Nitric oxide synthase does not contribute to cerebral autoregulatory phenomenon in anesthetized dogs

Nitric oxide (NO) is a potent vasodilator produced by nitric oxide synthase (NOS). We tested the following hypotheses: (1) cerebral blood flow (CBF) is NO dependent, (2) NO contributes to CBF autoregulation, and (3) NO participates in the neurohypophysial vasodilator response to hypotension. Three groups of sodium pentobarbital anesthetized dogs were studied using microspheres. In 7 dogs, N^ω-nitro-l-arginine methyl ester (l-NAME; 40 mg/kg, i.v.) increased mean arterial pressure (MAP) by 12%. Cerebrovascular resistance (CVR) increased more than MAP, resulting in a 20 ± 4% reduction (range 12--;67%) in baseline CBF. In unblocked conditions, actively autoregulated regions (e.g. cortex, white matter, median eminence) demonstrated a correlation between CVR and MAP whereas passive regions (neural lobe) did not. NOS block did not effect the relationship between MAP and CVR in most brain regions. However, a significant relationship between CVR and MAP developed in neural lobe after NOS block. Abrupt hypotension increased neural lobe blood flow to 239 ± 37% control at 3 min, despite NOS block. These results show that baseline cerebral vessel tone depends upon NOS activity. Enhanced NO release cannot explain either cerebral autoregulation or the transient hyperperfusion seen in neural lobe immediately following rapid hemorrhage.