Potassium transfer from brain to blood during sustained hyponatraemia in the calf.

1. The effects of hyponatraemia on cerebral blood flow, oxidative metabolism, and transfer of Na and K from the brain--c.s.f. compartment to blood have been examined in anaesthetized calves 2--6 weeks after birth. 2. Dilutional hyponatraemia was produced by administration of a long-acting antidiuretic hormone analogue (desmopressin) and the infusion of hexose solutions of various concentrations. Cerebral blood flow was measured using a hydrogen clearance technique, and metabolism and cation transfer quantified by simultaneous determination of arterio-cerebral venous concentration differences. 3. Sustained hyposmolar hyponatraemia (plasma osmolality, 232 +/- 1 m-osmole/kg; plasma Na, 117 . 1 +/- 0 . 5 m-mole/l.) was associated with a fall in cerebral blood flow, and increase in measured net transfer of K from the brain-c.s.f. compartment to the circulation. C.s.f. Na concentration and osmolality were both decreased. 4. No alterations in these variables occurred during sustained isosmolar hyponatraemia (plasma osmolality, 284 +/- 2 m-osmole/kg; plasma Na, 119 . 9 +/- 0 . 2 m-mole/l). 5. The results are discussed in relation to the route, mechanism and time course of K loss from brain during hyponatraemia.