LOCALIZATION OF THYROTROPIN-RELEASING-HORMONE MESSENGER-RNA EXPRESSION IN THE RAT-HEART BY IN-SITU HYBRIDIZATION HISTOCHEMISTRY

Our laboratory has recently detected mRNA of thyrotropin-releasing hor mone (TRH) in the rat heart. The density of mRNA for TRH is five-fold higher in the atria than in the left and right ventricle. We also foun d TRH receptor mRNA and H-3-TRH-binding sites in both ventricles. Card iac contractility was stimulated after intracoronary administration of TRH. This study was performed to investigate the localization of TRH in the heart. We utilized in situ hybridization histochemistry (ISHH) to localize TRH mRNA expression in the rat heart. ISHH was performed o n fresh frozen heart tissue sections, which were hybridized with a spe cific S-35-TRH oligo probe and subsequently processed by autoradiograp hy. The autoradiographic signals corresponding to TRH mRNA were analyz ed with an image program. For positive controls, TRH mRNA was identifi ed in the hypothalamic paraventricular nucleus. This test confirms the specificity of the TRH oligo probe. Cardiac hybridization signals wer e observed predominantly in the atria and localized preferentially in atrial connective tissues, vascular adventitia and atrial cardiomyocyt es. No hybridization signals were found in ventricular cardiomyocytes. These observations suggest that TRH is synthesized in atrial myocytes and atrial vascular structures. Based on studies which show synthesis of the TRH receptors in ventricular cardiomyocytes, we hypothesize th at atrial TRH is an endocrine source for the stimulation of ventricula r contractility and that endothelial and adventitial TRH may play a ro le(s) in the regulation of the growth and/or vasomotor tone of the car diac vascular system.