A method for using microspheres to measure muscle blood flow in exercising rats

A catheter-implantation procedure allowing use of the radiolabeled microsphere (MS) technique for measuring skeletal muscle blood flow (BF) in rats during high-speed treadmill running was desired. Attempts to use existing procedures were unsuccessful. We found that Silastic catheters (0.02 in. ID X 0.037 in. OD) placed in the ascending aorta (for MS infusions) and the renal artery (for reference sample withdrawal) minimized these exercise performance problems. It was then necessary to establish that aortic MS infusions result in good MS-blood mixing. We tested the method with the following: 1) the radioactivities found in reference withdrawal samples taken from two locations in the aorta were compared after left ventricular (LV) infusion and after aortic infusion; 2) BFs to bilaterally paired tissues were compared in anesthetized and conscious rats with LV and aortic infusions; 3) the distribution of MSs in the muscles was studied histologically; and 4) BFs in bilaterally paired tissues were compared in rats with aortic MS infusions during treadmill running. The results indicate that 1) the percent difference between the radioactivities found in the proximal and distal reference withdraw samples was the same for LV and aortic MS infusions; 2) BF to bilaterally paired tissue samples was the same with both LV or aortic MS infusions; 3) the MSs were distributed uniformly within muscles, and MS aggregation was not a significant problem; and 4) BFs to bilaterally paired tissue samples were the same in exercising rats. We conclude that this technique can be used to measure muscle BF in rats running on a treadmill.