[Hypernatremia and kidney function]

Hypernatremia is caused by a water deficit. Cases with hypernatremia and dehydration appear to cluster among children and the elderly with alterations in the level of consciousness thus with no independent access to water. In general, central nervous symptoms prevail. However, thorough examination reveals impaired renal function in many such cases. Animal experiments have shown that rapid increases of the sodium concentration in the renal artery will cause a reduction of renal blood flow (RBF), glomerular filtration rate (GFR) and inhibition of renin secretion, particularly during states of sodium chloride or volume depletion (i.e. with high plasma renin activity). In any other organ hypernatremia leads to vasodilation. The kidney, however, responds with vasoconstriction which can be reversed by the adenosine antagonist theophylline. This finding led to the hypothesis that adenosine mediates the renal response to hypernatremia. Adenosine is generated by the tubules at a higher rate when the kidney is forced to reabsorb large amounts of sodium. In this concept adenosine links metabolic processes of sodium reabsorption with the regulation of organ blood flow causing vasoconstriction via adenosine receptors on the vasa afferentia. This mechanism can explain impaired renal function during acute hypernatremia. It is concluded from experimental evidence that-apart from other therapeutic measures-the recovery of impaired renal function can be improved by administration of the adenosine antagonist theophylline.