The effect of parathyroid hormone and atrial natriuretic peptide on cyclic nucleotides production and proliferation of avian epiphyseal growth plate chondroprogenitor cells

Cells derived from avian tibia epiphyseal growth plate were cultured in vitro. The cells which exhibited a polygonal phenotype and are termed chondroprogenitor cells, developed in culture as a monolayer with a doubling time of 40-48 h in 5% fetal calf serum. Production of cAMP by the chondroprogenitor cells was stimulated by human and bovine native (1-84) PTH. The effect of PTH on cAMP production could be blocked by the (3-34) PTH analog, suggesting interaction with specific receptors. cAMP production by avian chondroprogenitor cells was also stimulated by cholera toxin, forskolin, and prostaglandin E2 but not by ACTH or prostaglandin F2 alpha. PTH, cholera toxin, and forskolin also stimulated proliferation of the chondroprogenitor cells. In contrast, neither cAMP production nor proliferation of avian skin fibroblasts was affected by PTH. Human (1-28) and rat (5-28) atrial natriuretic peptide stimulated cGMP production by avian chondroprogenitor cells and also by skin fibroblasts. Atrial natriuretic peptide inhibited the basal and PTH-stimulated [3H]thymidine incorporation into DNA of chondroprogenitor cells, but did not affect avian skin fibroblast proliferation. These results suggest that the proliferation of avian epiphyseal growth plate chondroprogenitor cells is modulated by opposing mechanisms induced by PTH and ANP, probably mediated by cAMP and cGMP, respectively.     

Atrial natriuretic peptide inhibits spontaneous rat oocyte maturation

We report results of experiments demonstrating a dose-dependent inhibition of spontaneous maturation (resumption of meiosis) in rat oocyte-cumulus complexes by atrial natriuretic peptide (ANP). The inhibition was persistent over the time period studied. The ANP analog Tyr8-ANP, which mediates smooth muscle relaxation in other organs without elevating cGMP levels, did not inhibit the spontaneous maturation. ANP, but not Tyr8-ANP, dose-dependently stimulated cGMP accumulation in oocyte-cumulus complexes. Furthermore, sodium nitroprusside (SNP), that stimulates a soluble form of guanylate cyclase, inhibited spontaneous maturation in oocyte-cumulus complexes and stimulated cGMP accumulation in oocyte-cumulus complexes. Neither ANP nor SNP stimulated cAMP accumulation. In oocytes where the surrounding cumulus cells had been removed neither ANP nor SNP inhibited the spontaneous maturation. These results demonstrate that cumulus cells, but not the oocyte itself, have ANP receptors and guanylate cyclases. Furthermore, ANP, via cGMP, can influence oocyte meiosis, suggesting a possible involvement of ANP and cGMP in the control of the meiotic process in rat oocytes.     

Atrial natriuretic peptide, oxytocin, and vasopressin increase guanosine 3',5'-monophosphate in LLC-PK1 kidney epithelial cells

The effect of atrial natriuretic peptide (ANP), arginine vasopressin (AVP), and oxytocin (OT) on cAMP and cGMP accumulation was investigated in LLC-PK1 kidney epithelial cells. The addition of ANP, AVP, and OT to intact cells produced a time- and concentration-dependent increase in cGMP accumulation. ANP produced a 1.7-fold increase in cGMP at 10 pM and a maximal 28-fold increase in cGMP at 1 microM. ANP had no effect on basal or AVP-induced stimulation of cAMP accumulation. OT was 10-fold more potent than AVP at increasing cGMP levels, producing a 2.1-fold increase in cGMP at 0.1 nM, whereas AVP was 100-fold more potent at increasing cAMP levels. At a concentration of 1 microM, AVP and OT produced a maximal 12 to 14-fold increase in cGMP, while OT and AVP produced 50- and 90-fold increase in cAMP, respectively. The selective OT agonist [Thr4, Gly7]oxytocin was very effective at increasing cGMP, but not at increasing cAMP levels. The V2-vasopressin agonist [deamino-Pen1,Val4, D-Arg8]vasopressin did not increase cGMP levels, but produced a 20-fold increase in cAMP levels. The addition of ANP together with either AVP or OT produced an additive increase in cGMP content. Simultaneous addition of AVP and OT did not lead to a greater increase in cAMP or cGMP levels. These results suggest that the AVP- and OT-induced increase in cGMP is mediated by OT receptors, whereas the increase in cAMP is probably mediated by vasopressin receptors. ANP increased the activity of particulate guanylate cyclase by 6-fold, while AVP and OT has no effect on particulate guanylate cyclase activity. The relatively selective inhibitor of soluble guanylate cyclase, methylene blue, had no effect on the ANP-induced increase in cGMP content in intact cells, but produced a 50% inhibition of the increase in cGMP by AVP and OT. Methylene blue did not alter the stimulation of cAMP by AVP or OT. These results demonstrate that ANP, AVP, and OT increase cGMP in LLC-PK1 kidney epithelial cells. The increase in cGMP by ANP is mediated by particulate guanylate cyclase, whereas AVP and OT probably increase cGMP by interacting with OT receptors coupled to soluble guanylate cyclase.     

Atrial and brain natriuretic peptides enhance dopamine accumulation in cultured rat hypothalamic cells including dopaminergic neurons.

Dopamine accumulation in hypothalamic cells by atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) was analyzed using newborn rat hypothalamic cells in culture. Both ANP and BNP caused a dose-dependent increase in [3H]-dopamine accumulation in the cells. ANP increased [3H]-dopamine accumulation significantly within 20 min. The effects of ANP and BNP on dopamine accumulation paralleled an increase in intracellular cGMP concentration. (Bu)2-cGMP and sodium nitroprusside, a stimulator of the soluble form of guanylate cyclase, also enhanced [3H]-dopamine accumulation. ANP had no effect on efflux of [3H] radioactivity after [3H]-dopamine uptake. These results suggest that a change in cGMP is one of the intermediate steps in dopamine accumulation in hypothalamic cells by ANP and BNP.     

Sodium nitroprusside inhibition of parathyroid hormone release is not mediated through cyclic GMP

Sodium nitroprusside effected a rapid, dose-dependent increase in intracellular cGMP accumulation in freshly dispersed bovine parathyroid cells. The effect was half-maximal between 10(-4) and 3 X 10(-4)M, maximal at 3 X 10(-3)M nitroprusside and could be amplified (approximately 50%) by the addition of methylisobutylxanthine (4 X 10(-4)M). The dose-response characteristics were similar to those previously described for the inhibition of cAMP accumulation and PTH release by this agent. Neither dibutyryl cGMP (10(-3)M) nor 8'-bromo-cGMP (10(-3)M) mimicked the inhibitory effect of nitroprusside on cAMP accumulation or PTH release. Dose-dependent stimulation of guanylate cyclase was found in a particulate preparation of parathyroid cells; activity was maximal at 10(-4)M nitroprusside while higher concentrations appeared to inhibit the enzyme. Nitroprusside significantly reduced both (-)isoproterenol and guanine nucleotide-stimulated adenylate cyclase activity in the particulate preparation with maximum inhibition between 10(-3)-10(-2)M. cGMP concentrations as high as 10(-4)M did not affect agonist-stimulated cAMP synthesis. Thus, although the kinetic and dose-response characteristics of the nitroprusside effect on cGMP suggest a linkage to its previously described effects on cAMP and PTH secretion, no direct evidence was found to indicate a causal relationship between the two. Rather it would appear that the effects on the adenylate and guanylate cyclase enzymes occur in parallel, possibly the result of some common primary perturbation of cellular physiology.     

Adrenocorticotropic hormone-responsive guanylate cyclase in the particulate fraction of rat adrenal glands

Low concentrations of ACTH, 7 x 10(-12) M, caused a marked stimulation of the 100,000 x g particulate guanylate cyclase without any detectable change in the adenylate cyclase activity. The lowest concentration of the hormone that elicited adenylate cyclase stimulation was 7 x 10(-10) M, a concentration 100--fold higher than that required to stimulate the guanylate cyclase. Although calcium was found to be obligatory in the hormonally--dependent guanylate cyclase activity, calcium alone could not duplicate the ACTH effect. Sodium nitroprusside and ascorbic acid inhibited the particulate guanylate cyclase activity. While ACTH was unable to stimulate the soluble guanylate cyclase, sodium nitroprusside markedly stimulated this enzyme. From these data, we conclude that the adrenal guanylate cyclase exists in two forms, particulate and soluble. The particulate form is specifically responsive to ACTH, and calcium is one of the essential coupling factors of this hormonally--responsive guanylate cyclase.     

Regulation of aldosterone secretion by avian adrenocortical cells

Dispersed chick adrenocortical cells were incubated with mammalian and avian angiotensin-II, Ca2+, K+, verapamil, nifedipine, Ca2+ ionophore (A23187), protein kinase-C activator (phorbol 12-myristate 13-acetate; TPA), atrial natriuretic peptide (ANP), sodium nitroprusside (SNP) and ACTH. Secretion of aldosterone and corticosterone, and accumulation of cyclic nucleotides were assessed. Secretion of aldosterone was not affected by angiotensin-II, Ca2+ channel blockers, Ca2+ ionophore or TPA. ANP stimulated production of cyclic GMP (cGMP), and inhibited aldosterone secretion with a similar dose-response relationship. SNP also stimulated cGMP production and inhibited the ACTH-stimulated aldosterone secretion. The results indicate that ANP is an inhibitor of aldosterone secretion in birds and suggest that this inhibition is mediated by cGMP. In contrast to mammalian glomerulosa cells, angiotensin-II and the calcium-inositol phosphate-protein kinase C pathway appear not to be involved in the regulation of aldosterone secretion by avian adrenal cells.     

Correlation of nitric oxide and atrial natriuretic peptide changes with altered cGMP homeostasis in liver cirrhosis.

BACKGROUND: Cyclic GMP (cGMP) concentration is increased in plasma of patients with liver cirrhosis. Three possible mechanisms may contribute: increased cGMP synthesis by soluble (activated by nitric oxide), or particulate (activated by atrial natriuretic peptide (ANP)) guanylate cyclase or increased release from cells. AIM: The aim of this work was to analyze the possible contributors to increased plasma cGMP and to assess whether changes in the parameters of the system vary with the degree of liver disease (Child Pugh score) or by the presence of ascites. METHODS: We measured cGMP in plasma and lymphocytes, soluble guanylate cyclase activation by nitric oxide in lymphocytes, nitrates and nitrites and ANPs (activator of particulate guanylate cyclase) in plasma. We analyzed the correlation between changes in different parameters to discern which parameters contribute to increased plasma cGMP. RESULTS: The plasma content of nitrates+nitrites, ANP and cGMP are increased. Activation of soluble guanylate cyclase by nitric oxide is increased in patients while basal cGMP in lymphocytes is decreased. CONCLUSIONS: Both increased ANP and increased activation of soluble guanylate cyclase by nitric oxide contribute to increased plasma cGMP in patients. The concentrations of ANP and cGMP in plasma increase with the degree of disease and are higher in patients with ascites.     

Immunocytology on microwave-fixed cells reveals rapid and agonist-specific changes in subcellular accumulation patterns for cAMP or cGMP.

We developed a method for cAMP and cGMP immunocytology based upon fixation by microwave irradiation. Fixation by microwave irradiation prevented three problems found with other fixation methods: nucleotide loss from cells, nucleotide diffusion within cells, and chemical modification of immunologic epitopes. Six agonists (four that stimulate adenylate cyclase and two that stimulate guanylate cyclase) produced cAMP or cGMP accumulation patterns that were agonist-specific, dose-dependent, detectable at physiologic concentrations of hormone, and time-dependent within 15 sec to 30 min. cAMP accumulation after 1 mM forskolin was greatest in the nucleus. Isoproterenol, prostaglandin E2, or calcitonin caused initial accumulation of cAMP along the plasma membrane, but later accumulation was greater in the cytoplasm. With calcitonin the later accumulation of cAMP was selectively perinuclear and along the nuclear membrane. Sodium nitroprusside stimulated cGMP accumulation diffusely throughout the cytoplasm. Atrial natriuretic peptide initiated cGMP accumulation near the plasma membrane, and cGMP accumulation moved from there into the cytoplasm. In conclusion, microwave irradiation preserved cell structure and allowed visualization of expected as well as unsuspected changes in intracellular accumulation patterns of cAMP and cGMP.     

Cell biology of atrial natriuretic peptide.

Atrial natriuretic peptide (ANP) exhibits a wide spectrum of cardiovascular, endocrine, metabolic and renal actions. cGMP is the major mediator of ANP at the cellular level and only tissues possessing particulate guanylate cyclase appear to present ANP-induced actions. Three types of ANP receptors have recently been cloned. Two of them (A and B receptors) are homologous and contain guanylate cyclase catalytic domains. The C receptor could possibly regulate the metabolic fate of ANP. Data obtained by the radiation inactivation method suggest the presence of an inter- or intramolecular inhibitory component of nearly 90 kilodaltons that represses the catalytic activity of guanylate cyclase within its membrane environment. The mechanism of guanylate cyclase stimulation by ANP could involve this inhibitory component. Preliminary data suggest that the hyperresponsiveness of the particulate guanylate cyclase/cGMP system in hypertension occurs through modulation of the inhibitory component.     

Atrial natriuretic factor elicits an endothelium-independent relaxation and activates particulate guanylate cyclase in vascular smooth muscle.

A 26 amino acid synthetic peptide fragment of atrial natriuretic factor (ANF) relaxed isolated rabbit aortic segments in which the endothelium was either intact or functionally destroyed. The relaxations were temporally associated with increases in levels of cGMP with no change in the levels of cAMP. The ANF-induced increases in cGMP were also observed in aortic segments pretreated with calcium-free buffer or the cGMP phosphodiesterase inhibitor M&B 22,948. Qualitatively similar results were obtained for sodium nitroprusside. ANF selectively activated particulate guanylate cyclase, having no effect on the soluble form of the enzyme. Thus, the direct (endothelium-independent) vasodilator effect of ANF may be mediated via increased tissue levels of cGMP. ANF appears to increase vascular cGMP levels by activation of particulate guanylate cyclase.     

Cyclic GMP phosphodiesterase and guanylate cyclase activities in rabbit ovaries and the effect of in-vivo stimulation with LH

The activities of guanylate cyclase and cyclic GMP (cGMP) phosphodiesterase, enzymes that are responsible for maintaining tissue levels of cGMP, were determined in the ovaries of rabbits killed without treatment or 4 h after administration of LH. Ovarian activities of the two enzymes were determined in the 100 000 g supernatant fraction (cytosol) and the resulting pellet (particulate fraction). Significant phosphodiesterase and cyclase activities were detected in both the cytosol and particulate fractions. Administration of LH had no significant effect on phosphodiesterase activity in either of the tissue fractions. On the other hand, LH caused a significant drop in guanylate cyclase activity in the cytosol and particulate fractions. This drop in the cyclase activity may be the cause of the decreased rabbit ovarian concentrations of cGMP that we have previously observed after LH stimulation.     

Nitric oxide as a signal in thyroid.

It is now well established that agonist activation of the PIP2/calcium cascade in the thyroid results in the enhancement of cGMP accumulation presumably by activation of the soluble guanylate cyclase. In many tissues the physiological signal controlling soluble guanylate cyclase is nitric oxide (NO) and its synthesis from arginine is controlled by the intracellular Ca2+. In this report we show results that suggest that NO may be the intermediate of the cGMP response to the activation of the PIP2/calcium cascade. In dog thyroid slices, incubation with carbamylcholine or A23187 increases significantly free intracellular Ca2+ levels and the cGMP content of the slices. NG-Monomethyl-L-arginine (NMMA), a competitive inhibitor of arginine for nitric oxide synthase, inhibited these cGMP responses but not the action of sodium nitroprusside which activates soluble guanylate cyclase directly. The inhibition was relieved by arginine. Methylene blue, which blocks the activation of soluble guanylate cyclase by NO, also decreased the three stimulatory effects. NMMA and methylene blue also decreased the basal levels of cGMP. NO may therefore be an important autocrine and paracrine factor in thyroid.     

Characterization by affinity cross-linking of a receptor for atrial natriuretic peptide in cultured human thyroid cells associated with reductions in both adenosine 3',5'-monophosphate production and thyroglobulin secretion

We have previously identified specific atriopeptin (ANP) receptors in cultured human thyroid cells and demonstrated that ANP reduced thyroglobulin (Tg) secretion. In this report the relationship of Tg inhibition to cyclic nucleotide intermediate pathways was explored, and the thyroidal ANP receptor was characterized by affinity cross-linking. Concentrations of Tg, cGMP, and cAMP were measured in medium from thyroid cells cocultured with ANP. ANP significantly inhibited cAMP production at the lower concentration of 0.1 nmol/L and stimulated cGMP levels at a higher concentration of 10 nmol/L. The percentage of inhibition of Tg release over the ANP range of 0.01-10 nmol/L appeared to parallel cAMP, but not cGMP, levels, suggesting that ANP acts via a cAMP pathway in the thyroid. Affinity cross-linking studies characterizing the ANP receptor in thyrocytes and a bovine endothelial cell line known to be cGMP responsive to ANP indicated a single unit ANP receptor of 140 kD coupled to guanylate cyclase in endothelial cells, while a 70-kD receptor was found in thyroid cells which specifically binds to ANP, atriopeptin-I, and atriopeptin-III. These studies in thyrocytes suggest that reduced Tg release may be mediated by a specific single 70-kD ANP receptor associated with an inhibitor cAMP pathway and provide additional insight into the nature of a newly described thyroid-ANP interaction.     

YC-1, a novel activator of platelet guanylate cyclase

YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole] inhibited the aggregation of and ATP release from washed rabbit platelets induced by arachidonic acid (AA), collagen, U46619, platelet-activating factor (PAF), and thrombin in a concentration-dependent manner. YC-1 also disaggregated the clumped platelets caused by these inducers. The thromboxane B2 formation caused by collagen, PAF, and thrombin was inhibited by concentrations of YC-1 that did not affect formation of thromboxane B2 and prostaglandin D2 caused by AA. YC-1 suppressed the increase of intracellular Ca2+ concentration and generation of inositol 1,4,5-trisphosphate caused by these five aggregation inducers. Both the cAMP and cGMP contents of platelets were increased by YC-1 in a concentration- and time-dependent manner. Like sodium nitroprusside, YC- 1 potentiated formation of cAMP caused by prostaglandin E1 but not that by 3-isobutyl-1-methylxanthine. Adenylate cyclase and cAMP phosphodiesterase activities were not altered by YC-1. Activity of cGMP phosphodiesterase was unaffected by YC-1. Activities of guanylate cyclase in platelet homogenate and cytosolic fraction were activated by YC-1, whereas particulate guanylate cyclase activity was unaffected. The antiplatelet effect of sodium nitroprusside but not that of YC-1 was blocked by hemoglobin and potentiated by superoxide dismutase. After intraperitoneal administration for 30 minutes, YC-1 prolonged the tail bleeding time of conscious mice. These data indicate that YC-1 is a direct soluble guanylate cyclase activator in rabbit platelets. It may also possess antithrombotic potential in vivo.     

Nitric oxide activates guanylate cyclase and increases guanosine 3′:5′-cyclic monophosphate levels in various tissue preparations

Nitric oxide gas (NO) increased guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] activity in soluble and particulate preparations from various tissues. The effect was dose-dependent and was observed with all tissue preparations examined. The extent of activation was variable among different tissue preparations and was greatest (19- to 33-fold) with supernatant fractions of homogenates from liver, lung, tracheal smooth muscle, heart, kidney, cerebral cortex, and cerebellum. Smaller effects (5- to 14-fold) were observed with supernatant fractions from skeletal muscle, spleen, intestinal muscle, adrenal, and epididymal fat. Activation was also observed with partially purified preparations of guanylate cyclase. Activation of rat liver supernatant preparations was augmented slightly with reducing agents, decreased with some oxidizing agents, and greater in a nitrogen than in an oxygen atmosphere. After activation with NO, guanylate cyclase activity decreased with a half-life of 3-4 at 4 degrees but re-exposure to NO resulted in reactivation of preparations. Sodium azide, sodium nitrite, hydroxylamine, and sodium nitroprusside also increased guanylate cyclase activity as reported previously. NO alone and in combination with these agents produced approximately the same degree of maximal activation, suggesting that all of these agents act through a similar mechanism. NO also increased the accumulation of cyclic GMP but not cyclic AMP in incubations of minces from various rat tissues. We propose that various nitro compounds and those capable of forming NO in incubations activate guanylate cyclase through a similar but undefined mechanism. These effects may explain the high activities of guanylate cyclase in certain tissues (e.g., lung and intestinal mucosa) that are exposed to environmental nitro compounds.     

Ovarian cyclic GMP concentration and guanylate cyclase activity in immature rats after treatment with pregnant mare serum gonadotrophin

We have previously reported that the LH-induced decrease in the concentration of ovarian cyclic GMP (cGMP) in the rabbit was accompanied by a drop in ovarian guanylate cyclase activity. The present experiments were carried out to see if the increase in cGMP concentration that occurs in immature rat ovaries after stimulation with pregnant mare serum gonadotrophin (PMSG) is also accompanied by changes in guanylate cyclase activity. Total ovarian cGMP, along with ovarian weight, was found to be increased at 16 h after PMSG treatment. Ovarian concentrations of cGMP, however, increased only after that period (at 20, 24 and 48 h) and the increase was progressive. Guanylate cyclase activity was found in both the cytosol and 100 000 g particulate fractions of the immature rat ovaries. Forty-three hours after PMSG treatment, activity in the particulate fraction was found to be significantly increased. This increase in guanylate cyclase activity was also found at 20 h but not at 16 h. Thus, the increase in ovarian cGMP concentration in immature rats after PMSG treatment was accompanied by increased guanylate cyclase activity.     

Growth hormone-releasing factor increases somatostatin release and mRNA levels in the rat periventricular nucleus via nitric oxide by activation of guanylate cyclase.

Previous work has shown that growth hormone-releasing factor (GRF) stimulates cGMP production and somatostatin [somatotropin (growth hormone)-release-inhibiting factor, SRIF] release without altering cAMP accumulation by fragments of median eminence incubated in vitro. Therefore, this study was undertaken to evaluate the effect of GRF and cGMP on SRIF mRNA and SRIF release in the periventricular nuclei of male rats in vitro. SRIF mRNA levels were determined in explants of periventricular nuclei incubated for 6 hr in Waymouth's medium in the presence of various substances. Steady-state levels of SRIF mRNA were measured by an S1 nuclease protection assay using a 32P-labeled rat SRIF RNA probe. SRIF release and cGMP formation were measured at 30 min and 6 hr by RIA. SRIF mRNA levels and SRIF release were significantly (P < 0.025) increased (approximately 2-fold) by 1 microM dibutyryl cGMP, whereas sodium butyrate had no effect. This augmentation was not influenced by cycloheximide, an inhibitor of protein synthesis. Sodium nitroprusside (10 microM), an activator of the guanylate cyclase pathway via its release of nitric oxide, augmented (P < 0.001) SRIF mRNA levels and significantly increased (P < 0.05) SRIF release. GRF (1 nM) increased SRIF mRNA (P < 0.001) and stimulated the release of SRIF at 30 min (P < 0.05) and 6 hr (P < 0.01). This stimulation was abolished by 10 microM NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of nitric oxide synthase, but not by NG-monomethyl-D-arginine (D-NMMA, the inactive isomer). GRF also increased cGMP formation. This effect was completely blocked by incubation with L-NMMA but not D-NMMA. These results indicate that GRF releases nitric oxide. The nitric oxide diffuses to the adjacent SRIF neurons, where it activates guanylate cyclase, leading to increased formation of cGMP. This cGMP increases SRIF mRNA and SRIF release in the periventricular nuclei of male rats.     

Atrial natriuretic peptide effects in AtT-20 pituitary tumour cells.

Whether atrial natriuretic peptide (ANP)-evoked inhibition of corticotrophin-releasing factor (CRF)-stimulated ACTH secretion was also manifest in ACTH secreting AtT-20 pituitary tumour cells was investigated. ANP stimulated increases in cGMP accumulation at concentrations of the peptide above 10(-8) M which indicates the presence of the ANP receptors on these cells. CRF stimulated a concentration-dependent increase in ACTH secretion from AtT-20 cells which was unaffected by ANP, 8-bromo-cGMP, or sodium nitroprusside (SNP). Calcium stimulated a concentration-dependent increase in ACTH secretion from electrically permeabilised cells which was unaffected by co-incubation with cGMP but potentiated by cAMP. These results reveal the presence of ANP receptors on AtT-20 cells but suggest that an incomplete expression of the stimulus-secretion coupling mechanisms for ANP, at some point after cGMP production, prevents the effects of natriuretic peptides upon ACTH secretion being manifest in these cells.     

Cyclic GMP may serve as a second messenger in peptide-induced muscle degeneration in an insect.

At the end of metamorphosis, the intersegmental muscles of the moth Antheraea polyphemus undergo rapid degeneration in response to the peptide eclosion hormone (EH). Muscle death was preceded by a 22-fold increase in muscle guanosine-3',5'-cyclic monophosphate (cGMP) titers, which peaked 60 min after peptide exposure; adenosine-3'5'-cyclic monophosphate (cAMP) titers remained unchanged. EH induced a dose-dependent increase in muscle cGMP content with a threshold dose similar to that needed to induce cell death. Exogenous cGMP, but not cAMP, mimicked the action of EH. Sodium nitroprusside, a potent stimulator of guanylate cyclase, and methylated xanthines, a class of 3',5'-cyclic-nucleotide phosphodiesterase inhibitors, also induced the selective death of these muscles. It is concluded that an elevation of cGMP level is involved in EH-induced muscle degeneration. The intersegmental muscles become sensitive to EH at the end of adult development in response to the declining titers of the steroid molting hormones, the ecdysteroids. At earlier times, treatment with EH, exogenous cGMP, sodium nitroprusside, or methylated xanthines was ineffective in causing cell death. Nevertheless, treatment with EH at this time resulted in a marked increase in intersegmental-muscle cGMP. Thus, the onset of physiological responsiveness to the peptide hormone presumably results from biochemical changes distal to the EH receptors and guanylate cyclase.