PRKD1 and hydrops fetalis: By exploiting this culturing method, the following important findings were made in adult mouse SAN cells: (1) discovery of β-AR-mediated discrete cAMP pools consistent with compartmentalization; (2) detection and examination of cGMP production and activity of PKA, CaMKII, and PKD, thus facilitating the study of signaling networks; and (3) reduction in cAMP and cGMP levels that could contribute to SAN dysfunction during HF.