Expression of heart oxytocin receptor and its mRNA in two rat strains with different activity of HPA axis.

OBJECTIVES: Oxytocin (OT) is a neuropeptide acting both as a peripheral hormone and in the brain as neurotransmitter and neuromodulator. In addition to its well-known effects on milk-ejection and uterine contraction, OT was shown to exert neuroendocrine regulation of heart functions. The aim of this study was to investigate the expression of mRNA of OT receptors (OTR) in rat hearts by real-time quantitative PCR (qPCR). The study was performed in Sprague-Dawley (SD) and Lewis (LE) rat strains, the latter having lower activity of HPA axis.

METHODS: We used adult male SD and LE rats. OTR mRNA expression was detected in all heart chambers by comparing their threshold cycle values (CT) to CT of reference gene β-actin. The relative expression ratios were calculated using the 2-ΔΔCT method. The specificity of reaction of primary antibody with OTRs was tested by Western Blot and localization of OTR in the heart compartments was performed by immunofluorescence with commercial OTR specific antibodies.

RESULTS: We found expression of OTR mRNA in all heart compartments. The expression of OTR mRNA in both atria (LA, RA) was much higher than in the ventricles (RV, LV). By using two-way ANOVA we found no statistical differences between corresponding compartments of SD and LE rats. Immunohistochemical studies showed that OTR staining is not related to neuronal tissue and findings from left atrium indicate that prevalent localization of OTR is on cell membranes of cardiomyocytes.

CONCLUSIONS: The finding of expression of OTR mRNA by real-time qPCR and proof of OTR staining by immunohistochemistry in all heart compartments indicate that OT and its receptors may have function as a cardiovascular hormone. The differences in the HPA axis activity, as is exemplified in Sprague-Dawley and Lewis rat strain, do not project in the expression of OTR mRNA under basal condition. The effect of activity of HPA on OTR expression should be studied under stimulated conditions as it was performed in the behavioral studies.