Cardiac Thyrotropin-Releasing Hormone Mediates Left Ventricular Hypertrophy in Spontaneously Hypertensive Rats

Abstract

Abstract-Local thyrotropin-releasing hormone (TRH) may be involved in cardiac pathophysiology, but its role in left ventricular hypertrophy (LVH) is still unknown. We studied whether local TRH is involved in LVH of spontaneously hypertensive rats (SHR) by investigating TRH expression and its long-term inhibition by interference RNA (TRH-iRNA) during LVH development at 2 stages (prehypertrophy and hypertrophy). SHR and their control rats (WKY) were compared. Cardiac hypertrophy was expressed as heart/total body weight (HW/BW) ratio. TRH content (radioimmuno assay), preproTRH, TRH receptor type I, brain natriuretic peptide (BNP), and collagen mRNA expressions (real-time polymerase chain reaction) were measured. For long-term inhibition of TRH, TRH-iRNA was injected into the left ventricle (LV) wall for 8 weeks. Hearts were processed for morphometric studies and immunohistochemical analysis using antibodies against ␣-smooth muscle actin and collagen type III. LV preproTRHmRNA abundance was similar in both strains at 7 weeks of age. At the hypertrophic stage (18 weeks old), however, there was a 15-fold increase in SHR versus WKY, consistent with a significant increase in tripeptide levels and the expression of its receptor. Specific LV-TRH inhibition at the prehypertensive stage with TRH-iRNA, which decreased Ͼ50% preproTRH expression and tripeptide levels, prevented LVH development as shown by the normal HW/BW ratio observed in TRH-iRNA-treated SHR. In addition, TRH-iRNA impeded the increase in BNP and type III collagen expressions and prevented the increase in cardiomyocyte diameter evident in mismatch iRNA-treated adult SHR. These results show for the first time that the cardiac TRH system is involved in the development of LVH in SHR. (Hypertension. 2011;57:103-109.) • Online Data SupplementKey Words: TRH Ⅲ cardiac hypertrophy Ⅲ rat Ⅲ SHR Ⅲ interference RNA T hyrotropin-releasing hormone (TRH), a small neuropeptide (p-Glu-His-Pro-NH2) initially identified in the hypothalamus, is amply distributed in the central nervous system 1 and in other extraneural tissues 2 and has been shown to have central and peripheral biological effects independent of thyroid hormone production. 3 TRH also acts on the cardiovascular system of rodents. 4,5 Many groups have identified preproTRH-mRNA by Northern blot analysis and RNAse protection assay in rat cardiac tissues and have referred the presence of specific type I TRH receptors (TRH-R1) in ventricles, establishing that a TRH system is present in the rat heart. 6 -8 In contrast to the hypothalamic TRH system, cardiac preproTRH-mRNA may be augmented by glucocorticoids and by testosterone but may not be regulated by T 3 . 8 In addition, Hasegawa et al 9 reported for the first time an inotropic effect of TRH on the guinea pig myocardium, implying that this effect could be mediated by an increase in a slow inward Ca 2ϩ current. Furthermore, Socci et al 7 found similar results and reported that TRH modulates cardiac contractility of isolated rat hearts as an autocrine factor in a conce...

Thyrotropin-releasing hormone overexpression induces structural changes of the left ventricle in the normal rat heart

“…As expected, TRH overexpression for 4 wk was not enough to induce a 10-fold TRH increase, as has been observed in adults SHR (23); however, a twofold TRH increase was able to induce expression of fetal genes and fibrosis, suggesting that chronic LV TRH could lead to the development of fibrosis and hypertrophy in a normal rat.…”

“…In this regard, the SHR model has been extensively studied, and the participation of many systems in LV hypertrophy development has been described (the renin-angiotensin system, endothelin system, adrenergic system, etc.) (2,23,18). On this basis, we speculated whether single ventricle TRH overexpression would be enough to induce cardiac structural changes in a normal rat independent of any other alteration.…”

“…The probe was aligned with the needle, and special care was taken to visualize the region of the myocardial wall. Injection was performed in three different sites (from the base to apex) to efficiently surround the entire LV as previously described (23,16).…”

“…Reactions (in duplicate) were carried out in a total volume of 20 l. A three-step protocol (95°C for 30 s; 60, 64, or 66°C annealing for 30 s; and 72°C extension for 40 s) was repeated for 40 cycles. Rat primers (Invitrogen) for rat preproTRH, rat BNP, rat collagen type III, and rat ␤-actin were as previously described (23). Additional primers were designed as follows: rat Bax, forward 5=-CTGCAGAGGATGATTGCTGA-3= and reverse 5=-GGGCACTTTAGTGCACAGG-3=; rat Bcl-2, forward 5=-GATAACG-GAGGCTGGGATG-3= and reverse 5=-CTCACTTGTGGCCCAGG-TAT-3=; and rat caspase-3, forward 5=-CAAGTCGATGGACTCTG-GAA-3= and reverse 5=-TGACATTCCAGTGCTTTTATGG-3=.…”

“…We have recently reported that adult SHR shows LV TRH hyperactivity and that long-term inhibition of LV TRH by naked small interfering (si)RNA prevents LV fibrosis and cardiomyocyte enlargement and attenuates the expression of brain natriuretic peptide (BNP), demonstrating successful inhibition of cardiac hypertrophy in a rat model by cardiactargeted ablation of TRH production (23).…”