Protein kinase A (PKA) is activated during sympathetic stimulation of the heart and phosphorylates key proteins involved in cardiac Ca 2+ handling, including the L-type Ca 2+ channel (Ca V 1.2) and phospholamban (PLN). This results in acceleration and amplification of the beat-to-beat changes in cytosolic Ca 2+ in cardiomyocytes and, in turn, an increased rate and force of contraction. PKA is held in proximity to its substrates by protein scaffolds called A kinase anchoring proteins (AKAPs). It has been suggested that the short and long isoforms of AKAP7 (also called AKAP15/18) localize PKA in complexes with Ca V 1.2 and PLN, respectively. We generated an AKAP7 KO mouse in which all isoforms were deleted and tested whether Ca 2+ current, intracellular Ca 2+ concentration, or Ca 2+ reuptake were impaired in isolated adult ventricular cardiomyocytes following stimulation with the β-adrenergic agonist isoproterenol. KO T he key determinants of cardiac output-the force of contraction and rate of relaxation-are rooted in the amplitude and kinetics of Ca 2+ transients that occur in individual cardiomyocytes. Adrenergic stimulation initiates cAMP-dependent signaling pathways that activate PKA leading to phosphorylation of numerous proteins that are critical for Ca 2+ entry, release, and reuptake, as well as sarcomeric proteins more closely associated with contraction, such as myosin-binding protein C and troponin I. This phosphorylation amplifies Ca 2+ influx through voltagegated Ca 2+ channels (Ca V 1.2 in the ventricle) and the corresponding increase in Ca 2+ -induced Ca 2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptors augments contractility. Equally important is the enhanced removal of Ca 2+ from the cytosol that allows the heart to relax more quickly during diastole, which is accomplished primarily by phosphorylating phospholamban (PLN), which in turn relieves PLN inhibition of the sarcoplasmic reticulum Ca 2+ ATPase (SERCA). Distinct, localized actions of PKA are coordinated in two ways: (i) cAMP production and hydrolysis are restricted by the subcellular localization of cyclases and phosphodiesterases, respectively, and (ii) PKA is directed to specific subcellular sites by binding to an assortment of protein scaffolds known as A kinase anchoring proteins (AKAPs) (1). By directing PKA to specific subcellular sites, AKAPs determine not only the specificity of protein phosphorylation, but also the speed with which these systems respond to adrenergic stimulation. Some AKAPs are implicated in clinically relevant cardiac signaling events (2-4). For example, regulation of potassium channel current in the heart depends on formation of complexes containing AKAP9 (yotiao), PKA, and the I Ks potassium channel α subunit (KCNQ1); an inherited single point mutation in AKAP9 impairs AKAP9-KCNQ1 interaction and ultimately leads to long QT syndrome (4).AKAP7 is expressed as a family of alternatively spliced anchoring proteins that bind all isoforms of PKA regulatory subunit, albeit with different aff...