Cerebral microvascular dilation during hypotension and decreased oxygen tension: A role for nNOS

thelial (eNOS) and neuronal nitric oxide synthase (nNOS) are impli-cated as important contributors to cerebral vascular regulation through nitric oxide (NO). However, direct in vivo measurements of NO in the brain have not been used to dissect their relative roles, particularly as related to oxygenation of brain tissue. We found that, in vivo, rat cerebral arterioles had increased NO concentration ([NO]) and diam-eter at reduced periarteriolar oxygen tension (PO2) when either bath oxygen tension or arterial pressure was decreased. Using these pro-tocols with highly selective blockade of nNOS, we tested the hypoth-esis that brain tissue nNOS could donate NO to the arterioles at rest and during periods of reduced perivascular oxygen tension, such as during hypotension or reduced local availability of oxygen. The decline in periarteriolar PO2 by bath manipulation increased [NO] and vessel diameter comparable with responses at similarly decreased PO2 during hypotension. To determine whether the nNOS provided muc