Wall stress: a determinant of regional differences in response of frog microvessels to norepinephrine.

GORE, ROBERT Mr. Wall stress: a determinant of regional di$erences in response of frog microvessels to norepinephrine. Am. J. Physiol. 222(l): 82-91. 1972.-It has been reported that arterioles respond more vigorously to norepinephrine (NE) than terminal arteries and arteries, but the cause of the phenomenon has not been resolved. This paper demonstrates that segmental differences in the response of frog mesenteric vessels to NE can be explained in terms of initial tangential stress (St) states normally extant in the vasculature. Simultaneous measurements of pressure (P) and internal radius-to-wall thickness ratios (ri/a) were made at points along the vascular tree from arteries to capillaries. Pressures were recorded with a servo-null system and dimensions were measured with either an image-splitting eyepiece or photographically. The distribution of St throughout the arterial vasculature was calculated from these measurements (St = P(ri/s)). Mean St was 1.2 •t 0.1 X lo5 dynes/cm2 in arterioles, 2.8 rf: lo5 in terminal arteries, and 3.7 % 0.1 X lo5 in arteries. Next, pressures and diameters were measured in 33 arterioles during iontophoretic application of constant, supramaximal doses of NE. Maximum constriction occured when St was in an optimum range (St*) 1 1.5 X lo5 dynes/cm2, but the magnitude of constriction was less in arterioles with St greater or less than St*. Further experiments, in which St was experimentally varied from 0 to 3 X lo5 dynes/cm2 in individual segments of the vascular tree, revealed that St* was constant for arterioles, terminal arteries, and presumably arteries. These data show that St in arterioles is normally equal to St*, so they develop maximum force to overcome distending pressure; whereas St in terminal arteries and arteries is normally greater than St*, so these larger vessels cannot develop their full constrictive potential.