Regulation of cerebrovascular compliance compared to forearm vascular compliance in humans: a pharmacological study.

Increasing evidence indicates that cerebrovascular compliance contributes to the dynamic regulation of cerebral blood flow but the mechanisms regulating cerebrovascular compliance in humans are unknown. This retrospective study investigated the impact of neural, endothelial, and myogenic mechanisms on the regulation of vascular compliance in the cerebral vascular bed compared to the forearm vascular bed. An index of vascular compliance (Ci) was assessed using a Windkessel model applied to blood pressure waveforms (finger photoplethysmography) and corresponding middle cerebral artery blood velocity or brachial artery blood velocity waveforms (Doppler ultrasound). Data were analyzed during a 5-minute baseline period (10 waveforms) under control conditions and during distinct sympathetic blockade (Experiment 1: phentolamine; 10 adults), cholinergic blockade (Experiment 2: glycopyrrolate; 9 adults), and myogenic blockade (Experiment 2: nicardipine; 14 adults). Experiment 1: Phentolamine increased Ci similarly in the cerebral vascular bed (131 ± 135%) and forearm vascular bed (93 ± 75%; P = 0.45). Experiment 2: Glycopyrrolate increased cerebrovascular Ci (72 ± 61%) and forearm vascular Ci (74 ± 64%) to a similar extent (P = 0.88). Experiment 3: Nicardipine increased Ci but to a greater extent in the cerebral vascular bed (88 ± 88%) and forearm vascular bed (20 ± 45%; P = 0.01). Therefore, adrenergic, cholinergic, and myogenic mechanisms contribute to the regulation of cerebrovascular and forearm vascular compliance. However, myogenic mechanisms appear to exert more specific control over vascular compliance in the brain relative to the forearm.