Metabolic vasodilatation in skeletal muscle and the problem of the direct or indirect action of oxygen on resistant blood vessels.

1. Vasodilatation induced by venous blood infusion and reactive hyperaemia following the reduction of perfusion pressure for different lengths of time (1 to 128 sec) were studied in the haemodynamically isolated and denervated vascular bed of the dog gracilis muscle. The two local regulatory situations differed chiefly in respect to the oxygen concentration in the precapillary part of the blood bed (51.4% and 97.6% respectively) and in the size of the blood flow (100% and 48.5% of the resting value). 2. The interval between the time when venous blood entered the resistant precapillary vessels and the onset of the local regulatory response was 25.1 +/- 1.9 seconds. In prolongation of the duration of reduced perfusion pressure, the continuous increase in the maximum reactive hyperaemia value was interrupted, on reducing it for 16-32 seconds, by a significant (P less than 0.001) abrupt increase in this value. 3. We conclude from the good agreement of these two time values that: 1. reduction of the blood oxygen concentration to 51.4% does not directly affect the smooth muscle cells in the wall of resistant vessels; 2. an interval of 25.1 +/- 1.9 sec (16-32 seconds) is needed for interference with the metabolism of skeletal muscle to attain a critical value and for the relevant chemical signal to produce a dilatation response in the arterioles.