DIAMETER DEPENDENCE OF MYOGENIC TONE OF HUMAN PIAL-ARTERIES - POSSIBLE RELATION TO DISTENSIBILITY

Background and Purpose Responses to changes in intraluminal pressure o f isolated human pial arteries (200 to 1200 mu m i.d.) obtained from p atients undergoing neurosurgery were measured. Methods The vessels wer e cannulated and pressurized (60 mm Hg); vascular diameter and intralu minal pressure were recorded simultaneously, After spontaneous develop ment of steady state tone, intraluminal pressure was changed to both h igher and lower levels in random sequence. Results Human pial arteries exhibited myogenic responses and maintained their diameter over the p ressure range of 20 to 100 mm Hg. The level of myogenic tone observed at 30 mm Hg did not vary significantly with artery diameter. In contra st, at 60 and 90 mm Hg, the extent of myogenic tone increased as the d iameter decreased (up to 70% to 80% of maximum in 200-mu m i.d. arteri es). The arteries contracted to KCI 30 mmol/L, norepinephrine 1 mu mol /L, and vasopressin 0.1 mu mol/L and relaxed to acetylcholine 3 mu mol /L. The extent of these responses did not vary with the diameter of th e artery. Arterial distensibility, represented by the slope of the tan gent of the passive pressure-diameter curve at lower pressures (5 to 5 0 mm Hg), increased as arteries became smaller, This is consistent wit h the possibility that the level of myogenic tone is related to vessel distensibility. Human omental arteries of comparable size did not dev elop myogenic tone but contracted to KCl and norepinephrine and relaxe d to acetylcholine to an extent similar to pial arteries. Conclusions There is a specific gradient of myogenic responsiveness in human pial arteries that varies inversely with their diameter. This tone does not develop in all vascular beds. These levels of tone in the pial circul ation would be expected to he of profound functional significance by a llowing blood flow to vary widely.