The main goal of this application is to define the role of vitamin D and parathyroid hormone (PTH) in the regulation of the renin-angiotensin system in vivo, which plays a crucial role in the regulation of blood pressure, electrolyte and volume homeostasis. Recently, we found that disruption of the vitamin D signaling in mice results in a deregulated elevation of renin expression and plasma angiotensin II production, leading to hypertension and increased water intake. We demonstrated that the renin up-regulation is independent of the hypocalcemia associated with vitamin D receptor (VDR) ablation in VDR knockout (VDRKO) mice, but whether the secondary hyperparathyroidism associated with VDR inactivation also contributes to the increase in renin expression remains unclear. We also demonstrated that 1,25-dihydroxyvitamin D3 directly suppresses renin expression in cell culture by a VDR-dependent mechanism, but in vivo evidence is still required to prove that vitamin D is a negative endocrine regulator of the renin-angiotensin system. To address these questions, we proposed two Specific Aims: (1) Verify the role of vitamin D as a negative regulator of renin expression by targeting human VDR (hVDR) expression in renin-producing cells in transgenic (Tg) mice using the 4.1 kb renin gene promoter. In contrast to VDRKO mice, the Tg mice will allow us to directly assess the role of VDR in vivo in the regulation of renin expression under a normal calcium and PTH condition. A down-regulation of renin expression in the Tg mice will confirm that vitamin D negatively regulates renin expression. (2) Clarify the role of vitamin D and PTH in the regulation of renin expression by generating and analyzing VDRKO mice that express the IIVDR transgene (VDRKO/hVDR). The hVDR will be introduced into VDRKO mice through breeding, and the VDRKO/hVDR mice will express hVDR only in renin-producing cells, and still develop hypocalcemia, secondary hyperparathyroidism and a high concentration of circulating 1 ,25-dihydroxyvitamin D3 like the VDRKO mice. Thus, if VDRKO/hVDR mice still show the same renin up-regulation as VDRKO littermates, it will be concluded that PTH has a stimulatory role and vitamin D does not play an inhibitory role; If renin expression in VDRKO/hVDR mice is lower than in wildtype littermates, it will indicate that PTH does not have a stimulatory effect and vitamin D is indeed a negative regulator; If renin expression in VDRKO/hVDR mice is lower than VDRKO littermates but higher than wildtype littermates, it will indicate that PTH stimulates and vitamin D suppresses renin expression.