1. It has recently been shown that obstructive sleep apnoea (OSA) patients have increased urinary water and salt excretion during sleep which tends to normalize with nasal continuous positive airway pressure (CPAP) treatment.
2. To investigate the mechanisms of these changes in renal function, nocturnal urinary excretion of catecholamines and guanosine 3′:5′-cyclic monophosphate (cyclic GMP), which reflects atrial natriuretic factor (ANF) release, and next-morning plasma active renin concentrations were studied in 21 OSA patients on 2 consecutive nights, either untreated or treated with nasal CPAP.
3. In keeping with previous results, fractional urine flow and fractional Na+ and Cl− excretions were higher during untreated than during CPAP-treated nights.
4. No difference in plasma active renin concentration or in urinary excretion of noradrenaline, adrenaline, free dopamine and total dopamine could be demonstrated, but cyclic GMP excretion was significantly higher during untreated than during CPAP-treated nights.
5. The data are consistent with the hypothesis that the increased water and salt excretion in OSA patients is due to increased ANF release.
6. The proposed mechanism is an atrial distension due to increased (more negative) intrathoracic pressures during ineffective inspiratory efforts against the occluded upper airways which have been found in OSA.