Dependence of smooth muscle tone upon pulsatility in the iliac artery of the anaesthetised pig

The aim of the study was to examine features of the myogenic response of a conduit artery to the presence and absence of pulsatile pressure. The iliac arteries of 16 anaesthetised pigs (10 in control conditions, 6 under sympathetic blockade) were instrumented with flowmeter, sonomicrometry crystals for diameter measurement, a micro-tip manometer for pressure measurement and snares placed proximally and distally to the crystals to isolate a test segment from the remainder of the arterial system. When the snares were tightened to occlude the test segment, systemic arterial pressure remained constant. There was a large shift in the pressure-diameter relationship, in that there was a rapid decline in test segment pressure for the same diameter. This indicated arterial wall smooth muscle relaxation in response to removal of pulsatility of arterial pressure. The difference in mean pressure between pulsatility present and absent was significant (p<0.0001, paired t test, n=10). Before proximal and distal occlusion, test segment pressure was (mean±SD) 92.26±12.39 mmHg, whereas after distal and proximal occlusion at the same diameter, it was 42.34±10.87 mmHg. We conclude that in the presence of pulsatile pressure, there is a large proportion of arterial wall smooth muscle tone related to stretch of the arterial wall during the cardiac cycle, indicating that, under normal pulsatile pressure conditions, much of the normal tone can be attributed to the pulsatile component of the arterial myogenic response.