Results from these studies provided new insight into the mechanisms controlling colonic motility during pregnancy and may form the basis for drug treatments of colonic motility disorders

Results from these studies provided new insight into the mechanisms controlling colonic motility during pregnancy and may form the basis for drug treatments of colonic motility disorders. In conclusion, oxytocin inhibit the contractile activity of proximal colonic smooth muscle of rabbits em in vitro /em . Hexamethonium (10 molL-1) partly blocked the inhibition of oxytocin (1 UL-1) on the contractile frenquency of CM. N-nitro-L-arginine-methylester (L-NAME, 1 molL-1), progesterone (32 molL-1) and estrogen (2.6 molL-1) had no effects on OT-induced responses. CONCLUSION: OT inhibits the motility of proximal colon in rabbits. The action is partly relevant with N receptor, but irrelevant with that of NO, progesterone or estrogen. INTRODUCTION Oxytocin (OT) is a very abundant neuropeptide. The structure of the OT gene was elucidated in 1984[1], and the sequence of the OT receptor was reported in 1992[2]. OT exerted a wide spectrum of central and peripheral effects[3-5]. It was reported that hypothalamic paraventricular nucleus is a site of controlling gastric function[6], oxytocin facilitated the manifestation of inhibitory effects of hypothalamus on the motor function of gastrointestinal tract[7]. The experiments on rats had shown that gastric motility was inhibited by microinjection of oxytocin into the dorsal motor nucleus of the vagus (DMN), and that the inhibition of gastric motility after electrical stimulation of the hypothalamic paraventricular nucleus was blocked by microinjection of an oxytocin receptor antagonist directly into the DMN. These results suggested that oxytocin acted on the gastric motility DMN[8]. The reports on peripheral action of oxytocin to influence gastrointestinal (GI) motility were controversial: OT decreases the contractions of the guinea pig stomach both genomic and nongenomic pathways[12]. Estrogen induces the OTR mRNA expression, and then increases the OTR density on the membrane of the uterus smooth muscle and central nervous system[13,14]; on the other hand, progesterone inhibits the nuclear OTR mRNA expression, and decreases the sensitivity of the target cell on OT stimulation[15,16]; progesterone was also reported to bind to OTR with high affinity and inhibit the receptor function[17]. In this study, we investigated the effect of OT on proximal colonic motility of rabbits; We also investigated if the OT-induced responses were relevant with NO, steroid hormones or N receptor. MATERIALS AND METHODS Animal preparation Rabbits of both sexes, weighing 1.5-2 kg, were fasted for 24-hour and sacrificed. The proximal colon (1 cm from the cecocolonic junction) was removed. The segment of the colon was opened along the mesentery. Muscle strips (8 2 mm) were cut, parallel to either the circular or the longitudinal fibers, and named circular muscle (CM) and longitudinal muscle (LM). The mucosa on each strip was carefully removed. Experiments The muscle strip was suspended in a tissue chamber containing 5 mL Krebs solution (37 C) and bubbled continuously with 950 mLL-1 O2 and 50 mLL-1 CO2[18]. One end of the strip was fixed to a hook on the bottom of the chamber. The other end was connected to an external isometric force transducer HD3 (JZ-BK, BK). Motility of colonic strips (under an initial tension of 1 1 g) in 2 tissue chambers were simultaneously recorded on ink-writing recorders (LMS-ZB, Cheng-Du). After 1 h equilibration, OT (0.1, 1, 10 UL-1) was added in the tissue chamber to observe their effects on proximal colon; N-nitro-L-arginine-methylester (L-NAME, 1 molL-1), hexamethonium (10 molL-1, progesterone (32 molL-1) or estrogen (2.6 molL-1), given 3 min before the administration of OT (1 U L-1), was added separately to investigate whether the actions of OT were relevant with NO, N receptor or steroids. The resting tension, the contractile frequency, and the mean contractile amplitude of LM and CM were measured. Drugs preparation The following agents were used: oxytocin (Biochemical Pharmaceutical Company, Shanghai, China), N-nitro-L-arginine-methylester (L-NAME) and hexamethonium (Sigma Chemical Company), progesterone and estrogen (The Ninth Pharmaceutical Factory in Shanghai). Data analysis The results were presented as test, 0.05 was considered to be significant. RESULTS Effect of OT on the spontaneous contraction of colonic smooth muscle strips OT (0.1 UL-1) failed to elicit significant effects on the contractile activity of proximal colonic smooth muscle strips ( 0.05). OT (1 to 10 UL-1) decreased the mean contractile amplitude and the contractile frequency of CM and LM (Figure ?(Figure1,1, Figure ?Figure2).2). It had no significant effects on the resting tension of CM and LM. Open in a separate window Figure 1 A: Effect of oxytocin on the mean contractile amplitude of longitudinal muscle (LM) of proximal colon in rabbits. B: Effect of oxytocin on the mean contractile amplitude of circular muscle (CM) of proximal colon in rabbits. a 0.05 control, = 10. Open in a separate window Figure 2.The structure of the OT gene was elucidated in 1984[1], and the sequence of the OT receptor was reported in 1992[2]. in 1984[1], and the sequence of the OT receptor was reported in 1992[2]. OT exerted a wide spectrum of central and peripheral effects[3-5]. It was reported that hypothalamic paraventricular nucleus is a site of controlling gastric function[6], oxytocin facilitated the manifestation of inhibitory effects of hypothalamus on the motor function of gastrointestinal tract[7]. The experiments on rats had shown that gastric motility was inhibited by microinjection of oxytocin into the dorsal motor nucleus of the vagus (DMN), and that the inhibition of gastric motility after electrical stimulation of the hypothalamic paraventricular nucleus was blocked by microinjection of an oxytocin receptor antagonist directly into the DMN. These results suggested that oxytocin acted on the gastric motility DMN[8]. The reports on peripheral action of oxytocin to AZ3451 influence gastrointestinal (GI) motility were controversial: OT decreases the contractions of the guinea pig stomach both genomic and nongenomic pathways[12]. Estrogen induces the OTR mRNA expression, and then increases the OTR density on the membrane of the uterus smooth muscle and central nervous system[13,14]; on the other hand, progesterone inhibits the nuclear OTR mRNA expression, and decreases the sensitivity of the target cell on OT stimulation[15,16]; progesterone was also reported to bind to OTR with high affinity and inhibit the receptor function[17]. In this study, we investigated the effect of OT on proximal colonic motility of rabbits; We also investigated if the OT-induced responses were relevant with NO, steroid hormones or N receptor. MATERIALS AND METHODS Animal preparation Rabbits of both sexes, weighing 1.5-2 kg, were fasted for 24-hour and sacrificed. The proximal colon (1 cm from the cecocolonic junction) was removed. The segment of the colon was opened along the mesentery. Muscle strips (8 2 mm) were cut, parallel to either the circular or the longitudinal fibers, and named circular muscle (CM) and longitudinal muscle (LM). The mucosa on each strip was carefully removed. Experiments The muscle strip was suspended in a tissue chamber containing 5 mL Krebs solution (37 C) and bubbled continuously with 950 mLL-1 O2 and 50 mLL-1 CO2[18]. One end of the strip was fixed to a hook on the bottom of the chamber. The other end was connected to an external isometric force transducer (JZ-BK, BK). Motility of colonic strips (under an initial tension of 1 1 g) in 2 tissue chambers were simultaneously recorded on ink-writing recorders (LMS-ZB, Cheng-Du). After 1 h equilibration, OT (0.1, 1, 10 UL-1) was added in the tissue chamber to observe their effects on proximal colon; N-nitro-L-arginine-methylester (L-NAME, 1 molL-1), hexamethonium (10 molL-1, progesterone (32 molL-1) or estrogen (2.6 molL-1), given 3 min before the administration of OT (1 U L-1), was added separately to investigate whether the actions of OT were relevant with NO, N receptor or steroids. The resting tension, the contractile frequency, and the mean contractile amplitude of LM and CM were measured. Drugs preparation The following providers were used: oxytocin AZ3451 (Biochemical Pharmaceutical Organization, Shanghai, China), N-nitro-L-arginine-methylester (L-NAME) and hexamethonium (Sigma Chemical Organization), progesterone and estrogen (The Ninth AZ3451 Pharmaceutical Manufacturing plant in Shanghai). Data analysis The results were presented as test, 0.05 was considered to be significant. RESULTS Effect of OT within the spontaneous contraction of colonic clean muscle mass pieces OT (0.1 UL-1) failed to elicit significant effects within the contractile activity of proximal colonic clean muscle strips ( 0.05). OT (1 to 10 UL-1) decreased the mean contractile amplitude and the contractile rate of recurrence of CM and LM (Number ?(Number1,1, Number ?Number2).2). It experienced no significant effects within AZ3451 the resting pressure of CM and LM. Open in a separate window Number 1 A: Effect of oxytocin within the mean contractile amplitude of longitudinal muscle mass (LM) of proximal colon in rabbits. B: Effect of oxytocin.