Effects of parathyroid hormone and cyclic AMP analogues on the activity of ornithine decarboxylase and expression of the differentiated phenotype of chondrocytes in culture

Parathyroid hormone (PTH) greatly increased the level of adenosine 3', 5' cyclic monophosphate (cAMP) in rabbit costal chondrocytes in culture 2 minutes after its addition. PTH, as well as N6 O2' dibutyryl adenosine 3', 5' cyclic monophosphate (DBcAMP) and 8 Bromo adenosine 3', 5' cyclic monophosphate (8 Br-cAMP) induced ornithine decarboxylase (ODC; L-ornithine carboxylyase; EC 4.1.1.17), which reached a maximum 4 hours after their addition. Neither cAMP, N6 O2' dibutyryl guanosine 3', 5' cyclic monophosphate (DBcGMP), nor sodium butyrate increased the activity of the enzyme. PTH had no effect on DNA synthesis, while DBcAMP and 8 Br-cAMP decreased DNA synthesis. Expression of the differentiated phenotype of chondrocytes in culture was also induced by PTH, DBcAMP, and 8 Br-cAMP, but not by cAMP, DBcGMP, or sodium butyrate, as judged by morphological change. Glycosaminoglycan synthesis, a characteristic of the cartilage phenotype, began to increase 8 hours after addition of PTH or DBcAMP, reaching a plateau 32 hours after their addition. These findings suggest that PTH induces increase of ODC activity and expression of the differentiated phenotype of chondrocytes through increase of cAMP and that induction of OCD is closely related to expression of the differentiated phenotype of chondrocytes.     

Long acting cAMP analogues enhance sulfate incorporation into matrix proteoglycans and suppress cell division of fetal rat chondrocytes in monolayer culture.

The relationship between replication and the synthesis of matrix sulfated proteoglycans was investigated with fetal rat chondrocytes grown in monolayer culture. The effect of N6 O2' dibutyryl adenosine 3', 5' cyclic monophosphate (DBcAMP), adenosine 3', 5' cyclic monophosphate (cAMP), 8 Bromo adenosine 3', 5' cyclic monophosphate (8 Br-cAMP), sodium butyrate and hydroxyurea was examined. Between 0.05 and 0.5 mM DBcAMP, a dose related inhibition of cell division and stimulation of [35SO=/4] incorporation into matrix proteoglycans was demonstrated. At the higher concentrations of DBcAMP, cell division was completely inhibited and the enhancement of [35SO=/4] incorporation into matrix proteoglycans ranged between 40 and 120% (P less than 0.01). Utilizing 14C-glucosamine and photometric determination of proteoglycans with Alcian Blue, it was demonstrated that the increase in sulfate incorporation reflected enhanced accumulation of extracellular matrix. The effects of DBcAMP were mimicked by 8 Br-cAMP, suggesting they were mediated by the adenylyl cyclase system. cAMP (0.05-0.5 mM), sodium butyrate (0.1-0.5 mM) and hydroxyurea (0.5-5 mM) partially or fully inhibited cell division, but either failed or only slightly enhanced sulfate incorporation. The enhanced sulfated proteoglycan deposition promoted by DBcAMP began 8 to 12 hours after serum stimulation, its onset occurred prior to thymidine incorporation and the effect persisted for 28 hours. Determination of cell volume demonstrated an increase in size of DBcAMP treated chondrocytes between 8 to 12 hours, coincident with the onset of increased sulfate incorporation. These results are consistent with a model where matrix sulfated proteoglycan deposition by chondrocytes is mediated by intracellular cAMP levels and occurs in the G1 phase of the cell cycle.     

The role of intracellular cAMP in renal gluconeogenesis in view of differential action of various cAMP analogues

Effects of various cAMP analogues on gluconeogenesis in isolated rabbit kidney tubules have been investigated. In contrast to N(6),2'-O-dibutyryladenosine-3',5'-cyclic monophosphate (db-cAMP) and cAMP, which accelerate renal gluconeogenesis, 8-bromoadenosine-3',5'-cyclic monophosphate (Br-cAMP) and 8-(4-chlorophenylthio)-cAMP (pCPT-cAMP) inhibit glucose production. Stimulatory action of cAMP and db-cAMP may be evoked by butyrate and purinergic agonists generated during their extracellular and intracellular metabolism resulting in an increase in flux through fructose-1,6-bisphosphatase and in consequence acceleration of the rate of glucose formation. On the contrary, Br-cAMP is poorly metabolized in renal tubules and induces a fall of flux through glyceraldehyde-3-phosphate dehydrogenase. The contribution of putative extracellular cAMP receptors to the inhibitory Br-cAMP action is doubtful in view of a decline of glucose formation in renal tubules grown in the primary culture supplemented with forskolin. The presented data indicate that in contrast to hepatocytes, in kidney-cortex tubules an increased intracellular cAMP level results in an inhibition of glucose production.     

cGMP-mediated inhibition of cardiac L-type Ca(2+) current by a monoclonal antibody against the M(2) ACh receptor

The effects of a monoclonal antibody (B8E5) directed against the second extracellular loop of the muscarinic M(2) receptor were studied on the L-type Ca(2+) currents (I(Ca,L)) of guinea pig ventricular myocytes using the whole cell patch-clamp technique. Similar to carbachol, B8E5 reduced the isoproterenol (ISO)-stimulated I(Ca,L) but did not significantly affect basal I(Ca,L). Atropine blocked the inhibitory effect of B8E5. The electrophysiological parameters of ISO-stimulated I(Ca,L) were not modified in presence of B8E5. Inhibition of I(Ca,L) by B8E5 was still observed when intracellular cAMP was either enhanced by forskolin or maintained constant by using a hydrolysis-resistant cAMP analog (8-bromoadenosine 3',5'-cyclic monophosphate) or by applying the phosphodiesterase inhibitor IBMX. The effect of B8E5 was mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate, a potent stimulator of cGMP-dependent protein kinase, and prevented by a selective inhibitor of nitric oxide-sensitive guanylyl cyclase [1H-(1,2,4)oxadiazolo[4,3-a]quinoxaline-1-one]. These results indicate that the antibody B8E5 inhibits the beta-adrenergic-stimulated I(Ca,L) through activation of the M(2) muscarinic receptor and further suggest that the antibody acts not via the classical pathway of decreasing intracellular cAMP, but rather by increasing cGMP.     

cGMP and cyclic nucleotide-gated channels participate in mouse sperm capacitation

During capacitation of mammalian sperm intracellular [Ca(2+)] and cyclic nucleotides increase, suggesting that CNG channels play a role in the physiology of sperm. Here we study the effect of capacitation, 8Br-cAMP (8-bromoadenosine 3',5'-cyclic monophosphate) and 8Br-cGMP (8-bromoguanosine 3',5'-cyclic monophosphate) on the macroscopic ionic currents of mouse sperm, finding the existence of different populations of sperm, in terms of the recorded current and its response to cyclic nucleotides. Our results show that capacitation and cyclic nucleotides increase the ionic current, having a differential sensitivity to cGMP (cyclic guanosine monophosphate) and cAMP (cyclic adenosine monophosphate). Using a specific inhibitor we determine the contribution of CNG channels to macroscopic current and capacitation.     

Regulation of Ig-induced eosinophil degranulation by adenosine 3',5'-cyclic monophosphate.

We have investigated the effects of cAMP on Ig-induced human eosinophil activation. Stimulation of human normodense eosinophils with IgG- or secretory IgA (sIgA)-coated Sepharose beads induced cellular degranulation, as measured by the release of the granule protein, eosinophil-derived neurotoxin (EDN). Pretreatment with cAMP analogs (N6,O2,-dibutyryl adenosine-3,':5' cyclic monophosphate; 8-bromoadenosine 3':5' cyclic monophosphate; or N6-benzoyladenosine 3':5' cyclic monophosphate) or cAMP phosphodiesterase-inhibitors (theophylline or isobutylmethyl xanthine (IBMX] strongly inhibited Ig-induced human eosinophil degranulation. The beta-adrenoceptor agonists, isoproterenol and salbutamol, induced relatively low level increases in intracellular cAMP, and weakly suppressed EDN release induced by IgG-coated beads. However, cellular pretreatment with IBMX synergistically enhanced the inhibitory effects of isoproterenol or salbutamol on both IgG and sIgA-induced eosinophil degranulation. Similarly, PGE2 treatment increased intracellular cAMP concentrations in eosinophils and correspondingly inhibited the Ig-dependent cellular degranulation response: co-incubation with IBMX further enhanced both effects of PGE2. Finally, cholera toxin, which irreversibly activates the stimulatory guanine nucleotide-binding protein linked to adenylyl cyclase, strongly inhibited the release of EDN from IgG- or sIgA-stimulated eosinophils. The time-dependent accumulation of cAMP in cholera toxin-treated cells closely paralleled the time courses of inhibition of IgG- and sIgA-induced EDN release after toxin exposure. These data indicate that the cAMP-dependent signal transduction mechanism in eosinophils exerts a negative modulatory effect on the cellular degranulation responses induced by sIgA or IgG. The inhibitory effects of cAMP on eosinophil activation may provide an important physiologic and a clinically relevant therapeutic mechanism for limiting the release of eosinophil-derived cytotoxic proteins during certain allergic or inflammatory responses in vivo.     

Prostaglandin E2 potentiates granulocyte-macrophage colony stimulating factor-induced histamine synthesis in bone marrow cells: role of cAMP.

Histamine synthesis in response to Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) by murine hematopoietic cells is strikingly potentiated by prostaglandin E2 (PGE2). This synergy is mediated by an increase in intracellular adenosine 3':5'-cyclic monophosphate (cAMP), since: (a) exogeneous and endogeneous cAMP generated either by forskolin or IBMX potentiate GM-CSF-induced histamine synthesis, (b) the maximal potentiating effects of PGE2 and cAMP are not cumulative, and (c) GM-CSF together with PGE2 enhances intracellular cAMP content in a bone marrow population enriched for GM-CSF target cells. cAMP and PGE2 enhance histidine decarboxylase activity induced by GM-CSF showing that both factors act on histamine synthesis rather than on its release. Conversely, histamine synthesis promoted by Interleukin 3 (IL-3), the unique cytokine sharing this property with GM-CSF, is not modulated by PGE2 or cAMP, suggesting two distinct mechanisms for the induction of this biological activity in hematopoietic cells.     

Effects of intracellular cyclic AMP and cyclic GMP levels on DNA synthesis of young-adult rat hepatocytes in primary culture.

Possible roles of dibutyryladenosine 3',5'-cyclic monophosphate (cAMP) and dibutyryl-guanosine 3',5'-cyclic monophosphate (cGMP) in regulation of hepatocyte DNA synthesis were examined using primary cultures of young-adult rat hepatocytes maintained in arginine-free medium. Throughout the experimental period, nonparenchymal cells were hardly observed in the selective medium. When epidermal growth factor (EGF) was added to the cultures, a transient increase in the intracellular cAMP level preceded the elevation of hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was remarkably enhanced by the elevation of the intracellular cAMP level induced by treatment with cAMP alone or a combination of cAMP and theophylline, an inhibitor of cyclic nucleotide phosphodiesterase. Furthermore, the early elevation of intracellular cAMP alone, which was induced by treatment with the combination of cAMP and theophylline, caused a remarkable increase in hepatocyte DNA synthesis. On the other hand, addition of EGF to the cultures caused a rapid decrease in the intracellular cGMP level followed by an increase in hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was severely suppressed or completely inhibited by the elevation of the intracellular cGMP level induced by treatment with cGMP alone or a combination of cGMP and dipyridamole, a specific inhibitor of cGMP phosphodiesterase. These findings indicate that cAMP and cGMP act oppositely on the regulation of DNA synthesis of young-adult rat hepatocytes in primary culture: cAMP plays a positive role, whereas cGMP plays a negative role. Also it is strongly suggested that an early elevation of the intracellular cAMP level is essential for the onset of DNA synthesis in hepatocyte primary cultures.     

Independent regulation of pyruvate kinase expression by cyclic AMP and prostaglandin F2 alpha in mouse mastocytoma cells

P-815 mouse mastocytoma cells express the K isozyme of pyruvate kinase and the specific activity of this enzyme is increased in response to N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate, 8-bromoadenosine 3':5'-cyclic monophosphate, cholera toxin, and epinephrine, all of which also elevate the intracellular concentration of adenosine 3':5'-cyclic monophosphate. Prostaglandin F2 alpha also increases the cellular activity of this enzyme, but does not increase the adenosine 3':5'-cyclic monophosphate levels. Under all these conditions, the increase in enzymatic activity is accompanied by an equivalent increase in the pyruvate kinase protein level. However, neither the rate of enzyme synthesis nor the level of pyruvate kinase mRNA is elevated by N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate. On the other hand, it does increase the enzyme's half-life. In contrast, prostaglandin F2 alpha increases the rate of synthesis and the level of pyruvate kinase K mRNA, but has no influence on the rate of degradation. Therefore, these cells have two mechanisms which increase pyruvate kinase K levels. One operates via an increase in cAMP level and results in a decrease in the rate of degradation, whereas the other minimizes an upsurge in cAMP levels but still increases pyruvate kinase K activity by increasing its rate of synthesis.     

Permeability of human endothelial monolayers: effect of vasoactive agonists and cAMP

Permeability coefficients of human umbilical vein endothelial cell monolayers cultured on polycarbonate filters were determined by monitoring transendothelial albumin transport. Permeability was determined as a function of time in culture and in the presence of vasoactive agonists. Permeability decreased with increasing time in culture. All agonist experiments were performed with 15-day cultures because this time point best modeled the in vivo permeability barrier function. Permeability of endothelial monolayers decreased significantly in the presence of the stable prostacyclin analogue iloprost (6 nM), dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP, 0.5 mM)-3-isobutyl-1-methylxanthine (IBMX, 0.1 mM), 8-bromo cAMP (0.5 mM)-IBMX, dibutyryl cAMP-theophylline (0.5 mM), or IBMX. A 9.6-fold increase in permeability resulting from thrombin [0.15 U/ml (1 nM)] treatment was inhibited by pretreating the monolayers with dibutyryl cAMP-IBMX, 8-bromo cAMP-IBMX, dibutyryl cAMP-theophylline, dibutyryl cAMP, IBMX, iloprost, or D-Phe-Pro-Arg-CH2-alpha-thrombin (1 nM). The thrombin-induced permeability increase was not significantly altered by pretreating monolayers with aspirin (5 microM) or indomethacin (50 microM). Inactivated forms of thrombin, diisopropylflurophosphate-alpha-thrombin (1 nM) and D-Phe-Pro-Arg-CH2-alpha-thrombin, did not significantly affect permeability. Monolayer permeability was not altered in response to bradykinin (1 microM). These results suggest a mediating role for intracellular cAMP in the permeability barrier function of endothelial monolayers.     

Growth hormone inhibition of glucagon- and cAMP-induced lipolysis by chicken adipose tissue in vitro

The ability of growth hormone (GH) to inhibit the early (first hour) lipolytic response to glucagon and cAMP was investigated using chicken adipose tissue explants in vitro. In the first hour of incubation, GH inhibited glucagon, 8-bromo-3',5'-cyclic adenosine monophosphate (8-bromo-cAMP), and 1-isobutyl-3-methyl-xanthine (IBMX) induced glycerol release. The antilipolytic effect of GH was dose dependent, with inhibition of glucagon and 8-bromo-cAMP observed in the presence of as little as 100 ng/ml GH. In the fourth hour of incubation (late lipolytic response), GH (10, 100, or 1000 ng/ml) enhanced the lipolytic action of glucagon.     

Compounds that increase cAMP prevent ischemia-reperfusion pulmonary capillary injury.

This study evaluated the physiological effects of compounds that increase adenosine 3',5'-cyclic monophosphate (cAMP) on changes in pulmonary capillary permeability and vascular resistance induced by ischemia-reperfusion (I-R) in isolated blood-perfused rabbit lungs. cAMP was elevated by 1) beta-adrenergic stimulation with isoproterenol (ISO, 10(-5) M), 2) post-beta-receptor stimulation of adenylate cyclase with forskolin (FSK, 10(-5) M), 3) and dibutyryl cAMP (DBcAMP, 1 mM), a cAMP analogue. Vascular permeability was assessed by determining the capillary filtration coefficient (Kf,c), and capillary pressure was measured using the double occlusion technique. The total, arterial, and venous vascular resistances were calculated from measured pulmonary arterial, venous, and capillary pressures and blood flow. Reperfusion after 2 h of ischemia significantly (P less than 0.05) increased Kf,c (from 0.115 +/- 0.028 to 0.224 +/- 0.040 ml.min-1.cmH2O-1.100 g-1). These I-R-induced changes in capillary permeability were prevented when ISO, FSK, or DBcAMP was added to the perfusate at reperfusion (0.110 +/- 0.022 and 0.103 +/- 0.021, 0.123 +/- 0.029 and 0.164 +/- 0.024, and 0.153 +/- 0.030 and 0.170 +/- 0.027 ml.min-1.cmH2O-1.100 g-1, respectively). I-R significantly increased total, arterial, and venous vascular resistances. These increases in vascular resistance were also blocked by ISO, FSK, and DBcAMP. These data suggest that beta-adrenergic stimulation, post-beta-receptor activation of adenylate cyclase, and DBcAMP prevent the changes in pulmonary vascular permeability and vascular resistances caused by I-R in isolated rabbit lungs through a mechanism involving an increase in intracellular levels of cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cyclic AMP and serum arrest of the mitotic activity of human diploid fibroblasts.

Mitotic activity in confluent cultures of human diploid fibroblasts was arrested by the reduction of the serum concentration of the incubation medium to 0.5% or by the addition of 0.5 mM 6-N, 2'-O-dibutyryl-adenosine 3':5'-cyclic monophosphate (db cAMP). Under either of these conditions, cultures maintained a constant cell number for 14 days; cultures continuously exposed to medium containing 10% serum doubled their cell number during this 14-day period. The protein cotent per cell decreased by 20% when cells were maintained with 0.5% serum whereas that of cells exposed to db cAMP remained constant. Ultrastructural studies revealed that cells exposed to db cAMP exhibited a morphology typical of cells cultures with 10% serum alone, whereas cells incubated with 0.5% serum showed the ultrastructural changes in mitochondria, endoplasmic reticulum and Golgi complex previously identified with low-serum arrest. Cellular adenosine 3':5'-cyclic monophosphate (cAMP) levels remained constant during the 7-day growth period in which confluency was attained, as well as during the 14-day arrested period with 0.5% serum. These results indicated that the mitotic inhibition induced by reducing the serum concentration of the incubation medium was not mediated by increased intracellular levels of cAMP and differed from that induced by the addition of exogenous db cAMP.     

Inhibition of DNA synthesis of adult rat hepatocytes in primary culture by dibutyrylcytidine 3', 5'-cyclic monophosphate

The effect of dibutyrylcytidine 3',5'-cyclic monophosphate (Bt2cCMP) on DNA synthesis of adult rat hepatocytes in primary culture was examined. Bt2cCMP caused dose-dependent inhibition of the DNA syntheses stimulated by various growth factors including human hepatocyte growth factor (hHGF). Dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP) inhibited the DNA synthesis more effectively than Bt2cCMP, but dibutyrylguanosine 3',5'-cyclic monophosphate (Bt2cGMP) and n-butyrate had a slight or null inhibitory effect. When added at the onset of DNA synthesis, Bt2cAMP was much less effective, but Bt2cCMP was still effective. Thus Bt2cCMP is able to inhibit growth factor-stimulated hepatocyte proliferation.     

Effects of dibutyryl cAMP on pulmonary air embolism-induced lung injury in awake sheep

To assess the role of intracellular adenosine 3',5'-cyclic monophosphate (cAMP), we tested the effects of dibutyryl cAMP (DBcAMP), an analogue of cAMP, on lung injury induced by pulmonary air embolism in awake sheep with chronic lung lymph fistula. We infused air (1.23 ml/min) in the pulmonary artery for 2 h in untreated control sheep. In DBcAMP-pretreated sheep DBcAMP was infused (1 mg/kg bolus and 0.02 mg.kg-1.min-1 constantly for 5 h); after 1 h from beginning of DBcAMP administration the air infusion was started. After the air infusion, pulmonary arterial pressure (Ppa) and lung lymph flow rate (Qlym) significantly increased in both groups. DBcAMP-pretreated sheep showed significantly lower responses in Qlym (2.7 X base line) compared with untreated control sheep (4.6 X base line); however, Ppa, left atrial pressure, and lung lymph-to-plasma protein concentration ratio were not significantly different between the two groups. Although plasma and lung lymph thromboxane B2 and 6-ketoprostaglandin F1 alpha concentrations increased significantly during the air infusion, DBcAMP-pretreated sheep showed significantly lower responses. Thus DBcAMP infusion attenuated pulmonary microvascular permeability induced by air embolism. We conclude that pulmonary vascular permeability is in part controlled by the intracellular cAMP level.     

Three new potential cAMP affinity labels. Inactivation of human platelet low Km cAMP phosphodiesterase by 8-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate

Three new analogues of cAMP have been synthesized and characterized: 2-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate (2-BDB-TcAMP), 2-[(3-bromo-2-oxopropyl)thio]-adenosine 3',5'-cyclic monophosphate (2-BOP-tcAMP), and 8-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate (8-BDB-TcAMP). The bromoketo moiety has the ability to react with the nucleophilic side chains of several amino acids, while the dioxobutyl group can interact with arginine. These cAMP analogues were tested for their ability to inactivate the low Km (high affinity) cAMP phosphodiesterase from human platelets. The 2-BDB-TcAMP and 2-BOP-TcAMP were competitive inhibitors of cAMP hydrolysis by the phosphodiesterase with Ki values of 0.96 +/- 0.12 and 0.70 +/- 0.12 microM, respectively. However, 2-BDB-TcAMP and 2-BOP-TcAMP did not irreversibly inactivate the phosphodiesterase at pH values from 6.0 to 7.5 and at concentrations up to 10 mM. These results indicate that although the 2-substituted TcAMP analogues bind to the enzyme, there are no reactive amino acids in the vicinity of the 2-position of the cAMP binding site. In contrast, incubation of the platelet low Km cAMP phosphodiesterase with 8-BDB-TcAMP resulted in a time-dependent, irreversible inactivation of the enzyme with a second-order rate constant of 0.031 +/- 0.009 min-1 mM1. Addition of the substrates, cAMP and cGMP, and the product, AMP, to the reaction mixture resulted in marked decreases in the inactivation rate, suggesting that the inactivation was due to reaction at the active site of the phosphodiesterase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Direct effects of prolactin on corticosterone release by zona fasciculata-reticularis cells from male rats

The role of prolactin (PRL) in the male is not fully defined. The aim of this study was to investigate the function and mechanism of PRL on the production of corticosterone by zona fasciculata-reticularis (ZFR) cells in vitro. The ZFR cells were obtained from male rats under normal, hyperprolactinemic, or hypoprolactinemic situation. PRL stimulated the corticosterone release in a dose-dependent pattern in the ZFR cells from normal male rats. The cellular adenosine 3'-5'-cyclic monophosphate (cAMP) concentration positively correlated with PRL concentration in the presence of forskolin or 3-isobutyl-1-methylxanthine (IBMX). PRL enhanced the stimulatory effects of cAMP mimetic reagents, i.e., forskolin, 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP), and IBMX on the release of corticosterone. The adenylate cyclase inhibitor (SQ22536) inhibited the corticosterone release in spite of presence of PRL. Nifedipine (L-type calcium channel blocker) did not inhibit corticosterone release. The hyperprolactinemic condition was actualized by transplantation of donor rat anterior pituitary glands (APs) under kidney capsule. By comparison with the cerebral cortex (CX)-grafted group, AP-graft resulted in an increased release of corticosterone, 3beta-hydroxysteriod dehydrogenase (HSD) activity and cAMP production by ZFR cells. Acute hypoprolactinemic status was induced by bromocriptine for 2 days. The results showed the productions of corticosterone were lower in hypoprolactinemic group than in control group, which were persistent along with different ACTH concentrations. These results suggest that PRL increase the release of corticosterone by ZFR cells via cAMP cascades and 3beta-HSD activity.     

cAMP-positive regulation of angiotensinogen gene expression and protein secretion in rat adipose tissue

The adipose renin-angiotensin system (RAS) has been assigned to participate in the control of adipose tissue development and in the pathogenesis of obesity-related hypertension. In adipose cells, the biological responses to beta-adrenergic stimulation are mediated by an increase in intracellular cAMP. Because cAMP is known to promote adipogenesis and because an association exists between body fat mass, hypertension, and increased sympathetic stimulation, we examined the influence of cAMP on angiotensinogen (ATG) expression and secretion in rat adipose tissue. Exposure of primary cultured differentiated preadipocytes to the cAMP analog 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) or cAMP-stimulating agents (forskolin and IBMX) results in a significant increase in ATG mRNA levels. In adipose tissue fragments, 8-BrcAMP also increases ATG mRNA levels and protein secretion, but not in the presence of the protein kinase A inhibitor H89. The addition of isoproterenol, known to stimulate the synthesis of intracellular cAMP via beta-adrenoreceptors, had the same stimulatory effect on ATG expression and secretion. These results indicate that cAMP in vitro upregulates ATG expression and secretion in rat adipose tissue via the protein kinase A-dependent pathway. Further studies are required to determine whether this regulatory pathway is activated in human obesity, where increased sympathetic tone is frequently observed, and to elucidate the importance of adipose ATG to the elevated blood pressure observed in this pathological state.     

Regulation of atrial natriuretic factor-(99-126) secretion from neonatal rat primary atrial cultures by activators of protein kinases A and C

Atrial natriuretic factor (ANF) is stored in atrial myocytes as a prohormone (ANF-(1-126] and is cosecretionally processed to the circulating ANF-related peptides, ANF-(1-98) and ANF-(99-126). Recently, we have shown that the cosecretional processing of ANF can be replicated in primary cultures of neonatal rat atrial myocytes maintained under serum-free conditions and that glucocorticoids are responsible for supporting this processing activity. Activators of protein kinase C (phorbol esters and alpha-adrenergic agonists) and of protein kinase A (cAMP analogs, forskolin, and beta-adrenergic agonists) were tested for their abilities to alter the rate of ANF secretion from the primary cultures. ANF secretion was stimulated approximately 4-fold after a 1-h incubation of the cultures with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA); maximal release occurred at about 100 nM TPA. Reversed-phase high performance liquid chromatography analysis of secreted material indicated that the cells efficiently cosecretionally processed ANF under both basal and TPA-stimulated conditions. However, incubating the cultures for more than 1 h with TPA resulted in a blunted secretory response to further TPA challenge and a 40-50% decrease in the quantity of ANF in the cells. The alpha-adrenergic receptor agonist phenylephrine was also capable of stimulating ANF secretion by about 4-fold at a half-maximal dose of about 1 microM. Phenylephrine-stimulated ANF secretion was inhibited by the alpha 1-adrenergic antagonist prazosin with half-maximal inhibition occurring at approximately 1 nM. Forskolin, 8-bromoadenosine 3':5'-cyclic monophosphate, and N6-2(1)-O-dibutyryladenosine 3':5'-cyclic monophosphate inhibited basal, TPA- and phenylephrine-stimulated ANF secretion. The beta-adrenergic agonist isoproterenol partially inhibited phenylephrine-stimulated ANF secretion with the maximal effect occurring at 1 nM. These results indicate that ANF secretion from the neonatal rat atrial cultures is enhanced by activators of protein kinase C, and decreased by activators of protein kinase A, and that these secretory effects may be mediated through the actions of alpha- and beta-adrenergic receptors, respectively.     

Activation of signal transduction pathways protects quiescent Balb/c-3T3 fibroblasts against death due to serum deprivation.

Platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), and insulin protect density-inhibited murine Balb/c-3T3 fibroblasts against death by distinctive mechanisms. Determination of the cell survival-enhancing activity of growth factors by cell enumeration and neutral red uptake measurement gives equivalent results. PDGF displays a steep dose-response relationship in the 1-5 ng/ml range. The other factors display shallow log-linear relationships in the following ranges: EGF: 0.2-5 ng/ml; IGF-1: 2-80 ng/ml; and insulin: 57-4,500 ng/ml. Agonists that lead to the activation of protein kinase A, including forskolin, 8-bromoadenosine 3':5'-cyclic monophosphate (Br-cAMP) and N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (db-cAMP), markedly increase both short-term (5-h) and long-term (20-h) survival of cells. 2-Isobutyl-1-methylxanthine (IBMX) markedly enhances short-term survival, but its effect decays with time. The protein kinase C agonist 12-O-tetradecanoyl phorbol-13-acetate (TPA) has a moderate protective effect at concentrations of 16-32 nM, and 64 nM TPA is highly effective. The synthetic diaclglycerols 1,2-dioctanoylglycerol (DiC8) and 1-oleoyl-2-acetylglycerol (OAG) and the calcium ionophore ionomycin show low activity. Supplementation of EGF with a protein kinase A or C agonist results in a varying additive increase in short-term (5-h) cell survival and supplementation of EGF + insulin or PDGF + EGF + insulin increases further the already high level of protection given by the growth factor combinations. Combining a protein kinase A and a protein kinase C agonist in the absence of growth factors gives an approximately additive increase in cell survival. Results obtained with kinase, RNA, and protein synthesis inhibitors suggest that: 1) activated protein kinase C catalyzes one or more phosphorylation events in quiescent Balb/c-3T3 cells that lead to gene expression with the protein product(s) mediating protection of quiescent cells against death, and 2) phosphorylation events catalyzed by protein kinase A largely serve to protect cells by a mechanism not requiring de novo RNA and protein biosynthesis.