Microvascular flow response to localized application of norepinephrine on capillaries in rat and frog skeletal muscle.

Recently, Dietrich (1989, Microvasc. Res. 38, 125-135) demonstrated that a local application of a minute amount of norepinephrine (NE, 5.5 mM, 0.01-88 pmole) on a capillary in rat mesentery can elicit constriction of the feeding arteriole 0.5-1.0 mm away. This constriction can reduce or even stop blood flow in capillaries supplied by the arteriole. The main objective here was to show that the phenomenon of reduced flow occurs not only in the rat mesentery but also in other tissues and species. We chose to study the rat tibialis anterior and frog sartorius muscles. Using the same intravital video-microscopic approach as in the mesentery, strong NE stimuli (3 mM) were applied iontophoretically 48 times to 19 capillaries in 10 rats anesthetized with pentobarbital. They resulted in significant reductions (average: 80%) of the red blood cell velocity (VRBC) in capillaries. The onset of these reductions (i.e., 10% decrease from control) occurred within 3-52 sec (average: 20.9 sec) from the time of NE application. Reductions lasted 6.0 min. The same stimuli were applied 42 times to 15 capillaries in 6 frogs anesthetized with urethane. The average VRBC reduction was 86%. The onset occurred within 30.6 sec while the reduction lasted 6.6 min. Under the same conditions, arteriolar diameters in the sartorius muscle decreased significantly from 28.5 to 22.5 microns (n = 8). We also used local microinjection of small droplets of NE (30 mM) to 13 capillaries in 7 frogs. This resulted in a significant VRBC reduction of 64% with an onset time of 44.2 sec and a reduction duration of 17.2 min. Weak NE stimuli (3 microM) applied iontophoretically to 10 capillaries in 5 frogs resulted in marginal, but significant, VRBC reductions (9%). The present study demonstrates that the phenomenon of reduced flow after local application of NE may be a general phenomenon as it occurs also in skeletal muscle in both rat and frog. Our accompanying paper addresses the hypothesis that the phenomenon reflects communication of a NE-induced signal along the capillary.