Mechanical Load Opposes Angiotensin-Mediated Decrease in Vascular α1-Adrenoceptors

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Abstract
α1-Adrenergic receptor contraction of vascular smooth muscle is augmented by increases in angiotensin II and also in several forms of hypertension. Whether angiotensin directly modulates α1-adrenoceptor subtype expression to contribute to this effect is unknown. In a previous study, we demonstrated that increased mechanical load (pressure) per se does not alter expression of α1B- and α1D-adrenoceptors in rat aortic smooth muscle in cell culture, in vitro or in vivo. However, findings in aortic coarctation hypertension suggested that a humoral factor, possibly angiotensin, selectively reduces α1B-adrenoceptors and that increased mechanical load opposes this decrease. The present study examined this hypothesis by determining the effect of angiotensin alone and in the presence of mechanical loading on the expression of α1D- and α1B-adrenergic receptor mRNAs and α1-receptor density in cultured aortic smooth muscle cells. α1D mRNA content, per smooth muscle cell, concentration-dependently decreased after 3 hours of exposure to 0.3 nmol/L to 1 μmol/L angiotensin but by 24 hours had returned to control levels. In contrast, α1B mRNA concentration-dependently declined at a later time (24 hours) and remained decreased at 48 hours to 27±6% of control with 1 μmol/L angiotensin. Angiotensin also decreased α1-adrenoceptor density in a dose-dependent manner. Angiotensin had no effect on cell number in these confluent, quiescent cells but did increase cell protein and total RNA. This cellular hypertrophy and the decreases in α1-adrenoceptor mRNAs were blocked by the angiotensin type 1 receptor antagonist losartan. Cyclic mechanical loading of smooth muscle cells opposed the angiotensin-mediated hypertrophy and decrease in α1B mRNA expression and α1-adrenergic receptor density. These data suggest that angiotensin and intravascular pressure interact to affect cell growth and expression of α1B-adrenergic receptors by vascular smooth muscle.