The Effect And Mechanism Of Botulinum Toxin Type A On Endo- And Exo-genous Substance P-induced Contractile Response In Pyloric Smooth Muscle In Rats In Vitro

Objective:To investigate the effect and mechanism of botulinum toxin type A (BTX-A)on endo- and exo-genous substance P(SP)-induced contractility in the pyloric muscle strips in rats.Methods:(1)Electrical field stimulation(EFS)was used to induce contraction of pyloric circular muscle strips incubated in Krebs bicarbonate buffer via enhancing neurotransmitter release.An antagonist of NK1-receptor([D-Arg¹,D-Phe⁵,O-Trp^7,9, Leu¹¹]-substance P,1μmol.L^-1,DDDL-SP,n=10)or BTX-A(10 U·mL^-1,n=10)was respectively administrated following atropine(1μmol·L^-1)treatment for observing these inhibitory effect on endogenous SP-induced contractibility.(2)SP(1μmol·L^-1) was added once every 30 min into organ bath in which pyloric circular muscle strips were suspended and contained the different concentration of BTX-A(4 U·mL^-1,n=8 and 10 U·mL^-1,n=8)in Krebs bicarbonate buffer for 4 hours in order to estimate the inhibitory effects of BTX-A on exogenous SP-induced contractile response.Results:(1)Pyloric strips contractility was enhanced including the amplitude(P＜0.05),frequency and tension(P＜0.01)by EFS.The EFS-induced pyloric contractile response was inhibited partly by atropine,an antagonist of cholinergic muscarinic receptor.The residual contractility in pyloric muscle strips was further decreased by subsequent administration of DDDL-SP(1μmol·L^-1),an antagonist of NK₁-receptor, or BTX-A(10 U·mL^-1).(2)The pyloric muscle strips were incubated in BTX-A with low concentration(4 U·mL^-1)or higher concentration(10 U·mL^-1),and SP(1μmol·L^-1)was added once every 30 min during 4 hours.SP-induced contractile response was gradually decreased along with time in two groups.In addition,the decrease volumes of SP-induced contractile tension between two groups of BTX-A were significantly different at the 4^thhour,the inhibitory ratios were 73.36±11.46%in low dose group(P=0.011＜0.05)and 25.09±8.08%in high dose group(P=0.000＜0.01)respectively compared to SP-induced contractile tension without BTX-A.Conclusion:EFS-induced contraction in pyloric muscle strips is partly suppressed by atropine,residual contractile waves are similarly inhibited by DDDL-SP or BTX-A.These results suggest that EFS induces ACh and endo-genous SP release and pyloric contraction is the endo-genous SP-induced contractily is inhibited by BTX-A. Exo-genous SP-induced contractility of the pyloric smooth muscle strips is inhibited by BTX-A with a time- and dose-dependent.