Prostaglandin E2 Inhibits Histamine-Evoked Ca2+ Release in Human Aortic Smooth Muscle Cells through Hyperactive cAMP Signaling Junctions and Protein Kinase A

02 Nov 2017

In human aortic smooth muscle cells (ASMC), prostaglandin E2 (PGE2) stimulates adenylyl cyclase (AC) and attenuates the increase in intracellular free Ca2+ concentration ([Ca2+]i) evoked by activation of histamine H1 receptors. The mechanisms are not resolved. We show that cAMP mediates inhibition of histamine-evoked Ca2+ signals by PGE2. Exchange proteins activated by cAMP (EPACs) were not required, but the effects were attenuated by inhibition of cAMP-dependent protein kinase (PKA). PGE2 had no effect on the Ca2+ signals evoked by protease-activated receptors, heterologously expressed muscarinic M3 receptors, or by direct activation of inositol 1,4,5-trisphosphate (IP3) receptors by photolysis of caged IP3. The rate of Ca2+ removal from the cytosol was unaffected by PGE2, but PGE2 attenuated histamine-evoked IP3 accumulation. Substantial inhibition of AC had no effect on the concentration-dependent inhibition of Ca2+ signals by PGE2 or butaprost (to selectively activate EP2 receptors), but it modestly attenuated responses to EP4 receptors, activation of which generated less cAMP than EP2 receptors. We conclude that inhibition of histamine-evoked Ca2+ signals by PGE2 occurs through ‘hyperactive signalling junctions’, wherein cAMP is locally delivered to PKA at super-saturating concentrations to cause uncoupling of H1 receptors from phospholipase C. This sequence allows digital signalling from PGE2 receptors, through cAMP and PKA, to histamine-evoked Ca2+ signals.