Sarcoplasmic Reticulum Ca2+ Reuptake and Airway Smooth Muscle Inflammation

Abstract

Intracellular Ca2+ ([Ca2+]i) levels are increased in inflamed airway smooth muscle (ASM). Sarcoplasmic reticulum (SR) Ca2+ release and reuptake are key components of ASM [Ca2+]i regulation. Ca2+ reuptake occurs via the ATPase SERCA. We hypothesized that inflammation disrupts SERCA function, contributing to increased [Ca2+]i. We examined SERCA expression and regulation in human ASM exposed to pro‐inflammatory cytokines TNFα and IL‐13. Western blot analysis showed significantly decreased SERCA expression following overnight cytokine exposure. In other tissues, SERCA is regulated by the inhibitory protein phospholamban (PLB). We found that human ASM does not express PLB protein (although mRNA is detectable). Cytokine exposure did not induce PLB expression, raising the issue of how SERCA is regulated. We then found that direct SERCA phosphorylation (potentially via CaMKII) occurs in human ASM. In fura‐2 loaded human ASM cells, we examined [Ca2+]i responses to ACh or bradykinin (zero extracellular Ca2+) and found significant reduction in rates of fall of [Ca2+]i transients in cells exposed to TNF‐α or IL‐13. We further found that CaMKII antagonist KN‐93 significantly slowed the fall of [Ca2+]i transients in both controls and cytokine‐exposed human ASM cells. These data suggest that a) SERCA in human ASM is regulated by mechanisms such as CaMKII; and b) airway inflammation decreases SR refilling by blunting SERCA expression and function.Supported by NIH grants HL74309, the Flight Attendants Medical Research Institute (FAMRI), and the Department of Anesthesiology, Mayo Clinic.