Adenosine response on pial arteries, influence of CO2 and blood pressure.

The television image-splitting technique was used to study the influence of arterial pCO2 and blood pressure on the dilatatory response of pial arterioles to topically applied adenosine in chloralose anaesthetised cats. At normocapnia (pCO2 congruent to 35 mm Hg) 10(-5) adenosine caused pial arteriole dilatation of 29.2 +/- 2.7% (S.E.M.). This was significantly reduced to 14.5 +/- 1.6% (P < 0.001) at pCO2 25 mm Hg and to 8.5 +/- 1.6% (P Ø 0.001) at pCO2 48 mm Hg. Lowering the blood pressure to 65--85 mm Hg had no significant effect on the adenosine response, but raising the blood pressure to 140--160 mm Hg significantly reduced the adenosine response to 22.1 +/- 1.8% (P < 0.005). The response was independent of vessel size except at hypertension where vessels < 150 micrometer were significantly more reactive than the larger vessels (P < 0.01). These results indicate that adenosine induced vasodilatation of pial arterioles shows little change in the face of alterations in vessel tone induced by altering blood pressure, but is markedly decreased by the combination of changing perivascular pH and vascular resistance through moderate changes in arterial pCO2. The importance of these results in assessing the role of adenosine as a cerebral vasodilator is discussed.