Middle cerebral artery blood velocity, arterial diameter and muscle sympathetic nerve activity during post-exercise muscle ischaemia.

During exercise the transcranial Doppler determined mean blood velocity (Vmean) increases in the middle cerebral artery (MCA) and reflects cerebral flood flow when the diameter at the site of investigation remains constant. Sympathetic activation could induce MCA vasoconstriction and in turn elevate Vmean at an unchanged cerebral blood flow. In 12 volunteers we evaluated whether Vmean relates to muscle sympathetic nerve activity (MSNA) in the peroneal nerve during rhythmic handgrip and post-exercise muscle ischaemia (PEMI). The luminal diameter of the dorsalis pedis artery (AD) was taken to reflect the MSNA influence on a peripheral artery. Rhythmic handgrip increased heart rate (HR) from 74 +/- 20 to 92 +/- 21 beats min-1 and mean arterial pressure (MAP) from 87 +/- 7 to 105 +/- 9 mmHg (mean +/- SD; P < 0.05). During PEMI, HR returned to pre-exercise levels while MAP remained elevated (101 +/- 9 mmHg). During handgrip contralateral MCA Vmean increased from 65 +/- 10 to 75 +/- 13 cm s-1 and this was more than on the ipsilateral side (from 63 +/- 10 to 68 +/- 10 cm s-1; P < 0.05). On both sides of the brain Vmean returned to baseline during PEMI. MSNA did not increase significantly during handgrip (from 56 +/- 24 to 116 +/- 39 units) but the elevation became statistically significant during PEMI (135 +/- 86 units, P < 0.05), while AD did not change. Taken together, during exercise and PEMI, Vmean changed independent of an elevation of MSNA by more than 140% and the dorsalis pedis artery diameter was stable. The results provide no evidence for a vasoconstrictive influence of sympathetic nerve activity on medium size arteries of the limbs and the brain during rhythmic handgrip and post-exercise muscle ischaemia.