Cutaneous blood flow measurements: A standardization of the microsphere assay for vasoactive agents

Abstract

Other studies have shown that the number of isotopically labelled microspheres localized in a region, following injection into the left heart, is a function of the relative blood flow to that region. The present studies show that the number of 10 μ size¹¹³Sn-labelled microspheres impacted in skin and various organs of rabbits under urethane anesthesia is directly proportional to the number of injected, over a wide range, with no evidence of saturation of the microcirculatory bed. At the lowest dose tested, there were 300 microspheres per skin site and at the highest dose an equalsized skin site contained 15,000 microspheres. Saline-injected test sites assayed 45 min after injection were not significantly different from uninjected sites. It was found that a standardized 177 mm² area of skin received 0.023±0.001% of the cardiac output. To assay various agents for their effect on dermal blood flow and to determine the time course of the effects, it was convenient to express the radio-activity of the test site as a ratio relative to the average radio-activity of uninjected skin sites. Standard errors less than 10% of the mean could be obtained when 13×10⁶ microspheres were injected into the left heart via a catheter in the carotid artery, providing 4 to 6 replicate test sites were averaged. It was possible to directly compare lesions from groups of animals. Enhanced blood flow was produced by the injection of histamine and bradykinin. The effect was transient and subsided by 20 min. Prostaglandin E₁ was the most potent mediator tested and its action lasted between 1–2 h. Casein, calcium pyrophosphate, carrageenan, endotoxin and glycoge were injected and their effect assayed at the arbitrary time of 100 min. Casein produced a 3-fold enhancement of blood flow. Calcium pyrophosphate gave a positive but mild (less than 2-fold) effect. The other agents had no effect. The intradermal injection of adrenalin significantly reduced the blood flow to normal skin, but this effect was confined to the site and did not affect adjacent test sites. The most pronounced blood flows were obtained following the injection of Freund's complete adjuvant and the flow to such sites was 6 to 9 times normal, from 1 to 14 days after injection. Triprolidine-HCl administered systemically significantly depressed the blood flow to normal skin, but this was overcome by the intradermal injection of casein, bradykinin and PGE. This assay has potential application to study the kinetics and the inhibition of this fundamental component of the inflammatory response.