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Patients with chronic renal failure develop a “uremic” cardiomyopathy characterized by diastolic dysfunction, cardiac hypertrophy and systemic oxidant stress. Patients with chronic renal failure also are known to have increases in the circulating concentrations of the cardiotonic steroid marinobufagenin. On this background, we hypothesized that elevations in circulating marinobufagenin may be involved in the cardiomyopathy.First, we observed that administration of marinobufagenin (10 μg/kg/day) for 4 weeks caused comparable increases in plasma MBG as partial nephrectomy at 4 weeks. Marinobufagenin infusion caused increases in conscious blood pressure, cardiac weight and the time constant for left ventricular relaxation similar to partial nephrectomy. Decreases in the expression of the cardiac sarcoplasmic reticulum ATPase, cardiac fibrosis and systemic oxidant stress were observed with both marinobufagenin infusion and partial nephrectomy. Next, rats were actively immunized against a marinobufagenin -bovine serum albumin conjugate or bovine serum albumin control, and partial nephrectomy was subsequently performed. Immunization against marinobufagenin attenuated the cardiac hypertrophy, impairment of diastolic function, cardiac fibrosis and systemic oxidant stress seen with partial nephrectomy without significant effect on conscious blood pressure. These data suggest that the increased concentrations of marinobufagenin are important in the cardiac disease and oxidant stress state seen with renal failure.

It has been recognized that patients with chronic renal failure eventually develop diastolic dysfunction, cardiac hypertrophy and systemic oxidant stress along with increases in circulating concentrations of the cardiotonic steroid, marinobufagenin (MBG) in uremic cardiomyopathy. Because of this, we performed 5/6th partial nephrectomy in rats to study experimental renal failure (PNx), MBG infusion, PNx after immunization against MBG, and concomitant PNx and adrenalectomy. We also studied 5⁄6th partial nephrectomy as a potential experimental renal failure model in mice. Next, we speculated a relationship between decreases Fli-1 expression and increases in collagen production following exposure to MBG. Therefore, we examined Fli-1 knockdown mice and compared it to wild type mice. Physiological measurements with a Millar catheter and immunohistochemistry were performed. In vitro studies were then pursued with cultured isolated cardiac fibroblasts, human dermal fibroblasts, as well as a cell line derived from renal fibroblasts. First, in rats, we observed that PNx after immunization against MBG as well as concomitant PNx and adrenalectomy had similar blood pressure as PNx but less cardiac hypertrophy, diastolic dysfunction, and cardiac fibrosis. Second, in mice, the 5⁄6 nephrectomy resulted in impairment of both active and passive left ventricular relaxation at four weeks as well as progressive fibrosis in the heart. Third, the Fli-1 knockdown mice showed greater amounts of cardiac collagen expression and fibrosis compared to wild type before and after 4 weeks of experimental renal failure induced by 5/6th nephrectomy.We realized that stimulation of cultured cardiac fibroblasts with MBG could be prevented by administration of inhibitors of tyrosine phosphorylation, Src activation, EGFR transactivation, and N-acetyl cysteine. Furthermore, in response to MBG, decreases in nuclear Fli-1 accompanied increases in procollagen expression. Finally, we observed that exposure of cardiac fibroblasts to MBG was associated with a rapid translocation of PKCδ to the nucleus that appeared to peak at about 15 minutes as determined with confocal immunofluorescence and Western blot.Taken together, these data suggest that MBG directly induces increases in collagen expression by fibroblasts in a process involving Fli-1 and PKCδ, and is in part responsible for the cardiac fibrosis seen with experimental renal failure.