In the mammalian brain, nitric oxide (NO) is responsible for a vasodilatory tonus as well as the elevation of cerebral blood flow (CBF) induced by hypercapnia. There have been few comparative studies of cerebral vasoregulation in lower vertebrates. Using epi-illumination microscopy in vivo to observe CBF velocity on the brain surface (cerebral cortex), we show that turtles (Trachemys scripta) exposed to hypercapnia (inspired PCO2 = 4.9 kPa) displayed a 62% increase in CBF velocity, while systemic blood pressure remains constant. Exposing turtles to a PCO2 of 14.9 kPa caused an additional increase in CBF velocity, to 104% above control values, as well as a 30% increase in systemic blood pressure. The elevated CBF velocity during hypercapnia could not be blocked by a systemic injection of the NO synthase (NOS) inhibitor NG-nitro-L-arginine (L-NA). However, L-NA injection caused a temporary stop in CBF as well as a persistent increase in systemic blood pressure, suggesting that there is a NO tonus that is attenuated by the NOS inhibitor and that CBF is strongly dependent on this tonus, although compensatory mechanisms exist. Thus, although the cerebrovascular reaction to hypercapnia appeared to be NO-independent, the results suggest that there is a NO-dependent vasodilatory tonus affecting both cerebral and systemic blood circulation in this species.

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