The involvement of a calmodulin-dependent phosphodiesterase in the negative control of carbamylcholine on cyclic AMP levels in dog thyroid slices

Ca(II) Binding Regulates and Dominates the Reactivity of a Transition-Metal-Ion-Dependent Diesterase from Mycobacterium tuberculosis

The diesterase Rv0805 from Mycobacterium tuberculosis is a dinuclear metallohydrolase that plays an important role in signal transduction by controlling the intracellular levels of cyclic nucleotides. As Rv0805 is essential for mycobacterial growth it is a promising new target for the development of chemotherapeutics to treat tuberculosis. The in vivo metal-ion composition of Rv0805 is subject to debate. Here, we demonstrate that the active site accommodates two divalent transition metal ions with binding affinities ranging from approximately 50 nm for Mn(II) to about 600 nm for Zn(II) . In contrast, the enzyme GpdQ from Enterobacter aerogenes, despite having a coordination sphere identical to that of Rv0805, binds only one metal ion in the absence of substrate, thus demonstrating the significance of the outer sphere to modulate metal-ion binding and enzymatic reactivity. Ca(II) also binds tightly to Rv0805 (Kd ≈40 nm), but kinetic, calorimetric, and spectroscopic data indicate that two Ca(II) ions bind at a site different from the dinuclear transition-metal-ion binding site. Ca(II) acts as an activator of the enzymatic activity but is able to promote the hydrolysis of substrates even in the absence of transition-metal ions, thus providing an effective strategy for the regulation of the enzymatic activity.

Ca2+-calmodulin-dependent phosphodiesterase (PDE1): Current perspectives

Ca2+-calmodulin-dependent phosphodiesterases (PDE1), like Ca2+-sensitive adenylyl cyclases (AC), are key enzymes that play a pivotal role in mediating the cross-talk between cAMP and Ca2+ signalling. Our understanding of how ACs respond to Ca2+ has advanced greatly, with significant breakthroughs at both the molecular and functional level. By contrast, little is known of the mechanisms that might underlie the regulation of PDE1 by Ca2+ in the intact cell. In living cells, Ca2+ signals are complex and diverse, exhibiting different spatial and temporal properties. The potential therefore exists for dynamic changes in the subcellular distribution and activation of PDE1 in relation to intracellular Ca2+ dynamics. PDE1s are a large family of multiply-spliced gene products. Therefore, it is possible that a cell-type specific response to elevation in [Ca2+]i can occur, depending on the isoform of PDE1 expressed. In this article, we summarize current knowledge on Ca2+ regulation of PDE1 in the intact cell and discuss approaches that might be undertaken to delineate the responses of this important group of enzymes to changes in [Ca2+]i.

Cyclic GMP and regulation of cyclic nucleotide hydrolysis

Several of the different PDE isozyme families have the ability in vitro to hydrolyze cGMP. In particular they include the CaM-dependent PDEs, the cGMP-stimulated PDEs, and the cGMP binding, cGMP-specific PDEs. Existing evidence suggests or demonstrates that in different cell types, each of these can be important determinants for the control of cGMP steady-state levels. Each of these enzymes is differentially expressed and regulated; moreover, the amount of the enzyme expressed and the mode of regulation determine to a large extent the rate of rise, maximal level, rate of fall, and duration of the cGMP signal in the cell. In addition to enzymes that function to degrade cGMP at least two also are regulated by cGMP both in vitro and in the intact cell. The cGMP-stimulated PDE has the ability to decrease cAMP levels in response to cGMP and the cGMP-inhibited PDE can increase cAMP levels in response to cGMP. We are just beginning to define how many different isozymes of PDE exist in mammalian tissues, where they are located, and how they are regulated. Selective inhibitors to each are being developed and studies designed to define structural features that determine the mechanisms of action and regulation of the PDEs have been initiated. It is expected that in the next few years more PDEs will be discovered and the functions of the new an existing ones with be more clearly defined.

The role of protein phosphorylation in the regulation of cyclic nucleotide phosphodiesterases

The cyclic nucleotide phosphodiesterases constitute a complex superfamily of enzymes responsible for catalyzing the hydrolysis of cyclic nucleotides. Regulation of cyclic nucleotide phosphodiesterases is one of the two major mechanisms by which intracellular cyclic nucleotide levels are controlled. In many cases the fluctuations in cyclic nucleotide levels in response to hormones is due to the hormone responsiveness of the phosphodiesterase. Isozymes of the cGMP-inhibited, cAMP-specific, calmodulin-stimulated and cGMP-binding phosphodiesterases have been demonstrated to be substrates for protein kinases. Here we review the evidence that hormonally responsive phosphorylation acts to regulate cyclic nucleotide phosphodiesterases. In particular, the cGMP-inhibited phosphodiesterases, which can be phosphorylated by at least two different protein kinases, are activated as a result of phosphorylation. In contrast, phosphorylation of the calmodulin-stimulated phosphodiesterases, which coincides with a decreased sensitivity to activation by calmodulin, results in decreased phosphodiesterase activity.

Effect of prostaglandin E2 on agonist-stimulated cAMP accumulation in the distal convoluted tubule isolated from the rabbit kidney

The effects of calcitonin, vasoactive intestinal peptide (VIP), parathyroid hormone (PTH) and isoprenaline on intracellular cAMP accumulation were determined in the distal tubule (DCT) microdissected from collagenase-treated rabbit kidney. In DCTb (the initial "bright" portion) calcitonin (10 ng/ml) elicited a highly reproducible response 203.7 +/- 19.1 fmol cAMP mm-1 4 min-1 (SE,N = 13) whereas VIP-induced cAMP accumulation was less and more variable from one experiment to another (1 microM, 97.2 +/- 17.8 fmol mm-1 4 min-1, SE, N = 12). When used in combination, these two agonists were non-additive, indicating stimulation of a single pool of cAMP in DCTb. In DCTg, ("granular") which consists of at least two cell types, PTH (100 nM) elicited a marked, reproducible accumulation of cAMP (154.3 +/- 27.0 fmol mm-1 4 min-1; SE, N = 5). Isoprenaline (1 microM) and VIP (1 microM) induced much smaller increases in cAMP levels 20.9 +/- 2.7 and 29.4 +/- 4.1 fmol mm-1 4 min-1 (SE, N = 5) respectively, and, when used in combination, were non-additive, demonstrating that VIP and isoprenaline are active on the same cell type. In DCTb, prostaglandin E2 (PGE2) inhibited both calcitonin- and VIP-stimulated cAMP accumulation (calcitonin 57.8 +/- 2.7% inhibition, SE, N = 16; VIP, 80.6 +/- 2.1% inhibition, SE, N = 5). The EC50 values for calcitonin were 1.21 +/- 0.33 ng/ml and 1.83 +/- 0.25 ng/ml (SD, N = 3) in the absence and presence of PGE2 (300 nM) respectively with an IC50 for PGE2 of 26.3 +/- 6.3 nM (SE, N = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1-adrenergic regulation of thyrotropin-stimulated release of 3, 5, 3' -triiodothyronine and thyroxine from perifused mouse thyroid

The effect of methoxamine, a specific alpha 1-adrenergic agonist, on the release of T3, T4 and cAMP from perifused mouse thyroid was studied to clarify the role of the alpha 1-adrenergic receptor in the regulation of thyroid hormone secretion. TSH-stimulated T3 and T4 release was inhibited significantly by methoxamine. With regard to cAMP release, methoxamine inhibited TSH-stimulated cAMP release in the presence of 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone but did not inhibit TSH-stimulated cAMP release in the presence of 3-isobutyl-1-methylxanthine. Methoxamine did significantly suppress TSH-stimulated release of T3 and T4 in the presence of each phosphodiesterase inhibitor. Depletion of Ca2+ in the perifusion buffer abolished completely the inhibitory effect of methoxamine on TSH-stimulated T3 and T4 release. The present study suggests that activation of the alpha 1-adrenergic receptor inhibits TSH-stimulated T3 and T4 secretion through a Ca(2+)-dependent mechanism in the mouse thyroid gland.

Two mechanisms of inhibition by prostaglandin E2 of hormone-dependent cell cAMP in the rat collecting tubule

The effect of exogenous prostaglandin E2 (PGE2) on hormone-dependent adenosine 3',5'-cyclic monophosphate (cAMP) accumulation was investigated by microradioimmunoassay in collecting tubules microdissected from the cortex (CCT) or outer medulla (MCT) of the rat kidney. Two phosphodiesterase inhibitors were used: either a xanthine derivative (isobutyl-methylxanthine (IBMX, 1 mM] active on all forms of phosphodiesterase or Ro 20-1724 (50 microM) active on the phosphodiesterase type III. A prostaglandin synthesis inhibitor was added to all media. In the presence of IBMX, 0.3 microM PGE2 inhibited by 39.1% the response induced in the CCT by the beta-adrenergic agonist isoproterenol (1 microM). Under the same experimental conditions, arginine vasopressin (AVP)-stimulated cAMP accumulation in CCT or MCT was not affected by PGE2. In the presence of Ro 20-1724, 0.3 microM PGE2 did not modify the response to 1 nM AVP in CCT but inhibited this response in MCT samples (mean inhibition: 52.7%). The inhibition by PGE2 was dose dependent with a maximum at 0.3 microM, observed for all concentrations of AVP tested (from 50 pM to 1 nM) and did not affect the concentration of AVP inducing half-maximal cAMP accumulation. In a second experimental series performed in the presence of adenosine deaminase, an A1-adenosine agonist [theta)-N6-(R-phenylisopropyl)adenosine (PIA, 0.1 microM] also decreased the response to 1 nM AVP in the MCT. The addition of an A1-adenosine antagonist relieved the effect of PIA but did not modify the inhibition observed with PGE2. Thus PGE2 decreased the synthesis of cAMP in beta-adrenergic sensitive cells in rat CCT and might affect the catabolism of AVP-dependent cAMP level rather than its synthesis in rat MCT.

Involvement of a plasma membrane phosphodiesterase in the negative control of cyclic AMP levels by vasopressin in rat hepatocytes

Vasopressin has been shown previously to lower the glucagon-induced increase of cyclic AMP levels in isolated rat hepatocytes by way of an enhanced phosphodiesterase (EC 3.1.4.17) activity. Five phosphodiesterase inhibitors were tested for their ability to prevent vasopressin from lowering cyclic AMP levels in intact hepatocytes and for their inhibitory effect in vitro on soluble and particulate phosphodiesterase activities partially purified from hepatocytes. Three soluble activities have been separated by DEAE-cellulose chromatography: a phosphodiesterase hydrolyzing both cyclic AMP and cyclic GMP, a form stimulated by cyclic GMP and a cyclic AMP-specific activity. The absence of any statistically significant correlation between the in vivo (in intact cells) and the in vitro (on isolated phosphodiesterases) potencies of the inhibitors does not support a role for the cytosolic phosphodiesterases in mediating the vasopressin-induced decrease in cyclic AMP levels. No statistically significant correlation was observed between the inhibition of the vasopressin effect on cyclic AMP accumulation and the inhibition of phosphodiesterase activity either associated with the native plasma membranes or solubilized from these membranes with 0.4 M NaCl. In contrast, a statistically significant correlation was observed between the degree of inhibition of the vasopressin effect in the intact cells and the degree of inhibition of the intrinsic phosphodiesterase still associated with the plasma membranes after high-salt treatment. These data indicate that a phosphodiesterase activity integral to the plasma membrane is very likely involved in the negative control of cyclic AMP levels by vasopressin.

Stimulation of inositol phosphate formation in FRTL-5 rat thyroid cells by catecholamines and its relationship to changes in 45Ca2+ efflux and cyclic AMP accumulation

Catecholamines specifically stimulated the rapid formation of inositol phosphates, bisphosphates and trisphosphates in a concentration-dependent manner in FRTL-5 thyroid cells. Further analysis by high performance liquid chromatography revealed the presence of two isomers of inositol trisphosphate, 1,4,5- and 1,3,4-trisphosphate, suggesting that the 1,4,5-trisphosphate of inositol is further metabolized to the 1,3,4-trisphosphate isomer. The alpha 1-adrenoreceptor antagonist, prazosin, inhibited the effects of epinephrine, while the alpha 2-adrenoreceptor antagonist, yohimbine, was without effect. Treatment of FRTL-5 cells with pertussis toxin (to inhibit Ni) did not abolish the epinephrine effect on inositol trisphosphate formation. Carbachol, N6-[L-2-phenylisopropyl]-adenosine and forskolin were without effect on phosphoinositide metabolism. Both epinephrine and the calcium ionophore A23187 stimulated 45Ca2+ efflux from 45Ca2+-loaded FRTL-5 cells. The time-course of the epinephrine effect indicates that inositol 1,4,5-trisphosphate formation (t1/2 approximately 1 s) precedes both the efflux of 45Ca2+ (t1/2 approximately 30 s) as well as the reduction of cyclic AMP levels (t1/2 approximately 90 s) in response to epinephrine. These results strongly suggest that inositol 1,4,5-trisphosphate has the appropriate properties to act as a second messenger by which alpha 1-adrenergic hormones, through mobilization of intracellular Ca2+ and activation of cyclic AMP phosphodiesterase, reduce cyclic AMP levels in FRTL-5 cells.

Cholinergic control of cyclic nucleotide metabolism in human thyroid cells

In the presence of Ro 20-1724, a selective inhibitor of cyclic nucleotide phosphodiesterase, carbamylcholine increases cAMP and cGMP levels in human thyroid cells in primary culture. The increase of cAMP exhibited at concentrations of carbamylcholine between 10 fM and 10 pM, is dose- and time-dependent, it is maximum after 30 min and is abolished after 60 min. At higher carbamylcholine concentration (10 microM), cAMP increases rapidly, becoming maximum after 15 min, but returns to unstimulated values after 30 min. The increase of cGMP is also dose-dependent (0.1 nM-10 microM); it reaches the maximum after 30 min and returns to unstimulated values after 120 min. A significant increase of phosphodiesterase activity is observed at 10 microM carbamylcholine. Atropine, a muscarinic receptor antagonist, blocks carbamylcholine effects on both cAMP and cGMP production without affecting the thyrotropin-induced cAMP accumulation. Hexamethonium, a nicotinic receptor antagonist does not affect the cholinergic effects. In the presence of Ro 20-1724, 10 microM carbamylcholine significantly inhibits the effect of thyrotropin on cAMP production, while the combined addition of low doses of carbamylcholine and thyrotropin (0.1 nM and 10 pM, respectively) results in an additive effect on cAMP levels. Inhibition of thyrotropin activity on cAMP production, similar to that exerted by 10 microM carbamylcholine is produced by increasing free intracellular calcium; this inhibition is relieved by using a calmodulin-sensitive phosphodiesterase inhibitor, M and B 22948 at 50 microM dose. High concentrations (10 microM) of carbamylcholine increase the adenylate cyclase activity, without any significant effect on the thyrotropin-induced activation of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of thyroglobulin release by dog thyroid slices

Nonbutanol-extractable 131I (NBE131I) release by dog thyroid slices in vitro has been shown previously to be primarily thyroglobulin (Tg); it is stimulated by TSH. NBE131I (Tg) release has therefore been considered as an in vitro model of thyroglobulin secretion and was further characterized in this work. TSH-stimulated NBE131I (Tg) release, like TSH-stimulated BE131I (T4, T3 and iodide) release was reproduced by forskolin, an activator of adenylate cyclase. TSH-, (Bu)2cAMP- and forskolin-stimulated NBE131I (Tg) release was inhibited by 10(-5) M carbamylcholine, an effect relieved by 10(-5) M atropine, but not by 10(-4) M 1-methyl-3-isobutylxanthine. NBE131I (Tg) release was observed in the presence of 2 mM methimazole and 2 mM perchlorate. Cooling the slices to 20 degrees C or addition of 10(-5) M monensin completely blocked the formation of apical pseudopods and BE131I release but not NBE131I (Tg) release. Inhibition by 500 microM chloroquine of intralysosomal Tg hydrolysis and BE131I release did not enhance NBE131I (Tg) release. Cytochalasin B induced a concentration-dependent increase in basal and TSH-stimulated NBE131I (Tg) release at concentrations which depressed TSH-stimulated BE131I release. Removal of Ca2+ from the medium and slices by 10(-3) M or 10(-4) M EGTA increased NBE131I (Tg) release. In conclusion, in dog thyroid slices, TSH-stimulated NBE131I (Tg) release was mediated by cAMP and inhibited by 10(-5) M carbamylcholine at a step beyond cAMP. It was not neosynthesized Tg. It did not seem to require the formation of apical pseudopods or to result from the escape from lysosomes of undegraded thyroglobulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1-adrenergic regulation of TSH-stimulated cyclic AMP accumulation in rat thyroid cells

Addition of epinephrine to cultured FRTL-5 rat thyroid cells led to a concentration-dependent reduction of TSH- and forskolin-stimulated cAMP accumulation. Clonidine, which preferentially activates the alpha 2-adrenoreceptor, had no effect on cAMP levels. The reduction of cAMP levels by epinephrine was selectively blocked by prazosin, an alpha 1-adrenoreceptor antagonist, but not by yohimbine, an alpha 2-adrenoreceptor antagonist. Pretreatment of FRTL-5 cells with pertussis toxin failed to abolish the inhibitory effect of epinephrine on cAMP accumulation. The bioactivity of the pertussis toxin preparation in this cell line was verified by its ability to ADP-ribosylate the alpha-subunit of the inhibitory guanine nucleotide regulatory protein, Ni, as well as its ability to abolish the inhibitory effect of N6-[L-2-phenylisopropyl]-adenosine on TSH-stimulated cAMP formation. The inhibitory effect of epinephrine on cAMP levels was dependent on Ca2+ and was reversed by 3-isobutyl-1-methylxanthine. Taken together, these results suggest that epinephrine reduces cAMP levels via alpha 1-adrenoreceptors. The failure of pertussis toxin to abolish this alpha-adrenergic effect is consistent with the conclusion that epinephrine-induced attenuation of cAMP accumulation occurs through activation of a Ca2+-calmodulin-sensitive phosphodiesterase and does not involve Ni or Ni-like proteins.

The use of Xenopus oocytes for the study of ion channels

Recently, in addition to the "traditional" research on meiotic reinitiation and fertilization mechanisms, the oocytes of the African frog Xenopus laevis have been exploited for the study of numerous aspects of ion channel function and regulation, such as the properties of several endogenous voltage-dependent channels and the involvement of second messengers in mediation of neurotransmitter-evoked membrane responses. In addition, injection of these cells with exogenous messenger RNA results in production and functional expression of foreign membranal proteins, including various voltage- and neurotransmitter-operated ion channels originating from brain, heart, and other excitable tissues. This method provides unique opportunities for the study of the structure, function, and regulation of these channels. A multidisciplinary approach is required, involving molecular biology, electrophysiology, biochemistry, pharmacology, and cytology.

Modulation of cyclic AMP action in the dog thyroid by its agonist and antagonist Sp- and Rp-adenosine 3',5'-monophosphorothioate

The diastereoisomeric forms of adenosine 3',5'-monophosphorothioate, Sp-cAMPS and Rp-cAMPS, have been shown to mimic and to inhibit activation of protein kinase type I and type II by cyclic AMP. In the present work, Sp-cAMPS mimicked thyrotropin (TSH) action on thyroid hormone secretion and protein iodination in dog thyroid slices, whereas Rp-cAMPS antagonized those effects. The phosphorothioates have been tested as inhibitors or activators of the three major phosphodiesterases: the Ca2+/calmodulin-sensitive form, the cyclic GMP-stimulated form and the cyclic AMP-specific enzyme. At 100 microM Sp-cAMPS inhibited the three enzyme activities. In contrast, Rp-cAMPS failed to stimulate activity of the three enzymes. From a comparison of the biological properties of Sp- and Rp-cAMPS and 3-isobutyl-1-methyl xanthine, it is suggested that one site of action of the phosphorothioates is on the cyclic AMP-dependent protein kinases, i.e. the effects of Sp-cAMPS and Rp-cAMPS observed in intact cell can be ascribed to the agonistic and antagonistic effects on the cyclic AMP-dependent protein kinases. However, partial inhibition of phosphodiesterase activities by the phosphorothioates cannot be excluded.

Immunocytochemical demonstration of calmodulin in cells secreting by exocytosis

Calmodulin is a regulator of several calcium-dependent cellular processes. It has been suggested that it plays a role in the mechanism of secretion. Employing an indirect immunoperoxidase technique at the light microscope level, this study demonstrates the presence of calmodulin in several exocytotic cells (mast cells, thyroid follicular cells, neurohypophyseal neurosecretory terminals, pancreatic beta-cells and pancreatic acinus cells) in rat and man. The positive staining reaction for calmodulin was granular and at least in the case of rat mast cells it appeared to be associated with the granule membrane.

Acetylcholine and phorbol esters inhibit potassium currents evoked by adenosine and cAMP in Xenopus oocytes

In Xenopus laevis oocytes, adenosine and other purinergic agonists induce a K+-conductance increase that is fully mimicked by intracellular application of cAMP. Acetylcholine suppresses the K+-conductance increase caused by adenosine, by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, or by intracellular injection of cAMP. This effect of acetylcholine is not mimicked by intracellular injection of Ca2+ or of the Ca-mobilizing agent inositol 1,4,5-trisphosphate. However, adenosine and cAMP responses are inhibited by 4 beta-phorbol 12,13-dibutyrate and 4 beta-phorbol 12-myristate 13-acetate. These results suggest that, in Xenopus oocytes, the muscarinic inhibition of purinergic and cAMP responses is mediated through the activation of the phospholipid-dependent, Ca-activated protein kinase (protein kinase C).

Specificity of cGMP binding to a purified cGMP-stimulated phosphodiesterase from bovine adrenal tissue

The binding of [3H]cGMP (guanosine 3',5'-monophosphate) to purified bovine adrenal cGMP-stimulated phosphodiesterase was measured by Millipore filtration on cellulose ester filter. [3H]cGMP-binding activity was enhanced when the assay was terminated in buffer containing 70% of saturated ammonium sulfate to dilute the enzyme and wash the filters. The cGMP-binding activity was co-purified with the phosphodiesterase activity. The binding of [3H]cGMP to purified enzyme was measured in the presence or absence of the phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine. 1-Methyl-3-isobutylxanthine showed linear competitive inhibition with respect to cGMP as substrate in the phosphodiesterase reaction but stimulated the [3H]cGMP-binding activity in the binding assay. The stimulatory effect appeared not to be the result of preservation from [3H]cGMP hydrolysis; no cGMP phosphodiesterase activity has been measured under the cGMP-binding assay conditions, in the absence or presence of the inhibitor. Half-maximal stimulation by 1-methyl-3-isobutylxanthine occurred in the 5-7 microM concentration range. The specificity of binding of [3H]cGMP was investigated by adding increasing concentration of unlabeled analogs of cAMP (adenosine 3',5'-monophosphate) and cGMP. The binding of [3H]cGMP (50 nM) was displaced by unlabeled cGMP and cAMP with the following potency: 50% displacement was reached at the 0.1 microM cGMP range and only at a fiftyfold higher cAMP concentration. Our data with comparative series of analogs (e.g. 5'-amino-5'-deoxyguanosine 3',5'-monophosphate and 3'-amino-3'-deoxyguanosine 3',5'-monophosphate) showed that the potencies of stimulation of cAMP phosphodiesterase activity parallels displacement curves or [3H]cGMP binding to purified enzyme with no correlation with phosphodiesterase inhibition sequences. Those experiments suggest that the cGMP-binding activity is directly related to the non-catalytic (allosteric) cGMP-binding site.

Further characterization of the iodide inhibitory effect on the cyclic AMP system in dog thyroid slices

Iodide inhibits cyclic AMP accumulation in the thyroid by a process which is prevented by inhibition of iodide uptake and of thyroid peroxidase. By a similar process, it also exerts other independent effects such as the enhancement of iodinated protein release. Iodide inhibited the stimulation of adenylate cyclase by prostaglandin E1, cholera toxin and forskolin. The action of iodide was not relieved by phosphodiesterase inhibitors and was not additive with the effect of norepinephrine or adenosine. Iodide did not decrease the cellular level of ATP. The data are compatible with an inhibition of adenylate cyclase beyond the level of the receptor, presumably at the level of the catalytic unit or its interaction with the positive transducing unit NS. The effect of iodide required TSH for its expression but not for its installation. It was decreased under all conditions in which iodide organification was decreased: decreased iodide or increased methimazole concentration, absence of calcium in the medium, etc. However, the relation between iodide binding to proteins and effect was not linear. The effect was not relieved by washing in the absence of iodide and in the presence of perchlorate, but it was partly reversible in the presence of methimazole propylthiouracyl or thiourea. It was not relieved by cooling to 20 degrees C and cytochalasin b, which block stimulated thyroglobulin hydrolysis and iodothyronine release, nor by actinomycin D, cycloheximide, puromycin, mepacrine or indomethacin. The data suggest that iodide binds to a saturable cell component by a reaction which is reversible only in the presence of thiol-containing drugs.

Negative regulation of cyclic-AMP levels by carbamylcholine in dog thyroid is not mediated by cyclic-GMP

Carbamylcholine, through calcium, enhances cyclic-GMP accumulation and depresses cyclic-AMP accumulation in TSH stimulated dog thyroid. The results presented show that compounds which can be transformed to nitric oxide increase cyclic-GMP accumulation in the dog thyroid. These compounds do not require extracellular calcium for their action. In thyroid stimulated by TSH, these compounds do not depress AMP accumulation. Cyclic-GMP is not the main intracellular signal involved in the negative regulation of cyclic-AMP levels in dog thyroid.

A role for intracellular calcium and calmodulin in the release of triiodothyronine from human thyroid-cell monolayer cultures

Human thyroid cells in monolayer responded to acute stimulation by TSH with an increase in the secretion of T3. This process appeared to be dependent on a rise in the cytosolic calcium concentration since the antagonist of intracellular calcium mobilization, TMB-8, was found to inhibit the release of T3 in response to TSH. The importance of intracellular calcium was further shown using the agent veratridine which increases the free calcium level within cells; veratridine potentiated the stimulation of T3 secretion by TSH and itself stimulated the release of T3 to a level higher than that seen in the presence of TSH alone. The calcium ionophore A23187 produced a biphasic effect on T3 secretion from human thyroid monolayers; at low concentrations, A23187 caused a decrease in both unstimulated and TSH-stimulated T3 secretion but above a concentration of 1 microM, T3 secretion was increased. The calmodulin antagonist W7 was found to inhibit T3 release in response to TSH, indicating a role for calmodulin in mediating the effects of intracellular calcium on T3 secretion.