Effects of Ritodrine Hydrochloride, a Beta2-Adrenoceptor Stimulant, on Uterine Motilities in Late Pregnancy

Japan. J. Pharmacol. 35, 319-326 (1984) 319

Shigeru IKEDA, Hiroshi TAMAOKI, Masuo AKAHANE and Yoshifumi NEBASHI Central ResearchLaboratories, Kissei Pharmaceutical Co., Ltd. 19-48 Yoshino,Matsumoto 399-65, Japan Accepted April7, 1984

Abstract•\Ritodrine hydrochloride (ritodrine) is a beta2-adrenoceptor stimulant which has been effectively prescribed for the prevention of premature labor. The present studies were carried out to investigate the effects of ritodrine on uterine motility in rats and rabbits during gestation, as compared with those of isoproterenol and isoxsuprine. The results were as follows: 1) Spontaneous movements and evoked contractile responses of isolated rat uterus (19-20th days of gestation) were suppressed by 10-9-1 0-6 M ritodrine. The potency of ritodrine was approximately 10 times more than that of isoxsuprine and 100-1,000 times less than that of iso-

proterenol. 2) When these drugs were administered to pregnant rats or rabbits intravenously, the tocolytic potency was in the following order: isoproterenol> ritodrine>isoxsuprine. 3) Ritodrine induced hypotension and tachycardia, but these effects were less than those of isoproterenol and isoxsuprine. 4) The effects of isoproterenol and ritodrine were almost prevented by pretreatment with pro-

pranolol, but those of isoxsuprine were only partially or not affected. These results suggest that ritodrine is effective in preventing the uterine contractions in rats and rabbits and that it has less effect on the circulatory system than isoproterenol and isoxsuprine. It is also concluded that ritodrine produces these effects through activation of beta-adrenoceptors.

Ritodrine hydrochloride (ritodrine), a beta2-adrenoceptor stimulant, has been reported to be effective in inhibiting uterine contractions both in humans (1, 2) and in experimental animals (3, 4) and has the chemical structure shown in Fig. 1. Ritodrine has been introduced as a useful drug to prevent premature labor in obstetrics (5). The effects of ritodrine on uterine motility, the circulatory system and metabolism were mainly investigated using large animals, i.e., ewes (6, 7), and reports of investigations in rats or rabbits are limited (8). In the present study, we observed the effects of ritodrine on uterine activity, heart rate and blood pressure in rats and rabbits in comparison with isoxsuprine hydrochloride (isoxsuprine) and isoproterenol hydrochloride (isoproterenol). Fig. 1. Chemical structures of ritodrine hydrochloride and related compounds. 320 S. Ikeda et al.

quency for 5 min in rats and represented as Materials and Methods the integrated area under the records in Animals: Female Wistar strain rats, rabbits. weighing about 200 g, and female New The zero level was the bottom line after Zealand White rabbits weighing about 4 kg administration of 10 ƒÊg/kg (i.v.) of iso- were used. proterenol. When uterine contractions were Measurement of uterine motility in vitro: induced with oxytocin, it was infused at The experiments were carried out using 50 mU/kgimin in rats and 2.5 mU/kg/min uterine muscle strips from rats on the 19-20th in rabbits. When uterine contractions were days of gestation. The zero day of pregnancy induced with prostaglandin F2,6 (PGF2.), it was estimated by the presence of the plug was infused at 20 fig/kg/min in rats and after overnight housing with male rats. The administered at 500 fig into the intrauterine tissues were dissected to be 2-3 mm in width cavity in rabbits. and 18-20 mm in length, except for the Measurements of uterine motility, blood placental region. These preparations were pressure and heart rate in conscious rats: kept in an organ bath containing nutrient Pregnant rats on the 19-20th days of solution at 37•Ž (Krebs-Henselite bicarbonate gestation were anesthetized with ether, and a solution for spontaneous motility and balloon connected polyethylene catheter was modified Locke-Ringer solution for evoked inserted into the right uterine horn after respnses). The compositions of the nutrient removal of one fetus. solutions were as follows: Krebs-Henselite A polyethylene catheter for measurement bicarbonate solution (g/1) contained NaCI, of blood pressure was then inserted into the 34.5; KCI, 1.75; CaCl2, 2.75; KH2PO4, 0.8; left common carotid artery and the other one MgSO4.7H20, 1.45; Glucose, 5.0; NaHCO3, inserted for the administration of drugs into 10.5 and Modified Locke-Ringer solution the fore-stomach. All catheters were ex-

(g/1) contained NaCI, 8.8; KCI, 0.4; CaCl2, teriorized from a skin incision in the maternal 0.04; MgCl2, 0.018; NaHCO3, 0.4; KH2PO4, cranial site through a subcutaneous tunnel. 0.02; Na2HPO4, 0.08; Glucose, 0.5. The Measurements were performed at least 3 hr contractions of preparations were recorded after the operation. isometrically on an ink-writing rectigraph Drugs used in this experiment: The main

(Type 8-S, Sanei) through a force-displace- drugs used in this experiment were ritodrine ment transducer (SB-1TH, Nihon Kohden). HCI (Duphar), isoxsuprine HCI (Duvadilan, Measurements of uterine motility, blood Daiichi), isoproterenol HCI (Nakarai), pro- pressure and heart rate in anesthetized pranolol HCI (I nderal, ICI-Sumitomo), animals: Pregnant rats on the 19-20th days oxytocin (Syntocinon, Sandoz-Sankyo) and of gestation and rabbits on the 29-30th days prostaglandin F2ƒ¿ (Prostarmon-F, Ono). of gestation were anesthetized with urethane Results (1.5 g/kg, s.c.), and a balloon connected polyethylene catheter was inserted into the Effects on motilities of isolated rat uterus: right uterine horn after removal of one fetus. Spontaneous movements of isolated rat A polyethylene catheter was then inserted uterus were suppressed by ritodrine (10w- into the left common carotid artery. Blood 10-6 M), isoxsuprine (10-9-10-8 M) and pressure and uterine motility (balloon isoproterenol (10-11-10-8 M), in a dose- pressure) were measured with pressure dependent manner (Fig. 2). Contractile transducers (MPU-0.5A, Nihon Kohden), responses evoked by oxytocin (1 x10-2 U/ and the heart rates were taken from a cardio- ml), PGF2ƒ¿, (3 •~ 10-6 M), acetylcholine tachometer (2140. Sanei) triggered by the (1•~10-7 g/ml), KCI (40 mM) or electrical pulse pressure of the artery. These parameters field stimulation (20 V. 5 msec, 2 Hz of were recorded on an ink-writing rectigraph rectangular pulses for 10 sec) were also

(Type 8S, Sanei). depressed dose-dependently by the drugs The uterine activity was represented as tested. The pD2 values calculated from dose- the product of contractile height and fre- response relationships are shown in Table 1. Effects of Ritodrine on Uterine Motilities 321

The potency of ritodrine was about 10 times When ritodrine was administered at 10 mg/kg more than that of isoxsuprine and 1 00-1 000 (i.v.), the maximal suppression was about times less than that of isoproterenol. 60%, and this suppression was restored after Figure 3 shows the interaction of these 120 min. lsoxsuprine (1 mg/kg, i.v.) also three drugs with propranolol. Parallel shifts depressed uterine activity by about 50% for of the dose-response curves to the right were 2 hr. When isoxsuprine was administered at seen for ritodrine, isoxsuprine and iso- 10 mg/kg (i.v.), the suppressive response was proterenol in the presence of propranolol. similar to that of 1 mg/kg administration, but Effects on uterine activity in anesthetized some rats died after 2 hr. lsoproterenol (10 rats: Figure 4 shows the effects of ritodrine, μg/kg,i.v.) showed about 90% suppression isoxsuprine and isoproterenol on uterine of uterine activity, but this response was activity when administered intravenously to immediately restored. rats. Ritodrine (1 mg/kg, i.v.) suppressed The maximal inhibition by ritodrine of the uterine activity by approximately 50%, but uterine activity at 1000 ƒÊg/kgimin infusion this suppression was restored after 30 min. was about 60% at the end of administration.

Fig. 2. Effects of ritodrine, isoproterenol and isoxsuprine on spontaneous movements of isolated pregnant rat uterus (19-20th days of gestation).

Table 1. Tocolytic effects of the drugs tested which are represented as pD2 values in various preparations of pregnant rat uterus (19-20th days of gestation)

Spont: Spontaneous movement; Oxytocin: Oxytocin, 10-2 U/mI; PGF2a: Prostaglandin F2a, 3•~10-6 M;

Ach: Acetylcholine, 10-7 g/ml; KCI: KCI, 40 mM; E.S.: Electrical field stimulation; 20 V. 5 msec, 2 Hz, rectangular pulses for 10 sec. 322 S. Ikeda et al.

(B) Amplitude (A) Amplitude

Fig. 3. Concentration-response relationships of ritodrine, isoproterenol and isoxsuprine with and without propranolol in pregnant rat uterus (N=14-20).

Fig. 4. Effects of ritodrine, isoproterenol and isoxsuprine on uterine activity in pregnant rats (19-20th days of gestation), N=5. The drugs tested were administered intravenously.

Inhibition by isoxsuprine infusion (300 ƒÊg/ 80% depression of uterine activity immediately kg/min) was less than 40%, and no effect after the start of administration, but it was was seen at the end of infusion. Isoproterenol restored gradually even during infusion. infusion (1-10 ,ƒÊg/kg/min) showed about Ritodrine also depressed oxytocin (50 mU/ Effects of Ritodrine on Uterine Motilities 323 kg/min, infusion) induced uterine motility in even at 10 ƒÊg/kg/min infusion, while the a dose-dependent manner. A 10 mg/kg heart rate was markedly decreased. intravenous administration or 1000 ƒÊg/kg/ Table 3 shows the effects of the drugs min infusion of ritodine showed about 70% tested on uterine activity and heart rate in the inhibition. Isoproterenol showed about 80% presence of propranolol. inhibition at 10 ƒÊg/kg (i.v.) and about 90% The actions of ritodrine (1 mg/kg, i.v.) and with a 1 ƒÊg/kg/min infusion, but these isoproterenol (1 ƒÊg/kg, i.v.) were almost inhibitions recovered faster than that of completely abolished by pretreatment with ritodrine. On the other hand, the inhibitory propranolol (1 mg/kg, i.v.). However, the effects of isoxsuprine were less than 30%, effects of isoxsuprine were little affected by and no dose-response relationship was seen. propranolol. Ritodrine and isoproterenol depressed Effects on uterine activity in anesthetized PGF2ƒ¿ (20 ƒÊg/kg/min, infusion) induced rabbits: After 30 min control periods, rabbits uterine motility in a dose-dependent manner, received a sequential infusion of ritodrine but these effects were somewhat weaker than beginning at 1 ƒÊg/kg/min for the first 30 min those on oxytocin induced responses. The and increased to 30 ƒÊg/kg/min for the last inhibition by isoxsuprine of PGF2ƒ¿ induced 30 min. As shown in Fig. 5, ritodrine sup- motility was less than 20% at 1 mg/kg, by pressed uterine contractions in a dose- intravenous administration, and 100 ƒÊg/kg/ dependent manner. The maximum inhibition min infusion. When isoxsuprine was adminis- of the uterine activity at 10 ƒÊg/kg/min tered in higher doses, the opposite responses infusion of ritodrine was about 70%. After were sometimes seen. completion of a 30 ,ƒÊg/kg/min infusion, Effects on blood pressure and heart rate uterine contractions recovered to the control in anesthetized rats: Maximal changes of the level by 2 hr after. When ritodrine was infused blood pressure and heart rate when the at 10 ƒÊg/kg/min for 2 hr, uterine activity was drugs tested were infused are presented in suppressed by about 50% during infusion. Table 2. Two hours after the end of infusion, it Ritodrine and isoproterenol caused hypo- returned to 70% of the control level. tension and tachycardia dose-dependently. Ritodrine (1-1000 ƒÊg/kg, i.v.), isoxsuprine lsoxsuprine also induced marked hypotension (1-1000 ƒÊg/kg, i.v.) and isoproterenol (0.01-

Table 2. Cardiovascular changes in anesthetized rats (19-20th days of gestation), N=5

Each number represents the maximum response during drug infusion. 324 S. Ikeda et al.

Table 3. Effects of ritodrine, isoproterenol and isoxsuprine on spontaneous uterine activity and heart rate in pregnant rats (19-20th days of gestation) and interactions with pretreatment by propranolol, N=5

Fig. 6. Effects of ritodrine and isoxsuprine on spontaneous uterine activity in conscious rats (19- 20th days of gestation, N=8-10). Drugs were administered orally.

Fig. 5. Effects of sequential infusion of ritodrine tractions evoked by intrauterine adminis- on spontaneous uterine activity, blood pressure and tration of 1000 ƒÊg of PGF2ƒ¿. Maximal heart rate in pregnant rabbits (29-30th days of inhibitions after the highest dose of the drugs gestation, N=5). tested were 80% (ritodrine), 70% (isoxsuprine) and 75% (isoproterenol). 10 ƒÊg/kg, i.v.) suppressed oxytocin induced As shown in Fig. 5, blood prssure decreased uterine contractions dose-dependently, and in a dose-dependent manner and a 10 mmHg the maximal inhibition by each drug was 80%, fall of blood pressure was seen at 30 ƒÊg/kg/ 60% and 90%, respectively. Inhibitory effects min infusion. However, the heart rate was of isoproterenol (10 ƒÊg/kg, i.v.) disappeared little affected by ritodrine. 20 min after administration. On the other In conscious rats, ritodrine dose- hand, the inhibitory effects of ritodrine and dependently suppressed uterine contractions. isoxsuprine persisted at doses of 1000 ƒÊg/kg When it was administered at 100 mg/kg

(i.v.), and 50% inhibition occurred 30 min (p.o.), the maximal inhibition was about 50%. after administration. Isoxsuprine also depressed uterine con- Ritodrine (1-1000 ƒÊg/kg, i.v.), isoxsuprine tractions, but its effects tended to disappear

(1-1000 ƒÊg/kg, i.v.) and isoproterenol (0.01- faster than those of ritodrine (Fig. 6). 10 ƒÊg/kg, i.v.) also depressed uterine con- Ritodrine dose-dependently decreased blood Effects of Ritodrine on Uterine Motilities 325 pressure. The maximal fall of blood pressure rats. was about 35 mmHg when it was adminis- In this experiment, isoxsuprine caused the tered at 100 mg/kg (p.o.). Isoxsuprine most marked hypotension and isoproterenol decreased blood pressure by about 30 mmHg caused the most marked tachycardia among when it was administered at 30 mg/kg (p.o.). the three drugs tested. As shown in Table 3, The heart rate was little affected by ritodrine, the effects of ritodrine and isoproterenol were but isoxsuprine caused transient tachycardia, almost completely antagonized by pro- followed by marked bradycardia. pranolol. These results suggest that ritodrine acts through beta-adrenoceptors, which Discussion show close agreement with the reports of In this experiment, ritodrine showed dose- lzumi and Kishikawa (9) and Slimes and dependent suppressive effects on uterine Creasy (4). On the other hand, the effects of motility in pregnant rats and rabbits in the isoxsuprine were only partially or not same way as observed in large animals (4, 6). antagonized. In in vitro studies, the effects As shown in Table 1, the inhibitory of isoxsuprine were antagonized by pro- potency of ritodrine was about 10 times more pranolol. These discrepancies may be due than that of isoxsuprine and 100-1000 times to the non-adrenergic machanisms of less than that of isoproterenol in isolated isoxsuprine. Manley and Lawson (12) preparations. These results agreed well with reported that isoxsuprine showed the two the report of Izumi and Kishikawa (9). In actions both with and without beta- in vivo experiments on rats and rabbits, adrenoceptors. We also confirmed such ritodrine also suppressed uterine contractions actions of isoxsuprine in the hind limb in a dose-dependent manner. Its potency was perfusion test of dogs. about 100-1000 times less than that of From the present study, it is concluded isoproterenol, but the effective period of that ritodrine is effective in suppressing ritodrine was longer than that of isopro- uterine contractions through activation of terenol. It can be considered that since beta-adrenoceptors in rats and rabbits and ritodrine has no catechol components, it was that it has less effect on the circulatory not attacked by COMT as in the case of system than isoproterenol and isoxsuprine. isoproterenol. In some experiments, isoxsuprine showed inhibitory effects on uterine References motility in the same manner as that of ritodrine. 1 Barden,T.P.: Effect of ritodrineon human uterine On the other hand, isoxsuprine had little motility and cardiovascular response in term effect on responses evoked by oxytocin and labor and the early postpartum state. Am. J. PGF2ƒ¿ in anesthetized rats. Obstet. Gynecol.112, 645-652 (1972) In anesthetized rats, ritodrine and iso- 2 Baumgarten, K., Frohlich, H., Seidl, A. and proterenol caused hypotension and tachy- Sokol, K.: A new beta sympathomimeticpre- cardia dose-dependently, but in anesthetized paration for intravenous and oral inhibition of rabbits, ritodrine had little effect on the uterine contractions. Eur. J. Obstet. Gynecol.2, blood pressure and heart rate. In conscious 69-83 (1971) rats, ritodrine caused hypotension, but had 3 Larsen,J.J.: Beta-adrenoceptorsin the pregnant almost no effect on the heart rate. Isox- and non-pregnant myometrium of the goat suprine caused marked hypotension which and cow. Acta Pharmacol.Toxicol. 44, 1 32-1 38 brought about bradycardia as shown in (1979) 4 Siimes,A.S.I. and Creasy,R.K.: Effect of ritodrine Table 2. We confirmed that isoxsuprine on uterine activity,heart rate and blood pressure showed a negative chronotropic action at in the pregnant sheep: Combined use of alpha above 10-5 M in isolated guinea-pig atria, or beta blockade. Am. J. Obstet. Gynocol. 126, but these concentrations were not necessarily 1003-1010 (1976) correlated with that of low dose adminis- 5 Gerris, J., Thiery, M., Bogaert, M. and tration, in vivo. In humans (10), ewes (11) Schaeparyver,A.: Randomizedtrial of two beta- and dogs (12), isoxsuprine showed marked mimetic drugs (ritodrineand fenoterol) in acute tachycardia, and it might act differently on intrapartum tocolysis. Eur. J. Clin. Pharmacol. 326 S. lkeda et al.

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