Differential effects of antihypertensive drugs on nutritive and nonnutritive blood flow in anaesthetized rabbits

Arteriovenous (AV) shunt vessels connect arterioles directly to the venous side. These vessels play a role in thermoregulation and in the pathophysiology of migraine. Tracer microspheres are excellent tools to study drug effects on AV anastomoses. Microspheres with a diameter of 15 microns are unable to pass through capillary vessels, but they readily cross AV shunt vessels and are transported to the lungs and trapped there. We investigated the effects of antihypertensive agents on AV shunt flow in anaesthetized rabbits and we also measured nutritional blood flow in the ear, an organ known to have both nutritional and AV shunt vessels. Nitroprusside sodium tended to increase and prazosin strongly increased AV shunt flow. The calcium antagonist isradipine, by contrast, decreased AV shunt flow even though it is also a vasodilator. Spirapril, an angiotensin-converting enzyme inhibitor, elicited similar effects. The centrally acting antihypertensive agent guanfacine strongly decreased AV shunt flow. These effects did not correlate with the drug effects on nutritional blood flow, which were, however, so variable that none of them reached statistical significance. The following trends were seen: nitroprusside sodium, isradipine, and guanfacine increased nutritional blood flow, but prazosin, a dilator of AV shunts, did not. The amount of blood crossing through AV shunts from the arterial to the venous side is considerable (up to 15% of cardiac output before drugs). A drug-induced increase is not associated with an apparent benefit but causes a hyperdynamic circulation that might contribute to the hemodynamic load of the heart.