Understanding precisely how the heart can recognize and respond to many different extracellular signalling molecules, such as neurotransmitters, hormones and growth factors, will aid the identification of new therapeutic targets through which cardiovascular diseases can be combated. In recent years, we have learned more about the complex interactions that occur between the receptors and the signalling pathways of the heart and its environment. Most of these discoveries have focused on the most common type of cardiac receptor - the seven-transmembrane-spanning receptor or G-protein-coupled receptor.

Transgenic mice were created with cardiac-specific overexpression of the 1-adrenergic receptor kinase-1 (MARK1) or a PARK inhibitor. Animals overexpressing HARK1 demonstrated attenuation of isoproterenol-stimulated left ventricular contractility in vivo, dampening of myocardial adenylyl cyclase activity, and reduced functional coupling of 1-adrenergic receptors. Conversely, mice expressing the PARK inhibitor displayed enhanced cardiac contractility in vivo with or without isoproterenol. These animals demonstrate the important role of PARK in modulating in vivo myocardial function. Because increased amounts of PARK1 and diminished cardiac 1-adrenergic responsiveness characterize heart failure, these animals may provide experimental models to study the role of PARK in heart disease.W-adrenergic receptors (PARs) are the primary myocardial targets of the sympathetic neurotransmitter norepinephrine and the adrenal hormone epinephrine. Human myocardium contains 13I-and 132AR subtypes with the PI3AR being most abundant (1). Activation of PARs (PI and P2) in the heart by agonist binding causes stimulation of adenylyl cyclase, increased intracellular concentrations of adenosine 3',5 '-monophosphate (cAMP), and increased cytosolic calcium transients resulting in positive chronotropy and inotropy (increased rate and force of contraction). As is true for most heterotrimeric guanosine triphosphate (GTP) binding protein (G protein)-coupled receptors, prolonged exposure of PARs to agonist results in a rapid decrease in responsiveness. Agonist-dependent desensitization can be initiated by phosphorylation of activated receptors by members of the G protein-coupled receptor kinase (GRK) family (2). The P3-adrenergic receptor kinase-1 (PARKI) is a GRK that specifically phosphorylates activated P13 -(3) and P2ARs (2) in in vitro assays. In the case of the 13AR, these assays used cultured mammalian cells or highly purified membrane preparations

To study the mechanisms that activate expression of the atrial natriuretic factor (ANF) gene during pressure-induced hypertrophy, we have developed and characterized an in vivo murine model of myocardial cell hypertrophy. We employed microsurgical techniques to produce a stable 35-to 45-mmHg pressure gradient across the thoracic aorta of the mouse that is associated with rapid and transient expression of an immediate-early gene program (c-fos/cjun/junB/Egr-1/nur-77), an increase in heart weight/body weight ratio, and up-regulation of the endogenous ANF gene. These responses that are identical to those in cultured cell and other in vivo models of hypertrophy. To determine whether tissue-specific and inducible expression of the ANF gene can be segregated, we used a transgenic mouse line in which 500 base pairs of the human ANF promoter region directs atrial-specific expression of the simian virus 40 large tumor antigen (T antigen), with no detectable expression in the ventricles. Thoracic aortic banding of these mice led to a 20-fold increase in the endogenous ANF mRNA in the ventricle but no detectable expression of the T-antigen marker gene. This result provides evidence that atrial-specific and inducible expression of the ANF gene can be segregated, suggesting that a distinct set of regulatory cis sequences may mediate the up-regulation of the ANF gene during in vivo pressure overload hypertrophy. This murine model demonstrates the utility of microsurgical techniques to study in vivo cardiac physiology in transgenic mice and should allow the application of genetic approaches to identify the mechanisms that activate ventricular expression of the ANF gene during in vivo hypertrophy.In response to diverse stimuli, such as hypertension, valvular heart disease, and endocrine disorders, the myocardium adapts to increased workloads through the hypertrophy of individual muscle cells (for a review, see refs. 1 and 2). Although the signaling mechanisms that mediate the hypertrophic response of cardiac muscle cells remain unclear, transcriptional activation of cardiac target genes, including contractile proteins and embryonic markers, appears to play a pivotal role in this adaptive response (3, 4). In this regard, the reactivation of atrial natriuretic factor (ANF) gene expression in ventricular cells occurs in response to diverse hypertrophic stimuli (genetic, hormonal, volume overload, pressure overload, hypertension, etc.) in multiple species (5-11), including humans, and could be considered one of the conserved features of ventricular cell hypertrophy.To study the transcriptional regulation of cardiac genes, workers in our laboratory (3, 12, 13) and others (14-16) have extensively characterized cultured myocardial cell models in which several features of hypertrophy can be induced after stimulation with defined agents, such as a-adrenergic agonists (3, 12, 14-17) or endothelin 1 (13). In this model, the inducibility of a constitutively expressed contractile protein gene, myosin light chain 2 (MLC-2), is mediat...