Effect of Mechanical Loading on Vascular α 1D - and α 1B -Adrenergic Receptor Expression

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Abstract
Heterogeneous distribution and function of α 1 -adrenergic receptor subtypes on arterial and venous vessels, together with evidence for altered α-adrenergic receptor expression in hypertension, led us to examine whether mechanical load influences expression of α 1B -and α 1D -adrenergic receptors in rat aortic smooth muscle cells (SMCs). We used RNase protection and radioligand binding assays to measure mRNA and α 1 -adrenergic receptor density. In the first model, SMCs were subjected to phasic loading using flexible culture plates. As a positive control for the load stimulus, postconfluent, quiescent passage 5 cells demonstrated the expected load-dependent morphological realignment. However, no changes were detected in expression of either α 1D - or α 1B -adrenergic receptor mRNAs or receptor density after 24 to 48 hours of loading. β-Actin and SMC-specific α-actin mRNA, as well as cell number and per-cell total RNA and protein, were also unaffected. In a second model, intact thoracic aortas, in either the presence or absence of endothelial cells, were cultured for 48 hours under tonic load. Like cultured cells, 48 hours of load did not affect SMC expression of α 1 -adrenergic receptor mRNAs. We used suprarenal aortic coarctation to examine effects of increased pressure in vivo. As with the previous in vitro and in situ models, hypertension (30 days) had no effect on expression of α 1B - and α 1D -adrenergic receptor mRNAs in the suprarenal aorta compared with sham coarctation. To separate pressure per se from humoral influences, we also measured mRNAs in the subrenal, normotensive aorta. α 1B mRNA levels decreased to 68±14% of sham-coarcted controls in subrenal aorta exposed to normal blood pressure but also to systemic humoral changes induced by coarctation. As a positive control for a load effect, SMC-specific α-actin mRNA increased for loaded aorta in organ culture and in hypertensive aorta in vivo, whereas expression of β-actin mRNA was unaffected. These results from cell culture, organ culture, and in vivo models suggest that pressure (load) alone has no effect on α 1B - and α 1D -adrenergic receptor expression. In coarctation hypertension, smooth muscle protected from the hypertension showed a decline in α 1B mRNA that may be due to a humoral factor or factors.