1. NKCC1 is a widely expressed isoform of the Na,2Cl,K-cotransporter that mediates several direct and indirect vascular effects and regulates expression and release of renin. We have continued the evaluation of its function in NKCC1-deficient (NKCC1-/-) and wild type mice (WT). Specifically, we have assessed day/night differences of blood pressure (BP), locomotor activity, and renin release, and studied the effects of high (8%) or low (0.03%) dietary NaCl intake on BP, activity, and the renin/aldosterone system. On a standard diet, 24 h mean arterial blood pressure (MAP) and heart rate determined by radiotelemetry, and their day/night differences were not different in NKCC1-/- and WT mice. In NKCC1-/- mice on a high NaCl diet, MAP increased by 10 mm Hg in the night without changes in heart rate. In contrast, there was no salt-dependent blood pressure change in WT mice. MAP reductions by hydralazine (1 mg/kg) or isoproterenol (10 microg/mouse) were significantly greater in NKCC1-/- than WT mice. Plasma renin (ng Ang I/ml hr) and aldosterone concentrations (pg/ml) were higher in NKCC1-/- than WT mice (PRC: 3745 377 vs.1245 364; aldo: 763 136 vs. 327 98), and elevated levels of renin and aldosterone were found in NKCC1-/- mice during both day and night. High Na suppressed PRC and aldosterone in both NKCC1-/- and WT mice while a low Na diet increased PRC and aldosterone in WT, but not NKCC1-/- mice. These results permit us to conclude that 24 h MAP and MAP circadian rhythms do not differ between NKCC1-/- and WT mice on a standard diet, probably reflecting a balance between anti- and pro-hypertensive factors, but that blood pressure of NKCC1-/- mice is more sensitive to increases and decreases of Na intake.? ? 2. We have performed studies to determine the spatiotemporal development of renin expression and the development of the preglomerular arterial tree in mouse kidneys with renin-cell specific deletion of Gs, a core element for receptor activation of adenylyl cylases. Renin expression in the developing metanephric kidney begins in larger vessels, shifting to smaller vessels and finally remaining restricted to the terminal portions of afferent arterioles at the entrance into the glomerular capillary network. The mechanisms determining the successive expression of renin along the vascular axis of the kidney are not well understood. Since the cAMP signaling cascade plays a central role in the regulation of both renin secretion and renin expression in the adult kidney, it seemed feasible that this pathway may also be critical for renin expression during kidney development. We found that in the absence of Gs renin expression was largely absent in the kidneys at any developmental stage, whilst the development of the preglomerular arterial tree was apparently normal. These data indicate that both the initiation and the maintainance of renin expression along the preglomerular vasculature critically depends of the availability of Gs. We infer from these observations that the cAMP signaling pathway is not only critical for the regulation of renin synthesis and secretion in the mature kidney, but that it is also critical for establishing the juxtaglomerular expression site of renin during development.? ? 3. Cyclooxygenase-2 (COX-2) has been shown to be an important regulator of macula densa control of renin secretion. Furthermore, previous studies showing a marked reduction of basal plasma renin in COX-2-/- mice suggested tonic stimulation of renin release by COX-2. We have continued this line of investigation by assessing the role of COX-2 in regulation of renin and aldosterone release and of blood pressure by determining the effects of salt intake in conscious wild type (WT) and COX-2-deficient mice on a 129J background (COX-2-/-). Plasma renin concentration (PRC) in blood from the tail vein of conscious mice taken in the active phase between 11 PM and 1 AM was used as index of renin release. In WT, PRC (ng Ang I/ml hr) on normal, low salt (.003%, 1 week), and high salt diets (8%, 1 week) averaged 2252 plus/minus 406, 2923 plus/minus 471 (p=0.305), and 943 plus/minus 139 (p=0.011). In COX-2 -/- mice, PRC averaged 466 plus/minus 109, 569 plus/minus 89, and 430 plus/minus 54 on normal, low salt , and high salt diets respectively, values significantly lower than in WT on the same diet, but not significantly different between diets in the COX-2 deficient mice. Plasma aldosterone (pg/ml) in WT averaged 220 plus/minus 19, 292 plus/minus 49 (p=0.1), and 63 plus/minus 8 (p<.001) on normal, low salt, and high salt diets. While PRC did not appear to be well regulated in COX-2-/- mice, plasma aldosterone changes were similar to those in WT (229 plus/minus 33, 262 plus/minus 34, and 65 plus/minus 6.4). Thus, basal renin synthesis and release is markedly reduced in COX2-deficient mice, and the regulation of renin by dietary Na intake is blunted whereas basal levels and salt-dependent changes of plasma aldosterone are comparable in WT mice and COX-2-/- mice.? ? 4. Previous studies have indicated that an insufficient number of nephrons at birth is associated with the development of glomerulosclerosis in animals and humans, and that this may lead to the development of end-stage kidney disease later in life. Furthermore, a number of earlier studies have implicated glomerular hypertension in progressive renal disease following a number of different types of insults resulting in a reduced nephron number. Fvb Os/+ mice represent a mouse model of reduced renal mass and rapid development of marked albuminuria, glomerulosclerosis, and progressive renal failure. We have found that the presence of the Os allele in the Fvb background is associated with an 80% reduction in nephron number in contrast to C57Bl6 mice where nephron number is reduced by approximately 50%. Therefore, the genetic background is an important determinant of the renal response to the Os mutation. The reduction in nephron number was associated with a severe reduction in glomerular filtration rate, intraglomerular hypertension, albuminuria, and a dramatic compensatory hypertrophy of remaining glomeruli and tubules. This is in contrast to the C57Bl6 strain in which renal disease was minimal. The use of this mouse model indicates that a reduction in nephron number beyond a certain level is a condition predisposing to end stage renal failure.
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