The Functional Role of Protease-Activated Receptors on Contractile Responses by Activation of Ca 2+ Sensitization Pathways in Simian Colonic Muscles.

It has been known that activation of protease-activated receptors (PAR) affects gastrointestinal motility. In this study, we tested the effects of PAR agonists on electrical and contractile responses, and Ca2+ sensitization pathways in simian colonic muscles. The Simian colonic muscle was initially hyperpolarized by PAR agonists. After the transient hyperpolarization, simian colonic muscle repolarized to the control resting membrane potential (RMP) without a delayed depolarization. Apamin significantly reduced the initial hyperpolarization suggesting that activation of small conductance Ca2+ -activated K+ (SK) channels is involved in the initial hyperpolarization. In contractile experiments, PAR agonists caused an initial relaxation followed by an increase in contractions. These delayed contractile responses were not matched with the electrical responses that showed no after depolarization of the RMP. To investigate the possible involvement of ROCK pathways in the PAR effects, muscle strips were treated with ROCK inhibitors, which significantly reduced the PAR agonist-induced contractions. Furthermore, PAR agonists increased MYPT1 phosphorylation and ROCK inhibitors completely blocked MYPT1 phosphorylation. PAR agonists alone had no effect on CPI-17 phosphorylation. In the presence of apamin, PAR agonists significantly increased CPI-17 phosphorylation which was blocked by protein kinase C (PKC) inhibitors suggesting that Ca2+ influx is increased by apamin and is activating PKC. In conclusion, these studies show that PAR activators induce biphasic responses in simian colonic muscles. The initial inhibitory responses by PAR agonists are mainly mediated by activation of SK channels and delayed contractile responses are mainly mediated by the CPI-17 and ROCK Ca2+ sensitization pathways in simian colonic muscles.