Dependence of Skeletal Muscle Vascular Response to Serotonin Upon the Level of Vascular Resistance

Summary 5-Hydroxy tryptamine (5-HT; serotonin) was infused intra-arterially at sequentially faster rates in 52 isolated, collateral-free gracilis muscles of dogs. The rates chosen produced no systemic effects. Both constant (pump perfused) and natural flow experiments were conducted. Serotonin was first tested in muscles with unaltered initial resistance and subsequently in the same muscles after baseline resistance was increased by sympathetic nerve stimulation or lowered by exercise. 5-HT consistently increased vascular resistance in muscles with an initial low resistance. The resistance response (decrease) caused by 5-HT in muscles with an initial high resistance could be reversed by lowering baseline resistance metabolically (exercise). Also, the increase in resistance caused by 5-HT in muscles with an initial low resistance could be reversed by increasing baseline resistance via sympathetic nerve stimulation. These data show that the local vascular response of gracilis muscle to 5-HT is dependent upon the initial level of vascular resistance and imply that this dependency is not exclusively linked to neurogenic tone.