Mechanisms of Angiotensin II–Mediated Decreases in Intraneuronal Ca 2+ in Calcium-Loaded Stellate Ganglion Neurons

Abstract

Our laboratory has reported previously that angiotensin II, type-1 (AT(1)) receptor stimulation in isolated stellate ganglion neurons decreases intraneuronal calcium concentration ([Ca(2+)]i) acutely if baseline [Ca(2+)]i is high and increases [Ca(2+)]i if baseline [Ca(2+)]i is low. Part of the angiotensin II (Ang II) effect in high Ca(2+) neurons is mediated through stimulation of Na(+)-Ca(2+) exchange. Current experiments were conducted to identify additional steps in the signaling pathways. In Ca(2+)-loaded neurons, Ang II-induced decreases in [Ca(2+)]i were attenuated by phospholipase C inhibition (U73122) or nitric oxide (NO) synthase inhibition (L-NMMA) and were mimicked by the cGMP analogue 8-Br-cGMP. Protein kinase C (PKC) inhibition (bisindolylmaleimide I or Go6976) and protein kinase G (PKG) inhibition (KT5823) partially blocked Ang II-mediated decreases in [Ca(2+)]i, but complete blockade of Ang II effects was obtained with combined PKC and PKG inhibition. Modulation of inositol triphosphate (IP(3))-inducible ER Ca(2+) release by [Ca(2+)]i was investigated using furaptra, an ER-retaining dye. IP(3)-mediated ER Ca(2+) release in beta-escin-permeabilized neurons was measured after clamping of [Ca(2+)]i from 50 nM to 800 nM. Maximal ER Ca(2+) release was observed at approximately 200 nM [Ca(2+)]i, with noted blunting of release at higher [Ca(2+)]i. Steady-state mRNA transcript and protein levels revealed that the principal IP(3)R isoform expressed was IP(3)R-II. These results suggest that Ca(2+) loading in stellate ganglion neurons promotes Ang II-mediated decreases in [Ca(2+)]i via PKC and NO/cGMP/PKG pathways and inhibits IP(3)R-II-mediated ER Ca(2+) release.