The effect of exercise on the total water space and red blood cell volume of the gracilis muscle of the dog

The effect of increasing intensities of exercise on the water space and the red blood cell volume of the dog gracilis muscle was determined with ¹²⁵I-labelled iodoantipyrine (n = 8) and ^99mTc labelled red blood cells (n = 4). A sodium iodide scintillation probe attached to a two channel multiscaler and printer recorded the ¹²⁵I activity of the muscle as a measure of the water space and the ^99mTc activity as a measure of the red cell space. An electro-magnetic flow probe recorded muscle blood flow: aortic pressure was measured with a catheter in the mid-abdominal aorta. At least 30 min after i.v. administration of iodoantipyrine and red blood cells, the obturator nerve was electrically stimulated to produce muscle contraction at 1 contraction per second for 5 min. Data were recorded prior to exercise, during the 5 min exercise period, and for 10 min after exercise. During the greatest exercise period there was an eightfold increase in muscle blood flow, but only a small increase in [¹²⁵I]iodoantipyrine activity (2.7 ± 0.8%, 95% confidence limits) and ^99mTc red blood cell activity (3.2 ± 2.1 %). Both values promptly returned to baseline during recovery. These data show that there is a minimal change in the water space and blood volume of the exercising dog gracilis muscle.