ASGSB 1998 Annual Meeting Abstracts

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EFFECTS OF HINDLIMB UNLOADING ON THE VASO-CONSTRICTOR RESPONSIVENESS OF SKELETAL MUSCLE ARTERIOLES.   M.D. Delp. Departments of Health & Kinesiology and Medical Physiology, Texas A&M University, College Station.

Hindlimb unloading of rats results in elevations in blood flow at rest and during moderate intensity exercise in muscle composed primarily of type IIB fibers, and reductions in flow to muscle composed primarily of type I fibers during intense exercise. The purpose of this study was to determine whether the intrinsic responsiveness of arterioles from muscles composed of type IIB fibers (superficial portion of gastrocnemius muscle) and type I fibers (soleus muscle) to vasoconstrictor agonists is altered by hindlimb unloading. First order (1A) arterioles from the superficial gastrocnemius and soleus muscles of control (C, n=9) and 14 day hindlimb unloaded (HU, n=8) rats were isolated and cannulated with glass micropipettes in vitro. Intraluminal pressure was set at 60 cmH₂O. Spontaneous tone developed in all arterioles, but tone was greater in arterioles from the gastrocnemius muscle of C (61±8%) than HU (37±7%) rats. There was no difference in tone of arterioles from soleus muscle of C (55±8%) and HU (46±10%) animals. There was a decrease in the sensitivity (EC₅₀) of 1A arterioles from gastrocnemius muscle of HU rats to the vasoconstrictor agonists norepinephrine (10^-9-10^-4 M) and KCl (10-100 mM), but no difference in sensitivity of arterioles from soleus muscle of C and HU rats to these vasoconstrictors. These data indicate that HU diminishes spontaneous and agonist-induced constriction of 1A arterioles from muscle composed predominantly of type IIB fibers, but has no effect on contractile function of 1A arterioles from muscle composed primarily of type I fibers. This adaptation is consistent with the HU-induced elevations in blood flow observed in muscle composed of type IIB in rats and the diminished ability of astronauts to elevate peripheral resistance following spaceflight.
(Supported by NASA grants NAGW-4842 and NAG5-3754 and NSBRI grant NCC-9-58.)

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