Hindlimb unweighting induces changes in the RhoA-Rho-kinase pathway of the rat abdominal aorta

Hindlimb unweighting (HLU) in rats mimics the fluid shift experienced by astronauts and may serve as a model for ground-based orthostatic hypotension. It has been shown that the abdominal aorta of HLU rats exhibits a deficit in contractile response to adrenergic agonists. The hypothesis of the present study was that decreased activity in the RhoA/Rho-kinase pathway could contribute to that deficit. Wistar rats were subjected to 20 days of HLU treatment. Abdominal aorta rings from HLU and control rats were suspended in baths for measurement of contraction. Concentration response curves were obtained to the alpha adrenergic agonist, phenylephrine and the thromboxane-mimetic, U46619. HLU treatment caused decreased contraction in response to both. The Rho-kinase inhibitor, Y27632, caused a reduction in the phenylephrine-induced contraction in control, but not HLU aorta. Other rings were frozen after stimulation 1 microM U46619 or phenylephrine. Western analysis revealed a decreased expression of RhoA, but increased expression of both Rho kinase and MYPT1, the regulatory subunit of myosin light chain (MLC) phosphatase. MYPT1 and MLC phosphorylation was decreased by HLU in phenylephrine stimulated aorta. Decreased activity in the RhoA/Rho-kinase pathway may be involved in the decreased contraction seen in the HLU abdominal aorta.