Background— Cyclic guanosine monophosphate (cGMP) is the common second messenger for the cardiovascular effects of nitric oxide (NO) and natriuretic peptides, such as atrial or brain natriuretic peptide, which activate the soluble and particulate forms of guanylyl cyclase, respectively. However, natriuretic peptides and NO donors exert different effects on cardiac and vascular smooth muscle function. We therefore tested whether these differences are due to an intracellular compartmentation of cGMP and evaluated the role of phosphodiesterase (PDE) subtypes in this process. Methods and Results— Subsarcolemmal cGMP signals were monitored in adult rat cardiomyocytes by expression of the rat olfactory cyclic nucleotide–gated (CNG) channel α-subunit and recording of the associated cGMP-gated current ( I _CNG ). Atrial natriuretic peptide (10 nmol/L) or brain natriuretic peptide (10 nmol/L) induced a clear activation of I _CNG , whereas NO donors ( S -nitroso- N -acetyl-penicillamine, diethylamine NONOate, 3-morpholinosydnonimine, and spermine NO, all at 100 μmol/L) had little effect. The I _CNG current was strongly potentiated by nonselective PDE inhibition with isobutyl methylxanthine (100 μmol/L) and by the PDE2 inhibitors erythro-9-(2-hydroxy-3-nonyl)adenine (10 μmol/L) and Bay 60-7550 (50 nmol/L). Surprisingly, sildenafil, a PDE5 inhibitor, produced a dose-dependent increase of I _CNG activated by NO donors but had no effect (at 100 nmol/L) on the current elicited by atrial natriuretic peptide. Conclusions— These results indicate that in rat cardiomyocytes (1) the particulate cGMP pool is readily accessible at the plasma membrane, whereas the soluble pool is not; and (2) PDE5 controls the soluble but not the particulate pool, whereas the latter is under the exclusive control of PDE2. Differential spatiotemporal distributions of cGMP may therefore contribute to the specific effects of natriuretic peptides and NO donors on cardiac function.

Subcellular targeting of the components of the cAMP-dependent pathway is thought to be essential for intracellular signaling. Here we have identified a novel protein, named myomegalin, that interacts with the cyclic nucleotide phosphodiesterase PDE4D, thereby targeting it to particulate structures. Myomegalin is a large 2,324-amino acid protein mostly composed of α-helical and coiled-coil structures, with domains shared with microtubule-associated proteins, and a leucine zipper identical to that found in theDrosophila centrosomin. Transcripts of 7.5–8 kilobases were present in most tissues, whereas a short mRNA of 2.4 kilobases was detected only in rat testis. A third splicing variant was expressed predominantly in rat heart. Antibodies against the deduced sequence recognized particulate myomegalin proteins of 62 kDa in testis and 230–250 kDa in heart and skeletal muscle. Immunocytochemistry and transfection studies demonstrate colocalization of PDE4D and myomegalin in the Golgi/centrosomal area of cultured cells, and in sarcomeric structures of skeletal muscle. Myomegalin expressed in COS-7 cells coimmunoprecipitated with PDE4D3 and sequestered it to particulate structures. These findings indicate that myomegalin is a novel protein that functions as an anchor to localize components of the cAMP-dependent pathway to the Golgi/centrosomal region of the cell.