AJP - Heart and Circulatory Physiology, Vol 257, Issue 5 1360-H1368, Copyright © 1989 by American Physiological Society
Absent effect of plasma vasopressin on rat brain blood flow during hemorrhage
M. Nakai, Y. Yamane, Y. Umeda, M. Inada, J. Yamamoto and M. Kawamura
Department of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, Osaka, Japan.
We investigated whether a reflex increase in plasma vasopressin level due
to hemorrhagic hypotension affects brain blood flow. In 60 lightly
anesthetized, artificially ventilated rats, the flow was determined with
radiolabeled microspheres. We found excellent maintenance of blood flow
throughout all brain regions during the hypotensive state (71 mmHg on
average), and such maintenance of flow was not modulated at all by a
supramaximal intravenous dose of the selective vasopressin V1-receptor
antagonist [d(CH2)5 Tyr-(Me)]AVP. The latter finding also implies that the
V1 antagonist failed to unmask the vasodilator type actions of V2 receptors
on the maintenance of flow during hemorrhagic hypotension. These were true
also when the cervical sympathetic bundles were severed bilaterally. The
plasma level of endogenous vasopressin was increased during hypotension,
ranging from 118 to 973 pg/ml. Despite this increase, the brain blood flow
was entirely independent of the plasma vasopressin level in all the brain
regions studied. We conclude that the brain circulation of rats can
maintain its blood flow during hemorrhagic hypotension without any apparent
contribution from a concomitant reflex increase in plasma vasopressin.
Despite our negative results for the brain blood flow, the possible
segmental effects of circulating vasopressin on the brain arterial caliber
remain to be clarified under conditions of hemorrhagic hypotension.
