The influence of cortisol excess on kidney function in the dog.

The effects of cortisol excess on kidney function were studied in 8 normal conscious dogs. Cortisol was given orally until polyuria developed. Cortisol excess decreased urine osmolality (from 897 +/- 76 to 186 +/- 36 mosm. kg-1) and increased urine production (from 0.7 +/- 0.1 to 9.3 +/- 2.4 ml kg-1. h-1). The glomerular filtration rate increased by 23 +/- 9 per cent. Sodium and potassium concentrations in plasma were decreased. 66 Per cent of the increase in urine production was due to the increase in free water clearance and 34 per cent to the increased urea excretion. Cortisol excess apparently caused polyuria by inhibition of the action of ADH in the collecting duct, resulting in a decreased water and urea reabsorption. The decreased urea reabsorption possibly causes a smaller urea recirculation in the renal medulla and hence a decrease in concentrating capacity.