[The differences of the cerebral and spinal vessels in sensitivity to PaCO2 and vasoconstrictors]

We investigated the differences in the response to arterial CO2 tension and vasoconstrictors between the cerebral and spinal vasculatures using cranial and spinal window technique which provided the direct observation of pial vessels. Pentobarbital anesthetized dogs (n = 18) (CO2 tension; n = 6 and vasoconstrictor; n = 12) were instrumented for measurement of pial vessel diameters by intravital microscopy in the cranial and spinal window preparation simultaneously. We achieved hypocarbia (20-25 mmHg) followed by adjusting CO2 levels for normocarbia (35-40 mmHg) and for hypercarbia (55-60 mmHg) using CO2 gas addition. After obtaining the desired PaCO2, the measurements were made. In the next experiment, we administered 2 different concentrations of epinephrine or phenylephrine solutions (1:2 x 10(6), 1:2 x 10(5)) through the window, and the measurement was made sequentially. The response of cerebral and spinal vasculature to change in PaCO2 was almost similar. Topical application of both drugs produced a significant constriction of spinal pial arterioles compared with the cerebral ones, while epinephrine but not phenylephrine constricted cerebral pial venules compared with spinal ones. These results suggest that the responses to vasoconstrictor of cerebral and spinal pial vessels are not similar, and high sensitivity of spinal arterioles to vasoconstrictors may possibly contribute to a risk of ischemic damage of the spinal cord.