Rationale : Chronic elevation of 3′-5′-cyclic adenosine monophosphate (cAMP) levels has been associated with cardiac remodeling and cardiac hypertrophy. However, enhancement of particular aspects of cAMP/protein kinase A signaling seems to be beneficial for the failing heart. cAMP is a pleiotropic second messenger with the ability to generate multiple functional outcomes in response to different extracellular stimuli with strict fidelity, a feature that relies on the spatial segregation of the cAMP pathway components in signaling microdomains. Objective : How individual cAMP microdomains affect cardiac pathophysiology remains largely to be established. The cAMP-degrading enzymes phosphodiesterases (PDEs) play a key role in shaping local changes in cAMP. Here we investigated the effect of specific inhibition of selected PDEs on cardiac myocyte hypertrophic growth. Methods and Results : Using pharmacological and genetic manipulation of PDE activity, we found that the rise in cAMP resulting from inhibition of PDE3 and PDE4 induces hypertrophy, whereas increasing cAMP levels via PDE2 inhibition is antihypertrophic. By real-time imaging of cAMP levels in intact myocytes and selective displacement of protein kinase A isoforms, we demonstrate that the antihypertrophic effect of PDE2 inhibition involves the generation of a local pool of cAMP and activation of a protein kinase A type II subset, leading to phosphorylation of the nuclear factor of activated T cells. Conclusions : Different cAMP pools have opposing effects on cardiac myocyte cell size. PDE2 emerges as a novel key regulator of cardiac hypertrophy in vitro and in vivo, and its inhibition may have therapeutic applications.

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Both benzodiazepines and ␥-hydroxybutyric acid (GHB) are used to treat alcohol withdrawal syndrome. The molecular basis for this therapeutic efficacy was investigated with primary cultures of rat cerebellar granule cells. Long-term exposure of these cells to ethanol (100 mM, 5 days) reduced the abundance of mRNAs encoding the ␥ 2 L and ␥ 2 S subunits of the GABA type A receptor (Ϫ32 and Ϫ23%, respectively) but failed to affect that of ␣ 1 , ␣ 4 , or ␣ 6 subunit mRNAs. Subsequent ethanol withdrawal resulted in decreases in the amounts of ␣ 1 (Ϫ29%), ␣ 6 (Ϫ27%), ␥ 2 L (Ϫ64%), and ␥ 2 S (Ϫ76%),subunit mRNAs that were maximal after 6 to 12 h. In contrast, 3 h after ethanol withdrawal, the abundance of the ␣ 4 subunit mRNA was increased by 46%. Ethanol withdrawal did not affect neuronal morphology but reduced cellular metabolic activity. The increase in ␣ 4 subunit was confirmed by functional studies showing a positive action of flumazenil in patch clamp recordings of GABA-stimulated currents after ethanol withdrawal. Diazepam (10 M) or GHB (100 mM) prevented the increase in the amount of the ␣ 4 subunit mRNA, the metabolic impairment, and the positive action of flumazenil induced by ethanol withdrawal but failed to restore the expression of the ␣ 1 and ␥ 2 subunits. The antagonism by GHB seems not to be mediated by a direct action at GABA A R because GHB failed to potentiate the effects of GABA or diazepam on Cl Ϫ currents mediated by GABA type A receptor.

It is becoming increasingly evident that the freely diffusible second messenger cAMP can transduce specific responses by localized signalling. The machinery that underpins compartmentalized cAMP signalling is only now becoming appreciated. Adenylate cyclases, the enzymes that synthesize cAMP, are localized at discrete parts of the plasma membrane, and phosphodiesterases, the enzymes that degrade cAMP, can be targeted to selected subcellular compartments. A-kinase-anchoring proteins then serve to anchor PKA (protein kinase A) close to specific targets, resulting in selective activation. The specific activation of such individual subsets of PKA requires that cAMP is made available in discrete compartments. In this presentation, the molecular and structural mechanisms responsible for compartmentalized PKA signalling and restricted diffusion of cAMP will be discussed.