The cyclic purine nucleotides cAMP and cGMP are well-characterized second messengers and activators of PKA and PKG, respectively. In contrast, the functions of the cyclic pyrimidine nucleotides cCMP and cUMP are poorly understood. cCMP induces relaxation of smooth muscle via PKGI, and phosphodiesterases differentially hydrolyze cNMPs. Here, we report that cNMPs differentially activate PKA isoforms and PKGI alpha. The combination of cCMP with cAMP reduced the EC(50) of cAMP for PKA. PKGI alpha exhibited higher specificity for the cognate cNMP than PKA. Our data support a role of cCMP and cUMP as second messengers. (C) 2011 Elsevier Inc. All rights reserved.