Mechanism of action of cholera toxin on the opossum internal anal sphincter smooth muscle

Lack of nitrate tolerance in isosorbide dinitrate- and sodium nitroprusside-induced relaxation of rabbit internal anal sphincter

AIM: To investigate the tolerance development against the relaxant effect of nitric oxide donating drug isosorbide dinitrate (ISDN) and sodium nitroprusside (SNP) in internal anal sphincter (IAS) smooth muscle.

METHODS: Relaxation responses of ISDN, and electrical field stimulation (EFS) were obtained before and after tolerance induction by ISDN incubation.

RESULTS: ISDN (10^-7-10^-4 mol/L) and SNP (10^-8-10^-4 mol/L) caused a concentration-dependent relaxation on the basal tonus of the isolated rabbit IAS strips. After a period of 2 h incubation of the 6 x 10^-4 mol/L ISDN the relaxation effects of ISDN and SNP did not change compared to control strips. EFS evoked frequency-dependent relaxation in internal anal sphincter smooth muscle and E_max obtained from control strips were not changed in ISDN tolerance-inducing condition. In this study nitrate tolerance was not observed in rabbit IAS smooth muscle.

CONCLUSION: This result shows that nitric oxide donating drugs relaxes the internal anal sphincter of the rabbits without the development of tolerance.

Cellular regulation of basal tone in internal anal sphincter smooth muscle by RhoA/ROCK

Sustained contractions of smooth muscle cells (SMC) maintain basal tone in the internal anal sphincter (IAS). To examine the molecular bases for the myogenic tone in the IAS, the present studies focused on the role of RhoA/ROCK in the SMC isolated from the IAS vs. the adjoining phasic tissues of the rectal smooth muscle (RSM) and anococcygeus smooth muscle (ASM) of rat. We also compared cellular distribution of RhoA/ROCK, levels of RhoA-GTP, RhoA-Rho guanine nucleotide dissociation inhibitor (GDI) complex formation, levels of p(Thr696)-MYPT1, and SMC relaxation caused by RhoA inhibition. Levels of RhoA/ROCK were higher at the cell membrane in the IAS SMC compared with those from the RSM and ASM. C3 exoenzyme (RhoA inhibitor) and Y 27632 (ROCK inhibitor) caused a concentration-dependent relaxation of the IAS SMC. In addition, active ROCK-II (primary isoform of ROCK in SMC) caused further shortening in the IAS SMC. C3 exoenzyme increased RhoA-RhoGDI binding and reduced the levels of RhoA-GTP and p(Thr696)-MYPT1. ROCK inhibitor attenuated PKC-induced contractions in IAS SMC. Conversely, a PKC inhibitor (Gö 6850, which causes partial relaxation of the SMC) had no significant effect on ROCK-II-induced contractions. Further experiments showed the highest levels of RhoA, active form of RhoA (RhoA-GTP), ROCK-II, 20-kDa myosin regulatory light chain (MLC(20)), phospho-MYPT1, and phospho-MLC(20) in the IAS vs. RSM and ASM SMC. However, the trend was the reverse with the levels of inactive RhoA (GDP-RhoA-RhoGDI complex) and MYPT1. We conclude that RhoA/ROCK play a critical role in maintenance of spontaneous tone in the IAS SMC via inhibition of myosin light chain phosphatase.

Involvement of rho and rho-associated kinase in sphincteric smooth muscle contraction by angiotensin II

The tonic smooth muscles of lower esophageal sphincter (LES) and internal anal sphincter (IAS) are subject to modulation by the neurohumoral agents. We report that angiotensin (Ang) II-induced contraction of rat IAS and LES smooth muscle cells (SMC) was inhibited by Clostridium botulinum C3 exozyme, HA 1077 and Y 27632, suggesting a role for Rho kinase and a Rho-associated kinase (ROK). Ang II-induced contraction of the SMC was also attenuated by genistein, antibodies to the pp60(c-src), p(190) RhoGTPase-activating protein (p190 RhoGAP), carboxyl terminus of Galpha13, carboxyl terminus peptide, and ADP ribosylation factor (ARF) antibody. Ang II-induced increase in p(190) RhoGAP tyrosine phosphorylation was attenuated by genistein. Furthermore, Ang II-induced increase in smooth muscle tone and phosphorylation of myosin light chain (MLC; 20 kDa; MLC20-P) were attenuated by Y 27632 and genistein. The results suggest an important role for Galpha13 and pp60(c-src) in the intracellular events responsible for the activation of RhoA/ROK in Ang II-induced contraction of LES and IAS SMC.

Role of pp60(c-src) and p(44/42) MAPK in ANG II-induced contraction of rat tonic gastrointestinal smooth muscles

We examined the role of mitogen-activated protein kinase (p(44/42) MAPK) in ANG II-induced contraction of lower esophageal sphincter (LES) and internal anal sphincter (IAS) smooth muscles. Studies were performed in the isolated smooth muscles and cells (SMC). ANG II-induced changes in the levels of phosphorylation of different signal transduction and effector proteins were determined before and after selective inhibitors. ANG II-induced contraction of the rat LES and IAS SMC was inhibited by genistein, PD-98059 [a specific inhibitor of MAPK kinases (MEK 1/2)], herbimycin A (a pp60(c-src) inhibitor), and antibodies to pp60(c-src) and p(120) ras GTPase-activating protein (p(120) rasGAP). ANG II-induced contraction of the tonic smooth muscles was accompanied by an increase in tyrosine phosphorylation of p(120) rasGAP. These were attenuated by genistein but not by PD-98059. ANG II-induced increase in phosphorylations of p(44/42) MAPKs and caldesmon was attenuated by both genistein and PD-98059. We conclude that pp60(c-src) and p(44/42) MAPKs play an important role in ANG II-induced contraction of LES and IAS smooth muscles.

Animal model for angiotensin II effects in the internal anal sphincter smooth muscle: mechanism of action

Effect of ANG II was investigated in in vitro smooth muscle strips and in isolated smooth muscle cells (SMC). Among different species, rat internal and sphincter (IAS) smooth muscle showed significant and reproducible contraction that remained unmodified by different neurohumoral inhibitors. The AT(1) antagonist losartan but not AT(2) antagonist PD-123319 antagonized ANG II-induced contraction of the IAS smooth muscle and SMC. ANG II-induced contraction of rat IAS smooth muscle and SMC was attenuated by tyrosine kinase inhibitors genistein and tyrphostin, protein kinase C (PKC) inhibitor H-7, Ca(2+) channel blocker nicardipine, Rho kinase inhibitor Y-27632 or p(44/42) mitogen-activating protein kinase (MAPK(44/42)) inhibitor PD-98059. Combinations of nicardipine and H-7, Y-27632, and PD-98059 caused further attenuation of the ANG II effects. Western blot analyses revealed the presence of both AT(1) and AT(2) receptors. We conclude that ANG II causes contraction of rat IAS smooth muscle by the activation of AT(1) receptors at the SMC and involves multiple intracellular pathways, influx of Ca(2+), and activation of PKC, Rho kinase, and MAPK(44/42).