Effect of hemorrhagic hypotension on cerebrovascular reactivity and infrastructure in the cat

Background and Purpose: The goal of this study was to determine the alterations in contractile and dilatory responses and ultrastructure of the feline middle cerebral artery after hemorrhagic hypotension. Methods: In the sodium pentobarbital anesthetized cats, a steady 50 mm Hg level of hypotension was reached by bleeding into a reservoir and maintained at this level by further bleeding or autotransfusion for 2 hours. Rings of the arteries, from control animals and from animals after hypotension, were suspended for isometric tension recording in organ chambers filled with modified Krebs-Henseleit solution, aerated with 95% O2-5% CO2 at 37°C, and their reactions to contractile and relaxant agents were tested. Vascular ultrastructure was studied by electron microscope. Results: Endothelium-dependent relaxations induced by 10-8 M acetylcholine were enhanced, whereas there was a marked inhibition of the relaxation at 10-6 M. Relaxations induced by adenosine triphosphate and adenosine showed an impairment. Contractions induced by norepinephrine and prostaglandin F2a remained unchanged, whereas 5-hydroxytryptamine caused a more pronounced contraction after hypotension. No alterations in the morphology of endothelium or smooth muscle were found after hemorrhage. There was, however, a marked decrease in the number of transmitter vesicles in the perivascular nerve terminals. Conclusions: The present results show marked alterations in cerebrovascular reactivity and ultrastructure of the adventitia after hypotension. These alterations might play an important role in the development of cerebral vasoconstriction during and after this hemorrhagic state.