Effects of subchronic and chronic melatonin treatment on somatostatin binding and its effects on adenylyl cyclase activity in the rat frontoparietal cortex

Melatonin and somatostatin are known to exert similar effects on locomotor activity. We have previously demonstrated that acute melatonin treatment regulates somatostatin receptor function in the rat frontoparietal cortex. However, the effects of subchronic and chronic melatonin treatment on the somatostatin receptor-G protein-adenylyl cyclase system in the rat frontoparietal cortex are unknown. Melatonin was administered subcutaneously at a daily dose of 25 microg/kg for 4 days, 1 wk or 2 wk. Twenty-four hours after the last injection, the animals were sacrificed. Melatonin did not alter the somatostatin-like immunoreactivity content in the frontoparietal cortex from control and melatonin-treated rats during any of the previously indicated periods. Four days of melatonin administration induced both an increase in the number of [(125)I]-Tyr11-somatostatin receptors and a decrease in the affinity of somatostatin for its receptors in frontoparietal cortical membranes. The increased number of somatostatin receptors in the melatonin-treated rats was associated with an increased capacity of somatostatin to inhibit basal and forskolin-stimulated adenylyl cyclase activity. Melatonin administration for 4 days induced a higher adenylyl cyclase activity both under basal conditions and after direct stimulation of the enzyme with forskolin. No significant differences were observed in the function of Gi proteins in the 4-day melatonin-treated rats. Western blot analyses showed that the 4-day melatonin treatment reduced Gialpha(2) levels, without altering the amount of Gialpha(1). These melatonin-induced changes reverted to control values after 7 or 14 days of treatment. Altogether, the present findings suggest that subchronic melatonin treatment modulates the somatostatin receptor/effector system in the rat frontoparietal cortex.     

Acute modulation of somatostatin receptor function by melatonin in the rat frontoparietal cortex

Since melatonin (N-acetyl-5-methoxytryptamine) decreases locomotor activity and rearing and increases grooming behavior in a similar manner as somatostatin (SRIF), we examined if melatonin could induce these changes through somatostatinergic neurotransmission in the rat frontoparietal cortex. Male Wistar rats (200-250 g) received a single injection of melatonin (25 microg/kg per day) subcutaneously (s.c.) and were sacrificed 5 hr later. Melatonin treatment increased the number of 125I-Tyr11-SRIF receptors in frontoparietal cortical membranes without any changes in the dissociation constant (Kd). The capacity of SRIF to inhibit basal and forskolin (FK)-stimulated adenylyl cyclase (AC) activity was increased in melatonin-treated rats as compared to the control animals. Melatonin administration also induced a lower AC activity, both under basal conditions and after stimulation of the enzyme via stimulatory guanine nucleotide-binding proteins (Gs), or directly with FK. Functional inhibitory guanine nucleotide-binding protein (Gi) activity was increased in frontoparietal cortical membranes from melatonin-treated rats when compared to controls. Western blot analyzes showed that melatonin administration did not alter the amount of the Gialpha1, or Gialpha3 subunits, but reduced Gialpha2 levels in frontoparietal cortical membranes. No significant changes in SRIF-like immunoreactivity content and SRIF mRNA levels were detected in this brain area after melatonin treatment. Administration of the melatonin receptor antagonist luzindole (10 mg/kg, s.c.) 30 min before melatonin injection did not change the melatonin-induced effects on the SRIF receptor effector system. In conclusion, the present results show that acute melatonin administration increases the activity of the SRIF receptor effector system and decreases Gialpha2 levels in the rat frontoparietal cortex. In addition, the coupling of Gs to AC is disturbed by melatonin.     

Purification and kinetic properties of the soluble Mn2+-dependent adenylyl cyclase of the rat testis

The soluble Mn²⁺-dependent adenylyl cyclase (AC) of the rat testis was purified 1500-fold with some 19% yield of the initial activity. These results were accomplished by conventional separation techniques including (NH₄)₂SO₄ precipitation of testis cytosol (106 000 × g), gel filtration (Sephadex G-200) and ion-exchange chromatography (Sephadex DEAE-A50) followed by Sephadex G-100 gel filtration and isoelectric focusing.Analysis by polyacrylamide-gel electrophoresis (PAGE) of aliquots from each purification step revealed the following. (a) The Mn²⁺-dependent AC migrated with a R_f value of 0.40 irrespective of the degree of purification. (2) The AC peak from the isoelectric focusing column separated into 2 major protein bands; however, only one band (R_f 0.40) had AC activity.The molecular weight emerging from the position of migration on the Sephadex G-200 and G-100 columns appeared to be consistent at 47 000–48 000 D, as estimated from the relationship log MW versus elution volume.The purified enzyme fulfilled the requirements for a simple Michaelis-Menten kinetics with an apparent Km for Mn²⁺ and MnATP^2? of 6.7 and 2.5 mM, respectively. Varying the concentrations of ATP or Mn²⁺ separately did not alter the apparent affinity (K_m^app) for the other parameter.These and previous data from our laboratory show that the physico-chemical and kinetic properties (molecular weight and K_mapp for Mn²⁺ and MnATP^2?) do not alter during purification. Furthermore, the additional step of affinity chromatography seems obligatory if a homogeneous AC preparation is to be obtained.     

Prolactin desensitizes the prostaglandin e1-dependent adenylyl cyclase in the rat prostate gland

Prior exposure of the rat ventral prostate to a high dose of ovine prolactin in vivo or in vitro caused the specific desensitization of the prostaglandin E₁-dependent adenylyl cyclase. Maximal desensitization was achieved in vivo 6 h after exposure to 100 μg prolactin and complete resensitization occurred after 7 days. No heterologous desensitization of the response of the cyclase to other hormones tested was observed. Incubation of isolated secretory cells with prolactin and prostaglandin E₁ desensitized the subsequent response to the same prostaglandin. Simultaneous exposure of the cells to indomethacin prevented the prolactin-induced effect. Prolactin could also induce similar changes in the prostaglandin E₁-dependent cyclase of rat Leydig cell tumour cells in vivo, suggesting that this may represent a phenomenon shared by other prolacting target organs. This study supports the hypothesis that prostaglandins may constitute second messengers for prolactin effects on the prostate gland.     

Neuroprotective effects of melatonin on amphetamine‐induced dopaminergic fiber degeneration in the hippocampus of postnatal rats

Chronic amphetamine (AMPH) abuse leads to damage of the hippocampus, the brain area associated with learning and memory process. Previous results have shown that AMPH‐induced dopamine neurotransmitter release, reactive oxygen species formation, and degenerative protein aggregation lead to neuronal death. Melatonin, a powerful antioxidant, plays a role as a neuroprotective agent. The objective of this study was to investigate whether the protective effect of melatonin on AMPH‐induced hippocampal damage in the postnatal rat acts through the dopaminergic pathway. Four‐day‐old postnatal rats were subcutaneously injected with 5‐10 mg/kg AMPH and pretreated with 10 mg/kg melatonin prior to AMPH exposure for seven days. The results showed that melatonin decreased the AMPH‐induced hippocampal neuronal degeneration in the dentate gyrus, CA1, and CA3. Melatonin attenuated the reduction in the expression of hippocampal synaptophysin, PSD‐95, α‐synuclein, and N‐methyl‐D‐aspartate (NMDA) receptor protein and mRNA caused by AMPH. Melatonin attenuated the AMPH‐induced reduction in dopamine transporter (DAT) protein expression in the hippocampus and the reduction in mRNA expression in the ventral tegmental area (VTA). Immunofluorescence demonstrated that melatonin not only prevented the AMPH‐induced loss of DAT and NMDA receptor but also prevented AMPH‐induced α‐synuclein overexpression in the dentate gyrus, CA1, and CA3. Melatonin decreased the AMPH‐induced reduction in the protein and mRNA of the NMDA receptor downstream signaling molecule, calcium/calmodulin‐dependent protein kinase II (CaMKII), and the melatonin receptors (MT1 and MT2). This study showed that melatonin prevented AMPH‐induced toxicity in the hippocampus of postnatal rats possibly via its antioxidative effect and mitochondrial protection.     

Opposite effects of ethanol on the activation of adenylyl cyclase in human corpus luteum membranes

The influence of an acute exposure to ethanol on adenylyl cyclase activity in membrane fractions prepared from human corpus luteum was investigated. Ethanol up to a concentration of 5% (v/v) was without effect on basal luteal adenylyl cyclase activity, but markedly potentiated stimulation of NaF and hCG in a dose-dependent manner. In contrast, ethanol progressively inhibited forskolin stimulation at the same range of ethanol concentrations. Maximal NaF and hCG responsiveness of adenylyl cyclase activity was observed at 5% ethanol and reached values 80% and 100% higher than controls without ethanol, respectively. However, at the same ethanol concentration, forskolin-stimulated enzymatic activity was reduced by 40% relative to controls. Equilibrium binding studies involving [¹²⁵I]hCG interaction with luteal membranes in the presence of the concentration of ethanol showing maximal hCG responsiveness indicated that ethanol slightly affected (15% increase) the hCG binding compared to controls, without any appreciable change on the K_d for the hormone. This minor effect of ethanol on gonadotropin binding sites contrasted greatly with the extent at which ethanol maximally potentiated the gonadotropin-stimulated adenylyl cyclase. GTP was found to be less effective than GMP-P(NH)P in sustaining ethanol potentiation, suggesting that ethanol is unlikely to act by inhibiting GTPase activity. These data indicate that the acute effects of ethanol inhibit forskolin-stimulated adenylyl cyclase at concentrations potentiating stimulatory effects of NaF and of hCG, and that the synergistic interaction of ethanol and gonadotropin stimulation of adenylyl cyclase is, at least in part, due to an increase in the functional coupling of the occupied hCG-receptor complex with the components of the enzyme system.     

Chimeric Galphaq subunits can distinguish the long form of the Xenopus Mel1c melatonin receptor from the mammalian mt1 and MT2 melatonin receptors

The family of melatonin receptors is composed of the mt1, MT2, and Mel1c subtypes. The Mel1c is further divided into one long and two short isoforms. A recent study has shown that, unlike mt1 and MT2, the long form of Mel1c is incapable of activating the pertussis toxin-insensitive G16. Here we used three well-characterized Galphaq chimeras to explore the coupling specificity of the melatonin receptors. The qi5, qo5, and qz5 chimeras can link numerous Gi-coupled receptors to the stimulation of phosphoinositide-specific phospholipase C. Both mt1 and MT2 receptors interacted productively with the Galphaq chimeras, while the long form of Mel1c was totally ineffective. Among the Galphaq chimeras, qo5 was less efficiently coupled to the melatonin receptors. Such differential coupling is best explained by structural differences between the melatonin receptors as well as among the Galphaq chimeras. Since the long form of Mel1c receptor possesses an exceptionally large C-terminal tail, we tested the ability of four melatonin receptor C-terminal tail chimeras (Chi 1-4) to interact with the Galphaq chimeras. The presence of the large C-terminal tail of Mel1c in Chi 1 and Chi 3 markedly hindered their coupling to the Galphaq chimeras. On the other hand, the attachment of either the mtl or MT2 C-terminal tail to a Mel1c backbone produced chimeras (Chi 2 and Chi 4) that were capable of activating the Galphaq chimeras. These findings suggest the involvement of C-terminal regions of melatonin receptors in the recognition of G proteins.     

Anatomical and cellular localization of melatonin MT1 and MT2 receptors in the adult rat brain

The involvement of melatonin in mammalian brain pathophysiology has received growing interest, but information about the anatomical distribution of its two G‐protein‐coupled receptors, MT₁ and MT₂, remains elusive. In this study, using specific antibodies, we examined the precise distribution of both melatonin receptors immunoreactivity across the adult rat brain using light, confocal, and electron microscopy. Our results demonstrate a selective MT₁ and MT₂ localization on neuronal cell bodies and dendrites in numerous regions of the rat telencephalon, diencephalon, and mesencephalon. Confocal and ultrastructural examination confirmed the somatodendritic nature of MT₁ and MT₂ receptors, both being localized on neuronal membranes. Overall, striking differences were observed in the anatomical distribution pattern of MT₁ and MT₂ proteins, and the labeling often appeared complementary in regions displaying both receptors. Somadendrites labeled for MT₁ were observed for instance in the retrosplenial cortex, the dentate gyrus of the hippocampus, the islands of Calleja, the medial habenula, the suprachiasmatic nucleus, the superior colliculus, the substantia nigra pars compacta, the dorsal raphe nucleus, and the pars tuberalis of the pituitary gland. Somadendrites endowed with MT₂ receptors were mostly observed in the CA3 field of the hippocampus, the reticular thalamic nucleus, the supraoptic nucleus, the inferior colliculus, the substantia nigra pars reticulata, and the ventrolateral periaqueductal gray. Together, these data provide the first detailed neurocytological mapping of melatonin receptors in the adult rat brain, an essential prerequisite for a better understanding of melatonin distinct receptor function and neurophysiology.     

Effects of two melatonin analogues, S-20098 and S-20928, on melatonin receptors in the pars tuberalis of the rat

Abstract: By using quantitative autoradiography, we studied the effects of two drugs related to melatonin on the 2- ¹²⁵I-melatonin binding in the pars tuberalis (PT) of rats. The drugs tested were two naphthalenic analogues of melatonin, S-20098 ( N -[2-(7-methoxy-l-naphthyl) ethyl] acetamide), an agonist, and S-20928 ( N -[2-(l-naphthyl) ethyl] cyclobutyl carboxamide), a putative antagonist. Melatonin (s.c. and i.p.), S-20098 (s.c), and S-20928 (i.p.) were injected 4 hr before sacrifice. Acute administration of both melatonin and S-20098 decreased melatonin receptor density. In contrast, the putative antagonist S-20928, at a low dose (1 mg/kg), was ineffective on melatonin receptors. It neither affected the 2- ¹²⁵I-melatonin specific binding observed in the control group nor did it prevent the decrease in binding induced by melatonin when injected 5 min before the hormone. At a high dose (10 mg/kg), S-20928 totally blocked the effect of melatonin on melatonin receptor density and induced a decrease in binding capacity as melatonin did when injected alone. These results indicate that in the rat pars tuberalis, the melatonin agonist, S-20098, is able to down-regulate melatonin receptors, whereas S-20928 seems to behave as a partial agonist.     

Mechanisms underlying the effects of estrogen on nocturnal melatonin synthesis in peripubertal female rats: relation to norepinephrine and adenylate cyclase

We previously reported that estrogen modulates the nocturnal synthesis of melatonin in the pineal gland of peripubertal female rats. These effects appeared to be mediated by the modulation of N-acetyltransferase (NAT) activity. The present study assessed the mechanism underlying the effects of estrogen deficiency and stimulation on pineal melatonin synthesis in peripubertal female rats. We measured the norepinephrine levels and adenylate cyclase activity in pineal gland homogenates obtained from 4-10-wk-of-age female Sprague Dawley rats at mid-dark during the daily light/dark cycle. The animals were ovariectomized and daily s.c. administration of estradiol benzoate (E2B, 1.0 microgram/d) was initiated at 4 wk of age. Pineal norepinephrine levels increased significantly from Week 3 to 4 (P < 0.0001), and remained unchanged thereafter. Neither ovariectomy nor E2B administration significantly affected norepinephrine levels. Adenylate cyclase activity in the pineal gland peaked at 4 wk in untreated (control) rats. Ovariectomy at Week 4 led to a significant increase in adenylate cyclase activity at Week 8. At Week 10, adenylate cyclase activity returned to control levels. S.c. injection of E2B suppressed the ovariectomy-induced increase in adenylate cyclase activity to the level seen in control rats. These changes in mid-dark adenylate cyclase activity resembled those previously observed with NAT activity. The results suggest that estrogen modulates adenylate cyclase activity in the pineal gland of peripubertal female rats. The inhibitory effect of estrogen on melatonin synthesis appeared to be mediated in part, by changes in the norepinephrine-induced stimulation of pineal adenylate cyclase activity.     

Direct effect of glibenclamide on guanylate cyclase activity in the rat in vitro

Summary Glibenclamide enhanced the activity in the rat of guanylate cyclase in a number of extra-pancreatic tissues. Thus, glibenclamide enhanced guanylate cyclase activity in vitro two- to threefold in liver, kidney, heart, spleen and colon at a concentration of 1 mol/l. Dose-response curves of glibenclamide on hepatic guanylate cyclase revealed that more than half-maximal stimulation was observed at a concentration as low as 10 nmol/l (p< 0.001) and no stimulation of guanylate cyclase was seen when the concentration was decreased to 1 nmol/l. Maximal enhancement was seen at 100 nmol/l of glibenclamide. Varying the concentration of the guanylate cyclase co-factor manganese had no effect on the glibenclamide enhancement of guanylate cyclase. In addition to the increased insulin receptors found recently in monocytes and fibroblasts, the present findings may help explain the extra-pancreatic effects of glibenclamide and possibly of other sulphonylurea drugs.     

Effects of melatonin on macrophages/microglia in postnatal rat brain

The present study examined the response of macrophages/microglia to multiple injections of melatonin in the pineal gland and different regions of the brain. The macrophages/microglia showed a significant increase in cell numbers and upregulation of complement type 3 receptors (CR3), major histocompatibility complex class I (MHC I) and class II (MHC II) antigens, and antigens of monocyte/macrophage lineage, as detected by the antibodies OX-42, OX-18, OX-6, and ED1, respectively. The upregulation of the above antigens was observed in 1-d-old rats given daily injections of melatonin and killed at 7-11 d of age; no noticeable change was observed at earlier time intervals. The macrophages/microglia expressing the above antigens appeared round and showed a vacuolated cytoplasm compared with ramified cells in the control rats. Upregulation of CD4 antigens as detected with the antibody W3/25 was also observed in macrophages/microglia in the corpus callosum and epiplexus cells in the lateral ventricles, but not in the pineal gland and the cerebral cortex in the same age group. In rats killed between 2 and 5 d, and at 14 d of age after melatonin treatment, the immunoreactivities of macrophages/microglia with the above mentioned antibodies were comparable to cells in the control rats. Immunoreactive cells were not detected in any of the age groups in melatonin-treated or control rats with the antibodies W3/13 and OX-33, which are markers for T and B lymphocytes. It is concluded that CR3 receptors, MHC antigens, and CD4 antigens on macrophages/microglia are upregulated following melatonin administration. On the other hand, once the melatonin treatment is discontinued the expression of the various antigens/receptors returns to normal levels, suggesting that increased immune potentiality and its maintenance in these cells require the continuous action of the drug.     

Protective effect of melatonin on random pattern skin flap necrosis in pinealectomized rat

Random pattern skin flaps are still widely used in plastic surgery. However, necrosis in the distal portion resulting from ischemia is a serious problem, increasing the cost of treatment and hospitalization. Free oxygen radicals and increased neutrophil accumulation play an important role in tissue injury and may lead to partial or complete flap necrosis. To enhance skin flap viability, a variety of pharmacological agents have been intensively investigated. The aim of this study is to test the effects of melatonin, the chief secretory product of the pineal gland and a highly effective antioxidant, on random pattern skin flap survival in rats. Herein, to investigate the physiological and pharmacological role of melatonin on dorsal skin flap survival. Pharmacological (0.4, 4 and 40 mg/kg) levels of melatonin were given intraperitoneally (i.p.). For this, pinealectomized (Px) and sham operated (non-Px) rats were used. The effects of melatonin on levels of malondialdehyde (MDA), nitric oxide (NO), glutathione (GSH) and the activities of glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) were measured in the skin flap. The ratio of skin flap necrosis was compared among the experimental groups by using planimetry. MDA and NO levels were found to be higher in Px than non-Px rats; while GSH levels and GSH-Px, and SOD activities were reduced. Melatonin administration to Px rats reduced MDA and NO levels and increased GSH, GSH-Px, SOD levels. Melatonin also reduced the ratio of flap necrosis determined by using planimetry and supported through the photography. In conclusion, these results show that both physiological and pharmacological concentrations of melatonin improve skin flap viability.     

Immunohistological localization of melatonin in the pineal gland and retina of the rat

An incubation procedure for the demonstration of melatonin is described using freeze-dried, unfixed material embedded in vacuum directly in glycolmethacrylate. The results indicate the binding of melatonin in the nucleus of pineal parenchymal cells and in the outer nuclear layer of the retina of rats; binding is greater at night than during the day.     

In vitro autoradiographical localization of melatonin binding sites in the caprine brain

The recent development of a specific 2-[125I]-iodo-melatonin ligand has led to the identification of 125I-melatonin binding sites in the brains of numerous mammalian species. The present study reports the localization of 125I-melatonin binding sites in the brain of the dairy goat. Six previously untreated female goats, aged 5-7 years, were culled under natural light between 0900 and 1100. Brains and pituitaries were immediately dissected out and frozen on dry ice. Both transverse and sagittal sections of frozen brain were cut 20 microns thick and thaw-mounted onto gelatin-coated slides. Three consecutive sections were cut at intervals throughout the brain, mounted onto three slides, labeled A, B, and C, and thusly treated: (A) incubated for 2 hr at room temperature in a 50 pM solution of 125I-melatonin; (B) incubated for 2 hr at room temperature in a 50 pM solution of 125I-melatonin plus 1 microM cold melatonin; (C) fixed in Clarke's fluid and stained with toluidine blue. After incubation, A (specific) and B (nonspecific) slides were washed three times in ice-cold Tris-HCl buffer (pH 7.7), air-dried, exposed to an X-ray film for 2 weeks at -20 degrees C, and then fixed and stained. Specific 125I-melatonin binding sites were found in the pars tuberalis (PT), the area of the suprachiasmatic nucleus (SCN), preoptic area (POA), fornix/mediolateral septal areas, hippocampus, and the cerebral cortex. 125I-melatonin did not bind in the hindbrain, midbrain, neurohypophysis, pars intermedia or pars distalis of the adenohypophysis, or the pineal.     

The presence of melatonin receptors and inhibitory effect of melatonin on hydrogen peroxide-induced endothelial nitric oxide synthase expression in bovine cerebral blood vessels

Melatonin plays a key role in a variety of important physiological functions including influencing cerebral blood vessels. Therefore, in the present study, we have identified the existence of melatonin receptors and test the effect of melatonin on hydrogen peroxide-induced endothelial nitric oxide synthase (eNOS) expression in bovine cerebral arteries. The results indicate that mt1A melatonin receptor mRNA is expressed in bovine cerebral arteries. The relative levels of mt1A melatonin receptor mRNA expression in anterior, posterior, middle and vertebral cerebral arteries were compared. The data show the highest and lowest levels of mRNA expressions in the middle and vertebral cerebral arteries, respectively. The maximal number (B(max)) of different types of melatonin receptors in various regions of cerebral arteries were identified and further characterized by using the selective 2-[(125)I] iodomelatonin binding assay. Saturation studies revealed that the binding represented a single site of high affinity binding for the melatonin receptor with the highest and lowest binding capacities in the middle and vertebral arteries, respectively. In order to elaborate the functional significance of melatonin in cerebral blood vessels, hydrogen peroxide- induced induction in eNOS protein level and phosphorylation of calcium/calmodulain-dependent protein kinase II (phospho-CaMKII) were demonstrated in the bovine isolated cerebral arteries with these effect being abolished by melatonin. This is the first evidence showing expression of mt1A melatonin receptor in the bovine cerebral arteries. However, further studies are necessary to delineate the role of melatonin and its receptors in regulating physiology of the cerebral vessels.     

Effects of adrenalectomy and thyroidectomy on postnatal rat lung development and cytoplasmic factors modulating adenylate cyclase activity

Effects of adrenalectomy (ADX) and thyroidectomy (THX) on postnatal rat lung growth and development were investigated in relation to the cytoplasmic factors modulating adenylate cyclase activity. On day 21 ADX and THX in rats reduced body and lung weights on days 31 and 43. However, lung weight/body weight was not affected by ADX or THX. Treatment of ADX and THX rats with dexamethasone and thyroxine (T₄), respectively, for 10 days neither restored body growth to the control level nor had any effect on lung growth. Neither protein, DNA, or protein/DNA showed any significant changes in lungs of ADX and THX rats. However, lung glycogen increased in ADX rats and decreased on treatment with dexamethasone. On the other hand, THX and treatment of THX rats with T₄ essentially had no effect on lung glycogen level. ADX resulted in lower adenylate cyclase activity and its stimulation by the cytoplasmic factors in ADX rat lungs. Dexamethasone treatment of ADX rats restored the basal adenylate cyclase activity to the control level, but potentiated the cytoplasmic activation of adenylate cyclase much more than the control lung supernatant. THX, on the other hand, increased the basal adenylase cyclase activity, but its stimulation by the lung supernatant was diminished. Treatment of THX rats with T₄ did not alter the basal adenylate cyclase activity compared with THX rats, but the cytoplasmic activation of adenylate cyclase was restored to the control level. Alterations in adenylate cyclase activation by the lung supernatant from ADX and THX rats did not appear to be due to changes in either the amount of enzyme in membranes or its sensitivity to the cytoplasmic factors. While ADX did not alter cyclic AMP-phosphodiesterase activity in rat lung, THX reduced this enzyme activity, which was restored to the control level on treatment of THX rats with T₄. These data suggest that hormones such as glucocorticoids and thyroxine may influence lung growth and development by inducing alterations in the cytoplasmic factors activating adenylate cyclase, cyclic AMP, and glycogen metabolism.