H-89 dihydrochloride hydrate is widely used as a selective inhibitor of protein kinase A (PKA) in cellular and molecular biology research. It is commonly applied in studies investigating cAMP-dependent signaling pathways, helping researchers understand the role of PKA in processes such as cell proliferation, metabolism, gene expression, and apoptosis. Due to its ability to cross cell membranes, it is effective in cell-based assays where modulation of PKA activity is required. It is also used to examine cardiac function, neuronal signaling, and hormone action, where PKA plays a regulatory role. Caution is advised in interpretation of results, as high concentrations may affect other kinases.