AT2 receptors attenuate AT1 receptor-induced phospholipase D activation in vascular smooth muscle cells

Previous studies indicate that angiotensin (AT)(1) receptor-induced activation of phospholipase D (PLD) may importantly contribute to vascular hypertrophy, injury, and contraction. However, the role of AT(2) receptors in regulating AT(1) receptor-induced PLD activation is unknown. In this study, we identified angiotensin II receptors on cultured preglomerular vascular smooth muscle cells (PGSMCs) from spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) by reverse transcription-polymerase chain reaction (RT-PCR) and binding assays and examined their functional effects on angiotensin II-mediated PLD activity. Both RT-PCR and binding indicated that cultured SHR and WKY PGSMCs expressed AT(1) and AT(2) receptors, and the combined total of AT(1) and AT(2) receptors was similar between the strains. However, the number of AT(1) and AT(2) receptors differed between SHR and WKY PGSMCs in so much as the ratio of AT(1) to AT(2) receptors was approximately 1 to 1 and 3 to 1 in WKY and SHR PGSMCs, respectively. As previously reported, angiotensin II more potently activated PLD in SHR PGSMCs (SHR EC(50) = 4 nM; WKY EC(50) = 47 nM). Addition of an AT(2) receptor-specific antagonist or agonist shifted the angiotensin II-mediated PLD concentration-response curve of WKY PGSMCs in a manner consistent with AT(2) receptors producing an inhibitory signal. In contrast, in SHR little change was observed. Our findings indicate that the ratio of AT(1) to AT(2) receptors in vascular smooth muscle cells may be a determinant of the net effects of angiotensin II on PLD activity due to AT(2)-dependent inhibition of AT(1)-mediated PLD activity. Furthermore, cultured WKY PGSMCs provide an excellent model system to study endogenous AT(2) receptor signal transduction.