MUSCLE LENGTH DIRECTS SYMPATHETIC-NERVE ACTIVITY AND VASOMOTOR TONE IN RESISTANCE VESSELS OF HAMSTER RETRACTOR

Increased resistance to blood flow with muscle extension has been expl ained by the deformation of vessels within the muscle. In the present study, we developed a novel preparation of the hamster retractor muscl e to investigate whether passive changes in skeletal muscle length eli cit active vasomotor responses through a range of motion (85% to 130% of in vivo length; sarcomere length, 2.69+/-0.02 to 4.05+/-0.01 mu m) encompassing the classic length-tension relationship. Arterioles (diam eter, 32+/-3 mu m) and feed arteries (diameter, 75+/-4 mu m) were obse rved to progressively constrict (by 8+/-1 and 17+/-2 mu m, respectivel y) with muscle lengthening, reducing blood flow by >50%; reciprocal ch anges occurred with passive shortening. Sodium nitroprusside (10 mu mo l/L) dilated vessels (to 47+/-2 and 98+/-4 mu m, respectively) and abo lished vasomotor responses to changing muscle length. The coordination of vasomotor responses between arterioles and feed arteries maintaine d wall shear rate (control, 1764+/-200 s(-1)) and perfusion pressure ( 60+/-5 mm Hg) into the arteriolar network. Tetrodotoxin (TTX, 1 mu mol /L), phentolamine (1 mu mol/L), prazosin (0.1 mu mol/L), or 6-hydroxyd opamine (1 mmol/L) inhibited vasoconstrictor responses, indicating tha t action potentials initiated by muscle lengthening give rise to norep inephrine release from sympathetic nerves. As shown with glyoxylic aci d staining, sympathetic nerves formed a plexus encompassing arterioles and feed arteries. To test for a reflexive response initiated by intr amuscular mechanoreceptors, TTX was applied with micropipettes to prox imal segments of feed arteries, thereby neurally ''isolating'' the mus cle from the hamster. Whereas lengthening-induced vasoconstriction per sisted in arterioles and in feed artery segments distal to TTX, there was no vasomotor response central to the block. We conclude that passi ve lengthening stimulates the activity of periarteriolar sympathetic n erves; this activity propagates antidromically along nerve fibers into the feed arteries. These findings identify a mechanotransduction sequ ence by which the length of skeletal muscle actively governs vasomotor tone and the supply of oxygen to muscle fibers.