| Angiotensin receptor (AT) regulation was examined in renal medullary interstitial cells (RMIC); vascular smooth muscle cells (VSMC) isolated from 6 and 24 month old rats; and, an ovine model of left ventricular (LV) hypertrophy. AT1 receptors in RMIC were expressed at high levels when grown under iso-osmolar conditions. In contrast, hyperosmolar conditions, which mimicked the in vivo environment, caused a significant decrease in AT1 receptor expression. These data suggest that osmotic regulation of AT1 receptors alters interactions between Ang II and local vasoactive systems resulting in modulation of renal function under physiological conditions particularly, in the active regulation of the medullary osmotic gradient by RMIC located between the loops of Henle and vasa recta. AT1 receptors in VSCM from 24-month old rats were not down-regulated by cell growth to the same degree as in VSCM from 6 month old rats, suggesting that abnormal AT1 receptor regulation in aging VSMC may lead to the increased rate of proliferation and decreased elasticity and distensibility of aged vascular tissues observed in vivo. In both RMIC and VSMC, AT1 receptor regulation is controlled at the post-transcriptional level by RNA binding proteins that bind in the 5' leader sequence of the receptor mRNA and, which upon binding, inhibit AT1 receptor translation. In the sheep heart, the predominant AT receptor subtype is AT2. In a model of LV remodeling induced by anteroapical MI, AT2 receptor expression was up-regulated by the combination therapy of angiotensin converting enzyme inhibitors and AT1 receptor antagonists under conditions that significantly reduced LV remodeling compared to either treatment alone. These data suggest activation of AT2 receptors in myocardial tissue is cardioprotective and that the development of nonpeptide AT2 receptor agonists could have significant clinical utility in treating myocardial infarction and preventing LV remodeling. Taken together, these data suggest that AT receptors are dynamically regulated in a tissue-specific manner by physiological and pathophysiological factors including Ang II, which is generated from the local tissue renin-angiotensin system, in addition to the peripheral system. |
