Effect of ageing on ƒ 1A-adrenoceptor mechanisms in rabbit. Issei TAKAYANAGI, Mann MORIYA and Katsuo KOIKE

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1 J. Smooth Muscle Res. 28: 63-68, Effect of ageing on ƒ 1A-adrenoceptor mechanisms in rabbit isolated bronchial preparations Issei TAKAYANAGI, Mann MORIYA and Katsuo KOIKE Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences, Funabashi, Chiba 274 Japan Abstract Effect of ageing on ƒ 1A-adrenoceptor mechanisms in rabbit bronchial preparations was studied. The potency (pd2 value) of norepinephrine increased with age from 5 to 13 weeks and thereafter did not alter with age from 13 to 180 weeks. The affinity of norepinephrine (PKA value) and of a selective ƒ 1A-adrenoceptor blocker, 5-methylurapidil (pa2 value) did not alter with age. Efficacy of norepinephrine was proportional to receptor reserve (pd2-pka). These results suggest that age-related changes in aia-adrenoceptor mechanisms are due to changes in receptor reserve or total concentration of receptors, but not to changes in affinity of drugs to ƒ 1A-adrenoceptors and in contraction mechanisms. Introduction It is generally accepted that airway smooth muscle contains ƒ -adrenoceptors in addition to ƒà-adrenoceptors (Takagi et al., 1967; Fleish et al., 1970). Recently Takayanagi and Moriya (1991) reported that norepinephrine interacts with ƒ 1A-adrenoceptors to contract rabbit main bronchial preparations. The age-related change in the potency or pd2 value (the negative logarithm of the molar concentration required to produce a half-maximum response) of norepinephrine in rat isolated aortic strips and was deferens was found to be due to a change in receptor reserve or in the total number (density) of receptors but not to a change in drug affinity for ƒ 1-adrenoceptors (Takayanagi at al., 1986; 1987; 1989; Takayanagi and Koike, 1991). However, little is known about the effect of ageing on cria-adrenoceptor mechanisms in the rabbit bronchial preparations. Materials and Methods Male white rabbits (5-, 13-, 125-, and 180-week-old) were reared on a standard diet and tap water and libitum. They were kept in a temperature (24 }1 C) and humidity (55 }5%) controlled 12 hr day-night cycle. The rabbits were stunned by a blow on the back of the neck, exsanguinated and the lungs and airways removed. After removal of the trachea, a thin glass rod (1-2 mm o.d.) was passed through bronchus and the adhering lung parenchyma carefully removed. The bronchus in this study was the same part used as the main bronchus in our previous study (Takayanagi et al., 1984). Each preparation was placed in a 20 ml organ bath and attached via a thin thread to an

2 I. TAKAYANAGI et al. isotonic transducer under a tension of 0.5 g for isotonic recording of response. Responses to norepinephrine were recorded on a multi-pen recorder. The preparation was bathed in Krebs solution of the following composition (mm): NaCl 118, KC1 4.75, CaCl2 E2H2O 2.50, MgCl2 E 7H , KH2PO4 1.20, NaHCO and dextrose The solution also contained propranolol (10-6 M) and desmethylimipramine (10-7 M), corticosterone (3x10-5 M), ascorbic acid (10-3 M) and pargyline (10-6 M) to block ƒà-adrenoceptor and to inhibit the neural and extraneural uptake, oxidation of norepinephrine and monoamine oxidase (Takayanagi et al., 1987; Minneman et al., 1983). The organ bath was maintained at 37 C and constantly gassed with a mixture of 95% O2 and 5% CO2. The experiments were started after the preparation had been allowed for 90 min. Norepinephrine was applied to the bronchial preparation at intervals of 60 min. Agonistic activity was expressed as a pd2 value, a negative logarithm of the molar concentration producing half of the maximal response. Competitive antagonistic activity of 5- methylurapidil, a selective cria-adrenoceptor blocker, was expressed as a pa2 value, a negative logarithm of the dissociation constant of 5-methylurapidil to ƒ 1A-adrenoceptors. After determination of a control concentration-response curve of norepinephrine, the preparation was equilibrated with the antagonist for 15 min. The concentration-response curve of norepinephrine was then obtained in the presence of 5-methylurapidil and the procedure repeated with a high concentration of 5-methylurapidil in the same preparation. The pa2 value was calculated by the method of Arunlakshana and Schild (1959) as modified by Tallarida et al. (1979). The dissociation constant of norepinephrine was determined according to the method of Furchgott (1966), using an irreversible antagonist phenoxybenzamine to occlude a fraction of A-adrenoceptors. After determination of the control concentration-response curve of nor-ƒ 1 epinephrine, the strips were incubated with 10-8 M of phenoxybenzamine for 5 min. The strips were allowed to reequilibrate for 60 min with repeated washing every 10 min and cumulative concentration-response curves of norepinephrine were determined until constant responses were obtained. The agonist dissociaton constant was obtained from the equation: where [A] and [A L] are corresponding equieffective concentrations of norepinephrine before and after irreversible blockade of a fraction of receptors by phenoxybenzamine and q is the fraction of active receptors remaining after the treatment with phenoxybenzamine. If 1/[A] was plotted versus 1/[A L] and a dissociation constant (KA) was calculated by the equation: KA=(slope-1)/(intercept of Y-axis). Efficacy (e) of Stephenson (1956) was calculated using the equation: e=antilog (pd2-pka)+1, where pka is the negative logarithm of KA (Furchgott, 1966). Statistical analysis: Numerical results are expressed as means }S.E. and Duncan's new multiple range test was used to calculate statistical significant where appropriate. A P value less than 0.05 was considered a significant difference. Drugs used: ( ])-Norepinephrine hydrochloride, 5-methylurapidil hydrochloride and phenoxybenzamine hydrochloride (Sigma Chemical Co., St. Louis, MO, USA), all in powder form. Other chemicals used were of

3 Ageing on ƒ 1A-Adrenoceptors in Bronchus analytical grade. Results Norepinephrine contracted slowly the bronchial preparations from rabbits of different ages in a concentration dependent manner. The pd2 values of norepinephrine obtained from the curves were summarized in Table 1. The pd2 value significantly(p<0.05) increased with age from 5 to 13 weeks and thereafter did not alter with age from 13 to 180 weeks. Concentrationresponse curves for norepinephrine were shifted in a parallel fashion by an ƒ 1A-adrenoceptor blocker, 5-methylurapidil (10-9,3 V10-9 and 10-8M) in all strips from rabbits of different ages. The results obtained in the strips from 5-week-old rabbits were shown in Fig. 1, as the typical example. Schild plots yielded a straight line with a slope of unity (Table 2). The pa2 values of 5-methylurapidil against norepinephrine estimated by Schild plot analysis did not significantly differe (Table 2). All preparations lost 40 to 70% of maximum response to norepinephrine after 5 min treatment with phenoxybenzamine (10-8 M). Furchgott's double reciprocal plots of the results yielded a straight line. As the typical example, the results obtained in the strips from the 5- week-old rabbits were shown in Fig. 2. The negative logarithm of dissociation constant (pka value) of norepinephrine toƒ 1A-adrenoceptors, calculated according to the method of Furchgott Table 1. The pd2 and pka values for norepinephrine in bronchial preparations from rabbits of different ages. Each value is presented as a mean±s. E. (): No. of experiments. *: significantly different from the value obtained in the strips from 5-week-old rabbits(p<0.05). Fig. 1. Effect of 5-methylurapidil on the concentration-response curve for norepinephrine in bronchial preparation from 5-week-old rabbit. Abscissa: logarithm of molar concentration of norepinephrine. Ordinate: percent of maximum contractile response to norepinephrine(10-3m). œ,, and represent norepinephrine alone and with 5- methylurapidil at 10-9, 3 V10-9 and 10-8 M, respectively. Each value is presented as a mean }S. E. of 4 experiments.

4 I. TAKAYANAGI et al. Table 2. The pa2 values for 5-methylurapidil against norepinephrine and slope of Schild plot for antagonism between norepinephrine and 5-methylurapidil in bronchial preparations from rabbits of different ages. Each value is presented as a mean S. E. () : No. of experiments. Fig. 2. Inhibitory effect of phenoxybenzamine on the concentration-response curve for norepinephrine in bronchial preparation from 5-week-old rabbit (A) and Furchgott's double reciprocal plots of the data (B). A: Abscissa and ordinate are logarithm of molar concentration of norepinephrine and percent of maximum contractile response to norepinephrine (10-3 M), respectively. Each value is presented as a mean } S. E. of 4 experiments. B: Abscissa and ordinate are reciprocals of concentrations([a] and [A L]) of norepinephrine before and after irreversible blockade of a fraction of receptors by phenoxybenzamine (10-8M). (1966) is summarized in Table 1. The pka values estimated in preparations from rabbits of different ages did not significantly differ. However, the efficacy significantly (P<0.05) increased with age from 5 to 13 weeks and did not alter thereafter with age from 13 to 180 weeks (Table 1). Discussion Norepinephrine-induced contraction is considered to be mediated through ƒ 1A-adrenoceptors under the experimental conditions of this study as reported previously (Takayanagi and Moriya, 1991). Age-related changes were observed in the potency or pd2 value of norepinephrine in rabbit bronchial preparations, suggesting that there are changes in the contraction proccesses mediated via the ƒ 1A-adrenoceptors. In this study the pka values of norepinephrine did not alter with age. These data provided direct evidence that there is no age-related change in the affinity of ƒ 1A-adrenoceptors. This conclusion was supported by the fact that the pa2 value of 5-methylurapidil against norepinephrine did not alter with ageing.

5 Ageing on ƒ 1A-Adrenoceptors in Bronchus Fig. 3. Relationship between receptor reserve (pd2-pka) and efficacy for norepinephrine in rabbit bronchial preparations. Abscissa and ordinate: receptor reserve (pd2-pka) and efficacy, respectively. The number, 5, 13, 125 and 180 are the ages of rabbits in weeks, respectively. A correlation was found (r , p< 0.05). The efficacy (e) is an agonist- and tissue-dependent term, as defined by Stephenson (1956). Furchgott (1966) modified this model to differentiate the agonist and the tissue factors of efficacy by defining intrinsic efficacy (ƒã): e= (ƒã) [Rt.], where [Rt] refer to the total concentration of receptors. On the other hand, the difference between pd2 and pka values (pd2-pka) was used as an index for adrenoceptor reserve (Kenakin, 1984; Takayanagi et al., 1989; Takayanagi and Koike, 1991). The total concentration of receptors ([Rt]) is considered to be in proportion to the receptor reserve (Takayanagi and Koike, 1991). In this study we did not directly estimate the total concentration of ƒ 1A-adrenoceptors. Therefore, receptor reserve was used for the total concentration of receptors. The efficacy was proportional to the receptor reserve (Fig. 3), suggesting that changes inƒ 1A-adrenoceptor mechanisms with age are due to changes in receptor reserve but not to changes in affinity of drugs to ƒ 1A-adrenoceptors and also in contraction mechanisms. The present results were similar to the previous findings that age-related changes inƒ 1A-adrenoceptor processes in rat thoracic aorta and vas deferens were due to changes in the total concentration of receptors but not to changes in affinity of drugs to their receptors (Takayanagi et al., 1986; 1987; 1989; Takayanagi and Koike, 1991). Acknowledgements This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan, and by fund from Teijin Co. Ltd. Tokyo, Japan References. Arunlakshana, O. and Scild, H. O. (1959). Some quantitative use of drug antagonists. Brit. J. Pharmacol. 14, Fleish, J. H., Mailing, H. M. and Brodie, B. B. (1970). Evidence for existence of alpha-adrenergic receptors in the mammalian trachea. Am. J. Physiol. 218, Furchgott, R. F. (1966). The use of ƒà-haloalkylamines in the differentiation of receptors and in the determination of dissociation constants of receptor-agonist complexes. In; Advances in Drug Research, ed by N. J. Harper and A. B. Simons, Vol. 3, pp , Academic Press, London. Kenakin, T. P. (1984). The classification of drugs and drug receptors in isolated tissues. Pharmacol.

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