Pharmacological Research Communications, !/ol. 7, No. 2, 197'3

INDICATIONS FOR THE EXISTENCE OF TWO TYPES OF CARDIAC B-ADRENERGIC RECEPTORS

A.C.DreYer and J.Offermeier Dept. Pharmacology, Potchefstroom University, Potchefstroom 2520. RePublic of South Africa. Received 9 October 1974

SUMMARY The effects of a number of B-sympathomimetics and B-sympatholytics on the responses of spontaneouslybeating r-ight.atrium and e l e c t r i c a l l y driven l e f t atrium strips of guineaLpigs ' have• been•.determined. The a f f i n i t y values (PD2- and PA2-values ) for the chronotropic and inotropic effects were ~otably different for a number of these agonists and antagonists. These q'uantitative differences ~ive further support to the hypothesis that the chronotropic and inotropic-effects of B-adrenergic drugs are mediatedvia two different types of B-adrenergic receptors. INTRODUCTION In order to explain the various effects of sympathomimetics i t was necessary to postulate the existence of two types of-adrenergic receptors viz. ,~- and B-receptors-(Ahlquist,1948).. Using a s t a t i s t i c a l comparison of the effects o f a large number Of catecholamines.Lands et al concluded the existence of two d i s t i n c t types ofB-adrenergic~ receptors (Lands et a l . , 1967). The structure-activity relations of.B-adrenerg-ic stimulants mediating cardiac, stimulation, l i p o l y s i s a n d inhibition of small intestinal m o t i l i t y (called B1-adrenergic effects) differed s i g n i f i c a n t l y

fromthe structure-activity relationship for •-adrenergics responsible for bronchodilatation and vasodilation (called B2-adrenergic effects). This hypothesis of two types of B-adrenergic re'ceptors is now well documented and accepted by various :authors (Dunlop and Shanks, 1968; Collier and Dornhorst, 1969; LevY and Wilkenfeld, 1969; Farmer and Levy, 1970). Only quantitat~ive differences have thus far been demonstrated in the structureactivity relations for drugs acting onB1 and 62 receptors.

151

1 52

Pharmacological Research Communications, Vol.. 7, No. 2, 1975

Several investigators have indicated that the•quantitative influences on chronotropicandlinotropic effects of a number of sympathomimetic and sympatholytic amines differ Considerably-and have suggested that the BI receptors in the heart may not be homogenous (Youmans and Haney, 1939; Cotten and. Pincus, 1955; McDonald and Goldberg, 1963; Farmer and .Levy, 1970;

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Fig.l. - A typical registration of a drug which exhibits chronotropic selecti vi ty. Boissier. et a i . , - 1 9 7 3 ) .

• .Mostof the results,-however,

have been Obtained

on intact. animal s iand i t is uncertain I:o. what degree the •.apparent s e l e c t i v i t y m a y havebeen ~ influenced by differences inmetabolism or dis= t r i b u t i o n o f the.drugs .~nvestigated" " The. purpose of t h i s investigation was to determine whether d i f f e r e n t s t r u c t u r e - a c t i v i t y

relations hold ifor

B=adrenergic drugs with respect to -positive chronotropi c effects and. posi ti r e inotropi c" effects and :for B'adrenergic antagoni sts ;wi th respect .

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inotropic e f f e c t s induced by 6, ~

adrenergic agonists/.. ~ .Isolated •organ• preparations Were"employed in order to.excl ude. variabil ities such as distribution .and metabolism, METHODS• Left; atrial strips and right atrial pacemaker~preparations from the. hearts of guinea-pigs (0,3 0,4 kg)Were used... After • the animals were

Pharmacological Research Communications, Vol. 7, No. 2, 1975

153

killed by a sharp blow on the s k u l l , t h e i r hearts were rapidly removed and immersed in the isotonic solution mentioned below at room temperature for dissection of the a t r i a l preparations,

The preparations were mounted in a

I0 ml organ bath containing on isotonic solution with the following com: position: sodium chloride 8,0g; sodium bicarbonate l,Og; glucose l,Og; potassium chloride 0,4g; calsium chloride 0,2g; magnesium chloride O,Ig; insulin 1,6 U; d i s t i l l e d water to lO00ml.

The solution in the organ bath

was continuously oxygenated and s t i r r e d by a gas mixture of 95% 02 and 5% CO2.

The pH of this mixture was 7,4.

All experiments described were

carried out at 32,5°C. %E

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Chronotropic Inotropic

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10-8 10-7 10-6 10-4 M Fig.2o - Concentration-effect curves of a chronotropicselective drug. Note that the concentration-effect curve for chronotropic a c t i v i t y occurs at a lower range than that for inotropic a c t i v i t y .

The spontaneously beating r i g h t atrium was attached to a force displacement transducer (Grass mode] FT O,03)L

The amplified output from the force •

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displacement transducer was used to drive a.tachograph (Grass model 7 P4 AB).

This arrangement allowed continuous • monitoring of rate.

Before

drugs were investigated, the preparation was washed several times and allowed to s t a b i l i z e u n t i l the spontaneous rate did not change by more than + - 3% during a I0 minute observation p e r i o d . This usually required about 60 minutes.

Pharmacological Research Communications, Vol. 7, No. 2, 5975

154 The l e f t a t r i a l

strips were mounted in a similar way and attached to a force

displacement transducer (Grass model FT 0,03).

A driving stimulus was

applied through a pair of platinum electrodes in contact with the l e f t atrial

strip.

These strips were driven at one second intervals by square-

wave pulses of 5-msec duration.

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Stimulus i n t e n s i t y was j u s t above ~nres=

40

CHRONOTROPI C

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SELECTIVITY 30

Chronotropi c Inotropi c

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Fig. 3. - Chronotropic s e l e c t i v i t y exhibited in a series of phenylephrine derivatives with progressively larger substituents on the amino group.

hold in order to prevent release of endogenous noradrenaline from adrenergic nerve endings (Blinks, 1966).

The contractions were recorded on a Grass

model 7 polygraph. Cumulative concentrations-effect curves of the compounds investigatedwere obtained and pD2 values and r e l a t i v e i n t r i n s i c a c t i v i t i e s were calculated from these curves according t o the method of van, Rossum (Van Rossum, 19635. For competitive antagonists (8-blockers) pA2 values were also found accor= ding to the procedures described by van Rossum.

The values presented

are the means of at least six experiments in each case.

155

Pharmacological Researct~ Communications, Vol. 7, No. 2, 1975

TABLE I COMPARISON OF THE CHRONOTROPICAND INOTROPICEFFECTSOF A NUMBEROF B-SYMPATHOMIMETICS

CHRONOTROPIC EFFECT No

CHEMICAL STRUCTURE

HO~CH-CH2-NH-,CH-CH 3 HO~

2

OH

pD2

.....

(A)

R.I.A Relative potency

INOTROPIC EFFECT "

PD2

.i-(B)

.R. I .A IRelative! . .p_otency

A/B

8.5

I. 0

i00

8.0

i. 0

I00

1

7.9

0.9

25

6.1

0.9

1.3

19

6.1

0.9

5.3

1.0

0.2

2

5.7

0.9

0.5

20

CH3

HO~----~~-CH2-NH-CH 3 HO" -CH-NH-CH-CH3 H o ~ V / OH CH3

0,4

I 7.6

1.0

5.4

0.9

0.1

4.8

0.6

0. I

1

(n-C3H7)

7.1

0,7

4

4.9

0.7

0.1

40

HO~CH-CH2-NH (n-C4H9) OH

6.5

0.8

0.2

¢.7

0.8

0.5

HO~CH-CH2-NH (n-C3H7)

7.4

0.7

8

5.9

0.7

0.8

HO~_~_Y OH

5

CH3