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(Hypertension. 1997;29:1165-1172.)
© 1997 American Heart Association, Inc.

Articles

Mechanical Load Opposes Angiotensin-Mediated Decrease in Vascular ₁-Adrenoceptors

Mary L. Clements; ; James E. Faber

From the Department of Physiology, The University of North Carolina, Chapel Hill.

Abstract ₁-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 ₁-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 _1B- and _1D-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 _1B-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 _1D- and _1B-adrenergic receptor mRNAs and ₁-receptor density in cultured aortic smooth muscle cells. _1D 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, _1B 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 ₁-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 ₁-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 _1B mRNA expression and ₁-adrenergic receptor density. These data suggest that angiotensin and intravascular pressure interact to affect cell growth and expression of _1B-adrenergic receptors by vascular smooth muscle.

Key Words: muscle, smooth, vascular • mechanical stretch • angiotensin • receptors, adrenergic, alpha

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