Intact HEK293 cells and B103 neuroblastoma cells possess high basal concentrations of the established second messengers cAMP and cGMP and of the emerging second messengers cCMP and cUMP. We asked the question which nucleotidyl cyclase accounts for the high basal cNMP concentrations. Activators and inhibitors of soluble guanylyl cyclase had no major effects on cNMPs, and the activator of membranous adenylyl cyclase forskolin increased only cAMP. Addition of bicarbonate to medium increased, whereas removal of bicarbonate decreased levels of all four cNMPs. The inhibitor of soluble adenylyl cyclase, 2-(1H-benzo[d]imidazol-2-ylthio)-N'-(5-bromo-2-hydroxybenzylidene) propanehydrazide (KH7), reduced bicarbonate-stimulated cNMPs. In conclusion, bicarbonate-stimulated soluble adenylyl cyclase plays an important role in the regulation of basal cellular cNMP levels, most notably cCMP and cUMP. (C) 2014 Elsevier Inc. All rights reserved.