The action of sodium nitroprusside on the pial vessels.

This study in cats investigates the action of sodium nitroprusside on the pial vessels by systemic and local administration, with an intravital microscopic window technique and a photometric technique for graphic documentation of vessel diameter changes. Intravenous infusion caused vasodilatation parallel with decreasing blood pressure. Pial arterioles dilated more than venules, smaller vessels more than larger ones. Local administration caused maximal dilatation within 5--10 seconds. With blood pressure returning to normal after i.v. therapy, pial vessels remained wider than they were before hypotension at the same pressure level. From these data it is concluded that the substance acts longer on the brain vessels than on vessels elsewhere in the body, and that cerebrovascular autoregulation to blood pressure changes is disturbed during this period.