Relative contributions of passive and myogenic factors to diameter changes during single arteriole occlusion in the hamster cheek pouch.

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Abstract
To evaluate the relative influence of myogenic and passive effects on arteriolar dilation during interruption of blood flow, single arterioles of the suffused hamster cheek pouch were occluded while diameters were measured upstream and downstream from the occlusion. Arteriolar pressures, measured by the servo null method, increased 22.5% upstream from the occlusion and decreased 34% downstream. Dilation upstream from occlusions was invariably greater than that downstream, even when suffusion solution PO2 [partial O2 pressure] was increased to high levels that resulted in a decreased initial diameter and a reduction in the absolute increase in diameter during occlusion. In some experiments arterioles were initially occluded for 60 s upstream or downstream from the point of observation; then this occlusion was abruptly released and was replaced by another on the other side of the observation site. Diameter increases upstream from occlusions were again greater than those downstream. Abrupt changes in diameter, apparently of a passive nature, were correlated with the shift in pipette position; no secondary diameter changes indicating myogenic mechanisms were observed consistently during either high or low O2 suffusion. The average dilation for all experiments (single and double occlusions) was to 134% of control at upstream sites and to 122% of control at downstream sites. Passive effects apparently contributed 6-12% to the diameter changes during occlusion. Transient diameter changes following brief occlusions suggested that the arterioles may be capable of myogenic responses; passive and metabolic factors are apparently predominant in determining the steady state response to longer occlusions.