Endogenous nitric oxide influences arteriovenous anastomosis adrenergic tone in the conscious rabbit ear

The role of nitric oxide (NO) in the control of arteriovenous anastomoses (AVAs) has not been studied in vivo in a thermoregulatory end organ. In this study, the effect of local inhibition of NO synthesis by NG-nitro-L-arginine methyl ester (L-NAME) on the microvasculature in the rabbit ear (n=12) was observed in vivo through a chronically implanted ear microvascular chamber. Ear cutaneous blood perfusion (CBP), total auricular arterial flow (TAF), and ear temperature were monitored simultaneously with the direct microvascular observations. Results revealed that intrafacial artery infusion of L-NAME produced significant vasoconstriction of arterioles, AVAs, and venules (p < 0.05). A decrease of ear blood perfusion also was demonstrated by changes of CBP, TAF, and surface temperature. The data provide evidence that basal generation of NO influences the vascular resistance in the thermoregulatory end organ. Moreover, endogenous NO production may be more important in regulating the AVA flow than is flow in other parts of the rabbit ear microvasculature. The effects of NO inhibition on ear microvasculature were not abolished by superior cervical ganglionectomy, indicating that NO production in the rabbit ear is not a neurally mediated mechanism. Further study with a short-term rabbit ear preparation showed that inhibition of NO production with L-NAME enhanced microvascular constrictive responses to extraluminal application of norepinephrine. NO thus appears to play a role of basal vasodilator in opposition to the basal adrenergic vasoconstrictor tone in the rabbit ear.