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We tested the hypothesis that hindlimb unweighting (HLU) and the associated reduction in soleus muscle blood flow causes decreased expression of endothelial cell nitric oxide synthase (ecNOS) mRNA and protein and attenuated endothelium-dependent vasodilator responses in rat soleus feed arteries (SFA). Male Sprague-Dawley rats were exposed to HLU (n = 12) or cage control (Con; n = 12) conditions for 14 days. At the end of this period, SFA were isolated, removed, and cannulated with two glass micropipettes for examination of vasodilator responses or frozen for analysis of ecNOS mRNA and protein expression. RT-PCR of RNA from single SFA was used to measure ecNOS mRNA, and immunoblots on single SFAs were used to measure ecNOS protein content. Results revealed that both ecNOS mRNA and ecNOS protein expression were lower in SFA from HLU rats. Dilation to increased intraluminal flow was attenuated in SFA from HLU rats (Con: 88 +/- 8% vs. HLU: 53 +/- 8%) as was maximal vasodilation to acetylcholine (10(-9)-10(-4) M; Con: 88 +/- 5% vs. HLU: 73 +/- 5%). Sensitivity to the endothelium-independent vasodilator sodium nitroprusside (10(-10)-10(-4) M) was enhanced by HLU (EC(50): Con: 4.46 x 10(-7) M vs. HLU: 5.00 x 10(-8) M). Collectively, these data indicate that the chronic reduction in soleus blood flow associated with the reduced physical activity during HLU results in reduced expression of ecNOS mRNA and protein in SFA and attenuated endothelium-dependent vasodilation.
Jasperse et al. (Fri,) studied this question.