Renal Phosphodiesterase 4B Is Activated in the Dahl Salt-Sensitive Rat

Abstract

Abstract-Reduced ␤-adrenoreceptor signaling is associated with increased sympathoadrenal activity in hypertensive patients and animal models of hypertension. However, the mechanism that accounts for this characteristic decline in ␤-adrenergic signaling is unclear. In the present study, we investigated renal phosphodiesterase 4B, which metabolizes cAMP. Immunoblot analysis detected only the phosphodiesterase 4B4 isoform present in kidney tissue from spontaneously hypertensive rats, hypertensive Dahl salt-sensitive (SS) rats, and Dahl salt-resistant rats. The phosphorylated (activated) form of the protein was present at 2-fold greater levels in Dahl SS rats than in spontaneously hypertensive rats and Dahl salt-resistant rats, whereas the unphosphorylated form of the protein was reduced by approximately one half in SS animals. In accord with immunoblot data, rolipram-inhibitable cAMP hydrolyzing activity, a measure of PDE4 activity, was Ϸ3-fold greater in kidney cytosolic extracts from SS rats than in extracts from spontaneously hypertensive rats and salt-resistant rats. Phosphodiesterase 4B expression was detected by immunohistochemistry in the renal vasculature, proximal tubules, and distal tubules. These results raise the possibility that increased PDE4 activity, specifically phosphodiesterase 4B4 activity, reduces ␤-adrenergic signaling in the kidney and contributes to salt-sensitive hypertension in the Dahl SS rat. Key Words: phosphodiesterase 4B Ⅲ cAMP Ⅲ Dahl rat Ⅲ ␤-adrenergic signaling A ctivation of ␤-adrenoreceptors triggers the formation of cAMP through G protein-coupled adenylate cyclase, which subsequently stimulates cAMP-dependent protein kinase A (PKA). cAMP phosphodiesterase (PDE) negatively regulates ␤-adrenoreceptor signaling by hydrolyzing cAMP and reducing PKA activity. 1,2 Reduced vascular ␤-adrenergic responsiveness has been demonstrated in both hypertensive patients and rat models of genetic and acquired hypertension (see review 3 ). This reduced responsiveness has been hypothesized to play an important role in the pathogenesis of hypertension. However, the mechanism(s) underlying reduced ␤-adrenergic signaling remain unclear. ␤-Adrenergic receptor number has been measured in different animal hypertension models, and the results have varied, with receptor number increasing, decreasing, or remaining unchanged in the heart and vasculature (see review 4 ). Other evidence suggests that, in certain models of hypertension, expression of the stimulatory G protein Gs is reduced, and/or the expression of the inhibitory G protein Gi is increased. [5][6][7] Another possibility, which we have investigated here, is that PDE activity may be increased in hypertension.The PDE 4 family of PDEs, formerly known as cAMP-PDE and rolipram-sensitive PDE, is expressed in most cell types and is generally considered to account for most of the cAMP-hydrolyzing activity of cells (see review 8 ). PDE4s are generated from 4 distinct genes (PDE4A through D), and each gene generates long and short splice variants using alte...

Regulation of Renovascular Adenosine 3′,5′-Cyclic Monophosphate in Spontaneously Hypertensive Rats

“…Previous studies by Tawar et al21 report that the primary renal isoform of PDE4 is PDE4B4. It is conceivable that selective inhibition of PDE4B4 would minimize adverse effects while improving renal hemodynamics and blood pressure in genetic hypertension.…”

“…However, another interpretation is that these effects are observable in 5 weeks-of-age SHR because even very mild elevations in MABP may markedly alter the role of PDE4 in the renal vasculature. Because Tawar et al21 report that the primary renal isoform of PDE4 is PDE4B4, it would be interesting to determine whether this is the isoform of PDE4 responsible for masking the ability of isoproterenol to induced 3′,5′-cAMP in the renal vasculature of SHR.…”

Regulation of 3′,5′-cAMP in Preglomerular Smooth Muscle and Endothelial Cells From Genetically Hypertensive Rats

“…However, RO 20–1724 does not distinguish among the multiple PDE4 isoforms; indeed, presently no PDE4 inhibitors are available that block exclusively a specific PDE4 isoform. In this regard, there are 4 different PDE4 subtypes (PDE4A, PDE4B, PDE4C and PDE4D) with each subtype being encoded by a specific gene18 and being populated by a set of variants due to alternative mRNA splicing and alternative promoters16, 18. Our real time PCR results demonstrate that the dominant PDE4 mRNA subtype in both PGVSMCs and PGVECs is PDE4B.…”

“…At present, studies have identified 5 different variants of PDE4B16. Real time PCR was conducted using primers designed to recognize the 5 specific variants of PDE4B.…”

“…Tawar et al16 report that Western blotting of total renal tissue with a polyclonal PDE4B antibody against the common carboxyl terminus of the PDE4B family detects only PDE4B4, and not the other PDE4B variants, and that rolipram-inhibitable 3’,5’-cAMP hydrolyzing activity in the renal cytosol increases in proportion to the total renal expression of phosphorylated PDE4B4. Because rolipram inhibits all forms of PDE4, these data suggest that PDE4B is the major PDE4 subtype in the kidney with the PDE4B4 variant being dominant.…”