentage of the maximum of the first challenge to isoprenaline. If the maximum responses to electrical stimulation and isoprenaline between treated and untreated tissues were not significantly different response curves were calculated as a percentage of the maximum of the individual curves, i.e. normalized. If the maximum responses to electrical stimulation and isoprenaline between treated and untreated tissues were significantly different, response curves were calculated as a percentage of the maximum of the first curve.
Attenuation responses of the rat portal vein (method described by Doggrell, 1990b) Each portal vein was cleared of surrounding tissue and mounted under I g tension in 5 ml organ bath containing Krebs solution and allowed to equilibrate for 30rain. During the equilibration period, the tissues were washed by overflow. To prevent isoprenaline from stimulating ~-adrenoceptors, and to inhibit the extraneuronal uptake process, tissues were treated with phenoxybenzamine at 10-4M for 45 rain. Tissues were then rapidly washed for 20 min. The wash was then stopped, and the tissues were allowed to stabilize for 20 rain. During this period, the amplitude of the spontaneous contractions became constant and then a cumulative challenge to isoprenaline, 10 -9, 3 × 10 -9, 10 -8 M etc. was made to each portal vein on a 5 rain cycle. The cycle was continued until a maximum attenuation was obtained. Tissues were then rapidly washed in the presence of the drug for 45 rain. During this 45 rain about 500ml of drug-containing Krebs overflowed the
Actions of/bblocker stereoisomers tissue. Washing was stopped for 20 rain prior to a second challenge to isoprenaline. Tissues were then treated with a higher concentration of the same drug in a rapid wash. After a 45 rain/500 ml wash and a 20 rain stabilization, a third challenge to isoprenaline was initiated. On the portal vein the measurement of the contractile response was taken as the average of the amplitude of the final 3 contractions in a 5 min period. The attenuation in the presence of a drug or drugs was calculated as a percentage of the contractile response in the first stabilization period. If the maximum attenuations to isoprenaline between treated and untreated tissues were not significantly different response curves were calculated as a percentage of the maximum of the individual curves, i.e. normalized.
Assessment of data Slopes were determined for all isoprenaline response curves. In addition when responses to isoprenaline were normalized, PD2, concentration-ratio and PA2 values were determined. The slope (difference in percentage maximum of the response/ unit of logarithm of molar concentration of isoprenaline) and pD 2 value (negative logarithm of molar concentration of isoprenaline producing 50% of the maximum response) were computed by regression line analysis. The regression line analysis was performed on the steepest part of the response curve, which was usually over the range 20-80% of the maximum response. For each tissue, the concentration ratios (the antilog of the difference between'the PD2 values) were determined between each challenge with isoprenaline. Previous studies have shown that the sensitivity to isoprenaline decreased with successive challenges in the rat left atria (Doggrell, 1988) but not the rat portal vein (Doggreil, 1990b). Consequently concentration ratios obtained in the presence of drugs had to be corrected for changes occurring in untreated atria but not portal veins. Thus the difference in the mean pD2 values from untreated atrias were subtracted from the difference in the individual PD2 values from treated atrias. PA2 values (the negative logarithm of the molar concentration of drug that causes a twofold inhibition of the concentration-response curve for isoprenaline) were determined for concentrations of drugs that had no effect on the slope of the isoprenaline response curves by two methods. (i) The Sehild plot [log ( x - 1), where x is the concentration-ratio, ordinate, vs the logarithm of the molar concentration of drug, abscissa] was computed by regression line analysis of individual log (x - 1) values. This method was used when several concentrations of a drug inhibited the responses to isoprenaline. A least-squares analysis was performed to test whether the line of the Schild plot had a slope that was significantly different from 1. The pA 2 value was the abscissa intercept when the slope was not significantly different from 1. (ii) The formula, pA 2 = pAx+log (x - 1) where PAx is the negative logarithm of the molar concentration of drug. This method was used for single concentrations of drugs that had inhibitory effects on isoprenaline responses.
1171
( - ) - m e t o p r o l o l , ( + ) - ( + ) - and ( - ) - p i n d o l o l or (_+)-mepindolol at ~
0306-3623/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press plc
EFFECTS OF (+_)- (+)- AND (-)-METOPROLOL, (_)- (+)- AND (-)-PINDOLOL, (_)-MEPINDOLOL AND (+_)-BOPINDOLOL ON THE RAT LEFT ATRIA A N D PORTAL VEIN SHEILAA. DOGGRELL Department of Pharmacology, School of Medicine, University of Auckland, Private Bag, Auckland, New Zealand [Fax 649-770-956] (Received 22 February 1991)
Abstract--1. The effects of ( + )- (+)- and (-)-metoprolol, ( + )- ( +)- and ( -)-pindolol, ( d-)-mepindolol and (+)-bopindolol on the/J~-adrenoceptor mediated responses of the rat left atria and the fl2-adrenoceptot mediated responses of the rat portal vein to isoprenaline have been determined. 2. Racemic and (-)-metoprolol were selective /]l-adrenoceptor antagonists. (+)-Metoproiol was devoid of/Ladrenoceptor antagonistic activity. 3. Racemic and (-)-pindolol were potent and (+)-pindoioi was a modest fl-adrenoceptor antagonist. 4. (+)-Mepindolol and (+)-bopindolol were apparently competitive antagonists of the isoprenaline flradrenoceptor mediated responses of the rat left atria but non-competitive antagonists of the isoprenaline fl2-adrenoceptor mediated responses of the rat portal vein. 5. It is suggested that (±)-mepindolol and (±)-bopindoloi are slowly dissociating fl-adrenoceptor antagonists and the non-competitive antagonism can only be detected on tissues with modest receptor reserves for maximum responses to isoprenaline.
INTRODUCTION
lung (Walter et ai., 1984). (-)-Pindolol has a pA L value of 9.6, 9.2 and 8.6 at the fl2-adrenoceptors of the Racemic B-adrenoceptor antagonists (/Lblockers) guinea-pig trachea and soleus muscle (Jeppsson et al., are commonly used in the treatment of hypertension, 1984) and the rat aorta (Doggrell, 1990a), respectively. angina and cardiac arrhythmias. These drugs act as pA2 values for (+)- and (-)-pindolol at fll-adrenoantagonists at/~t" and ~2-adrenoceptors and may also ceptors have not been determined. have ancillary properties. The properties of racemic Mepindolol and bopindolol are r-blockers that are drugs are not always shared equally amongst the structurally related to pindolol and have asymmetric stereoisomers. There have been many studies centres (Fig. 1). (_+)-Mepindolol is a nonselective of racemic fl-blockers but only a few studies of the /~-blocker having a pA L value of 9.1, 8.8 and 8.3 at fl-blocker stereoisomers. the/]radrenoceptors of the rat atria (Beckmann et al., Metroprolol and pindolol (Fig. 1) are both fl-block- 1986) and guinea-pig atria and ventricles (Amerini et al., ers with asymmetric centres and ( + ) - and (-)-stereo1986), respectively and of 8.3 at the fl2-adrenoceptors isomers. (+)-Metropolol is a/]l-adrenoceptor selective of guinea-pig trachea (Amerini et al., 1986). (+_)antagonist having a pA2 value of 7.9 at the fl~-adreno- Bopindolol was a more potent flradrenoceptor ceptors of the rat right ventricle (Doggrell, 1989a) antagonist than propranolol on the guinea-pig atria and of 6.1 at the/]2-adrenoceptors of the guinea-pig (Berthold et al., 1981). However pA2 values for trachea (Fitzgerald, 1984). The effects of (+)- and bopindolol have not been reported. (-)-metoprolol at B-adrenoceptors are unknown. (_+)-Metoprolol and (+)-bopindolol have not been (+)-Pindolol is a nonselective/~-blocker with a pA L reported to have ancillary properties to/~-blockade. value of 8.7 at the fl~-adrenoceptors of the guinea-pig In contrast (+)-pindolol (Walter et al., 1984) and atria (Fitzgerald, 1984) and of 8.8 and 8.7 at the f12" (+)-mepindolol (Beckmann et al., 1986) have been adrenoceptors of the guinea-pig trachea (Fitzgerald, reported to be partial agouists at fl-adrenoceptors. 1984) and rat aorta (Doggrell, 1990a), respectively. The aim of the present study was to determine the Binding studies have demonstrated that (+)-pindolol pharmacodynamic profiles of ( + ) - and ( + ) - and is quite a potent ligand at ~-adrenoceptors with (-)-metoprolol, ( + ) - ( + ) - and (-)-pindolol, ( + ) selectivity for the fl2-adrenoceptors of the guinea-pig mepindolol and (_+)-bopindolol on isolated cardiolung over the Bm-adrenoceptors of the guinea-pig vascular preparations with special attention being ventricle (Walter et al., 1984). Contractility studies given to the effects of the drugs alone. Thus I have have produced pA2 values for (+)-pindolol at l]2- studied the effects of the drugs alone on the electrically adrenoceptors of 7.0 and 7.4 on the guinea-pig soleus driven rat left atria and on the spontaneous contractile muscle and trachea, respectively (Jeppsson et al., 1984). activity of the rat portal vein. Also I describe the effects (-)-Pindolol is a more potent figand than (+)-pindolol of (:t:)- (+)- and (-)-metoprolol, (+_)- ( + ) - and at ~-adrenoceptors but does not select between the/~m (-)-pindolol, (+)-mepindoloi and (+)-bopindolol and B2-adrenoceptors of the guinea-pig ventricles and on the ~radrenoceptor mediated contractions of the 1169
1170
SHEILA A. DOGGRELL
OH
H
X
'
,
H
CH s
H
I
I
O-CH,-C*-CH~-NHC-CH a
O-CH 2-C-CH 2-NHC-CHo ~
OH "
I
1
CH a
(-*)-METOPROLOL
( -+) - PINDOLOL !
CH2"CH 2"O'CH,~
OH I,
H
H
O
I
I
O-CH2-C-CH2-NHC-CH3 CH 3
H
CHs
( -+ ) - MEPINDOLOL
CH s I
O-CH2-C*-CH 2- NHC - CH3 H
H
CH s
H3
( +- ) - BOPINDOLOL
Fig. 1. Structure of (__+)-metoprolol, (_)-pindolol, (+)-mepindolol and (+)-bopindolol. *Asymmetric centre. electrically-driven rat atria a n d the fl2-adrenoceptor m e d i a t e d a t t e n u a t i o n s o f the rat portal vein to isoprenaline. MATERIALS AND M E T H O D S
General Male Wistar rats (250-350g) were stunned and exsangninated. The heart or portal vein was rapidly removed and placed in Krebs solution saturated with 5% CO2 in oxygen. All experiments were performed in the presence of a modified Krebs solution [composition (mM); NaC1, 116; KC1, 5.4; CaCI~, 2.5; MgCI 2, 1.2; NaH2PO 4, 1.2; NaHCO 3, 22.0; Dglucose, 11.2; Na2EDTA, 0.04] at 37°C which was bubbled with 5% CO2 in oxygen. Contractile responses were measured isometrically with force--displacement transducers (Grass model FTO3.C) and displayed on a polygraph (Grass model 79B). In each series of experiments, the individual values obtained were subject to Student's t-test. Differences were considered significant when P < 0.05. Mean values+ SEM were also obtained.
Contractile responses of the electrically driven rat left atria (method described by Doggrell, 1988) Left atria were removed from the heart and halved. Each atria half was mounted longitudinally between two platinum electrodes (approx. 3 era apart, above and below the tissue) under 1 g tension in 5 ml organ baths containing Krebs solution (with 10-5 M guanethidine to prevent the release of noradrenaline from nerve endings, and atropine at 10 -6 M) and allowed to equilibrate for 60 min. During the equilibration period, the tissues were washed by overflow. Tissues were electrically stimulated at 4Hz (5 msec, 10 V). After 9 rain of stimulation, a cumulative challenge with isoprenaline was initiated. Thus atria were challenged with isoprenaline at 3 rain intervals to give a final concentration of isoprenaline in the organ bath successfully of I 0 - " , 10 -~°, 10 -9 M, etc. This cycle was continued until a maximum response was obtained. One of the tissues was treated with a drug for 75 rain while the other tissue of the pair remained untreated throughout. During this 75 min about 1 litre of drug-free or -containing Krebs overflowed the tissue. The tissues were electrically
stimulated and cumulatively challenged with isoprenaline until an isoprenaline maximal response was obtained. This procedure was repeated with one tissue remaining untreated throughout and the paired tissue being treated with a higher concentration of the same drug. The contractile responses to electrical stimulation just prior to the second and third challenges with isoprenaline were calculated as a percentage of the response to stimulation prior to the first challenge with isoprenaline. For each challenge with isoprenaline, the response to electrical stimulation just prior to challenge with isoprenaline was subtracted from the combined response to electrical stimulation and isoprenaline. The maximal combined responses to electrical stimulation and isoprenaline were calculated as a percentage of the maximum of the first challenge to isoprenaline. If the maximum responses to electrical stimulation and isoprenaline between treated and untreated tissues were not significantly different response curves were calculated as a percentage of the maximum of the individual curves, i.e. normalized. If the maximum responses to electrical stimulation and isoprenaline between treated and untreated tissues were significantly different, response curves were calculated as a percentage of the maximum of the first curve.
Attenuation responses of the rat portal vein (method described by Doggrell, 1990b) Each portal vein was cleared of surrounding tissue and mounted under I g tension in 5 ml organ bath containing Krebs solution and allowed to equilibrate for 30rain. During the equilibration period, the tissues were washed by overflow. To prevent isoprenaline from stimulating ~-adrenoceptors, and to inhibit the extraneuronal uptake process, tissues were treated with phenoxybenzamine at 10-4M for 45 rain. Tissues were then rapidly washed for 20 min. The wash was then stopped, and the tissues were allowed to stabilize for 20 rain. During this period, the amplitude of the spontaneous contractions became constant and then a cumulative challenge to isoprenaline, 10 -9, 3 × 10 -9, 10 -8 M etc. was made to each portal vein on a 5 rain cycle. The cycle was continued until a maximum attenuation was obtained. Tissues were then rapidly washed in the presence of the drug for 45 rain. During this 45 rain about 500ml of drug-containing Krebs overflowed the
Actions of/bblocker stereoisomers tissue. Washing was stopped for 20 rain prior to a second challenge to isoprenaline. Tissues were then treated with a higher concentration of the same drug in a rapid wash. After a 45 rain/500 ml wash and a 20 rain stabilization, a third challenge to isoprenaline was initiated. On the portal vein the measurement of the contractile response was taken as the average of the amplitude of the final 3 contractions in a 5 min period. The attenuation in the presence of a drug or drugs was calculated as a percentage of the contractile response in the first stabilization period. If the maximum attenuations to isoprenaline between treated and untreated tissues were not significantly different response curves were calculated as a percentage of the maximum of the individual curves, i.e. normalized.
Assessment of data Slopes were determined for all isoprenaline response curves. In addition when responses to isoprenaline were normalized, PD2, concentration-ratio and PA2 values were determined. The slope (difference in percentage maximum of the response/ unit of logarithm of molar concentration of isoprenaline) and pD 2 value (negative logarithm of molar concentration of isoprenaline producing 50% of the maximum response) were computed by regression line analysis. The regression line analysis was performed on the steepest part of the response curve, which was usually over the range 20-80% of the maximum response. For each tissue, the concentration ratios (the antilog of the difference between'the PD2 values) were determined between each challenge with isoprenaline. Previous studies have shown that the sensitivity to isoprenaline decreased with successive challenges in the rat left atria (Doggrell, 1988) but not the rat portal vein (Doggreil, 1990b). Consequently concentration ratios obtained in the presence of drugs had to be corrected for changes occurring in untreated atria but not portal veins. Thus the difference in the mean pD2 values from untreated atrias were subtracted from the difference in the individual PD2 values from treated atrias. PA2 values (the negative logarithm of the molar concentration of drug that causes a twofold inhibition of the concentration-response curve for isoprenaline) were determined for concentrations of drugs that had no effect on the slope of the isoprenaline response curves by two methods. (i) The Sehild plot [log ( x - 1), where x is the concentration-ratio, ordinate, vs the logarithm of the molar concentration of drug, abscissa] was computed by regression line analysis of individual log (x - 1) values. This method was used when several concentrations of a drug inhibited the responses to isoprenaline. A least-squares analysis was performed to test whether the line of the Schild plot had a slope that was significantly different from 1. The pA 2 value was the abscissa intercept when the slope was not significantly different from 1. (ii) The formula, pA 2 = pAx+log (x - 1) where PAx is the negative logarithm of the molar concentration of drug. This method was used for single concentrations of drugs that had inhibitory effects on isoprenaline responses.
1171
( - ) - m e t o p r o l o l , ( + ) - ( + ) - and ( - ) - p i n d o l o l or (_+)-mepindolol at ~
