The interaction between cardiovascular system and thyroid: Atrial natriuretic peptide in the thyroid function of the lizard Podarcis siculus: In vivo experiments and immunolocalisation

Although the relationship between thyroid and heart has been extensively studied, especially in cases of heart disease, the hypothesis of a relationship between heart and thyroid is still far from being fully understood. The possible involvement of atrial natriuretic peptide (ANP) in thyroid regulation is supported by the immunohistochemical identification of the peptide in thyroid follicular cells of the lizard Podarcis siculus. The aim of this work was to study the action of ANP on the metabolism of thyroid hormones and to analyze the mechanism of ANF action in lizard thyroid follicles. Intraperitoneally administration of ANP (1–4 μg/100 g body weight) to Podarcis siculus lizards both after 2 h and after 24 h inhibited circulating plasma levels of T₃ and T₄ resulting in stimulation of TSH (Thyroid-Stimulating Hormone) and decrease of TRH (Thyrotropin Releasing Hormone). The inhibitory effect of ANP on the thyroid cells could be mediated by cGMP, which is one of the main mediators of ANP action. A stimulation of the level of 5-T4 ORD (type II) Monodeiodinase activity at the hepatic level by ANP increased hepatic T₃ levels and decreased hepatic T₄ levels, revealing an alternative mode of signalling by ANP on peripheral biosynthesis of thyroid hormones. In conclusion, our results indicate the ANP role in the regulation of thyroid hormone synthesis and secretion, supported by the immunohistochemical presence of the peptide in the apical region of thyroid follicular cells and in the fibres surrounding lizard follicles, which could underlie an ANP-mediated autocrine and paracrine regulatory pathway.