Inhibition of cyclic 3'5' monophosphate synthesis in rat testis by L-triiodothyronine M. M. Flawia*, A. R. Kornblihtt**, D. de Mendoza* and H. N. Torres* Instituto de Investigaciones Bioquimicas "Fundaci6n Campomar" and Facultad de Ciencias Exactas y Naturales, Obligado 2490, 1428 Buenos Aires, Argentina Summary Cytosolic adenylate cyclase activity from rat seminiferous tubules is inhibited by L-triiodothyronine (L-T3). In a typical dose-response curve, using Mn-ATP as substrate, no effect is observed at 10-10 M L-T3; about 15 to 25% inhibition is found in the range between 10-9 and 10 -6 M L-T3 and finally a sharp enzyme inhibition is evident at increasing hormone concentrations from 10 6 to 10 4 M. Incubation of decapsulated testes with L-T 3 leads to a decrease of intracellular cyclic AMP levels. Dose-response relationships for such effect are similar to those found for adenylate cyclase activity. In this case a clear response is observed at 10-8 M L-T 3. Abbreviations DEAE-cellulose, diethylaminoethyl cellulose; TRIS, Tris (hydroxymethyl) aminomethane; and EDTA, ethylenediamino tetracetate Introduction Evidence on the role of cyclic nucleotides as mediators of thyroid hormone actions is rather confusing. Stimulation by thyroid hormones of adenylate cyclase in myocardial (1, 2) and sperma- tozoal preparations (3) has been reported. An opposite effect was found by Friedman et al. (1977) in bovine thyroid membrane fractions (4). In this system, thyroid hormones, inhibited both basal and TSH-stimulated cyclase activities at unphysiolog- ical concentration. This inhibition could explain earlier findings on the ability of thyroid hormones * Career investigator of the Consejo Nacional de Investigaciones Cientificas y T6cnicas ** Fellow of the ConsejoNacional de Investigaciones Cientificas y T~cnicas and of the Comisi6n de InvestigacionesCientificas (Buenos Aires) to decrease intracellular cyclic AMP accumulation in thyroid gland (5, 6, 7). On the other hand, the inhibitory action of thyroid hormones and some polyhalogenated phenols on several Zn2+-con- taining dehydrogenases has been described (8, 9). As was pointed out by Braun and Dods (10) and confirmed by Neer (11), adenylate cyclase activity associated with rat seminiferous tubules has some relevant characteristics: 1) association to cytosolic proteins; 2) insensitivity to gonadotrophins; 3) insensitivity to fluoride and guanyl nucleotides; and 4) strong dependence on Mn 2+. This paper reports that thyroid hormones specifically inhibit this enzyme activity. Materials and methods Enzyme purification Decapsulated testes from Wistar rats were homogenized in three volumes of an ice-cooled solution containing 50 mM Trjs-HC1 buffer, pH 7.5, 1 mM mercaptoethan01 and 0.5 mM EDTA, using a SDT Tissuemizer (Tekmar Co) provided Molecular and Cellular Biochemistry34, 185-190 (1981). 0300-8177/81/0343-01855 01.20. 1981, Martinus Nijhoff/Dr. W. Junk Publishers B.V., The Hague. Printed in the Netherlands.