Specific inhibition by Na+ of TSH-stimulated thyroid cell adenylate cyclase

Inhibition of adenylate cyclase from luteinized rat ovary by monovalent cations: roles of the stimulatory guanine nucleotide-binding regulatory component and stimulatory hormone receptor

Sodium and other monovalent cations (added as chloride salts) inhibited adenylate cyclase of luteinized rat ovary. Sodium chloride (150 mM) inhibited basal enzyme activity by 20%. Sodium chloride inhibition was enhanced to 34-54% under conditions of enzyme stimulation by guanine nucleotides (GTP and its nonhydrolyzable analog 5'-guanylyl imidodiphosphate), fluoride anion, and agonists (ovine luteinizing hormone (oLH) and the beta-adrenergic catecholamine isoproterenol) acting at stimulatory receptors linked to adenylate cyclase. Sodium chloride inhibition was dependent on salt concentration over a wide range (25-800 mM) as well as the concentrations of GTP and oLH. Inhibition by NaCl was of rapid onset and appeared to be reversible. The order of inhibitory potency of monovalent cations was Li+ greater than Na+ greater than K+. The role of individual components of adenylate cyclase in the inhibitory action of monovalent cations was examined. Exotoxins of Vibrio cholerae and Bordetella pertussis were used to determine respectively the involvement of the stimulatory and inhibitory guanine nucleotide-binding regulatory components (Ns and Ni) in NaCl inhibition. Sodium chloride inhibited cholera toxin-activated adenylate cyclase activity by 29%. Ni did not appear to mediate cation inhibition of adenylate cyclase because pertussis toxin did not attenuate inhibition by NaCl. Enzyme stimulation by agents (forskolin and Mn2+) thought to activate the catalytic component directly was not inhibited by NaCl but was instead significantly enhanced. Sodium chloride (150 mM) increased both the Kd for high-affinity binding of oLH to 125I-human chorionic gonadotropin binding sites and the Kact for oLH stimulation of adenylate cyclase by sevenfold. In contrast, NaCl had no appreciable effect on either isoproterenol binding to (-)-[125I]iodopindolol binding sites or the Kact for isoproterenol stimulation of adenylate cyclase. The results suggest that in luteinized rat ovary monovalent cations uncouple, or dissociate, Ns from the catalytic component and, in a distinct action, reduce gonadotropin receptor affinity for hormone. Dissociation of the inhibitory influence of Ni from direct catalytic activation could account for NaCl enhancement of forskolin- and Mn2+-associated activities. On the basis of these results, the spectrum of divergent stimulatory and inhibitory effects of monovalent cations on adenylate cyclase activities in a variety of tissues may be interpreted in terms of differential enzyme susceptibilities to cation-induced uncoupling of N and catalytic component functions.

Polarization of thyroid cells in culture: evidence for the basolateral localization of the iodide "pump" and of the thyroid-stimulating hormone receptor-adenyl cyclase complex

When cultured in collagen gel-coated dishes, thyroid cells organized into polarized monolayers. The basal poles of the cells were in contact with the collagen gel, whereas the apical surfaces were facing the culture medium. Under these culture conditions, thyroid cells do not concentrate iodide nor respond to acute stimulation by thyroid-stimulating hormone (TSH). To allow the free access of medium components to the basal poles, the gel was detached from the plastic dish and allowed to float in the culture medium. After release of the gel, the iodide concentration and acute response to TSH stimulation were restored. Increased cAMP levels, iodide efflux, and formation of apical pseudopods were observed. When the thyroid cells are cultured on collagen-coated Millipore filters glued to glass rings, the cell layer separates the medium in contact with the apical domain of the plasma membrane (inside the ring) from that bathing the basolateral domain (outside the ring). Iodide present in the basal medium was concentrated in the cells, whereas no transport was observed when iodide was added to the luminal side. Similarly, an acute effect of TSH was observed only when the hormone was added to the basal medium. These results show that the iodide concentration mechanism and the TSH receptor-adenylate cyclase complex are present only on the basolateral domain of thyroid cell plasma membranes.

Inactivation of the guanine nucleotide regulatory site mediating inhibition of the adenylate cyclase in hamster adipocytes

The influence of N-ethylmaleimide and trypsin was studied on stimulatory and inhibitory regulations of the hamster adipocyte adenylate cyclase. Treatment of intact adipocytes or adipocyte ghosts with N-ethylmaleimide decreased basal and forskolin-stimulated adenylate cyclase activities. In the pretreated membrane preparations, inhibition of the enzyme by GTP and by stable GTP analogues was abolished. Concomitantly, activation of the adenylate cyclase by NaCl and its inhibition by the antilipolytic agents, prostaglandin E1 and nicotinic acid, were obliterated. In contrast, adenylate cyclase stimulation by ACTH and stable GTP analogues was not impaired but rather increased. Similarly, the NaCl-induced attenuation of the ACTH-stimulated enzyme activity was increased by the N-ethylmaleimide treatment. Limited proteolysis of hamster adipocyte ghosts with trypsin also obliterated GTP and prostaglandin E1-induced inhibitions and NaCl-induced activation of the adenylate cyclase. In contrast, adenylate cyclase activity stimulated by isoproterenol was increased after trypsin treatment. The data suggest that the activity of the adenylate cyclase is regulated via two distinct guanine nucleotide sites and that treatment with N-ethylmaleimide and limited proteolysis with trypsin functionally eliminates the regulatory site mediating adenylate cyclase inhibition, leading to a state where the enzyme activity is regulated only via the stimulatory site. The differential effects of these treatments on NaCl-induced activation and attenuation of the adenylate cyclase suggest that sodium acts on both regulatory sites in an inhibitory manner, and that by the functional elimination of the inhibitory site, only the sodium-induced attenuation of the adenylate cyclase via the stimulatory site is observed.