EXAMINATION OF THE ROLE OF NITRIC-OXIDE FOR THE HYPERCAPNIC RISE OF CEREBRAL BLOOD-FLOW IN RATS

We examined the effect of nitric oxide synthase (NOS) inhibition and tetrodotoxin (TTX) on the increase of cerebral blood flow (CBF) in parietal (CoBF) and cerebellar cortex (CeBF) in response to hypercapnia. Rats were anesthetized with halothane and artificially ventilated. Hypercapnia was induced by adding 5% CO2 to the inhalation mixture. CoBF and CeBF were measured by laser-Doppler flowmetry. NOS inhibition was achieved by intravenous (30 mg/kg) and/or topical application (1 mM) of N-G-nitro-L-arginine (L-NNA). Activity in perivascular nerves around pial and cortical vessels was inhibited by topical application of TTX (20 mu M). Under control conditions, hypercapnia (66 +/- 1 mmHg) increased CoBF by 70 +/- 4% and CeBF by 96 +/- 5%. Systemic L-NNA decreased the baseline level of CoBF and CeBF by 11 +/- 3%, but topical L-NNA did not affect baseline flow. Intravenous L-NNA. attenuated the hypercapnic increase of CoBF by 77 +/- 5% and CeBF by 63 +/- 4% within 10-20 min. Topical L-NNA attenuated the hypercapnic increase of CoBF by 52 +/- 6% and CeBF by 29 +/- 5% after 45-min exposure. Both CoBF and CeBF decreased rapidly when L-NNA was infused during sustained hypercapnia, but not when L-NNA was applied topically. Effect of intravenous L-NNA was partially prevented by pretreatment with intravenous L-arginine. Intravenous or topical L-NNA enhanced the rise of CBF elicited by cortical spreading depression, adenosine (1 mM), or sodium nitroprusside (300 mu M), except in the cerebellum where topical L-NNA attenuated the rise of CBF elicited by adenosine by 53%. Topical acetylcholine (0.1-10 mu M) increased CoBF by 22 +/- 4%. This response was abolished by both intravenous and topical L-NNA. Topical TTX (20 mu M) decreased baseline CoBF by 23 +/- 2% and CeBF by 9 +/- 3% and increased the CO2 reactivity of CoBF by 40 +/- 8% and CeBF by 19 +/- 4%. Our data are consistent with the hypothesis that NO modulates CBF in hypercapnia. The NOS inhibitor, when blood borne, had a rapid onset of action and was far more efficient than when topically applied. The findings suggest that the hypercapnic rise of CBF is modulated by L-NNA via interaction with NOS in cerebral blood vessels.