Hypokalemia induces renal injury and alterations in vasoactive mediators that favor salt sensitivity.

We investigated the hypothesis that hypokalemia might induce renal injury via a mechanism that involves subtle renal injury and alterations in local vasoactive mediators that would favor sodium retention. To test this hypothesis, we conducted studies in rats with diet-induced K+ deficiency. We also determined whether rats with hypokalemic nephropathy show salt sensitivity. Twelve weeks of hypokalemia resulted in a decrease in creatinine clearance, tubulointerstitial injury with macrophage infiltration, interstitial collagen type III deposition, and an increase in osteopontin expression (a tubular marker of injury). The renal injury was greatest in the outer medulla with radiation into the cortex, suggestive of an ischemic etiology. Consistent with this hypothesis, we found an increased uptake of a hypoxia marker, pimonidazole, in the cortex. The intrarenal injury was associated with increased cortical angiontensin-converting enzyme (ACE) expression and continued cortical angiotensin II generation despite systemic suppression of the renin-angiotensin system, an increase in renal endothelin-1, a decrease in renal kallikrein, and a decrease in urinary nitrite/nitrates and prostaglandin E(2) excretion. At 12 wk, hypokalemic rats were placed on a normal-K+ diet with either high (4%)- or low (0.01%)-NaCl content. Despite correction of hypokalemia and normalization of renal function, previously hypokalemic rats showed an elevated blood pressure in response to a high-salt diet compared with normokalemic controls. Hypokalemia is associated with alterations in vasoactive mediators that favor intrarenal vasoconstriction and an ischemic pattern of renal injury. These alterations may predispose the animals to salt-sensitive hypertension that manifests despite normalization of the serum K+.

Intravenous immunoglobulin protects against experimental thrombotic microangiopathy.

BACKGROUND: Intravenous immunoglobulin (IVIG) has been utilized in several forms of vasculitis and has many potential mechanisms of action, including the inhibition of C3 activation. We have previously demonstrated that IVIG can reduce glomerular injury in a model of membranous nephropathy mediated by C5b-9 [1]. C5b-9 has also been shown to mediate the thrombotic microangiopathy (TMA) induced by antibody to glomerular endothelial cells leading to a hemolytic uremic syndrome-type lesion [2]. METHODS: To test the hypothesis that IVIG might be effective in treating antibody-induced TMA, male uninephrectomized rats underwent right renal artery perfusion with goat anti-rat glomerular endothelial cell (GEN) antibody (20 mg/kg). Sheep IgG (200 mg/kg) was administered either 30 minutes before the renal artery perfusion (group I, N = 6) or 30 minutes postperfusion (group II, N = 9). A third control group received phosphate-buffered saline (PBS; group III, N = 12). A survival biopsy was performed at 15 minutes, and the animals were sacrificed on day 2. RESULTS: There were no significant differences in proteinuria or hematocrit between the groups. Animals pretreated with IVIG had significantly improved survival and renal function, which was associated with a decrease in glomerular C3 deposition. The protective effect of IVIG was abolished if the administration was delayed 30 minutes after perfusion. CONCLUSIONS: IVIG is effective in reducing injury in experimental TMA only if given prophylactically. The effect is mediated by inhibition of local intraglomerular complement activation.

Vascular endothelial growth factor accelerates renal recovery in experimental thrombotic microangiopathy.

BACKGROUND: Renal microvascular injury characterizes thrombotic microangiopathy (TMA). The possibility that angiogenic growth factors may accelerate recovery in TMA has not been studied. METHODS: TMA was induced in rats by the selective right renal artery perfusion of antiglomerular endothelial cell IgG (30 mg/kg). Twenty-four hours later, rats received vascular endothelial growth factor (VEGF121, 100 microg/kg/day) or vehicle (control) daily until day 14. To evaluate renal function, the unperfused left kidney was removed at day 14, and rats were sacrificed at day 17. RESULTS: The induction of TMA was associated with loss of glomerular and peritubular capillary endothelial cells and decreased arteriolar density at day 1. Some spontaneous capillary recovery was present by day 17; however, repair was incomplete, and severe tubulointerstitial damage occurred. The lack of complete microvascular recovery was associated with reduced VEGF immunostaining in the outer medulla. VEGF-treated rats had more glomeruli with intact endothelium, less glomerular ischemia (collapsed glomeruli), and greater peritubular capillary density with less peritubular capillary loss. This was associated with less tubulointerstitial fibrosis, less cortical atrophy, and improved renal function. CONCLUSIONS: VEGF accelerates renal recovery in this experimental model of TMA. These studies suggest that angiogenic growth factors may provide a new therapeutic strategy for diseases associated with endothelial cell injury.

Effects of intravenously administered C-type natriuretic peptide in humans: comparison with atrial natriuretic peptide.

We have previously reported that C-type natriuretic peptide (CNP), the third member of the natriuretic peptide family, is produced in vascular endothelial cells and suggested that CNP might be a local regulator of vascular tone and growth. To evaluate the biological actions of CNP as compared with human atrial natriuretic peptide (hANP), we intravenously administered synthetic CNP (0.43 nmol/kg) and alpha-hANP (0.43 and 0.043 nmol/kg) to healthy humans. The experiments were done on different days in the same five healthy volunteers (31+/-1 yr old). CNP injection caused a transient but significant decrease in both systolic and diastolic blood pressure (-4.3+/-1.3, -4.1+/-1.0 mmHg) with a significant increase in heart rate (+7.6+/-2.6 bpm), and exerted significant diuretic and natriuretic activities (+130+/-80%, +160+/-100% over the basal level). These effects of CNP (0.43 nmol/kg) were comparable to, or less than, those of alpha-hANP (0.043 nmol/kg). CNP injection also significantly suppressed aldosterone secretion (22% reduction as compared with the basal level). Our results demonstrate that intravenously-administered CNP acts as a natriuretic peptide with less potency than ANP.

Regulation of endothelial production of C-type natriuretic peptide by interaction between endothelial cells and macrophages.

We demonstrated endothelial production of C-type natriuretic peptide (CNP), the third member of the natriuretic peptide family, and its regulation by cytokines, including tumor necrosis factor-alpha (TNF alpha). We thus proposed that CNP can control vascular tone and growth as an endothelium-derived relaxing peptide. We also revealed the marked elevation of plasma CNP concentration in patients with septic shock, in which TNF alpha plays a significant part. As the interaction between endothelial cells (EC) and monocytes-macrophages plays a pivotal role in the pathogenesis of atherosclerosis, we investigated the effect of coculture of EC and macrophages on endothelial production of CNP. We used a human monocytic leukemia cell line, THP-1, which differentiates into macrophages when treated with phorbol 12-myristate 13-acetate. The coculture of EC and THP-1-derived macrophages enhanced CNP secretion by more than 10-fold compared with the single culture of EC or the coculture of EC and THP-1 without phorbol 12-myristate 13-acetate treatment. Prevention of direct contact between EC and THP-1-derived macrophages did not attenuate the increase in CNP secretion. Northern blotting revealed the augmentation of CNP messenger RNA expression in EC in the coculture. We detected TNF alpha in the conditioned medium from the coculture of EC and THP-1-derived macrophages. Furthermore, anti-TNF alpha antibody inhibited the stimulation of CNP secretion in the coculture. CNP at a concentration of 1 nM did not stimulate cGMP production in EC or THP-1-derived macrophages, but it elevated cGMP production significantly in vascular smooth muscle cells. These results indicate that endothelial production of CNP is stimulated mainly by TNF alpha released from THP-1-derived macrophages in the coculture. Endothelial CNP at the enhanced level may be one of the vascular mediators to regulate local vascular tone and growth through cGMP production by vascular smooth muscle cells, suggesting the potential significance of endothelial CNP in atherosclerosis.