Local temperature regulation of microtissue clearance from rat skeletal muscle.

• The tissue-clearance technique as a measure of effective blood flow through a tissue is obviously indirect, and the interpretation of results is necessarily based on the validity of several assumptions. One assumption is that injection into a "muscle" labels the fluid in a single uniform tissue, i.e., no connective tissue is involved. A second assumption is that clearance rate is relatively independent of the injection volume, and that the mechanics of injection do not measurably alter the circulatory features of the tissue immediately about the injection site. Warner et al. suggest that there is an inverse relationship between the size of the bleb and the clearance rate. A final assumption is that the tissue clearance is a valid, although indirect, measure of effective blood flow. A previous report from this laboratory validated some of these assumptions by a method extending the tissue-clearance technique to microinjection sites. The highly diffusible dye, water blue, was microinjected directly into the exposed mesentery of the frog, and the rate of disappearance was followed by means of a suitable microspectrophotometer. There was no directly observable disturbance of the capillaries in the area where the injection was made. Visual estimates of the blood flow correlated with measured clearance rates under control conditions and when the circulation was altered by topical applications of histamine or epinephrine. However, the absolute clearance in the frog mesentery (0.55 minr) was considerably higher than that reported from gross injection of Na and I in mammalian skeletal muscle (0.05 to 0.06 min.").' This discrepancy might be an artifact due to the smaller