Prostaglandin stimulation of adenosine 3′,5′-monophosphate accumulation in cultured chondrocytes in the presence or absence of parathyroid hormone

The effect of prostaglandin analogues on the cycle AMP level in cultured chondrocytes were examined. Prostaglandin E₁ at 0.4 to 30 μM, increased the intracellular concentration of cyclic AMP in chondrocytes. Its effect was rapid, being evident within 1 min and reaching a maximum in 10 to 20 min. The maximum level was sustained until 30 min after its addition and then decreased gradually. Prostaglandin D₂ and E₂ also increased the cyclic AMP level in chondrocytes, but they had less effect than prostaglandin E₁. Prostaglandin A₁ had no effect on the nucleotide level in chondrocytes, although they markedly increased the level in fibroblasts. The time course of stimulation of cyclic AMP accumulation in chondrocytes by prostaglandin E₁, D₂ or E₂ was quite different from that by parathyroid hormone (PTH): the effect of prostaglandin was slower and more sustained than that of PTH. PTH potentiated the effect of prostaglandin E₁, E₂, or D₂ on the cyclic AMP level in chondrocytes and that the combined effects of prostaglandin, PTH or both produced a synergistic effect on the accumulation of cyclic AMP in the chondrocytes. These findings suggest that prostaglandin E₁, E₂, and D₂ increase the synthesis of cyclic AMP and that the combined effect of the prostaglandins and PTH on the cyclic AMP level in chondrocytes is partly attributed to the synergistic synthesis of cyclic AMP in the cells.     

Cyclic AMP induces synthesis of prostaglandin E1 in platelets

Although platelets are known to synthesize small amounts of prostaglandin E1 the control of the formation of this prostanoid has not been investigated. Incubation of human platelet-rich plasma with various compounds which are known to increase cyclic AMP concentration in platelets and inhibit platelet aggregation also increased intracellular prostaglandin E1 synthesis. The prostaglandin E1 was isolated by high pressure liquid chromatography and definitively identified by negative and positive ionization mass spectroscopy. The amounts of prostaglandin E1 formed were proportional to the concentration of cyclic AMP in platelets. Prostacyclin (10 nM) which is the most potent stimulator of cyclic AMP formation increased intracellular cyclic AMP by 4.6 fold and prostaglandin E1 level by 3 fold over the basal levels. Addition of theophylline, a cyclic AMP phosphodiesterase inhibitor, together with prostacyclin increased cyclic AMP concentration 8.7-fold and prostaglandin E1 level 12-fold compared to basal concentrations. Dibutyryl cyclic AMP (2 mM) and 8-bromo cyclic AMP (0.1 mM) increased prostaglandin E1 levels by 3 fold and 2 fold over the basal level, respectively. Prostaglandin D2 (3 microM) when added to platelet-rich plasma increased the cyclic nucleotide levels by 2 fold concomitant with 2 fold increase in prostaglandin E1 concentration. In contrast prostaglandin E2 or prostaglandin F2 alpha which had no effect on cyclic AMP level did not affect the prostaglandin E1 synthesis. Addition of 2',5'-dideoxyadenosine, an inhibitor of adenylate cyclase, to platelet-rich plasma inhibited both the increase of intracellular prostaglandin E1 and cyclic AMP levels induced by prostacyclin.     

Prostaglandin E2 production in 3T3 cells transformed by polyoma virus raises the intracellular adenosine 3':5'-monophosphate levels.

High cellular adenosine 3':5'-monophosphate (cyclic AMP) were found in a polyoma-virus-transformed 3T3 fibroblast line, which produces comparatively high prostaglandin E2 concentrations. Prostaglandin E2 and cyclic AMP increased with time during 6 days of growth. Both effects were prevented by three different cyclo-oxygenase inhibitors. Addition of prostaglandin E2, at a concentration which would been synthesized in the absence of inhibitor, reversed the effect of indomethacin, one of the cyclo-oxygenase inhibitors, on cyclic AMP. It is concluded that endogenous prostaglandin E2 production in these transformed cells influences their cellular cyclic AMP levels.     

Formation of adenosine 3',5'-monophosphate from adenosine in mouse thymocytes.

The effect of adenosine on the mouse thymocyte adenylate cyclase-adenosine 3':5'-monophosphate (cyclic AMP) system was examined. Adenosine, like prostaglandin E1, can cause 5-fold or greater increases in thymocyte cyclic AMP content in the presence but not in the absence of certain cyclic phosphodiesterase inhibitors. Two non-methylxanthine inhibitors potentiated the prostaglandin E1 and adenosine responses, while methylxanthines selectively inhibited the adenosine response. Adenosine increased cyclic AMP content significantly within 1 min and was maximal by 10 to 20 min with approx. 2 and 10 muM adenosine being minimal and half-maximal effective doses, respectively. Combinations of prostaglandin E1, isoproterenol and adenosine were near additive and not synergistic. Of the adenosine analogues tested, only 2-chloro- and 2-fluoroadenosine significantly increased cyclic AMP. Thymocytes prelabeled with [14C]adenine exhibited dramatic increases in cyclic [14C]AMP 10 min after addition of adenosine or prostaglandin E1 which corresponded to simultaneously determined increases in total cyclic AMP. Using [14C]adenosine, the percent of total cyclic AMP increase due to adenosine was only 16%. Adenosine was also shown to elicit a 40% increase in particulate thymocyte adenylate cyclase activity. Therefore, the increased content of cyclic AMP seen in mouse thymocytes after incubation with adenosine was due primarily to stimulation of adenylate cyclase and only partially to conversion of adenosine to cyclic AMP. The increased cellular content of cyclic AMP may be, in part, responsible for various immunosuppressive effects of adenosine.     

Effect of prostaglandin E on the adenyl cyclase-cyclic AMP system and gluconeogenesis in rat renal cortical slices.

Prostaglandin E was found to increase the formation of cyclic acdenosine 3',5'-monophosphate (cyclic AMP) by renal cortical slices. This increased release of cyclic AMP was not influenced by the absence of Ca2+ in the incubating media. The enhanced production of cyclic AMP was probably mediated by stimulation of membrane-bound adenylate cyclase activity. An increase in adenyl cyclase activity was observed with increasing concentrations of prostaglandin E. Furthermore, prostaglandin E augmented glucose production from alpha-ketoglutarate. This effect on gluconeogenesis was abolished by the removal of Ca2+ from the incubating medium. These effects are similar to those described for parathyroid hormone and suggest that the renal cortex is a prostaglandin-dependent system. Prostaglandin E decreased cyclic AMP production and glucose production (from alpha-ketoglutarate) in response to submaximal doses of parathyroid hormone, suggesting that prostaglandin may be important in modulating the intracelluar action of parathyroid hormone in the kidney cortex.     

Platelet activating factor-mediated leukotriene biosynthesis in rat lungs: effect of prostaglandins E1 and F1 alpha

The effect of prostaglandins E1 and F1 alpha on peptidoleukotriene biosynthesis/release from rat chopped lung stimulated with platelet activating factor was studied. Prostaglandin E1, known to stimulate adenylate cyclase in airways, inhibited the biosynthesis of leukotrienes C4, D4 and E4 and total peptidoleukotrienes whereas prostaglandin F1 alpha, which has no effect on adenylate cyclase, did not exert any effect on total peptidoleukotriene release, though a small inhibition was found for leukotriene D4. Cyclic AMP itself inhibited peptidoleukotriene release from platelet activating factor-stimulated lung, suggesting that the effect of prostaglandin E1 is mediated by cyclic AMP.     

Effects of parathyroid hormone on cyclic AMP, cyclic GMP, and efflux of calcium in isolated renal tubules.

The effects of parathyroid hormone (PTH) on concentrations of cyclic AMP and cyclic GMP were investigated in isolated renal cortical tubules from hamsters. Efflux of 45Ca from tubules was compared to temporal changes in both cyclic nucleotide concentrations. A rapid increase in cyclic AMP occurred following addition of PTH which was maximal by 1 min but decreased over the next 4 min period. Cyclic GMP concentrations were not significantly altered at 1 min but increased between 1 and 5 min from basal levels. Concentrations of both nucleotides remained significantly elevated from basal levels between 5 and 15 min following PTH. Efflux of 45Ca was increased by PTH with time-course changes closely paralleling changes in cyclic GMP concentrations. Changes in both cyclic AMP and cyclic GMP were related to PTH concentrations of the incubation media and were increased by addition of theophylline. Increasing the calcium concentration from 1 to 3 mM did not significantly alter the effect of PTH on cyclic AMP, however, cyclic GMP concentrations were further increased.     

Effect of cyclic AMP on the intracellular degradation of newly synthesized collagen

Prostaglandin E1 and cholera toxin increased the intracellular levels of cyclic AMP of human lung fibroblasts. With prostaglandin E1, the increase in cyclic AMP occurred within 10 min followed by a decline to less than one-half of peak values in 6 h. With cholera toxin, the increase occurred within 60 min but the level of cyclic AMP remained increased for 6 h. Both agents caused a decrease in collagen production as expressed as the proportion of newly synthesized protein represented by collagen. The increase in cyclic AMP levels was accompanied by a marked increase in the proportion of newly synthesized collagen which was degraded intracellularly prior to secretion. Analysis of the degraded collagen showed it to be predominantly less than 1000 daltons in molecular mass, but still in peptide linkage. The data are consistent with the hypothesis that cyclic AMP levels in diploid fibroblasts regulate the amount of collagen produced by fibroblasts, at least in part, by modulating the level of intracellular collagen degradation.     

Regulation of cyclic AMP level and lactic acid production in Ehrlich ascites tumor cells.

1. Quercetin (3.3',4',5,7-pentahydroxy flavone) at the concentration of 10(-4) M, as well as 2-10(-2) M theophylline and 1.5 - 10(-4) M prostaglandin E2 caused maximal rise of cyclic AMP in Ehrlich ascites tumor cells. 2. No additional increase of cyclic AMP level in these cells was found when both quercetin (10(-4) M) and theophylline (2-10(-2) M) were present in the incubation medium, while combination of quercetin (10(-4) M) and prostaglandin E2 (1.5 - 10(-4) M) has a synergistic effect on the level of cyclic AMP. 3. Degradation of cyclic AMP by homogenate of Ehrlich ascites tumor cells was inhibited by both quercetin and theophylline. 4. Quercetin, and to a smaller but significant extent theophylline, inhibited the lactic acid production in Ehrlich ascites tumor cells while prostaglandin E2 did not change the glycolytic rate in these cells. No synergistic inhibitory effect on lactic acid production was found when combinations of quercetin and prostaglandin E2, quercetin and theophylline or prostaglandin E2 and theophylline were tested. 5. Treatment of Ehrlich ascites tumor cells with dextran sulfate abolished the inhibitory effect of quercetin on lactic acid production, while the effect of the bioflavonoid on cyclic AMP levels was not altered.     

Comparison of inhibition by a tumor promoter (12-O-tetradecanoylphorbol-13-acetate) of expression of the differentiated phenotype of chondrocytes in rabbit costal chondrocytes in culture with inhibition by retinoic acid

Both retinoids and the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibit expression of the differentiated phenotype by rabbit costal chondrocytes in culture, as judged by morphological changes and decreased sulfation of glycosaminoglycans (GAG). However, the inhibition of the differentiated phenotype of chondrocytes in TPA-treated cells is restored by parathyroid hormone (PTH), while the inhibition by retinoids is not [Takigawa et al. (1982) Mol. Cell. Biochem. 42, 145-153; Takigawa et al. (1983) Cell Differ. 13, 283-291]. In the present study, we examined the difference between TPA-treated chondrocytes and retinoic acid-treated chondrocytes to determine the mechanism of the restoration of the differentiated phenotype in de-differentiated cells treated with TPA. PTH increased the activity of ornithine decarboxylase [ODC; EC 4.1.1.17], a rate limiting enzyme of polyamine biosynthesis, and proteoglycan synthesis in chondrocytes pretreated with TPA as well as in normal chondrocytes. The maximal stimulations of ODC activity and GAG synthesis were observed 4 h and 24-36 h, respectively, after addition of PTH. The dose-response curve for ODC induction by PTH was parallel to that of PTH-stimulated proteoglycan synthesis both in TPA-treated chondrocytes and in normal chondrocytes. PTH also increased the intracellular cyclic AMP level after 2 min in TPA-treated cells as in normal cells. Addition of dibutyryl cyclic AMP (DBcAMP) induced ODC and restored proteoglycan synthesis in TPA-treated cells. The dose-response curve for induction of ODC by DBcAMP was parallel to that of DBcAMP-stimulated proteoglycan synthesis in both TPA-treated chondrocytes and normal chondrocytes. On the other hand, the increases by PTH in the intracellular cyclic AMP level, ODC activity, and proteoglycan synthesis were inhibited in chondrocytes pretreated with a combination of TPA and retinoic acid as well as in those pretreated with retinoic acid alone. TPA stimulated the syntheses of DNA and RNA in chondrocytes but did not increase the cyclic AMP level or ODC activity. PTH and DBcAMP inhibited the syntheses of DNA and RNA both in TPA-treated cells and in normal cells. These results suggest that ODC induction mediated by elevation of cyclic AMP plays an important role in re-differentiation of de-differentiated cells pretreated with these agents.     

Cyclic nucleotides and their relationship to complement-component-C2 synthesis by human monocytes.

The time courses of changes in cyclic nucleotide levels in monocytes have been studied. Histamine and prostaglandin E2 (PGE2) produced a rapid rise in cyclic AMP (peak 15 min) levels, which returned to normal within 4h, whereas cholera toxin, NaF and phosphodiesterase inhibitors produced slow sustained rises lasting over 24h. With the exception of isobutylmethylxanthine (10 mumol X 1(-1), none of these reagents altered cyclic GMP levels. alpha 1-Adrenergic and nicotinic cholinergic receptor-ligand interactions and imidazole produced rapid and relatively short-lived falls in cyclic AMP, and rises in cyclic GMP. In contrast, prostaglandin synthetase inhibitors produced delayed but more sustained falls in cyclic AMP but no rises in cyclic GMP. Agents that increased cyclic AMP decreased complement-component-C2 production, and those that decreased cyclic AMP increased C2 production. Agents that increased cyclic GMP alone (ascorbate, nitroprusside and prostaglandin F2 alpha) did not affect C2 production. Antigen-antibody complexes that stimulate C2 synthesis produced falls in cyclic AMP and rises in cyclic GMP similar to those produced by adrenergic and cholinergic ligands. Serum-treated complexes and anaphylatoxins, which inhibited C2 production, were associated with changes in cyclic AMP similar to those produced by histamine and PGE2. These data suggest that there are two transmembrane signals involved in the regulation of C2 production by monocytes. The inhibitory signal is adenylyl cyclase activation. The stimulatory signal is not so obvious, but may be Ca2+ influx, since the time courses of changes in cyclic nucleotides produced by agents that stimulate C2 synthesis are identical, and alpha 1-adrenergic agonists cause the formation of Ca2+ channels.     

Effects of prostaglandins and other drugs on the cyclic AMP content of cultured bone cells.

Prostaglandins of the E-series (PGE1 and PGE2) may be involved in disease-related, localized loss of bone. E-prostaglandins increase the cyclic AMP content of many cells; and, to determine if their effects on bone are mediated by cyclic AMP, we examined the effects of E-prostaglandins and of other agents on the cyclic AMP content of cultured bone cells. PGE2 produced a rapid, marked and dose-related increase in the cyclic AMP content of confluent monolayers of bone cells isolated from newborn rat calvaria. At 2.8 X 10(-6) M, PGE1 and PGE2 had approximately the same effect, while the effect of PGF2alpha was much less pronounced. In the presence of theophylline, PGE2 had a more marked effect than parathyroid hormone (PTH) and the combination of PGE2 and PTH had a synergistic effect. The divalent, cationic, ionophore, A23187, produced an increase in cellular cyclic AMP and had an additive effect in combination with PGE2. Synthetic salmon calcitonin (CT), which inhibits the bone resorptive effect of PGE2, increased cellular cyclic AMP and had an additive effect in combination with PGE2. A prostaglandin antagonist, SC-19220, partially inhibited the resorptive effect of PGE2 and reduced its effect on cellular cyclic AMP. The calcium antagonist, D600, inhibited the bone resorptive effects of PGE2 but had no effect on increased cellular cyclic AMP produced by PGE2. The marked effect of PGE2 on bone cell cyclic AMP suggests that this action is involved in the mechanism of PGE2-related bone loss. The fact that agents with different effects on PGE2-induced increases in cellular cyclic AMP can inhibit its resorptive actions, suggests that PGE2-induced changes in cyclic AMP may be related less to its resorptive actions than to its inhibitory effect on bone formation.     

Effects of prostaglandins F alpha on dog thyroid cyclic AMP level and function

Prostaglandins F1 alpha and F2 alpha, at high concentrations (greater than or equal to 28 microM) enhanced cyclic AMP accumulation in dog thyroid slices. At lower concentrations, they inhibited the cyclic AMP accumulation induced by thyrotropin (TSH), prostaglandin E1, and cholera toxin. This effect was rapid in onset and of short duration, calcium-dependent and suppressed by methylxanthines. Prostaglandin F alpha also inhibited TSH-induced secretion and activated iodide binding to proteins. These characteristics are similar to those of carbamylcholine action, except that prostaglandins F did not enhance cyclic GMP accumulation. The effect of prostaglandin F alpha was not inhibited by atropine, phentolamine and adenosine deaminase and can therefore not be ascribed to an induced secretion of acetylcholine, norepinephrine or adenosine. It is suggested that prostaglandins F act by increasing influx of extracellular Ca2+. Arachidonic acid also inhibited the TSH-induced cyclic AMP accumulation. However this effect was specific for TSH, it was enhanced in the absence of calcium and was not inhibited by methylxanthines or by indomethacin at concentrations which completely block its conversion to prostaglandin F alpha. Arachidonic acid action is sustained. This suggests that arachidonic acid inhibits thyroid adenylate cyclase at the level of its TSH receptor and that this effect is not mediated by prostaglandin F alpha or any other cyclooxygenase product.     

Correlation of the biosynthesis of prostaglandin and cyclic AMP in monolayer cultures of rabbit articular chondrocytes

We have utilized ionophores to test whether stimulation of chondrocyte prostaglandin biosynthesis is accompanied by an increase in cyclic nucleotide levels in these cells. Radioimmunoassay of prostaglandin E2, 6-oxo-prostaglandin F1 alpha (the stable metabolite of prostaglandin I2) and prostaglandin F2 alpha showed that synthesis of each was stimulated by the divalent-cation ionophore, A23187 after short-term incubation (1-7 min) in serum-free medium. No stimulation of thromboxane B2 was detected. Two monovalent ionophores, lasalocid and monensin failed to stimulate prostaglandin biosynthesis after short-term incubation. Ionophore A23187-stimulated prostaglandin biosynthesis was variably and partially inhibited by sodium meclofenamate, indomethacin and aspirin, but not by sodium salicylate. Ionophore A23187-stimulated prostaglandin biosynthesis was accompanied by a 7.5-fold increase in cyclic AMP levels after 15 min. Sodium meclofenamate, indomethacin and aspirin which inhibited prostaglandin E2 biosynthesis also reduced cyclic AMP levels. Exogenous prostaglandin E2 (1 microgram/ml) stimulated cyclic AMP biosynthesis, which was not inhibited by aspirin. These results indicated that prostaglandins can be considered as one of the local effectors controlling cyclic AMP production in articular cartilage.     

The role of cyclic AMP in the chemotactic responsiveness and spontaneous motility of rabbit peritoneal neutrophils. The inhibition of neutrophil movement and the elevation of cyclic AMP levels by catecholamines, prostaglandins, theophylline and cholera toxin.

Agents known to affect intracellular levels of cyclic AMP in many diverse systems have been tested for their effect on the chemotaxis induced by Escherichia coli culture filtrates, spontaneous motility and cyclic AMP levels of rabbit peritoneal neutrophils. Prostaglandin E1 and A1 but not prostaglandin F2alpha increased neutrophil cyclic AMP levels and, correspondingly, only the former two prostaglandins inhibited chemotaxis. Nevertheless, a quantitative relationship between prostaglandin stimulation of cyclic AMP and inhibition of chemotaxis could not be found. Epinephrine, isoproterenol, and, to a much lesser extent, norepinephrine increased neutrophil cyclic AMP through beta adrenergic stimulation. Only epinephrine and isoproterenol inhibited chemotaxis, but the inhibition was variable and not related to the ability of these catecholamines to increase intracellular cyclic AMP. Cholera toxin increased neutrophil cyclic AMP after a 30-min lag period which paralled its inhibitory effect on chemotaxis and spontaneous motility. However, the effect on chemotaxis require 50 ng/ml of toxin whereas the effect on cyclic AMP was manifested at 2 ng/ml of toxin. Prior to 30-min preincubation there was no effect of even 1250 ng/ml of toxin on either cyclic AMP or chemotaxis. Choleragenoid prevented the effects of toxin on both cyclic AMP and chemotaxis. The bacterial chemotactic factor obtained from E. coli culture filtrates did not effect a measurable change in levels of neutrophil cyclic AMP. The data indicate that even though cyclic AMP is not, in the main sequence of events, triggering the chemotactic response, increases in neutrophil cyclic AMP may modulate the movement and thus the chemotactic responsiveness of the neutrophil.     

Effects of various tumor promoters on expression of cartilage phenotypes in rabbit costal chondrocytes in culture

12-O-Tetradecanoylphorbol-13-acetate (TPA), a skin tumor-promoting phorbol ester, and teleocidin and aplysiatoxin, which are potent tumor promoters in mouse skin but are chemically unrelated to phorbol esters, induced change of cultured rabbit costal chondrocytes from a polygonal to a fibroblastic shape and inhibited glycosaminoglycan (GAG) synthesis and metachromatic matrix formation in these cells. The potencies of teleocidin and aplysiatoxin to inhibit GAG synthesis were almost the same as that of TPA. On the other hand, Tween 60 and cantharidin, weak mouse skin tumor promoters, phenobarbital, a liver tumor promoter, and saccharin, a bladder tumor promoter, had no effect on the morphology or GAG synthesis of cultured chondrocytes. Like TPA, teleocidin and aplysiatoxin increased DNA and RNA syntheses of chondrocytes. Parathyroid hormone (PTH) and dibutyryl cyclic AMP reversed the morphological and histochemical changes caused by a 4-day treatment with teleocidin or aplysiatoxin as well as with TPA, reversal being apparent after 2 days. PTH increased intracellular cyclic AMP after 2 min in chondrocytes pretreated with teleocidin or aplysiatoxin as well as with TPA. PTH also increased ornithine decarboxylase [ODC; EC 4.1.1.17] activity in these chondrocytes after 4 h. These results show that retention of responsiveness to PTH is a typical characteristic of chondrocytes dedifferentiated by treatment with TPA-type tumor promoters such as TPA, teleocidin and aplysiatoxin. The results also suggest that ODC induction mediated by elevation of cyclic AMP plays an important role in re-differentiation of teleocidin- and aplysiatoxin-treated chondrocytes.     

Cyclic AMP-dependent and -independent effects on tissue-type plasminogen activator activity in osteogenic sarcoma cells; evidence from phosphodiesterase inhibition and parathyroid hormone antagonists

The plasminogen activator (PA) in clonal osteogenic sarcoma cells of rat origin (UMR 106-01 and UMR 106-06) and in osteoblast-rich rat calvarial cells has been characterized using specific antibodies to be tissue-type PA (tPA). An Mr value of 75,000 by SDS-polyacrylamide gel electrophoresis and fibrin autoradiography supports this characterization. There was also evidence for an Mr 105,000 component, which could be due to a proteinase-inhibitor complex. The mechanism of regulation of this tPA activity has been studied in the clonal osteogenic sarcoma cells. Parathyroid hormone (PTH) and prostaglandin E2, which increase cyclic AMP production in the sarcoma cells, also increased tPA activity. The sensitivity and magnitude of the tPA response to PTH and prostaglandin E2 were increased by simultaneous treatment with isobutylmethylxanthine (IBMX) at drug concentrations which had little effect themselves on tPA activity. In UMR 106-06 cells, which unlike UMR 106-01 cells show a cyclic AMP response to calcitonin, tPA activity was also increased in response to calcitonin, and the effect was enhanced by IBMX. 1,25-Dihydroxyvitamin D-3 also increased tPA activity in the cells, but this response was not modified by IBMX. Synthetic peptide antagonists of PTH-responsive adenylate cyclase, [34Tyr]-hPTH (3-34) amide and [34Tyr]-hPTH (5-34) amide, inhibited the PTH-induced increase in tPA activity over the same concentration range at which they inhibited cyclic AMP production, but the antagonist peptides had no effect on the tPA responses to prostaglandin E2, calcitonin or 1,25-dihydroxyvitamin D-3. These data indicate that cyclic AMP mediates the actions of PTH, prostaglandin E2 and calcitonin in increasing tPA activity in the clonal osteogenic sarcoma cells. 1,25-Dihydroxyvitamin D-3, on the other hand, increases tPA activity through a mechanism independent of cyclic AMP.     

Regulatory role of cyclic adenosine 3',5'-monophosphate on the platelet cyclooxygenase and platelet function.

Thromboxane A2 plays an important role in arachidonic acid- and prostaglandin H2-induced platelet aggregation. Agents that stimulate platelet adenylate cyclase (prostaglandin I2, prostaglandin I1 and prostaglandin E1) and dibutyryl cyclic AMP inhibit both thromboxane A2 formation and arachidonate-induced aggregation in platelet-rich plasma. Despite complete suppression of aggregation with agents that elevate cyclic AMP, considerable thromboxane A2 is still formed. Prostaglandin H2-induced aggregations which bypass the cyclooxygenase regulatory step are also inhibited by agents that elevate cyclic AMP without any measurable effect on thromboxane A2 production. These data demonstrate that cyclic AMP can inhibit platelet aggregation by a mechanism independent of its ability to suppress the cyclooxygenase enzyme. Parallel experiments with washed platelet preparations suggest that they may be an inadequate model for studying the relationship between the platelet cyclooxygenase and platelet function.     

The effect of prostaglandins on ox pituitary content of adenosine 3′:5′-cyclic monophosphate and the release of growth hormone

1. An assay, based on competition between adenosine 3':5'-cyclic monophosphate (cyclic AMP) and cyclic [(3)H]AMP for binding to a rabbit skeletal muscle protein, has been used to measure tissue contents of cyclic AMP. The assay has a sensitivity of 0.05pmol of cyclic AMP. Cyclic GMP and cyclic CMP have 0.5%, and cyclic IMP 6.5%, of the ability of cyclic AMP to displace cyclic [(3)H]AMP from binding protein; AMP, ADP and ATP have no effect. 2. By using this method, the cyclic AMP content of ox pituitary slices exposed to prostaglandin was determined; release of growth hormone was measured by radioimmunoassay. 3. Release of growth hormone was increased by 45min incubation in 1mum-prostaglandin E(2) in the absence of theophylline, or in 10nm-prostaglandin E(2), 0.1mum-prostaglandin A(1) or 1mum-prostaglandin B(1) in the presence of 0.5mm-theophylline. 4. Pituitary cyclic AMP content was increased by 10min incubation in 1mum-prostaglandin E(2) in the absence of theophylline, or in 0.1mum-prostaglandin E(2) in the presence of 0.5mm-theophylline. 5. The maximum increase in cyclic AMP content was observed 10min, and significant changes in growth hormone release 30min, after introduction of prostaglandin E(2). 6. The increase in pituitary cyclic AMP content, but not in the rate of release of growth hormone, was observed in the absence of external Ca(2+). 7. The stimulation of release of growth hormone by prostaglandin was decreased by preincubation of tissue for 2h in colchicine (100mum) or cytochalasin B (10mug/ml). 8. These results support the suggestion that increased release of growth hormone after treatment with prostaglandin is the result of increased tissue cyclic AMP content, and possibly involves a microfilamentous or microtubular protein.     

Cardiac effects of prostaglandins E1 and F1 alpha

The effects of prostaglandin E1 (PGE1) and prostaglandin F1 alpha (PGF1 alpha) were studied on perfused rat hearts and isolated rat atria. Both PGE1 and PGF1 alpha produced dose-dependent increases in right atrial rate but had no effect on left atrial tension development. PGE1 (10(-4) M) increased right atrial cyclic AMP content without changing phosphorylase a activity. PGF1 alpha (10(-4) M) did not change right atrial cyclic AMP or cyclic GMP content. Both prostaglandins had no effect on left atrial cyclic nucleotide content. When infused at a rate of 1 microgram/min, PGE1 produced a time-dependent increase in cyclic AMP content in the Langendorff perfused hearts but did not alter contractile force development or phosphorylase a activity. An infusion of PGF1 alpha produced a dose-dependent increase in tension development which was secondary to a negative chronotropic effect. PGF1 alpha (1 microgram/min) did not produce any changes in cyclic nucleotide levels or phosphorylase a activity in the Langendorff perfused hearts. These results show that PGE1 can selectively increase myocardial cyclic AMP content without altering contractile force or phosphorylase activity and that PGF1 alpha does not increase rat cardiac AMP levels.