Heterotrimeric G proteins are comprised of a guanine nucleotide binding Got subunit and the G beta gamma dimers that link G protein-coupled receptors (GPCRs) to effectors. This study focuses on the expression and localization patterns for certain G beta and G gamma subunits in neonatal and adult cardiomyocytes. We identify developmental downregulation of G beta 1, G beta 2, and G gamma 2, and a switch in the molecular form of G gamma 3, in cardiomyocytes. G beta 1 is highly localized to caveolae membranes, whereas G 2 is identified in caveolae and other membrane fractions. G beta 3 is not detected in neonatal cardiomyocytes, but rather G beta 3 is upregulated in adult cardiomyocytes and detected in the caveolae and soluble fractions. The observation that cardiomyocytes co-express multiple G beta and G gamma subunits in a developmentally regulated manner, and that these G beta and G gamma subunits assume distinct subcellular localization patterns, provides for a high level of signaling specificity in the heart. (c) 2008 Elsevier Inc. All rights reserved.