Angiotensin AT 1B Receptor Mediates Calcium Signaling in Vascular Smooth Muscle Cells of AT 1A Receptor–Deficient Mice

Abstract

—Our studies on angiotensin II receptor subtype 1A (AT _1A ) knockout mice define how endogenous receptors other than AT _1A receptors stimulate changes in cytosolic calcium concentration ([Ca ²⁺ ] _i ) in cultured aortic vascular smooth muscle cells (VSMCs). Wild-type cells have a 1.7 ratio of AT _1A /AT _1B receptor mRNA as determined by semiquantitative reverse transcriptase–polymerase chain reaction. Mutant cells express AT _1B receptor mRNA but not that for the AT _1A receptor. In wild-type cells with AT _1A present, Ang II (10 ⁻⁷ mol/L) produces a characteristic rapid peak increase in [Ca ²⁺ ] _i of 150 to 180 nmol/L, followed by a plateau phase characterized by a sustained 70 to 80 nmol/L increase in [Ca ²⁺ ] _i . An unexpected finding was that the magnitude and time-dependent pattern of [Ca ²⁺ ] _i changes produced by Ang II were similar in cells that lacked AT _1A receptors but possessed AT _1B receptors. The response in mutant cells indicates effective coupling of an Ang II receptor to one or more second messenger systems. The similarity of response patterns between cells with and without AT _1A receptors suggests that non-AT _1A receptors are functionally linked to similar signal transduction pathways in mutant cells. The fact that mutant and wild-type cells exhibit similar patterns of calcium mobilization and entry supports the notion that AT _1A and non-AT _1A receptors share common signal transduction pathways. The AT ₂ receptor ligands PD-123319 and CGP-42112 do not alter Ang II effects in either VSMC type, suggesting a paucity of AT ₂ receptors and/or an absence of their linkage to [Ca ²⁺ ] _i pathways. The nonpeptide AT ₁ receptor blocker losartan antagonizes Ang II–induced [Ca ²⁺ ] _i increases in both cell groups, supporting mediation by native AT _1B receptors and effective coupling of this subtype to second messenger systems leading to calcium entry and mobilization. Our results demonstrate that Ang II causes calcium signaling in AT _1A –deficient VSMCs that is mediated by an endogenous losartan-sensitive AT _1B receptor.