The involvement of atrial natriuretic peptide (ANP) in the regulation of thyroid gland is supported by the presence of high-affinity ANP receptors and the identification of the peptide in thyroid follicular cells. The aim of this work was to study the action of ANP on parameters of thyroid hormone biosynthesis and analyze the intracellular mechanism of the ANP action in cultured bovine thyroid follicles. The addition of ANP (0.1-10 nM) to the culture medium for 24 h inhibited the TSH (thyroid-stimulating hormone)-stimulated iodide uptake with a maximal inhibition at 1 nM ANP. When thyrocytes were incubated with 10 nM ANP the inhibitory effect slightly increased from 24 to 72 h. Thyroglobulin (Tg) mRNA expression was reduced by 1 and 10 nM ANP. After 24 h of treatment with the cGMP analogue, N(2),2'-O-dibutyrylguanosine 3':5'-cyclic monophosphate [(Bu)(2)cGMP] (0.1 and 1 mM), an inhibition of iodide uptake and Tg mRNA expression was obtained, evidencing a cGMP-mediated inhibitory signal in the thyroid cell. A reduction of the cAMP production was induced by incubation of thyroid follicles with 1 and 10 nM ANP for 24 h. Under a similar treatment the cGMP accumulation was increased only by 10 nM ANP. The inhibitory effect of ANP on Tg mRNA level was reverted in the presence of pertussis toxin, an inhibitor of the G(i)-protein-mediated reduction of the adenylate cyclase activity. These results indicate an inhibitory action of ANP on parameters of thyroid hormone biosynthesis. A G(i)-protein-mediated reduction of the cAMP production seems to be the main factor involved in the ANP action although a role of the cGMP pathway should not be discarded specially at high ANP levels.