Regional distribution of blood flow in calf muscles of rat during passive stretch and sustained contraction.

The regional distribution and number of microspheres in the calf muscles of rat has been studied during isometric sustained contraction and in stretched uncontracted muscles in situ. Carbonized microspheres, 15 +/- 5 mum, were injected into the aortic arch and muscle blood flow arrested 6 sec later. The calf muscles were freeze sectioned (12 slices of 40 mum) and the microspheres counted microscopically. The microsphere concentration in the gastrocnemius and plantaris muscles during rest was 4.6 +/- 1.6 spheres/mm3 (mean +/- S.E.). One min after a standardized exercise programme the sphere concentration was increased to 20.5 +/- 3.9 spheres/mm3. At increasing force of contraction following the standard exercise programme, the microsphere concentration fell from 11.6 +/- 2.5 at 25% of maximal force of contraction (MFC) to 2.2 +/- 0.6 spheres/mm3 at 100% MFC (5.6 kg/cm2). Corresponding measurements in stretched, uncontracted muscles showed a similar fall in microsphere concentration in the central inner zone and in the peripheral outer zone was slightly higher than unity (1.08-1.16) in muscles at rest and at light postexercise hyperemia. At 75 and 100% MFC the ratio was 0.76 +/- 0.07 and 0.57 +/- 0.13, significantly lower than unity. Stretching of the uncontracted muscle group to 175% MFC reduced the ratio towards zero. The greater reduction in blood flow to the inner central zone of contracted calf muscles shown by microsphere distribution was confirmed by measurement of 125 I-antipyrine distribution. These results show an increased resistance against blood flow during active contraction or stretching of the calf muscles, most pronounced in the central inner zone at high tensions.