Sodium pump inhibitors in blood pressure regulation and in profibrotic signaling in salt sensitive hypertension and aging

National Institute On Aging

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Abstract

To test the hypotheses, we have performed the following experiments: We used cultured VSMC from the young (3-mo) and old (24-mo) male Sprague-Dawley rats, and young VSMC with silenced PKG-1 gene to treat them with 1 nmol/L ANP, or with 1 nmol/L MBG, or with a combination of ANP and MBG. Collagen-1, Fli1 and PKG-1 levels were assessed by Western blotting analyses. We have demonstrated, that vascular PKG1 and Fli1 levels in the old rats were reduced versus young counterparts. ANP prevented inhibition of vascular NKA by MBG in young VSMC, but not in old VSMC. In VSMC from the young rats, MBG induced down-regulation of Fli1 and an increase in collagen-1 level, whereas ANP blocked this effect. Silencing of PKG1 gene in young VSMC resulted in a reduction in the levels of PKG1 and Fli1. MBG additionally reduced Fli1 and increased collagen-1 level, and ANP failed to oppose these MBG effects, similar to VSMC from the old rats with the age-associated reduction in PKG1. Based on the results of our study we concluded, that age-associated reduction in vascular PKG1 and the resultant decline in cGMP signaling lead to the loss of the ability of ANP to oppose MBG-induced inhibition of NKA and fibrosis development. Silencing of PKG1 mimicked these effects of aging. In summary, in VSMC from young and old rats, preincubation with MBG was associated with an increased level of collagen-1, and this effect of MBG became stronger in advanced age, i.e., the older VSMC exhibited higher collagen level after MBG administration than did young VSMC. The observed age-associated effect of MBG was ANP/PKG-dependent, because silencing of PKG1 in young VSMC was associated with augmentation of the MBG effect of collagen-1 level. ANP failed to protect VSMC against the profibrotic effect of MBG following silencing of PKG1. Thus, the ANP/PKG-dependent mechanism of augmentation of the effects of MBG very likely contributed to the development of salt-sensitivity of blood pressure, accompanied by the development of MBG-induced cardiovascular fibrosis. Accordingly, following acute NaCl loading, 24-month-old rats exhibited a greater pressor response and a lesser natriuretic response to NaCl loading as compared to 3-month-old rats. Our present data are in agreement with a recent finding indicating that a group of single nucleotide polymorphisms located in the PKG1 gene is associated with the salt sensitivity of blood pressure in hypertensive human subjects. Our prior studies indicate that MBG, acting via NKA-dependent signaling, stimulates the synthesis of collagen in cardiac, renal, and aortic tissue, and that immunoneutralization of MBG reverses cardiac fibrosis in rats with renal failure. Because fibrosis is a hallmark of cardiovascular aging, the PKG agonists and natriuretic peptides exhibit anti-fibrotic effects, the impact of the age-associated reduction in cGMP/PKG signaling on the pro-fibrotic effects of MBG merits further investigation.