Release of an antidiuretic substance by bradykinin in the rat

1. Water-loaded rats under ethanol anaesthesia were used to determine whether bradykinin releases ADH and whether the release is a reflex response to hypotension or the result of direct stimulation of the central nervous system by the peptide.2. Intravenous injections of bradykinin caused hypotension followed by prolonged antidiuresis; both responses were dose dependent.3. The antidiuresis caused by bradykinin could not be accounted for by hypotension alone. Haemorrhage and intravenous injections of isoprenaline, which caused falls in arterial blood pressure similar to those caused by bradykinin, produced only short-lasting antidiuretic responses.4. Intracarotid injections of bradykinin caused antidiuresis with little or no preceding hypotension. No antidiuresis was produced by intracarotid injections of isoprenaline.5. After intravenous injection of bradykinin, small amounts of ADH were identified in whole blood and, on a few occasions only, in concentrated urine. However, ADH was not detected in blood extracts.6. After intravenous injection of nicotine in doses which caused antidiureses similar to that caused by bradykinin, ADH was regularly detected in large amounts in blood extracts and in unconcentrated urine.7. Bradykinin did not cause prolonged antidiureses when injected into a rat with congenital diabetes insipidus.8. ADH release was not increased when isolated rat neural lobes were incubated with bradykinin although release was elevated by an increase in the K(+) concentration of the incubation medium.9. It is suggested that bradykinin causes release of an antidiuretic substance by a direct action on the central nervous system. The possibility that this substance is ADH is discussed.