Activation of gene expression by the cAMP-dependent signaling pathway is regulated by members of the cAMP response element binding protein (CREB) family consisting of CREB, CREM, and ATF-1. It is decisively for the understanding of the heart function as to which type of heart cells expresses CREB and/or CREM. Ventricular myocytes and fibroblasts of young (3 months) and old (24 months) rat hearts were separately investigated to analyse CREB, CREM, and phospho-CREB. Western blot showed CREB expression exclusively in fibroblasts but CREM was predominantly detected in ventricular myocytes. CREB-positive nuclei in heart sections were only revealed in fibroblasts. CREB was activated by forskolin (10 mu M), PMA (500 nM), and cyclical mechanical strain (1 Hz, 5% elongation) in fibroblasts. The number of CREB-positive myocytes in old rats was larger than in young rats. But CREB could not be activated by forskolin (10 mu M) in all myocytes. Our results suggest that the expression of CREB depends on the cell type and the age of the animal. We discuss that modulation of gene expression as it occurs with a age could be affected by the change within the CREB family members. (c) 2005 Elsevier Inc. All rights reserved.