Renal function in hyperbaric environment.

During mixed gas saturation diving (to 3-49.5 ATA) daily urine flow increases by about 500 ml/day, with no changes in fluid intake and glomerular filtration rate. The diuresis is accompanied by a significant decrease in urine osmolality and increase in excretion of such solutes as urea, K+, Na+, Ca2+ and inorganic phosphate (Pi). The fall in urine osmolality is mainly due to a reduction of free water reabsorption which is associated with a suppression of insensible water loss and the attendant inhibition of antidiuretic hormone (ADH) system. The increase in urea excretion may be associated with a reduction of urea reabsorption at the collecting duct as a consequence of ADH suppression. The rise in K+ excretion is due to a facilitated K+ secretion at the distal tubule as a result of increased aldosterone, urine flow and excretion of impermeable anions such as Pi. The activation of aldosterone system is partly attributed to a transient dehydration induced by early hyperbaric diuresis. The increase in Na+ excretion in the face of enhanced aldosterone secretion indicates that the Na+ transport in the proximal tubule is markedly inhibited (by unknown mechanism). The Pi excretion increases with no changes in plasma level of parathyroid hormone (PTH), thus it may be due to an inhibition of Na(+)-Pi cotransport in the proximal tubule. The increase in Ca2+ excretion may be secondary to the inhibition of Na+ transport at the proximal tubule. Precise information on the proximal tubular Na+ transport is important to understand the mechanisms of impaired solute transport under hyperbaric conditions.