COMPARABLE EFFECTS OF ARTERIOLAR AND CAPILLARY STIMULI ON BLOOD-FLOW IN RAT SKELETAL-MUSCLE

Although the capillary wall represents an active interface between blo od and tissue, the potential role of the capillary in blood flow contr ol has not been determined. The goals were (i) to establish the presen ce of the capillary sensing and communication phenomenon (Dietrich and Tyml, Microvasc. Res. 43, 87-99, 1992) in mammalian microvasculature and (ii) to determine the relative sensitivity of the capillary and th e arteriole to locally applied vasoactive agents. Using intravital vid eo microscopy, norepinephrine (NE; 10(-7)-3 x 10(-3) M), acetylcholine (ACh; 10(-4)-10(-2) M), or bradykinin (BK; 10(-9)-10(-3) M) was appli ed via micropipettes on capillaries (300 mu m downstream from feeding arterioles) or on arterioles, at the surface of the extensor digitorum longus muscle of anesthetized rats. Red blood cell velocity (V-RBC) i n capillaries and arteriolar diameters was measured from video recordi ngs. The overall control V-RBC and control diameter were 190 mu m/sec and 8.3 mu m, respectively. NE applied on the capillary caused a dose- dependent reduction in V-RBC (UP to 100%, i.e., 0 mu m/sec) via a cons triction of the feeding arteriole. Both ACh and BK applied on the capi llary caused a dose-dependent increase in V-RBC (up to 115%) via arter iolar dilation. Based on two different approaches, these responses cou ld not be explained in terms of diffusion of agents from capillary to the arteriole. When testing for the relative sensitivity of the arteri ole and the capillary, application of NE and ACh on arterioles caused V-RBC and diameter responses similar to those of capillary stimulation s. When testing for the speed of response in these two microvessels, t he time of noticeable V-RBC change after NE (i.e., 10% from control) w as also similar. We concluded that (i) the rat skeletal muscle capilla ry could respond to a variety of locally applied materials and (ii) th e capillary could have as profound an effect on microvascular flow as the arteriole. Thus capillary could have the potential to participate in microvascular flow control. (C) 1997 Academic Press.