Adrenergic control of vascular resistance varies in muscles composed of different fiber types: influence of the vascular endothelium

Behnke BJ, Armstrong RB, Delp MD. Adrenergic control of vascular resistance varies in muscles composed of different fiber types: influence of the vascular endothelium. Am J Physiol Regul Integr Comp Physiol 301: R783-R790, 2011. First published June 15, 2011; doi:10.1152/ajpregu.00205.2011.-The influence of the sympathetic nervous system (SNS) upon vascular resistance is more profound in muscles comprised predominately of low-oxidative type IIB vs. high-oxidative type I fiber types. However, within muscles containing high-oxidative type IIA and IIX fibers, the role of the SNS on vasomotor tone is not well established. The purpose of this study was to examine the influence of sympathetic neural vasoconstrictor tone in muscles composed of different fiber types. In adult male rats, blood flow to the red and white portions of the gastrocnemius (Gast(Red) and Gast(White), respectively) and the soleus muscle was measured pre- and postdenervation. Resistance arterioles from these muscles were removed, and dose responses to alpha(1)-phenylephrine or alpha(2)-clonidine adrenoreceptor agonists were determined with and without the vascular endothelium. Denervation resulted in a 2.7-fold increase in blood flow to the soleus and Gast(Red) and an 8.7-fold increase in flow to the Gast(White). In isolated arterioles, alpha(2)-mediated vasoconstriction was greatest in Gast(White) (similar to 50%) and less in Gast(Red) (similar to 31%) and soleus (similar to 17%); differences among arterioles were abolished with the removal of the endothelium. There was greater sensitivity to alpha(1)-mediated vasoconstriction in the Gast(White) and Gast(Red) vs. the soleus, which was independent of whether the endothelium was present. These data indicate that 1) control of vascular resistance by the SNS in high-oxidative, fast-twitch muscle is intermediate to that of low-oxidative, fast-twitch and high-oxidative, slow-twitch muscles; and 2) the ability of the SNS to control blood flow to low-oxidative type IIB muscle appears to be mediated through postsynaptic alpha(1)- and alpha(2)-adrenoreceptors on the vascular smooth muscle.