CENTRALLY-INDUCED VASOPRESSOR RESPONSES

Dihydroxyouabain (ouabain) , 1.0-10 pg, per rat, injected intracerebroventricularly, produced dose-related vasopressor responses accompanied by corresponding increases in abdominal sympathetic n erve activity in 16 weeks old Wistar (NT) rats anesthetized with urethane. The heart rate then also increased, dose-dependently, to ouabain injected in doses up to 10 pg. However , 100 pg produced arrhythmia resulting in bradycardia. Pressor effects were appreciable within one minute after the ouabain injection, but did not become maximal until between 7-10 min later. Either the removal of sympathetic vasomotor tone by surgical s ection of the s pinal cord or intravenous pretreatment with a vasopressin antagonist significantly reduced the vasopressor r esponses in the NT rat. Ouabain, 10 pg, injected intraventricular ly in 16 weeks old Kyoto Wistar rats produced similar cardiovascular responses to those in the NT rat, but the magnitude of the blood pressure responses, along with the h eart rate and sympathetic r esponses, was larger in SHR than in WKY. These results suggest that dihydroxyouabain acts centrally to elevate the blood pressure by increasing n ot only the sympathetic discharge but also, perhaps, the secretion of vasopressin. In light of previous studies showing that SHRs exhibit both sympathetic hyperactivity and hypersecretion of vasopressin, the present results suggest that their enhanced responsiveness to ouabain could result from both the sympathetic hyperactivity and an enhanced vasopressin release as a result of the c entrally injected ouabain. 1499

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