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β1-adrenergic receptor (β1AR) stimulation activates the classic cAMP/protein kinase A (PKA) pathway to regulate vital cellular processes from the change of gene expression to the control of metabolism, muscle contraction, and cell apoptosis. Here we show that sustained β1AR stimulation promotes cardiac myocyte apoptosis by activation of Ca2+/calmodulin kinase II (CaMKII), independently of PKA signaling. β1AR-induced apoptosis is resistant to inhibition of PKA by a specific peptide inhibitor, PKI14-22, or an inactive cAMP analogue, Rp-8-CPT-cAMPS. In contrast, the β1AR proapoptotic effect is associated with non–PKA-dependent increases in intracellular Ca2+ and CaMKII activity. Blocking the L-type Ca2+ channel, buffering intracellular Ca2+, or inhibiting CaMKII activity fully protects cardiac myocytes against β1AR-induced apoptosis, and overexpressing a cardiac CaMKII isoform, CaMKII-δC, markedly exaggerates the β1AR apoptotic effect. These findings indicate that CaMKII constitutes a novel PKA-independent linkage of β1AR stimulation to cardiomyocyte apoptosis that has been implicated in the overall process of chronic heart failure

␤ 1 and ␤ 2 adrenergic receptors (AR) regulate the intrinsic contraction rate in neonatal mouse cardiac myocytes through distinct signaling pathways. It has been shown that stimulation of ␤ 1 ARs leads to a protein kinase A-dependent increase in contraction rate. In contrast, stimulation of ␤ 2 ARs has a biphasic effect on contraction rate, with an initial protein kinase A-independent increase followed by a sustained decrease that is blocked by pertussis toxin. The ␤ 2 AR undergoes agonist-induced endocytosis in cardiac myocytes while the ␤ 1 AR remains on the cell surface. It has been shown that a PDZ domain binding motif at the carboxyl terminus of ␤ 1 AR interacts with the postsynaptic density protein PSD-95 when both are expressed in HEK293 cells. We found that mutation of this PDZ binding motif in the ␤ 1 AR (␤ 1 AR-PDZ) enabled agonist-induced internalization in cardiac myocytes. Moreover, stimulation of ␤ 1 AR-PDZ had a biphasic effect on the myocyte contraction rate similar to that observed following stimulation of the ␤ 2 AR. The secondary decrease in the contraction rate was mediated by G i and could be blocked by pertussis toxin. Furthermore, a non-selective endocytosis inhibitor, concanavalin A, inhibited the internalization of wild type ␤ 2 AR and the mutated ␤ 1 AR-PDZ, and blocked the coupling of both receptors to G i . Finally, treating myocytes with a membrane-permeable peptide representing ␤ 1 AR PDZ motif caused the endogenous ␤ 1 AR to behave like ␤ 1 AR-PDZ. These studies suggest that association of the ␤ 1 AR with PSD-95 or a related protein dictates signaling specificity by retaining the receptor at the cell surface and preventing interaction with G i .␤ adrenergic receptors (␤ 1 AR, ␤ 2 AR, and ␤ 3 AR) 1 are heptahelical G protein-coupled receptors (GPCRs) that mediate physiological responses to the hormone epinephrine and the neurotransmitter norepinephrine. Both ␤ 1 and ␤ 2 ARs are expressed in the heart and play critical roles in regulating cardiac function in animals (1). In vitro studies show that ␤ 1 and ␤ 1 ARs have very similar signaling properties when expressed in undifferentiated cell lines (2). However, evidence also suggests that they have different signaling properties in differentiated cells in vivo (3, 4). Moreover, these receptors may differ in other functional parameters such as desensitization (5). ␤ 1 AR knockout mice lack the normal inotropic and chronotropic response to the adrenergic agonist, isoproterenol (6), whereas these responses are preserved in ␤ 2 AR knockout mice (7).We recently reported that ␤ 1 and ␤ 2 ARs regulate contraction rate in mouse neonatal myocytes through different signal transduction pathways (8). Activated ␤ 1 AR couples only to G s and leads to a PKA-dependent increase in contraction rate. Activated ␤ 2 AR undergoes sequential coupling to G s and G i and has a biphasic effect on contraction rate, with an initial PKAindependent increase followed by a sustained decrease that can be blocked by pertussis toxin (8). Recent biochemical evidence su...

In order to determine whether G protein-coupled receptors play a role in early embryogenesis, we looked for cDNA fragments amplified between primers located in consensus sequences of transmembrane segments. Using one such amplified fragment as a probe, we cloned a novel member of the G protein-coupled receptor superfamily in Xenopus. Alignment of the deduced protein sequence with that of other receptors discloses some homology with angiotensin receptors. A single transcript of 2.5 kb is detected at the late blastula stage and its expression increases during gastrulation. In situ hybridization reveals transcripts initially in the ventrolateral involuting marginal zone and later in the lateral plate mesoderm. At larval stages, the transcript is expressed in procardiac tube and forming blood vessels, where it is localized in the inner endothelial layer. Thus, this gene traces an endothelial lineage and represents a very early and unique marker in Xenopus of the specification of cardiac and vascular endothelia. We propose the name of X-msr for mesenchyme-associated serpentine receptor.