PKA Enhances the Sensitivity of IP3 Receptors in Swine Airway Submucosal Gland Cells.

Abstract

Increased sensitivity of airway submucosal gland cells (SMGC) to ACh by PKA activation results in increased transepithelial ion movement. The mechanism of the increased sensitivity induced by PKA is unclear. Since ACh‐induced transepithelial ion movement is dependent on the IP3/Ca2+ signaling pathway, we hypothesized that PKA enhances sensitivity of IP3 receptors in airway SMGC, resulting in increased [Ca2+]i. Patch clamp of swine airway SMGC was performed in these studies. We showed that forskolin decreased the EC50 for IP3 about 4‐fold measured as the peak KCa current induced upon whole cell formation with IP3 in the pipette. VIP (10 nM), a co‐neurotransmitter present in cholinergic airway nerve fibers, enhanced the response to 0.3 µM IP3. Neither VIP nor IP3 (0.3 µM) induced KCa current when applied alone. VIP induced a significant KCa current in SMGC only when the internal solution contained IP3. Removing external Ca2+ did not abolish the VIP‐induced sensitization of the peak IP3 response although the plateau response was lost. This suggests that Ca2+ release is sensitized as well as Ca2+ influx. Elevating external Ca2+ to 5 mM revealed a KCa current induced only in the presence of IP3 (0.3 µM). Thus Ca2+ entry may also be increased by IP3 (0.3 µM). These results suggest that PKA increases the sensitivity of IP3 receptors by phosphorylation, resulting in the increased intracellular Ca2+ release and Ca2+ entry.