Arachidonic Acid Activates the Noncapacitative Entry of Ca2+ during [Ca2+]i Oscillations

Trevor J Shuttleworth

doi:10.1074/jbc.271.36.21720

Abstract

Current models for agonist-activated Ca2+ entry in nonexcitable cells focus on the capacitative mechanism where entry is activated as a downstream result of the sustained depletion of agonist-sensitive stores without any direct requirement for inositol phosphates. This mechanism has been shown to be important for the sustained Ca2+ signals seen in a variety of nonexcitable cells under conditions of maximal stimulation. In contrast, relatively little attention has been given to Ca2+ entry under more physiological levels of agonist where, for example, oscillating Ca2+ responses are common. In recent studies using cells from the exocrine avian nasal gland, we have shown that agonist-activated Ca2+ entry under these conditions demonstrates properties that are inconsistent with current versions of the capacitative model. We now report that activation of this novel noncapacitative Ca2+ entry is via a distinct signaling pathway involving an agonist-induced, phospholipase A2-mediated generation of arachidonic acid.

Highlights

The agonist-induced entry of Ca2ϩ from the extracellular medium is a key component in the cytosolic Ca2ϩ signals that link activation of various receptors on the cell surface with the initiation and control of cell function
The addition of carbachol will result in the elevation of [Ca2ϩ]i, and this could activate a phospholipase A2 (PLA2) resulting in the release of arachidonic acid
As further evidence for a role in the inhibition of PLA2, we found that the inhibition of [Ca2ϩ]i oscillations induced by isotetrandrine could be reversed by simple addition of low concentrations of exogenous arachidonic acid (2–5 ␮M) despite the continued presence of isotetrandrine (Fig. 6A)

Summary

Introduction

The agonist-induced entry of Ca2ϩ from the extracellular medium is a key component in the cytosolic Ca2ϩ signals that link activation of various receptors on the cell surface with the initiation and control of cell function. Relatively little attention has been given to Ca2ϩ entry under more physiological levels of agonist where, for example, oscillating Ca2ϩ responses are common [8, 9] Current models for these phenomena generally emphasize special properties and modifications of inositol 1,4,5-trisphosphate (InsP3)1-induced Ca2ϩ release, and Ca2ϩ entry is assumed to play only a relatively minor role. This assumption is contradicted by our recent findings using cells from the exocrine avian nasal gland where we showed that such patterns of [Ca2ϩ]i signaling do not depend exclusively on the generated levels of InsP3 and, critically, on the activation of Ca2ϩ entry which acts by regulating the InsP3-mediated liberation of intracellular Ca2ϩ from agonist-sensitive stores [10, 11]. As carbacholinduced [Ca2ϩ]i oscillations in these cells require both the generation of InsP3 and the arachidonate-mediated Ca2ϩ entry, we conclude that the successful initiation and maintenance of an oscillatory [Ca2ϩ]i response involves a combined and coordinated activation of a PLC and a PLA2

Methods

Results

Conclusion