Renal potassium bicarbonate release in humans exposed to an acute volume load

Summary Cells of the renal medulla regulate their volume by transmembrane ion movements when exposed to large changes in osmolality. Since renal cells in culture release KHCO₃ in response to hypotonic stress 11], we investigated the effect of an acute water load on urinary KHCO₃ excretion in 5 healthy individuals. Water diuresis was induced by the ingestion of 1.51 hypoosmolal fluid (22 mosm/kg H₂O) over 15 min. The rate of urinary volume excretion increased from an initial value of 1.4 ml/min to 9.3 ml/min after 75 min. Urinary osmolality dropped from an initial value of 940±32 mosm/kg H₂O to 74±4 mosm/kg H₂O (n = 5). The decrease of osmolality was accompanied by the transient release of potassium and bicarbonate. Peak values of KHCO₃ excretion were observed between 30 and 45 min after the onset of the experiment corresponding to the drop of urinary osmolality. The magnitude of renal potassium release correlated significantly (r=0.93; P < 0.05) with endogenous plasma aldosterone concentrations measured prior to the experiment in the 5 volunteers. We conclude that medullary epithelial cells release KHCO₃ when exposed to hypotonic stress. The volume regulatory response is upregulated by aldosterone.Abbreviations ADH antidiuretic hormone - MDCK Madin-Darby canine kidney