Effects of acetylcholine on peripheral vascular protein permeability

It is common practice to assume that when a vasodilator such as acetylcholine (ACh) produces a decrease in lymph/plasma protein ratio (R) while lymph flow (L) increases, permeability-surface area product (PS) and reflection coefficient (sigma) are unchanged. However, if PS and sigma are unaltered by alpha stimulus that increases L, then alpha decreased R can be associated with an elevated, constant, or reduced sigma and PS. To test what the effect of ACh, alpha "pure vasodilator," is in the hindquarters of the anesthetized dog, we infused 127 micrograms ACh min-1 into the abdominal aorta of 6 female mongrel dogs while collecting lumbar trunk lymph in order to measure L and R. sigma and PS were computed by the method of fluctuations over 15 min collections. The rise or decrease in LR was well correlated with L (r = .951) as expected, but was much less than predicted if sigma and PS had been unaltered during ACh infusion. Computations indicated that sigma rose with ACh approximately 34% (P = .0007) and PS rose 54% (P = .042) above control. This can be interpreted as a decrease in the radial dimension of the protein transport channels and an increase in the number of such channels per unit area, with both changes induced either by altered capillary anatomy or redistribution in a heteroporous system. Such an analysis seems compatible with the results of other studies in a variety of tissues which indicate that ACh tightens membranes either directly or through an effect mediated by reduced arterial pressure.