Effects of Chronic Administration of Carteolol on 8-Adrenoceptors in Spontaneously Hypertensive Rat Heart

Koichi Ebii, Reiko Fukunaga, Takashi Taniguchi * , Motohatsu Fujiwara' °2, Sunao Nakayama3, Yuichi Saitoh3 and Yukio Kimura3

Department of Neurobiology, Kyoto Pharmaceutical University, Kyoto 607, Japan 'Department of Pharmacology , Faculty of Medicine, Kyoto University, Kyoto 606, Japan 2Department of Pharmacology, Faculty of Pharmaceutical Sciences, Mukogawa Women's University, Nishinomiya 663, Japan ;Tokushima Research Institute , Otsuka Pharmaceutical Co., Ltd., Tokushima 771-01, Japan

Received March 12, 1991 Accepted May 16, 1991

ABSTRACT-We studied the effects of chronic administration of 8-adrenoceptor antagonists with and without intrinsic sympathomimetic activity (ISA): carteolol (with ISA) and propranolol (without ISA), respectively, on the heart of spontaneously hypertensive rat (SHR) and Wistar Kyoto rat (WKY). Six-week-old SHRs and WKYs were orally given carteolol or propranolol for ten weeks. The heart rate was reduced in propranolol-treated SHR, but not in carteolol-treated ones. In WKY, carteolol treatment increased the heart rate. The number and affinities of 8-adrenoceptors were analyzed using [3H]dihydroalprenolol as a ligand. Propranolol at 30 mg/kg in creased the number of cardiac f8-adrenoceptors in both SHR and WKY. In contrast, 10 mg/kg carteolol significantly decreased the number of cardiac 8-adrenoceptors in SHR, but not in WKY. These data indicate that carteolol, a /3-adrenoceptor antago nist with ISA, does not cause up-regulation of the number of cardiac ,8-adrenoceptors in the rat and suggest that this fact is related to a possible lack of "rebound phe nomena" after sudden discontinuation of chronic carteolol-therapy in humans.

Propranolol is a 8-adrenoceptor antagonist (4, 5). It has been suggested that this increase which is widely used in the treatment of dis in the number of 18-adrenoceptors as a result eases such as hypertension and angina pecto of chronic propranolol treatment is associated ris. However, a number of untoward cardiac with the elevated sensitivity to catecholamines events, in particular "rebound phenomena", (6, 7). Such findings would provide a possible occurred after abrupt withdrawal of explanation for the rebound phenomena after propranolol-therapy (1-3). By radioligand withdrawal of chronic propranolol-therapy in binding studies, several investigators have dem humans. onstrated that chronic in vivo treatment with Several investigators found that in human propranolol leads to a significant increase in lymphocytes, chronic treatment with proprano the number of 8-adrenoceptors in rat hearts lol led to an increase in the number of /3 adrenoceptors, while chronic treatment with ,Q-adrenoceptor antagonists with intrinsic sym pathomimetic activity (ISA) did not lead to an the /3-blocking effect of 0.3 mg/kg carteolol increase (but rather led to a decrease) in the on isoproterenol-stimulated SHR heart rate number of /3-adrenoceptors (8-10). Based on was seen for about 12 hours (19). Age-match these results, they suggested that "rebound ed WKYs were orally given carteolol (10 phenomena" could be prevented by the use of mg/kg/day) or propranolol (30 mg/kg/day). /3-adrenoceptor antagonists with ISA. Thus, Control SHR and WKY rats were given cor the ISA may play an important role in a-adre responding volumes of water. Both systolic noceptor antagonist-induced regulation of the blood pressure and heart rate were measured number of /3-adrenoceptors. at 16 hr after drug administration by the tail Carteolol is a nonselective /3-adrenoceptor cuff method. Sixteen hours after the last dose, antagonist with ISA (11) used clinically for the animals were anesthetized with ether, sac mild to moderate hypertension, and this antago rificed by exsanguination, and then the hearts nist could cause vasodilation by facilitation of were removed for radioligand binding studies. the release of endothelium-derived relaxing factor (12) and by a presynaptic inhibitory ac Tissue preparation tion on adrenergic nerve ending (13, 14). To Hearts were carefully removed from the our knowledge, there is no report regarding animals, rinsed with ice-cold saline, and then the rebound phenomena after withdrawal of kept in a freezer (-80°C). The ventricles were chronic carteolol-therapy. To date, several re minced with scissors and homogenized in 10 ports regarding regulation of /3-adrenoceptors volumes of ice-cold 50 mM Tris-HC1 buffer after chronic treatment of 8-adrenoceptor (pH 7.4) with a glass homogenizer. The antagonists with ISA have appeared (15 18). homogenates were filtered through two layers However, the relationship between the ISA of of gauze, re-homogenized two times at setting these drugs and changes in physiological param 10 on a polytron (PT-10) with 20-second eters such as blood pressure and heart rate bursts, centrifuged at 1,000 X g for 10 min, during chronic treatment has not been fully and then the supernatant was carefully re charcterized. moved and centrifuged at 100,000 X g for 30 Therefore, we studied the effects of chronic min. The resulting pellet was resuspended in carteolol administration on the number of car 50 mM Tris-HC1 buffer (pH 7.4). Protein con diac /3-adrenoceptors in spontaneously hyper centration was determined by the method of tensive rat (SHR) and Wistar Kyoto rat Lowry et al. (20). (WKY). The systolic blood pressure and heart rate of the same animals were measured in an Binding assay attempt to correlate their physiological re The binding assays using [3H]dihydro sponses with the ISA of this drug during chron alprenolol ([3H]DHA) as the radioligand were ic treatment. performed by incubating aliquots of the heart homogenates at a temperature of 25°C for 20 MATERIALS AND METHODS min in 250 ,u l of the Tris-HCI buffer, contain ing [3H]DHA, in the absence or presence of a Chronic drug treatment high concentration of alprenolol (10 ,uM). The Male SHRs and WKYs supplied by Charles binding in the presence of 10,uM alprenolol, River Japan, Inc. were used. Since carteolol the non-specific binding, was subtracted from and proptanolol are usually administered oral that obtained in the absence of alprenolol, the ly to humans, we also gave these drugs to the total binding, to obtain the specific binding. animals by the oral route. Six-week-old SHRs The assay was terminated by the addition of 3 were orally given, twice a day, carteolol ml of ice-cold buffer followed by rapid filtra (0.3, 3, 10 mg/kg/day) or propranolol (30 tion through the Whatmann GF/C glass fiber mg/kg/day) dissolved in water for 10 weeks as filters under suction. After washing twice with 3 ml of the buffer, the filters were transferred ligand binding studies, the maximal number of to counting vials and 5 ml of scintillation fluid [3H]DHA binding sites (Btmjx)and the equilib was added. Radioactivity was counted in a rium dissociation constant (Kd) were calcu Packard Tri-Carb scintillation spectrometer lated from Scatchard plots (21). (model 2000 CA). All assays were performed in triplicate. RESULTS

Drugs used Effects of carteolol and propranolol on systolic [3H]DHA (specific activity, 92.2 Ci/mmol) blood pressure and heart rate was purchased from New England Nuclear, In SHR, the oral administration of carteolol Boston, MA, U.S.A. 1-Alprenolol d-tartrate (0.3, 3 and 10 mg/kg) and propranolol (30 and dl-propranolol hydrochloride were pur mg/kg) for 10 weeks did not significantly chased from Sigma Chemical Co., St. Louis, affect systolic blood pressure (Fig. 1). The MO, U.S.A. dl-Carteolol hydrochrolide was ISA of carteolol and propranolol was deter obtained from Otsuka Pharmaceutical Co., mined by changes in the heart rate of SHR. Ltd., Tokushima, Japan. All other chemicals Propranolol (30 mg/kg) produced a significant were of reagent grade or of the purest grade decrease in the heart rate of 12-week and 16 commercially available. week old rats (drug administration for 6 and 10 weeks, respectively) (Fig. 2). Carteolol (0.3 Statistical analysis and 3 mg/kg) had a tendency to reduce the The experimental data given in the text are heart rate, but the changes were not statisti means ± S.E. of N experiments. The signifi cally significant. cance of difference was estimated by Dun In WKY, the administration of carteolol (10 nett's test, and values of P < 0.05 were con mg/kg) and propranolol (30 mg/kg) for 10 sidered statistically significant. Comparisons of weeks did not have any significant influence the changes of systolic blood pressure and on systolic blood pressure; however, carteolol heart rate were performed between pre (10 mg/kg) produced a significant increase in (6-week old) and drug-administration (8-, 12 the heart rate of 16-week old rats (Table 1). and 16-week old, respectively). In the radio

Fig. 1. Age-related changes in systolic blood pressure (SBP) in SHR. SBP was measured by the tail-cuff method. Drugs were administered as described in Materials and Methods; (•) carteolol (0.3 mg/kg/day) (n = 13), (/) carteolol (3 mg/kg/day) (n = 15), (•) carteolol (10 mg/kg/day) (n = 14), (A) propranolol (30 mg/kg/day) (n = 11). Control animals (()) (n = 13) were given the corresponding volume of water. The values are given as means + S.E. Statistical analyses were performed by Dunnett's test. Fig. 2. Age-related changes in heart rate (HR) in SHR. HR was measured by the tail-cuff method. For symbols, see the legend to Fig. 1. The values are given as means ± S.E. Statistical analyses were performed by Dunnett's test, * * P < 0.01

Table 1. Effects of carteolol and propranolol on systolic blood pressure (SBP) and heart rate (HR) in 6-week and 16-week old WKY

Characterization of [3H]DHA binding to 8 able (Fig. 4). Scatchard analysis indicated a adrenoceptors in SHR heart single class of binding sites. A typical Scatch Specific [3H]DHA binding increased linearly ard plot is shown in the inset of Fig. 4. with increasing protein concentration up to 0.4 mg protein per assay (data not shown). There Effects of chronic administration of carteolol fore, binding assays were conducted with less and propranolol on 8-adrenoceptors than 0.4 mg of protein per assay. At 25°C, In SHR, 10 mg/kg carteolol significantly de specific [3H]DHA binding was rapid, reached creased the number of ,8-adrenoceptors. In equilibrium within 15 min and was displaced contrast, 30 mg/kg propranolol significantly by the addition of 10,uM alprenolol (Fig. 3). increased the number of ,8-adrenoceptors The specific binding of increasing concentra (Table 2). In WKY, 10 mg/kg carteolol did tion of [3H]DHA (1.3-14.0 nM) was satur not significantly affect the number of 8-adre Fig. 3. Time course for association (*) and dissociation (A) of specific [3H]DHA binding to SHR heart. Incubation was performed with [3H]DHA, 2.1 nM, at 25 C. Displacement of the labeled ligand was induced by the addition of 10 ,M alprenolol after 20-min incubation. All points were determined in triplicate.

Fig. 4. Saturation and Scatchard plots of [3H]DHA binding to SHR heart. Heart homegenates were incu bated with increasing concentrations of [3H]DHA (1.3 14.0 nM) at 25 C. Results from a typical experiment are shown. The inset shows the Scatchard plot. The regression line (r = 0.97) indicates a Kd of 4.1 nM and a B,,,~,,of 41 fmol/mg protein. All points were determined in triplicate.

noceptors and 30 mg/kg propranolol signifi drug-treated groups in both SHR and WKY cantly increased the number of /3-adre hearts. There was also no significant differ noceptors (Table 3). There was no significant ence in wet weight of the ventricles among the difference in the Kd values of the control and groups (data not shown). Table 2. Effects of /3-adrenoceptor antagonists on Kd and B,a, of [3H]DHA bind ing to SHR heart

Table 3. Effects of /3-adrenoceptor antagonists on K,1and B,,,,,x of I3H]DHA bind ing to WKY heart

DISCUSSION and 3). This finding is in agreement with several other reports (22-24). On the other In the present study, we observed a de hand, it has been reported that the number of crease in heart rate in propranolol (30 mg/kg) cardiac fl-adrenoceptors in SHR is smaller treated SHR, but not in carteolol (10 mg/kg) than that in WKY (25-27). The reason for treated SHR. Although the fl-blocking po this discrepancy remains unknown. tency of carteolol was 30 to 300 times larger We observed an increase in the number of than that of propranolol (11, 19), the decrease /3-adrenoceptor binding site in the heart taken in heart rate was not observed in carteolol from 30 mg/kg propranolol-treated SHR and treated SHR, suggesting that the ISA of this WKY, without any significant changes in the drug masked the fl-blocking effect on the Kd values. Such increases in the number of heart rate. A tendency for the heart rate to cardiac 8-adrenoceptors are in agreement with decrease following smaller doses (0.3 and 3 other reports (4, 5). On the other hand, 0.3 mg/kg) of carteolol may be due to the weaker mg/kg carteolol (which is nearly equipotent to ISA in such doses. In fact, the heart rate was 30 mg/kg propranolol) had no effect on /8 increased in carteolol (10 mg/kg)-treated 16 adrenoceptors in SHR heart, and 10 mg/kg week old WKY. carteolol decreased the number of a-adre In the radioligand binding studies using noceptors in SHR heart without significant [3H]DHA, there was no difference in the changes in the Kd values. Our results are in number of cardiac fl-adrenoceptors between agreement with those of other investigators the SHR control and WKY control (Tables 2 who suggested that the "rebound phenomena" after withdrawal would not occur following /3 4 Glaubiger, G. and Lefkowitz, R.J.: Elevated / adrenoceptor antagonists with ISA (8-10). 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