The use of ultrastructural and histochemical techniques to correlate sympathetic activity with blood flow through cutaneous arteriovenous anastomoses in conscious sheep.

In conscious, chronically prepared sheep, blood flow through cutaneous arteriovenous anastomoses (AVAs) was varied by manipulating spinal cord temperature. Ultrastructural examination of nerve terminals or varicosities adjacent to the media of the AVA wall showed that they contained numerous small clear or dense-cored vesicles typical of noradrenergic vesicles in sympathetic nerves. During spinal cooling about 1% of femoral flow passed through AVAs, the AVAs showed more intense catecholamine fluorescence and the percentage of dense-cored vesicles and their density was significantly greater than during spinal warming; up to 50% of femoral flow passed through AVAs during spinal warming. Our results conform with pharmacological and electrophysiological evidence for alpha-adrenergic involvement in control of AVAs. This correlation between structure and function could provide a useful method for (a) obtaining relative indices of blood flow through AVAs in body regions where techniques to measure blood flow cannot be applied, and (b) studying sympathetic control of cutaneous AVAs in experimental conditions under which electrophysiological techniques cannot be applied.