Evidence for the involvement of central alpha 2 mechanisms in intrathoracic volume expansion--induced diuresis: a study with clonidine and propranolol.

Intravenous (10 micrograms/kg) or intracisternal (1 microgram/kg) clonidine inhibited the diuretic response to negative pressure breathing (NPB) and left atrial distension (LAD) in chloralose anesthetized dogs. The drug reduced the induced tachycardia, but not the increase in respiratory rate caused by NPB, and did not change the blood pressure. Propranolol (1 mg/kg i.v.) did not change the NPB-induced diuresis. Intravenous yohimbine (1 mg/kg i.v.) opposed the effects of intravenous or intracisternal clonidine, whereas prazosin (0.05 mg/kg i.v.) had no effect. These results show that the adrenergic receptor implicated in the volumetric control of vasopressin secretion could be of the alpha 2 subtype. This alpha 2 adrenoceptor could be centrally located. Clonidine might therefore be proposed to combat the dehydration observed after long-term weightlessness.