Ionic responsiveness in third ventricular hypertonic stimulation of antidiuresis in ducks

Domestic ducks were chronically equipped with a device probing the third cerebral ventricle (V_III) for localized intracerebroventricular (i.c.v.) perfusion. In conscious animals made diuretic by intravenous water loading with 1.0 ml/min hypoosmotic glucose solution (200 mOsm/kg), hyperosmotic i.c.v. stimulations were tested for antidiuretic actions. Artificial cerebrospinal fluid made hypertonic (400 mOsm/kg) by adding sucrose, mannitol, NaCl, LiCl, choline chloride, NaI, NaNO₃, LiNO₃, CaCl₂ or MgCl₂ was perfused i.c.v. for 10-15 min at rates of 10-15 μl/min. Arterial pressure and heart rate were monitored continuously. Hyperosmotic stimulations with non-electrolytes did not induce antidiuresis. Approximately equivalent degrees of antidiuresis were elicited by Na⁺-, Li⁺- and choline salts with a tendency for moderate rises in arterial pressure. Compared to Cl^-- and I^--salts, the effects of NO₃^--salts were attenuated. Divalent cations caused prolonged antidiuresis, sometimes preceded by initial diuresis, with circulatory side effects unrelated to the changes in renal fluid excretion. It is concluded that the observed antidiuretic effects were mediated by cation-sensitive, rather than osmosensitive neurons on the brain side of the blood-brain-barrier. Their transduction mechanism might consist of poorly selective membrane channels permeable to cations but not to anions.