Effect of locally applied epinephrine and norepinephrine on blood flow and diameter in capillaries of rat mesentery.

Epinephrine (5.5 mM; quantity: 0.01-87 pmole) and norepinephrine (5.9 mM; quantity: 0.01-88 pmole) were applied to 144 and 26 mesenteric capillaries, respectively, in 39 rats. Neither of these agents had a significant effect on the mean capillary diameter (control diameter before epinephrine application: 9.6 +/- 3.6 microns SD; before norepinephrine application: 11.4 +/- 6.6 microns SD). Unexpectedly, in 36% of the epinephrine applications and 49% of the norepinephrine applications, capillary blood flow was either reduced (epinephrine: within 6-61 sec; norepinephrine: within 3-36 sec) or stopped completely (epinephrine: within 8-124 sec; norepinephrine: 11-124 sec). These reductions (duration: 12 sec-32 min for epinephrine; 21 sec-12 min for norepinephrine) were associated with flow reductions in all neighboring capillaries fed by a common supplying arteriole. In order to check the possible direct effect of drug diffusion norepinephrine was applied at increasing distances up to 400 microns from the arteriole. For all such distances arterioles constricted much quicker than predicted by diffusion. Fluorescent staining using glyoxylic acid showed the presence of adrenergic nerves around arterioles, along capillaries as well as in the avascular tissue. It is concluded that conduction along nerves, or possibly via endothelial cell-to-cell communication, could explain the intriguing retrograde effect of locally applied drugs on the supplying arteriole.