Abstract: The goal of this study was to find out, whether Cx43 and Cx40 expression may be chronically regulated via adenylyl cyclase (AC)/cAMP/protein kinase A (PKA) and protein kinase C (PKC) pathways. Therefore confluent cultures of neonatal rat cardiomyocytes were incubated 24 h with various concentrations of either dibutyryl-cAMP (10-1000 μM), forskolin (0,1-10 μM) or the protein kinase C (PKC) activator phorbol-12,13-didecanoate (PDD) (1 nM-10 μM) in absence or in presence of the p38-MAP-kinase inhibitor SB203580 (10 -5 M) or the MEK1-inhibitor PD98059 (10 -5 M). Connexin 40 and 43 expression was investigated by Western blotting and PCR. Incubation with either db-cAMP, forskolin or PDD led to a dose-dependent significant (p<0.05) increase in Cx43 expression as revealed by Western blot. These results were also confirmed by PCR studies indicating de-novo synthesis of Cx43. In contrast Cx40 expression was not altered by activation of the PKA pathway. However the PKC activator PDD enhanced both connexins Cx43 and Cx40 on the protein level. In additional PCR experiments only the PDD-induced Cx43 increase was associated with an increase in Cx43-mRNA, whereas the Cx40-mRNA remained unchanged. The increases in Cx43 protein content could be completely suppressed by SB203580 (p<0.05) but not by PD98059. In absence of a stimulating drug, these inhibitors (SB203580 or PD98059) did not affect Cx43 content. In PCR studies the increase in Cx43-mRNA under the influence of db-cAMP, forskolin or PDD could also be completely suppressed by SB203580. From these results it can be concluded, that (a) Cx43 and Cx40 are differentially regulated and (b) the Cx43 expression can be regulated via AC/cAMP/PKA and PKC dependent pathways, (c) from the PCR results that the up-regulation in Cx43 seems to be attributable to enhanced de-novo synthesis and that (d) the activation of p38 MAP kinase is a common pathway for regulation of Cx43 expression in rat cardiomyocytes.