Bicarbonate Regulation of Intracellular cAMP in Pulmonary Endothelial Cells

Abstract

Transmembrane adenylyl cyclases can be stimulated by isoproterenol or forskolin to generate a subplasma membrane cAMP pool in pulmonary endothelial cells. In contrast, toxic bacterial adenylyl cyclases are injected into endothelial cells and require a eukaryotic factor to generate a cytosolic cAMP pool. Recently, a mammalian adenylyl cyclase has been described which lacks transmembrane domains, is insensitive to heterotrimeric G‐protein modulation, and is uniquely activated by bicarbonate and calcium to generate cAMP. We sought to determine whether this bicarbonate‐sensitive adenylyl cyclase is expressed in pulmonary endothelial cells, and whether the enzyme contributes to baseline cAMP concentrations. Using RT‐PCR we demonstrated the endogenous expression of this bicarbonate‐sensitive soluble adenylyl cyclase in pulmonary microvascular and pulmonary artery endothelial cells. Utilizing a bicarbonate free system we determined that activity of this soluble adenylyl cyclase contributes to basal cAMP levels. Further, acetazolamide inhibition of bicarbonate synthesis leads to a dose dependent decrease in basal cAMP levels in pulmonary microvascular endothelial cells. Therefore, bicarbonate stimulation of the soluble adenylyl cyclase contributes to the basal cAMP levels in pulmonary endothelial cells. Supported by HL66299