AJP - Heart and Circulatory Physiology, Vol 253, Issue 2 402-H411, Copyright © 1987 by American Physiological Society
Length-dependent sensitivity of vascular smooth muscle in normotensive and hypertensive animals
E. Coskinas and J. M. Price
Ring segments were obtained from the aorta of spontaneously hypertensive
(SHR), Wistar-Kyoto (WKY), and Wistar (WST) rats and from the anterior
tibial artery of normotensive and hypertensive dogs. Norepinephrine
dose-response curves were generated at lengths relative to that for maximum
force (Lmax) and at a specific preload (in grams). The doses of stimulus
necessary to elicit 10 and 50% maximum force of contraction (ED10 and ED50)
are greater at lengths less than Lmax than at Lmax for all groups. When SHR
rings are compared with WKY or WST rings, ED10 or ED50 is the same at
either length. The same result was found in comparing normal and
hypertensive dogs. ED10 and ED50 from hypertensive groups were the same as
their respective control groups when compared at the same preload. We
conclude that sensitivity of the rat aorta and dog anterior tibial artery
to norepinephrine depends on muscle length, this relationship is not
altered in hypertension, and the conclusion on sensitivity in hypertension
is the same for excised rings compared at the same length or at the same
preload. Contractility (active stress) was lower (rats) or the same (dogs)
but thickness was greater in the hypertensive groups than in their
controls. The results show that a change in the vessel wall that can cause
increased flow resistance and pressure is not in sensitivity or
contractility but in wall thickness. Apparent differences in sensitivity
from normal and hypertensive animals can be explained by an unchanged
length-dependent sensitivity. The results also show that differences in
body weight will affect a comparison of wall tension but not of
sensitivity.
