Excitatory nerve stimulation and agonist stimulation induce gastric fundus smooth muscle contraction via stimulus dependent Ca2+ sensitization pathways‐not via myosin light chain phosphorylation

Abstract

Ca2+ sensitization of smooth muscle contraction occurs by myosin light chain phosphatase (MLCP) inhibition. CPI‐17 phosphorylation or MYPT‐1 phosphorylation by Rho kinase (ROK) inhibits MLCP. Studies have shown that bath application of acetylcholine (Ach) induces Ca2+ sensitization in the gastrointestinal smooth muscles. We addressed a question; does excitatory nerve stimulation also cause Ca2+ sensitization in fundus smooth muscle? We stimulated fundus smooth muscle at a series of frequencies and determined the phosphorylated levels of MYPT‐1 at Thr 696 and 853 (pT696 and pT853), CPI‐17 (pT38) and LC‐20 at Ser 19 (pS19) by western blotting at different phases of contractions. Nerve stimulation robustly increased levels of pT38 but not pT696, pT853, and pS19. In contrast, bath application of Ach increased phosphorylation of pT696, pT853, and pT38, but not pS19. The L‐type Ca2+ channel blocker nicardipine completely blocked Ach induced contraction with a partial block in the increase of pT696 (but not pT853) and pT38 levels. The ROK inhibitor SAR1x partially blocked Ach induced contraction with a complete block of the increase in pT853 (but not pT696) and a partial block of the increase in pT38. At present, we conclude that excitatory nerve stimulation induces Ca2+ sensitization in gastric fundus smooth muscle by a different pathway than bath application of Ach. Supported by NIH grant RR018751 and DK40569.