MYOGENIC RESPONSES OF ISOLATED RAT SKELETAL-MUSCLE VENULES - MODULATION BY NOREPINEPHRINE AND ENDOTHELIUM

The pressure-induced myogenic response of large venules of skeletal mu scle and its possible interactions with adrenergic receptor activation and endothelial factors have not yet been elucidated. Therefore, firs t-order venules of rat gracilis muscle were isolated, cannulated, and placed in an organ chamber. Changes in internal diameter of the vessel s as a function of perfusion pressure (PP) were obtained. In response to increases in PP (0.5-17.5 mmHg), the diameter of venules increased from 197.1 +/- 23.96 to 369 +/- 14.1 mu m. In passive conditions (in C a2+-free solution), the pressure-diameter curve of venules shifted sig nificantly upward. In the presence of norepinephrine (NE; 10(-6) M) in the bath solution, the pressure-diameter curve of active venules shif ted significantly downward, and in the pressure-normalized diameter cu rve, a negative slope developed (-6.1 +/- 4.6). In both the absence an d presence of NE, removal of endothelium significantly reduced venular diameters in the pressure ranges of 3-5 and 2-5 mmHg, respectively, b ut did not change significantly the characteristics of the pressure-di ameter curves. These findings indicate that the smooth muscle of venul es actively responds to changes in intraluminal pressure. This respons e is greatly facilitated by NE and modulated by the endothelium. The m yogenic response of skeletal muscle venules, especially in the presenc e of NE, could have a role in the regulation of the resistance and cap acitance of venules and, consequently, blood flow and tissue exchange in skeletal muscle.