CAPILLARY ADRENOCEPTORS IN RAT SKELETAL-MUSCLE

The purpose of this study was to examine whether functional alpha- and beta-adrenoceptors exist on capillaries of rat skeletal muscle, and f urther to determine which subtype of these receptors predominates on t hese capillaries. Using intravital video microscopy, we measured red b lood cell velocity (V-RBC) responses in capillaries of rat extensor di gitorum longus muscle (EDL) following a local application of these ago nists: norepinephrine (NE; alpha(1), alpha(2); 10(-7) to 3 x 10(-3) M) , phenylephrine (PE; alpha(1); 3 x 10(-4) to 10(-2) M), clonidine (CLO ; (alpha(2); 3 x 10(-3) to 10(-2) M), UK14304 (alpha 2; 3 x 10(-4) to 10(-2) M), and isoproterenol (IPR; beta 1, beta 2; 10(-7) to 3 x 10(-3 ) M). Responses to NE (10(-5) M) were also measured after a local pret reatment with prazosin (alpha(1) antagonist; 10(-5) to 10(-3) M) and r auwolscine (alpha(2) antagonist; 3 x 10(-3) to 3 x 10(-2) M), while re sponses to IPR (10(-5) M) were measured after local atenolol (ATE; bet a(1) antagonist; 10(-3) to 10(-2) M) and butoxamine (BUT; beta(2) anta gonist; 10(-3) to 10(-2) M) pretreatment. The overall control V-RBC wa s 226 mu m/sec. NE, PE, CLO, and UK14304 resulted in concentration-dep endent decreases of V-RBC (from -12 to -89%) from the control level, w hile IPR caused concentration-dependent increases (17 to 174%). PE red uced V-RBC to a larger degree than CLO and UK14304. NE-induced V-RBC r esponses tended to be attenuated more by prazosin than by rauwolscine. Both ATE (10(-2) M) and BUT (10(-3) and 10(-2) M) alone decreased V-R BC However, only ATE significantly attenuated the IPR-induced V-RBC re sponses. These results suggest that the capillary of rat EDL muscle ha s alpha- and beta-adrenoceptors. From the two alpha-adrenoceptor subty pes, the capillary may be predominated by the alpha(1)-adrenoceptors. (C) 1997 Academic Press.