Nitric oxide-sensitive guanylyl cyclase signaling affects CO2-dependent but not pressure-dependent regulation of cerebral blood flow

Abstract

Cerebrovascular CO₂ reactivity is affected by nitric oxide (NO). We tested the hypothesis that sildenafil selectively potentiates NO-cGMP signaling, which affects CO₂ reactivity. Fourteen healthy males (34 ± 2 yr) were enrolled in the study. Blood pressure (BP), ECG, velocity of cerebral blood flow (CBF; measured by transcranial Doppler), and end-tidal CO₂ (EtCO₂) were assessed at baseline (CO₂ ~39 mmHg), during hyperventilation (CO₂ ~24 mmHg), during hypercapnia (CO₂ ~46 mmHg), during boluses of phenylephrine (25-200 µg), and during graded head-up tilting (HUT). Measurements were repeated 1 h after 100 mg sildenafil were taken. Results showed that sildenafil did not affect resting BP, heart rate, CBF peak and mean velocities, estimated regional cerebrovascular resistance (eCVR; mean BP/mean CBF), breath/min, and EtCO₂: 117 ± 2/67 ± 3 mmHg, 69 ± 3 beats/min, 84 ± 5 and 57 ± 4 cm/s, 1.56 ± 0.1 mmHg·cm^-1·s^-1, 14 ± 0.5 breaths/min, and 39 ± 0.9 mmHg, respectively. Sildenafil increased and decreased the hypercapnia induced in CBF and eCVR, respectively. Sildenafil also attenuated the decrease in peak velocity of CBF, 25 ± 2 vs. 20 ± 2% (P < 0.05) and increased the eCVR, 2.5 ± 0.2 vs. 2 ± 0.2% (P < 0.03) during hyperventilation. Sildenafil did not affect CBF despite significant increases in the eCVRs that were elicited by phenylephrine and HUT. This investigation suggests that sildenafil, which potentiates the NO-cGMP signaling, seems to affect the cerebrovascular CO₂ reactivity without affecting the static and dynamic pressure-dependent mechanisms of cerebrovascular autoregulation.

Keywords: CO2 reactivity; PDE5 inhibitor; autoregulation; cerebral blood flow; sildenafil.