Angiotensin II Type 2 Receptor mRNA Expression in the Developing Cardiopulmonary System of the Rat

Abstract

Recent studies have shown that angiotensin II has a trophic action on the heart. The presence of two types of angiotensin II receptors, type 1 (AT ₁ ) and type 2 (AT ₂ ), has been reported in the rat heart. This in situ hybridization study describes the tissue and cell location of AT ₂ receptor mRNA in the developing rat cardiopulmonary system, from 15 days of gestation to adulthood. Expression of AT _1A receptor mRNA was studied in parallel for direct comparison. The aortic arch and pulmonary artery expressed high levels of AT ₂ receptor mRNA from 15 days of gestation up until 15 days postpartum, whereas expression of this mRNA was observed only just before and after birth in the coronary arteries. AT ₂ receptor mRNA was not detected in any cardiac muscle of the fetus, neonate, or adult. The annulus of all four heart valves expressed AT ₂ mRNA from 21 days of gestation until 10 days postpartum, but no labeling was seen in the valve leaflets. The subendocardial atrial tissue showed a high level of AT ₂ receptor mRNA expression during the early postnatal period, but no expression was observed in the atrial myocytes from fetal stages to adulthood. The bronchi and trachea, but not the lung parenchyma, showed a high level of AT ₂ receptor mRNA expression starting from 17 days of gestation until 10 days postpartum. AT ₂ receptor mRNA expression in the cardiopulmonary system is therefore transient, developmentally regulated, and mostly located in vascular structures. By these three characteristics, its expression contrasts with that of AT _1A , which is continuously expressed in the cardiac muscle to adulthood. This spatiotemporal pattern of expression of angiotensin II receptor mRNAs during development suggests a possible role for angiotensin II in organogenesis.