Brief hypercapnia enhances somatosensory activation of blood flow in rat

Activation of CBF by hypercapnia or functional stimulation has been attributed to multiple mediators, most of which are thought to interfere with cerebrovascular reactivity in a closely time-related manner. Here we describe that brief hypercapnia produces marked up-regulation of somatosensory activation of blood flow that outlasts carbon dioxide exposure for at least 60 min. In chloralose-anesthetized, mechanically ventilated rats, somatosensory activation was carried out by electrical stimulation of the forepaw. Blood flow was measured in the contralateral primary somatosensory cortex by laser-Doppler flowmetry (LDF). Under control conditions, somatosensory stimulation increased LDF by 38.8 +/- 11.0%. Ventilation with 6% CO2 for 3 min caused a rise of LDF by 28.0 +/- 8.7%. Baseline CBF and PaCo2 returned to control values within 20 min. Repetition of somatosensory stimulation after hypercapnia revealed a long-lasting up-regulation of the flow response: 25 min after hypercapnia, functional stimulation increased LDF by 86.0 +/- 18.1%, and 60 min after hypercapnia even by 96.0 +/- 26.0%. This is the first demonstration of CO2-induced up-regulation of functional activation of blood flow and an example of the importance of general physiological variables for the modulation of the coupling process.