Do vasoregulatory mechanisms in exercising human muscle compensate for changes in arterial perfusion pressure?

We tested the hypothesis that vasoregulatory mechanisms completely counteract the effects of sudden changes in arterial perfusion pressure on exercising muscle blood flow. Twelve healthy young subjects (7 female, 5 male) lay supine and performed rhythmic isometric handgrip contractions (2 s contraction/ 2 s relaxation 30% maximal voluntary contraction). Forearm blood flow (FBF; echo and Doppler ultrasound), mean arterial blood pressure (arterial tonometry), and heart rate (ECG) were measured. Moving the arm between above the heart (AH) and below the heart (BH) level during contraction in steady-state exercise achieved sudden approximately 30 mmHg changes in forearm arterial perfusion pressure (FAPP). We analyzed cardiac cycles during relaxation (FBF(relax)). In an AH-to-BH transition, FBF(relax) increased immediately, in excess of the increase in FAPP (approximately 69% vs. approximately 41%). This was accounted for by pressure-related distension of forearm resistance vasculature [forearm vascular conductance (FVC(relax)) increased by approximately 19%]. FVC(relax) was restored by the second relaxation. Continued slow decreases in FVC(relax) stabilized by 2 min without restoring FBF(relax). In a BH-to-AH transition, FBF(relax) decreased immediately, in excess of the decrease in FAPP (approximately 37% vs. approximately 29%). FVC(relax) decreased by approximately 14%, suggesting pressure-related passive recoil of resistance vessels. The pattern of FVC(relax) was similar to that in the AH-to-BH transition, and FBF(relax) was not restored. These data support rapid myogenic regulation of vascular conductance in exercising human muscle but incomplete flow restoration via slower-acting mechanisms. Local arterial perfusion pressure is an important determinant of steady-state blood flow in the exercising human forearm.