Repeated constriction to respiration-induced hypocapnia is not mimicked by isocapnic alkaline solution in rabbit basilar artery in situ.

The purpose of this study was to test whether constriction of the cerebral vasculature in response to respiration-induced hypocapnia was mimicked by isocapnic alkaline solution. Since the regulation of the cerebral vasculature by hypocapnia necessitates vessels to constrict repeatedly in response to hypocapnic challenge, we tested whether repeated challenge with isocapnic alkaline solution was also associated with constriction. In contrast to our previous demonstration that repeated hypocapnic challenge elicited constrictions of similar magnitudes in rabbit basilar artery in situ, repeated challenge with isocapnic alkaline solution resulted in reduced constriction. Constriction to hypocapnia was also reduced following isocapnic alkaline solution. Since we previously demonstrated that constrictions to hypocapnia and isocapnic alkaline solution were endothelin-1 dependent, we tested whether the inhibition of hypocapnia- and isocapnic alkaline solution-induced constrictions following isocapnic alkaline solution was due to reduced endothelin-1 constriction. Endothelin-1 constriction was not reduced following isocapnic alkaline solution. Thus, constriction to isocapnic alkaline solution does not mimic constriction to hypocapnia. The results further suggest that the decreased constriction to isocapnic alkaline solution is due to blockade of endothelin-1 release, and that both hypocapnia and isocapnic alkaline solution share a common step in their endothelin-1 release pathways that can be inhibited by isocapnic alkaline solution.