Atrial endothelin-1 signalling in hypertensive rats

Abstract

Purpose: Hypertension induces cardiac remodelling at the structural and functional level. Endothelin-1 (ET-1) may contribute to cardiac remodelling. It is not known, however, whether atrial ET-1 signalling is altered in hypertension. Here we tested the hypothesis that ET-1 signalling is augmented in the atria of spontaneously hypertensive rats (SHR) and that this may contribute to hypertension-induced atrial arrhythmias. Methods: SHR and control Wistar-Kyoto (WKY) rats were studied at 6-8 months of age. Hearts were removed for morphometric analysis. Atria were snap frozen and analyzed for mRNA and protein expression by qPCR and Western blotting. Isolated atrial myocytes were field stimulated in the presence of 50 nM ET-1, and Ca transients (CaTs) were characterized at the global or subcellular level by epifluorescence (Fura-2/AM) and line scan confocal microscopy (Fluo-4/AM). To mimic atrial tachycardia, atrial tissue slices were stimulated at 5 Hz for up to 20 h and used for Western blotting. Results: At 6-8 months of age, SHR exhibited compensated LV hypertrophy, but left atrial hypotrophy, despite increased atrial mRNA levels of BNP (2-fold) and β-MHC (3.6-fold), while α-MHC was unaffected. There were no differences in global CaTs between atrial myocytes from WKY and SHR. Exposure of atrial myocytes to 50 nM ET-1 caused a 35% increase in systolic Ca in SHR (n=14), but only a 16% increase in WKY (n=21; P<0.05). Moreover, in SHR atrial myocytes, ET-1 induced a larger fractional SR Ca release (P<0.05 vs WKY). The ET-1-induced increase in CaTs occurred in the subsarcolemmal as well as in the central cytoplasm. Notably, ET-1 also induced arrhythmogenic Ca waves both in SHR and WKY atrial myocytes. Further analysis at the molecular level indicated that the atrial mRNA expression of ET-1 was increased (2-fold) in SHR. ETA receptor expression was unaltered at the mRNA level but upregulated (4-fold) at the protein level in the left atria. Gαq and PLCβ (isoforms 1 and 4) mRNA expression were also upregulated in SHR. Simulated atrial tachycardia in atrial tissue slices caused a 2-fold upregulation of ETA receptor expression in both SHR and WKY. Conclusions: At 6-8 months, SHR exhibit increased atrial expression of ET-1, ETA receptors, Gαq and PLCβ. Atrial tachycardia further increases ETA receptor expression. At the functional level, atrial myocytes from SHR respond to ET-1 with increases in systolic Ca and the development of arrhythmogenic Ca waves. Thus, atrial ET-1 signalling is enhanced already at early stages of hypertension and may contribute to the initiation of atrial tachycardia in hypertension.