Prolonged calcium transients and myocardial remodelling in early experimental uraemia.

BACKGROUND: Cardiovascular disease is the most common cause of premature death in patients with end-stage renal disease, possibly due to a specific 'uraemic cardiomyopathy'. This study was designed to investigate the cardiac changes induced by a moderate impairment of renal function in a model of uraemia. METHODS: Male Wistar rats (n=11) were rendered uraemic by 5/6 nephrectomy or sham operated (n=11). After 4 weeks, cardiac dimensions were measured from fixed tissue sections using a digital image analysis technique. In parallel groups of animals, cardiac myocytes were isolated and studied for evidence of functional changes attributable to uraemia. After steady-state field stimulation at 0.5 Hz, intracellular Ca(2+) handling (using Fura-2) was investigated. Up to 20 consecutive transients were averaged as the extracellular Ca(2+) was increased. RESULTS: The 5/6 nephrectomy group had a 75% reduction in glomerular filtration rate, and a 2- to 3-fold increase in serum urea and creatinine compared with sham-operated control animals (P<0.0001). However, the blood pressure was found to be similar in each group. Histology of the intact hearts (five pairs) showed a significant increase in tissue cross-sectional area (14%; P<0.04), cross-sectional area of the left ventricle (22%; P<0.04), and a significant increase in left ventricular wall thickness (15%; P<0.03). In the single cardiac cell study, under basal conditions (1-2 mM extra-cellular Ca(2+)) no significant differences in intracellular Ca(2+) were observed, but in high extracellular Ca(2+) the uraemic cells were slower to return to diastolic intracellular Ca(2+) levels (P<0.05). CONCLUSIONS: The data provide evidence of altered myocardial structure and function in early experimental uraemia. The changes described are consistent with concentric hypertrophy of the left ventricle, which occurs in the absence of hypertension.

Altered L-arginine metabolism results in increased nitric oxide release from uraemic endothelial cells.

Results regarding the nitric oxide (NO) system in uraemia are contradictory. L-arginine, the precursor of NO, is also metabolized by arginase to form ornithine and urea. In the present study, endothelial NO production and arginine metabolism in uraemia were assessed. In addition an in vivo model was used to examine excess consumption of NO in uraemia. NO and amino acid measurements were made from basal and stimulated (by bradykinin) uraemic and control endothelial cells. Reverse-transcriptase PCR was used to assess endothelial NO synthase (eNOS) and inducible NOS (iNOS) expression. Finally, aortae of uraemic rats were stained for nitrotyrosine (a marker of peroxynitrite). Basal uraemic cells produced more NO than the control cells. L-arginine levels were greater in uraemic (supernatants/cells), but ornithine levels were higher in control (supernatants/cells). Following stimulation, NO levels in supernatants were similar, but the rise in NO production was greater in control compared with uraemic cells; l-arginine levels still remained higher in uraemic supernatants/cells. Differences in ornithine concentration (supernatants/cells) disappeared following bradykinin stimulation, due to a rise in ornithine levels in the uraemic group. There was no difference in eNOS expression, nor was iNOS detected in either group. Only aortae from uraemic rats showed evidence for nitrotyrosine staining. These studies demonstrated increased basal NO release in uraemic endothelial cells, perhaps by inhibition of arginase and hence diversion of arginine to the NO pathway. The increased NO produced under basal conditions may be inactive due to excessive consumption, resulting in peroxynitrite formation. Interestingly, bradykinin appears to restore arginase activity in uraemia, resulting in normalization of NO production.