Analysis of the calcium paradoxon of renin secretion

The secretion of the protease renin from renal juxtaglomerular cells is enhanced by subnormal extracellular calcium concentrations. The mechanisms underlying this atypical effect of calcium have not yet been unraveled. We therefore aimed to characterize the effect of extracellular calcium concentration on calcium handling of juxtaglomerular cells and on renin secretion in more detail. For this purpose we used a combination of experiments with isolated perfused mouse kidneys and direct calcium measurements in renin secreting cells in situ. We found that lowering of the extracellular calcium concentration led to a sustained elevation of renin secretion. Electron-microscopical analysis of renin secreting cells exposed to subnormal extracellular calcium concentrations revealed big omega-shaped structures resulting from the intracellular fusion and subsequent emptying of renin storage vesicles. The calcium concentration dependencies as well as the kinetics of changes were rather similar for renin secretion and...