Atenolol, Sotalol, and Amosulalol on the Rat Left Atria and Portal Vein

CHIRALITY 58-14 (1993)

The Effects of (+, (+)-, and (-)-Atenolol, Sotalol, and Amosulalol on the Rat Left Atria and Portal Vein

SHEILA A. DOGGRELL Department of Pharmacology, School of Medicine, University of Auckland, Auckland, New Zealand

ABSTRACT The effects of ( f)-, (+)-, and (-)-atenolol, sotalol, and amosulalol alone on the rat left atria and portal vein and on the respective P1- and PZ-adrenoceptor-mediated responses to isoprenaline have been determined. ( f )-Atenolo1at 10 -6 M had no effect whereas high concentrations of (+)- and (- )-sotalol, 10 -5-10 -4 M,and ( f )-, ( + )-, and ( - )-amosula- lo1 depressed the response of the rat left atria to cardiac stimulation which indicates membrane stabilizing activity. None of the drugs tested had any effect alone on the rat portal vein. The order of potency as antagonists was ( f )-amosulalol > (*)-atenolo1 > (+ ))-sotalolat P1 - adrenoceptors and ( f)-amosulalol > ( f)-sotalol > ( f )-atenolo1 at P2 -adrenoceptors. ( f )-Atenolo1and ( f )-amosulalolare P1 -selectivewhereas ( f)-sotalol is P2 -selective.For each of the racemic P-blockers, the P1 - and Pz-adrenoceptorblocking activity was predominantly due to the ( - )-enantiomer. o 1993 Wiley-Liss, Inc.

KEY WORDS: ( f )-, ( + )-, and ( - )-atenolol, sotalol, amosulalol, enantiomers, P1 - and P2- adrenoceptors, rat left atria and portal vein

INTRODUCTION pig atria,9 rat jugular vein, lo and rat right ventricle l1 and of Racemic P-adrenoceptor antagonists (P-blockers) are com- 7.04 and 7.90 at the 02-adrenoceptors of the guinea pig tra- monly used in the treatment of hypertension, angina, and car- chea12 and of the rat aorta,13 respectively. The activity of diac arrhythmias. These drugs act as antagonists at P1 - and ( f )-amosulalolat the P1 -adrenoceptors of the rat right ventri- Pz -adrenoceptors and may also have ancillary properties. The cle14 and the rat atrium and at the P2-adrenoceptorsof the properties of racemic drugs are not always shared equally guinea pig tracheal2 is predominantly due to the (-)-enanti- among the stereoisomers. There have been many studies of omer. racemic P-blockers but only a few studies of the P-blocker ( f )-Atenolo1 is reported to lack whereas ( f)-amosulalol l1 enantiomers. has membrane stabilizing activity on cardiac tissue. The mem- Recently we have studied the effects of ( f)-propranolol and brane stabilizing activity of ( f ))-amosulalolon the rat right its enantiomers2 on the rat left atria and have shown that low ventricle is predominantly due to the (- )-enantiomer.l4 In contrast ( )-sotalol lengthens cardiac action potential dura- concentrations of ( f )-propranolol have both P1 -adrenoceptor f blocking and membrane-stabilizing activity (defined as the ti~n'~,'~and this causes a positive inotropic effect.l5!l7The ability to depress the response to cardiac stimulation) and that positive inotropic effect of (+ )-sotalol on canine ventricular these properties are predominantly due to the ( - )-enantiomer. trabecula muscles and Purkinje strands was common to the It is not known whether this membrane stabilizing activity of ( + )- and ( - )-enantiomen.l5 In contrast the ( + )-, but not ( - )-, (-)-propran0101 on the rat left atria is a property shared by the enantiomer of sotalol had a positive inotropic effect on the rat enantiomers of other 0-blockers. left atria. l7 Atenolol, sotalol, and amosulalol (Fig. 1) are all P-blockers The aim of the present study was to determine the phar- with asymmetric centres. ( *)-Atenolo1 is a P1-adrenoceptor macodynamic profiles of ( f )-, ( + )-, and ( - )-atenolol, sotalol, selective antagonist having a pA2 value of 6.90 at the P1- and amosulalol on 2 isolated cardiovascular preparations with adrenoceptors of the cat atria3 and of 5.91 at the p2- special attention being given to the effects of the drugs alone. adrenoceptors of the guinea pig pulmonary artery.4 The ef- Thus we have studied the effects of the drugs alone on the fects of ( + )- and ( - )-atenolo1 at P-adrenoceptors are electrically driven rat left atria and on the spontaneous contrac- unknown. ( f )-Sotalol is a nonselective P-blocker5 and has a tile activity of the rat portal vein. Also I describe the effects of pA2 value of 6.3-6.7 at the pl-adrenoceptors of kitten and ( f )-, ( +)-, and (-)-atenolol, sotalol, and amosulalol on the guinea pig atria.6 Experiments in anaesthetized dogs have P1 -adrenoceptor mediated contractions of the electrically illustrated that the P1 -adrenoceptor blocking activity of driven rat atria and the P2-adrenoceptor mediated attenuations ( f)-sotalol resides predominantly in the ( - )-enantiomer. of the rat portal vein to isoprenaline. The effects of (+)- and ( - )-sotalol at PZ-adrenoceptorshave not been characterized. (*)-Amosulalol is considered to be a nonselective Received for publication April 13, 1992; accepted August 10, 1992. Address reprint requests to Sheila A. Doggrell, Department of Pharmacology, P-adrenoceptor antagonist having pA2 values of 7.60, 7.30, School of Medicine, University of Auckland, Private Bag, Auckland, New Zea- 7.70, and 7.50 at the PI -adrenoceptors of rat right atria,8 guinea land. o 1993 Wiley-Liss, Inc. (i)-, (+ )-, AND ( - )-ATENOLOL/SOTALOL/Ah4OSULALOL 9 the other tissue of the pair remained untreated throughout. During this 75 min about 500 ml 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 proce- dure was repeated with one tissue remaining untreated throughout and the paired tissue being treated with a higher concentration of the same drug. cn3s02Nn ~ycn2-Nn-cn ,c H3 The contractile responses to electrical stimulation just prior to the second and third challenges with isoprenaline were cal- \ n culated as a percentage of the response to stimulation prior to = n3 the first challenge with isoprenaline. For each challenge with isoprenaline, the response to electrical stimulation just prior (+)-SOTALOL 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 n3ce-cn2-Nn-- cn2- c n2-o to electrical stimulation and isoprenaline between treated and untreated tissues were not significantly different response n ocn3 curves were calculated as a percentage of the maximum of the individual curves, i.e., normalized. If the maximum re- (2)-AMOSULALOL sponses to electrical stimulation and isoprenaline between treated and untreated tissues were significantly different, re- Fig. 1. Structures of ( f )-atenolol, ( i )sotalol, and ( f )-amosulalol. Asterisk indicates asymmetric carbon. sponse curves were calculated as a percentage of the maximum of the first curve.

METHODS Attenuation Responses of the Rat Portal Vein (Method General Described by Lloggrel118) Male Wistar rats (250-350 g) were stunned and exsan- Each portal vein was cleared of surrounding tissue and guinated. The heart or portal vein was rapidly removed and mounted under 1g tension in 5 ml organ bath containing Krebs placed in Krebs solution saturated with 5% CO2 in oxygen. All solution and allowed to equilibrate for 30 min. During the experiments were performed in the presence of a modified equilibration period, the tissues were washed by overflow. To Krebs solution (composition (mM): NaCl, 116; KCl, 5.4; CaC12, prevent isoprenaline from stimulating a-adrenoceptors,and to 2.5; MgClz, 1.2; NaH2P04, 1.2; NaHCO3, 22.0 D-glucose, 11.2; inhibit the extraneuronal uptake process, tissues were treated Na,EDTA, 0.04) at 37°C which was bubbled with 5% C02 in with phenoxybenzamine at 10 -4 M for 45 min. Tissues were oxygen. Contractile responses were measured isometrically then rapidly washed for 20 min. The wash was then stopped, with force displacement transducers (Grass model FT03.C) and and the tissues were allowed to stabilize for 20 min. During this displayed on a polygraph (Grass model 79B). In each series of period, the amplitude of the spontaneous contractions became experiments, the individual values (percentages, slopes, pD2 constant and then a cumulative challenge to isoprenaline, values, concentration-ratios, and PA, values) obtained were 10 -g, 3 x 10 -9, 10 -* M etc. was made to each portal vein on subject to Student's t test. Differences were considered signifi- a 5 min cycle. The cycle was continued until a maximum cant when P < 0.05. Mean values f SEM were also obtained. attenuation was obtained. Tissues were then rapidly washed in the presence of drug for 45 min. During this 45 rnin about 500 Contractile Responses of the Electrically Driven Rat ml of drug-containing Krebs overflowed the tissue. Washing Left Atria (Method Described by LbggreU1) was stopped for 20 min prior to a second challenge to isoprena- Left atria were removed from the heart and halved. Each line. Tissues were then treated with a higher concentration of atria half was mounted longitudinally between two platinum the same drug in a rapid wash. After a 45 min/500 ml wash and electrodes (approximately 3 cm apart, above and below the a 20 min stabilization, a third challenge to isoprenaline was tissue) under 1 g tension in 5 ml organ baths containing Krebs initiated. solution (with 10 -5 M guanethidine to prevent the release of On the portal vein the measurement of the contractile re- noradrenaline from nerve endings, and atropine at 10 -6 M) sponse was taken as the average of the amplitude of the final and allowed to equilibrate for 60 min. During the equilibration 3 contractions in a 5 rnin period. The attenuation in the pres- period, the tissues were washed by overflow. Tissues were ence of a drug or drugs was calculated as a percentage of the electrically stimulated at 4 Hz (5 msec, 10 V). After 9 min of contractile response in the first stabilizationperiod. If the maxi- stimulation, a cumulative challenge with isoprenaline was initi- mum attenuations to isoprenaline between treated and un- ated on a 3 rnin cycle. This cycle was continued until a maxi- treated tissues were not significantly different response curves mum response was obtained. were calculated as a percentage of the maximum of the individ- One of the tissues was treated with a drug for 75 min while ual curves, i.e., normalized. 10 DOGGRELL

Assessment of Data dine sulphate, and ( - )-isoprenaline bitartrate (Sigma Chemical Slopes were determined for all isoprenaline response curves. cO.), PhenoxYbenzamine hydrochloride (donated bY SmithKline In addition when responses to isoprenaline were normalized, Beecham), and ( * ), ( + )-, and ( - )-amosulalol (donated by pD2, concentration-ratio, and pA2 values were determined. The Yamanouchi slope (difference in percentage maximum of the response/unit RESULTS of logarithm of molar concentration of isoprenaline) and pD2 Rat Left Atria value (negative logarithm of molar concentration of isoprenaline producing 50% of the maximum response) were computed Direct muscle stimulation (4 Hz, 5 msec, 10 V) contracts the by regression line analysis using a computer. The regression rat left atria and the force of contraction is increased by iso- line analysis was performed on the steepest part of the re- prenaline acting at P1 -adrenoceptors. sponse curve, which was usually over the range 20-80% of the The responses to cardiac stimulation were not altered by maximum response. For each tissue, the concentration ratios (f)-,(+)-, and (-)-atenolo1 at 10 -7-10 -6 M, (+)-sotalol at (the antilog of the difference between the pD2 values) were 10 -5-10 -4 M, and (f)-, (+)-, and (-)-amosulalol at 10 determined between each challenge with isoprenaline. Previous 10 -7-10 -6 M. (f)-Sotalol at 10 -5-10 -4 M and studies have shown that the sensitivity to isoprenaline de- ( f )-amosulalol at 10 -5 M inhibited the responses to cardiac creased with successive challenges in the rat left atria' but not stimulation (Table 1). The inhibitory effect with (f)-sotalol was solely due to the (- )-enantiomer (Table 1).In contrast the the rat portal vein. l8 Consequently concentration-ratios obtained in the presence of drugs had to be corrected for changes inhibitory effect with ( f )-amosulalol was equally due to the occurring in untreated atria but not portal veins. Thus the (+)- and (- )-enantiomers (Table 1). difference in the mean pD2 values from untreated atrias were The responses to isoprenaline were not altered by (+)-ateno- subtracted from the difference in the individual pD2 values lo1 at 10 -7 M, (+ )-sotalol at 10 -5 M, and (+ )-amosulalol at from treated atrias. pA2 values (the negative logarithm of the 10 -8-10 -7 M. (f)- and (-)-atenolo1 at 10 -7-10 -6 M, molar concentration of drug that causes a twofold inhibition of (+)-atenolo1 at 10 -6 M, (f)- and (-)-sotalol at 10 -5-10 -4 the concentration-response curve for isoprenaline) were deter- M, (+ )-sotalol at 10 -4 M, ( f )-amosulalol at 10 -8-10 -5 M, mined for concentrations of drugs that had no effect on the (+ )-amosulalol at 10 -6-10 -5 M, and (- )-amosulalol at slope of the isoprenaline response curves by use of the formula, 10 -7-10 -5 M inhibited the responses to isoprenaline (Fig. 2) without altering the slopes of the isoprenaline response curves pA2 = PA, + log(x - 1) where PA, is the negative logarithm of the molar concentration of drug and x is the concentration. Or the isoprenaline responses (data not shown). ratio. The pA2 values for ( f )-atenolol, ( f)-sotalol, and ( f)-amosulalol and their enantiomers were derived from the Drugs formula and are shown in Table 2. At PI-adrenoceptors the order of potency was (f)-amosulalol > (*)-atenolo1 > The drugs used were ( f)-, ( + )-, and ( - )-sotalol hydrochlo- ( f )-sotalol. For each of the racemic P-blockers, the - ride (donated by Bristol-Myers Squibb), ( f)-atenolo1 (donated PI by ICI Pharmaceutical Division), (+ )- and (-)-atenolo1 (Re- adrenoceptor blocking activity was predominantly due to the search Biochemicals Inc.), atropine sulphate (Serva), guanethi- ( - )-enantiomer (Table 2). Rat Portal Vein TABLE 1. Effect of atenolol and amosulalol on the The rat portal vein has spontaneous contractile activity responses of the rat left atria to cardiac stimulation which is attenuated by isoprenaline stimulating P2 -adrenoceptors. l8 Cardiac (f)-,(+)-, and (-)-Atenolol, I 5 x lop5M, (f)-,(+)-, stimulation" and ( - )-sotalol, I 10 -4 M, and ( f)-, ( + )-, and ( - )-amosula- 101, I M, had no effect on the spontaneous contractile Control 70 (7) activity of the rat portal vein. (*)-sotaioi, 10-5 M 42 * (7)* Three successive challenges of the rat portal vein to iso- Control 59 4 (7) (*)-Sotalol, M 38 (7)* prenaline produced identical attenuation curves. l8 The responses to isoprenaline were not altered by ( + )-sotalol at 3 x Control 63 * (8) 10 M or (+ )-amosulalol at 10 -7 M but were inhibited by (-)-Sotalol, 10-5 M 50 f 4 (8)* ( )-atenolo1at 5 x 10 -6-5 x 10 -5 M, (+ )- and (-)-atenolo1 Control 61 (8) f (-)-Sotalol, M 45 (8)* at 5 x 10 M, (f)-sotalol at 3 x 10 -6-10 -4 M, (+)-sotalol at 10 -5-10 -4 M, (-)-sotalol at 10 -6-10 -4 M, (f)-and Control 62 f 10 (7) (- )-amosulalol at 10 -7-10 -5 M, and (+ )-amosulalol at (* )-Amosulalol, M 37 * 10 -6-10 -5 M (Fig. 3) without altering the slopes of the iso- Control 55 * (7) prenaline response curves or the isoprenaline maximum re- (+)-Amosulalol, M 32 * (8)* sponses (data not shown). Control 58 f 7 (7) The pA2 values for ( f )-atenolol, ( f )-sotalol, and (-)-Amosulalol, 10-5 M 29 * (8)* ( f)-amosulalol and their enantiomers were derived from the formula and are shown in Table 3. At P2-adrenoceptors the 'Calculated as a percentage of the first response. All values are mean i SEM. The number in parentheses is the number of observations. order of potency was (f)-amosulalol > (f)-sotalol > *P < 0.05, unpaired t test with own control. ( f )-atenolol. For each of the racemic P-blockers, the Pz- ( f )-, ( + )-, AND ( - )-ATENOLOL/SOTALOL/AMOSULMosuLALOL 11 TABLE 2. PA, values at PI-adrenoceptors(rat left atria).

PA, PA,

( f )-Atenolol, M 7.14 f 0.18 (8) ( f )-Amosulalol, M 8.30 f 0.19 (12) ( f)-Atenolol, M 6.96 f 0.20 (10) (f)-Amosulalol, M 8.36 f 0.24 (11) ( f )-Amosulalol, M 7.98 f 0.39 (11) (+)-Atenolol, M 6.47 f 0.19 (10) ( f )-Amosulalol, 10-5 M 8.53 f 0.26 (10) (-)-Atenolol, M 8.02 f 0.15 (10) (+)-Amosulalol, 10-6 M 6.35 f 0.21 (7) (-)-Atenolol, M 8.03 f 0.16 (8) (+)-Amosulalol, M 6.19 f 0.16 (8) (f)-sotaioi, 10-5 M 6.13 f 0.11 (7) (- )-Amosulalol, 10-7 M 8.82 f 0.18 (8) (f)-sotaioi, 10-4 M 6.19 f 0.09 (7) (- )-Amosulalol, M 8.59 f 0.12 (8) (-)-Amosulalol, 10-5 M 8.70 f 0.21 (8) (+ )-Sotalol, M 4.74 f 0.13 (7) (- )-sotaiOi, 10-5 M 5.94 * 0.11 (8) (-)-Sotalol, M 5.91 f 0.13 (8)

nAll values are mean f SEM. The number in parentheses is the number of observations.

ISOPRENALINE (MI

Fig. 2. Effects of (&)-, (+)-, and (-)-atenolo1 (left), sotalol (centre), and the absence (A) and presence (A)of (-)-sotalol at M. Right, top: re- amosulalol (right) on the responses of the rat left atria to isoprenaline. Left, top: sponses in the absence (0)and presence (.) of ( f)-amosulalol at Mand responses in the absence (0)and presence (0)of ( f )-atenolo1at 10 -6 M; middle: in the absence (0) and presence (0)of (*)-amosulalol at M; middle: responses in the absence (0)and presence (0)of ( + )-atenolo1at 10 -6 M; bottom; responses in the absence (0)and presence (0)of (+)-amosulalol at M; in the absence (0)and presence (0)of (- )atenolo1 at M. Centre, top: bottom: responses in the absence (0)and presence (.) of ( - )-amosulalol at responses in the absence (0)and presence (.) of ( * )-sotalol at 10 -5 M and in M and in the absence (0)and presence (0)of (-)-amosulalol at M. the absence (A)and presence (A)of (+)sotalol at M; middle: responses Responses are calculated as a percentage of the maximum and plotted against in the absence (A) and presence (A)of (+)-sotalol at M; bottom: re- the log of the molar concentration of isoprenaline. Each value is the mean from sponses in the absence (0)and presence (.) of (-)-sotalol at M and in 7-11 tissues; vertical lines show SEM. 12 DOGGRELL TABLE 3. PA, values at p,-adrenoceptors (rat portal vein)

~~ ____ PA, PA,

(f)-Atenolol, 5 x M 5.65 f 0.15 (8) (-)-sotalol, 10-6 M 6.79 f 0.15 (8) (f)-Atenolol, 5 x M 5.47 f 0.15 (8) (-)-Sotalol, 3 x M 7.06 f 0.08 (8) (-)-Sotalol, 10-5 M 6.69 f 0.17 (8) (+)-Atenolol, 5 x M 5.28 f 0.25 (7) (-)-sotaioi, 10-4 M 6.82 f 0.13 (8) (-)-Atenolol, 5 x M 5.99 f 0.10 (8) ( f )-Amosulalol,10-7 M 7.38 f 0.12 (8) (f)-Amosulalol, 10-6 M 7.52 f 0.12 (8) (f))-Amosulalol, M 7.58 f 0.11 (8) (*)-sotalol, 3 x 10-6 M 6.61 f 0.14 (8) (*)-sotaioi, 10-5 M 6.55 f 0.13 (8) (+)-Amosulalol, 10-6 M 5.96 f 0.20 (8) (+)-sotai~i,3 x 10-5 M 6.80 f 0.11 (8) (+)-Amosulalol, 10-5 M 5.82 f 0.09 (8) (f)-sotaioi, 10-4 M 6.57 f 0.14 (8) (-)-Amosulalol, 10-7 M 7.54 f 0.14 (8) (+)-sotaioi, 10-5 M 5.23 f 0.16 (7) (-)-Amosulalol, M 7.76 f 0.12 (8) (+)-sotaioi, 3 x 10-5 M 5.45 f 0.16 (7) (-)-Amosulalol, 10-5 M 7.45 rf: 0.21 (8) (+ )-sotai~i,10-4 M 5.18 f 0.13 (7)

“All values are mean f SEM. The number in parentheses is the number of observations.

ISOPRENALINE (M)

-9 -8 -7 -6 -5 ,;4 lo9 12 ro7 12 ti5 1i4 10 10 10 10 10 1’ 1, 1.

Fig. 3. Effects of (f)-,(+)-, and (- )-atenolo1 (left),sotalol (centre),and (0)and presence of ( f)-amosulalol at M (V)and 10 -5 M (H)and, from amosulalol (right) on the responses of the rat portal vein to isoprenaline. Left, other tissues, in the absence (0)and presence of ( f)-amosulalol at M (A); top: responses in the absence (0)and presence of (*)-atenolo1 at 5 x 10 -6 M middle: responses in the absence (0)and presence of (+)-amosulalol at (H) and 5 x M (A);middle; responses in the absence (0)and presence M(A) and, from other tissues, in the absence (0)and presence of ( + )-amosula- (W) of (+)-atenolo1 at 5 x M; bottom; responses in the absence (0)and lo1 at M (mh bottom: responses in the absence (0) and presence of presence (H) of (+)-atenolo1 at 5 x low6M. Centre, top: responses in the ( - )-amosulalol at 10 -7 M ( V)and 10 -5 M (H)and, from other tissues, in the absence(o)andpresenceof(*)-sotalolat3 x 10-6M(H)and3 x 10-5M absence (0)and presence of (- )-ammulalo1 at 10 -6 M (0).Responses are (A);middle; responses in the absence (0)and presence of (+)-sotalol at calculated as a percentage of the maximum and plotted against the log of the M (0)and 10 -4 M (A); bottom: responses in the absence (0)and presence of molar concentration of isoprenaline. Each value is the mean from 7-8 tissues; ( - )sotalol at 10 -5 M (0)and 10 -4 M(A). Right, top: responses in the absence vertical lines show SEM. ( & )-, ( + )-, AND ( - )-ATENOLOL/SOTALOL/AMOSMosULALOL 13 adrenoceptor blocking activity was predominantly due to the left atria is more sensitive to the membrane stabilizing activity ( -)-enantiomer (Table 3). of ( f )-propranololthan that of ( f )-amosulalol.This may indi- cate that ( f )-amosulaloland ( f )-propranololcause membrane Selectivity for PI - or Pz-Adrenoceptors stabilizing activity by different mechanisms with the ventricle The pA2 values for each of the drugs at the P1 -adrenocep- being more sensitive to the ( f )-amosulalolmechanism and the tors of the rat left atria (Table 2) and at the P2-adrenoceptors atria being more sensitive to the ( f)-propranolol mechanism. of the rat portal vein (Table 3) were compared. The comparison The pAg values for ( f)-atenolo1 obtained in the present shows that ( f )-, ( + )-, and ( - )-atenolo1 and amosulalol are study (7.05 at the P1-adrenoceptors of rat atria and 5.56 at the selective P1 - whereas ( f )-, ( + )-, and ( - )-sotalol are selective P2 -adrenoceptors of the rat portal vein) are in agreement with P2-adrenoceptor antagonists. those published previously (see Introduction). In addition we have shown that both the P1- and P2-adrenoceptor blocking DISCUSSION activity of ( f )-atenolo1is predominantly due to the ( - )-enanti- Low concentrations of ( f )- and ( - )-propranololhave mem- omer. brane-stabilizing activity (defined as the ability to depress the The pA2 values for ( f )-sotalol obtained in the present study response to cardiac stimulation alone) on the rat left atria (see (6.16 at PI- and 6.63 at P2 -adrenoceptors) are also in broad Introduction). The present study shows that this property of agreement with those published previously (see Introduction). ( - )-propranololis not shared by low to modest concentrations We have confirmed that the P1 -adrenoceptorblocking activity of ( f )-, ( + )-, or ( - )-atenolol, ( f )- and ( - )-sotalol, and ( f )-, of ( f )-sotalol is predominantly due to the ( - )-enantiomer and, ( + )-, and ( - )-amosulalol or by high concentrations of in addition, we have also shown that the P2-adrenoceptor (+)-sotalol as these concentrations of the drugs had no effect blocking activity of ( f )-sotalol is mainly due to the ( - )-enanti- on the response to cardiac stimulation alone. Only high concen- omer. trations of (&)- and (-)-sotalol and (f)-,(+)-, and There is a major difference between the pAZ value for ( - )-amosulalol exert membrane stabilizing acting on the rat ( f )-amosulalol at the PI -adrenoceptorsof the rat left atria and left atria. those reported previously for ( f)-amosulalol at a wide range of The ability of ( f )- and ( - )-sotalol to decrease the response cardiac pI-adrenoceptors. Thus the pA2 value for to cardiac stimulation in the rat left atria observed in the ( f )-amosulalol obtained in the present study with the rat left present study was an unexpected finding as ( f )- and (-)-soh atria is 8.29 which is 0.59-0.99 greater than that previously 101 have previously been shown to increase the response to reported (see Introduction). This difference is unlikely to be due cardiac stimulation in the rat left atria as a consequence of to our experimental technique as the pA2 values derived in the action potential prolongation, probably due to potassium chan- present study for ( f )-atenolo1 and ( f )-sotalol are consistent nel blockade.17 A possible explanation for the difference be- with those reported previously (discussed above). One possible tween the two studies is that (+)-sotalol acts both as a potas- explanation for this difference is that the PI -adrenoceptors of sium channel blocker to increase and as a membrane the rat left atria are not identical with those in other tissues. stabilizing agent to decrease the response to cardiac stimula- Moreover ( f )-atenolo1 and ( f )-sotalol do not select between tion in the rat left atria and that the effect observed depends on the P1 -adrenoceptorsof cardiac tissues but that ( f )-amosulalol the frequency of stimulation. Tande and Refsum17 stimulated shows selectivity for the P1-adrenoceptors of the rat left atria the rat left atria at a low frequency (100/min) which was proba- over other cardiac P1 -adrenoceptors. bly ideal for demonstrating positive inotropism as a result of The pA2 values for ( f)-amosulalol obtained in the present action potential prolongation whereas we stimulated at a study at the P2- of the rat portal vein, 7.49, is in agreement with higher, more physiological, frequency (240/min) which was that reported previously at the P2-adrenoceptorsof other tis- more suited to demonstrating membrane stabilizing activity. It sues (see Introduction). ( f ))-Amosulalolis classified as a nonse- was not unexpected that ( f )-, ( + )-, and (- )-amosulalolcaused lective P-blocker. However, because of greater pA2 values ob- membrane stabilizing activity on the rat left atria as we have tained at the pl-adrenoceptor of the rat left atria, our study previously demonstrated membrane stabilizing activity with suggests that (f)-amosulalol is selective for some P1- these drugs on the rat right ventricle. 11,14 adrenoceptors. As with ( f )-atenolo1 and ( f )-sotalol, both the The mechanism underlying membrane stabilizing activity PI- and j32 -adrenoceptorblocking activity of ( f )-amosulalolis with P-blockers on the heart is poorly defined. The concentra- predominantly due to the ( - )-enantiomer. tion of ( f )-amosulalol and ( f )-propranolol that causes membrane-stabilizing activity is dependent on the cardiac tissue ACKNOWLEDGMENTS studied. In studies using the rat right ventricle we have demon- This study was supported by the National Heart Foundation strated membrane stabilizing activity with modest concentra- of New Zealand and performed with the skilful technical assist- tions of ( f )-amosulalol (10 -6 M”) and (-)-amosulalol, 10 -7 ance of Mr. David N. Jensen. M, but not ( + )-amosulalol, 10 -6 M14 and with higher concentrations of (*)-propranolol and 3 x M19). In LITERATURE CITED contrast on the rat left atria membrane stabilizing activity is observed with low concentrations of ( f )-, ( + )-, and ( - )-pro- 1. Doggrell, S.A. Simultaneous assessment of membrane-stabilizing and p-adrenmptor blocking activity of drugs with the rat isolated left atria. J. lv2 ( )-, ( )-, pranolol but only with a high concentration of f + Pharmacol. Methods 19:93-107, 1988. ( - and )-amosulalol(present study). Thus the rat right ventricle 2. Doggrell, S.A. The membrane stabilizing and p1-adrenoceptor blocking is more sensitive to the membrane stabilizing activity of activity of ( + )- and ( - )-propranolol on the rat left atria. Gen. Pharmacol. ( - )-amosulalol than that of ( - )-propranolol whereas the rat 21:677-680,1990. 14 DOGGRELL

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