Role of CPI‐17 phosphorylation in calcium sensitization in smooth muscle of small intrapulmonary airways

Abstract

Phosphorylation of protein‐kinase C (PKC)‐potentiated myosin phosphatase inhibitor (CPI‐17) plays a role in Ca2+ sensitization in different smooth muscle cell (SMC) types; however its role in small intrapulmonary airways is mostly unknown. We tested the hypothesis that Ca2+ sensitization is accompanied by CPI‐17 phosphorylation. We used phase‐contrast microscopy and Western blots to assess airway contraction, Ca2+ sensitivity and protein phosphorylation in mouse lung slices. We found that stimulation of lung slices with PKC activator phorbol‐12‐myristate‐13‐acetate (PMA, 10 μM) induced strong airway contraction in Ca2+ permeabilized airways due to Ca2+ sensitization. This contraction was reverted by addition of a selective PKC inhibitor (GF109203X, 1 μM) but not by a ROK inhibitor (Y‐27632, 10 μM). Phosphorylation of CPI‐17 increased after stimulation with PMA and CPI‐17 dephosphorylation was observed after subsequent addition of GF109203X. Furthermore, acetylcholine (ACh, 0.5 μM) and serotonin (5‐HT, 0.5 μM) induced strong airway contraction in Ca2+ permeabilized airways which was resistant to GF109203X and blocked by Y‐27632. Phosphorylation of CPI‐17 did not increase significantly after ACh or 5‐HT stimulation. Our results suggest that CPI‐17 phosphorylation is an important mechanism of Ca2+ sensitization but 5‐HT and ACh do not use this pathway in airway SMCs.