Effects of prostaglandin H2, prostaglandin E2, and arachidonic acid on parathyroid hormone and antidiuretic hormone activation of rat kidney adenylate cyclase

These experiments examine interactions of arachidonic acid; the substrate for prostaglandin cyclooxygenase, prostaglandin (PG)H₂, a key endoperoxide intermediate in prostaglandin synthesis; and prostaglandin (PG)E₂, an important prostaglandin produced within the kidney; with adenylate cyclase activity in renal cortex, outer medulla, and inner medulla. In addition, the effects of arachidonic acid, PGH₂, and PGE₂ on parathyroid hormone (PTH) activation of adenylate cyclase in cortex, and of antidiuretic hormone (ADH) activation of that enzyme in outer and inner medulla are examined. Arachidonic acid elicited a concentration-dependent inhibition of basal and PTH-stimulated adenylate cyclase activity in renal cortex. Concentration-dependent inhibition by arachidonic acid of basal and ADH-stimulated adenylate cyclase activity was observed in outer and inner medulla. PGH₂ inhibited basal activity in all three areas of the kidney. There was also inhibition by PGH₂ of medullary ADH and cortical PTH stimulation. PGE₂ stimulated adenylate cyclase in all three areas. PGE₂ had no effect upon PTH stimulation in cortex and was additive with ADH in outer and inner medulla. PGE₂ stimulation was inhibited by arachidonic acid, and this inhibition seemed competitive. Inhibition by both arachidonic acid and PGH₂ was not destructive. Experiments with [1-¹⁴C]arachidonic acid and indomethacin suggest that the inhibition by arachidonic acid was actually mediated by arachidonic acid and not a metabolite. Both PGH₂ and arachidonic acid inhibition was independent of phosphodiesterase. This activation by product, PGE₂, and inhibition by its precursors, arachidonic acid and PGH₂, provide a possible mechanism by which the prostaglandin system could modulate adenylate cyclase responsiveness to hormonal activation.     

Effects of trinucleotides on hormonally responsive adenylate cyclase activity in rat kidney

The effects of trinucleotides on basal and hormonally stimulated adenylate cyclase activity in rat kidney homogenates was evaluated. In the absence of added trinucleotides, parathyroid hormone (PTH) increased cortical adenylate cyclase activity, while antidiuretic hormone (ADH) increased medullary activity; however, prostaglandin (PG)E₂ did not stimulate adenylate cyclase activity in the renal cortex, outer medulla, or inner medulla. However, with exogenous guanosine 5′-triphosphate (GTP) at concentrations as low as 8 × 10^?7M, PGE₂ did activate adenylate cyclase. This effect was seen over a concentration range of 8 × 10^?7 to 8 × 10^?4M PGE₂. In addition, deoxyGTP and cytosine 5′-triphosphate (CTP) were also effective in the cortex, and deoxyGTP was effective in the inner medulla. GTP also augmented PTH stimulation slightly in the cortex, but had no effect on ADH stimulation in medullary tissue. Therefore, a diference in the GTP requirement was observed for prostaglandins compared to polypeptide hormones. When stimulation by PTH or ADH was examined over a wide concentration range, no dependency upon GTP for stimulation was observed. Furthermore, when the ATP concentration was reduced from 2 to 0.2 mM and Mg⁺⁺ concentration was varied from 1 to 5 mM, a dependency of either PTH or ADH stimulation upon GTP was still not observed. Guanylyl 5′-imidodiphosphate (8 μM) elevated both basal and polypeptide hormone activity in all three tissues but did not elevate PGE₂ above basal values for that group. GTP appears to be important in renal regulation of PGE₂-stimulated adenylate cyclase activity.     

Impaired inner medullary production of prostaglandin E2 in the kidney of the myxedematous rat.

Renal cAMP concentration and renal prostaglandin (PG) E2 and PGF2alpha production were determined in control rats and rats with surgically induced myxedema. Slices from the kidneys of each group were incubated in vitro. cAMP concentration of inner medullary slices from the kidneys of myxedematous rats was decreased in comparison to slices from control rats. There was no difference between the two groups when slices from outer medulla and cortex were compared for cAMP level. In myxedema, there was decreased inner medullary production of PGE2, but not PGF2alpha. The difference in PGE2 production between myxedema and control rat kidneys was not corrected by addition of either arachidonic acid (170 micronM) or phospholipase A2 (5 microgram/ml). Arachidonic acid, however, increased inner medullary cAMP concentration in myxedema kidneys to a value not measurably different from control. Thus, there appears to be a decreased PGE2 production in the renal inner medulla of rats with myxedema. In view of the fact that PGE2 has previously been shown to increase inner medullary cAMP content, the reduced inner medullary content of cAMP would appear to be attributable to reduced PGE2 production.     

Shell gland responsiveness to prostaglandins F2α and E1 and to arginine vasotocin during the laying cycle of the domestic hen (Gallus domesticus)

Contractile responses of isolated shell gland (SG) strips from laying hens displayed no significant differences 6 hrs before oviposition, at oviposition, and 6 hrs after oviposition when stimulated with arginine vasotocin (AVT), prostaglandin E₁ (PGE₁), or prostaglandin F_2α (PGF_2α). Dose-response curves show that the sensitivity of the SG to these agents, in vitro, is: AVT > PGF₂ > PGE₁. PGF_2α, however, produces the largest contractile response, while PGE₁ appears to be a poor agonist of contractile activity in vitro. These results are discussed in relation to the known hormonal patterns during the ovulatory cycle of the hen and the physiological roles attributed to these oxytocics in the control of oviposition.     

Modulation of insulin secretion by cyclic AMP and prostaglandin E: the effects of theophylline, sodium salicylate and tolbutamide

Cyclic AMP (cAMP) and prostaglandin E (PGE) are acknowledged to be local modulators of insulin secretion in response to physiologic stimuli. We have attempted to define the mechanisms of action of distinct classes of pharmacologic insulin secretagogues (nonsteroidal anti-inflammatory drugs, methylxanthines and sulfonylureas) by examining their dependence on changes in cAMP and PGE accumulation in monolayer cultures of neonatal rat pancreatic cells. As we have previously observed, sodium salicylate (SS, 20 mg/dl) stimulated insulin secretion in proportion to the ambient glucose concentration. SS potently inhibited PGE synthesis and decreased cAMP accumulation; exogenous PGE₁ reversed the effect on insulin secretion. Similar results were found using ibuprofen, a structurally dissimilar inhibitor of prostaglandin synthesis. Theophylline (1.5 mM) also stimulated insulin secretion in a fashion dependent on glucose concentration. However, in contrast to SS, theophylline (THEO) augmented cAMP accumulation without reducing PGE synthesis. Furthermore, the effects of THEO and SS on insulin release were additive. Exogenous PGE₁ did not reverse the effects of THEO on insulin secretion. The effects of sulfonylureas were markedly different from those of both SS and THEO. Stimulation of insulin release by tolbutamide (TOLB, 2 mg/dl) was accompanied by only minor and inconsistent inhibition of PGE synthesis. The effect of TOLB was not reversed by exogenous PGE₁ and was largely independent of ambient glucose concentration. Additionally, the stimulatory effect of SS on insulin secretion was additive to that of a maximally effective concentration of TOLB. Accumulation of cAMP could also not be implicated in TOLB action. Tolbutamide failed to augment cAMP generation, and its stimulatory action on insulin secretion was additive to that of a maximally stimulatory concentration of THEO. Virtually identical results were observed with a second sulfonylurea, chlorpropamide. We conclude that stimulation of insulin secretion by certain distinct classes of pharmacologic agents may be defined by changes in cAMP synthesis in some cases and in PGE availability in others. In the case of the sulfonylureas, other mechanisms must be sought.     

Induction of IgM and IgM-rheumatoid factor synthesis in vitro by indomethacin

Summary Indomethacin, which is thought to exert its therapeutic effect by inhibiting the synthesis of PGE₂, is a commonly used first-line agent in the treatment of rheumatoid arthritis (RA). However, the effect of this drug on the humoral immune response in RA remains unclear. In this study, modulation of the in vitro synthesis of IgM and IgM-rheumatoid factor (RF) by indomethacin and prostaglandin E₂ was examined in 11 patients with active RA and 10 normal controls. Indomethacin at a final concentration of 1 g/ml significantly enhanced IgM production (P<0.01) and RF production (P<0.02) inStaphylococcus aureus Cowan I (SAC) stimulated RA cultures when compared to controls in whom no net enhancement effect was observed. In the patients, this increase in IgM production was more pronounced than the corresponding increase in RF synthesis (P=0.078), suggesting that IgM and IgM-RF-secreting RA plasma cells have different susceptibilities to PGE₂ mediated suppression. Nonetheless, addition of PGE₂ (10^–8 M final concentration) to the cultures inhibited IgM and RF production to a similar degree in the patient and control cultures. These findings demonstrate that PGE₂ causes suppression of IgM and IgM auto-antibody production in vitro and that inhibition of endogenous PGE₂ synthesis in RA patients treated with indomethacin results in a marked increase in the production of these antibodies.     

Rheumatoid synovial cell morphologic changes induced by a mononuclear cell factor in culture

Adherent rheumatoid synovial cells in culture produce large amounts of prostaglandin E₂ (PGE₂) and collagenase. When exposed to a monocyte-derived factor, such cells exhibit marked increases in PGE₂ and collagenase production. In addition, cellular morphology becomes more stellate. In the presence of this factor, indomethacin inhibits both PGE₂ production and the stellate changes, whereas collagenase production usually continues at a high rate. Addition of PGE₂ to cultures reproduces the stellate change as does the cyclic adenosine monophosphate (cAMP) analog 8-bromo-cAMP. Colchicine inhibits morphologic transformation induced by the monocyte-derived factor, whereas cytochalasin B has no effect. It appears that the stellate morphology is dependent upon PGE₂-induced cAMP stimulation and is not related to collagenase production per se.     

Independent effects of bradykinin on adenosine 3',5'-monophosphate and prostaglandin E2 metabolism by rabbit renal medulla

Bradykinin-stimulated increases in renal prostaglandin (PG) synthesis are thought to result in subsequent increases in cAMP content. This study assesses the relationship between bradykinin-stimulated increases in PGE2 and cAMP syntheses in renal inner medullary slices. Bradykinin-mediated increases in cAMP (2 min) preceded those in PGE2 (5 min) synthesis. Forskolin, an activator of adenylate cyclase, increased cAMP, while 2',5'-dideoxyadenosine, an adenylate cyclase inhibitor, reduced cAMP. However, neither agent altered bradykinin-stimulated PGE2 synthesis. Aspirin decreased basal and abolished bradykinin-stimulated PGE2 production, but did not alter bradykinin-induced increases in cAMP content. Maximal stimulatory concentrations of 1-methyl-3-isobutylxanthine, a cyclic nucleotide phosphodiesterase inhibitor, and bradykinin were additive in their capacity to increase inner medullary cAMP content. These results suggest that 1-methyl-3-isobutylxanthine and bradykinin increase cAMP by separate mechanisms and that bradykinin increases inner medullary cAMP by a direct effect on the production of that cyclic nucleotide. Bradykinin-mediated increases in cAMP and PGE2 syntheses by renal medullary slices are independent effects of this renally acting hormone.     

Calcium and O2-dependent control of inner medullary cGMP: possible role for Ca2+-dependent arachiodonate release and prostaglandin synthesis in expression of the action of osmolality on renal inner medullary guanosine 3'5' monophosphate

The present study examined the effects of osmolality on basal cGMP metabolism and cGMP responses to carbamylcholine in rat inner medulla. The basal cGMP content of inner medullary slices and cGMP responses to carbamylcholine fell as media osmolality was increased from 305 to 1650 mosmole/liter by the addition of urea plus NaCl to standard Krebs bicarbonate buffer. Exclusion of extracellular Ca2+ or addition of tetracaine abolished the effects of both a reduction in osmolality and carbamylcholine to increase cGMP. Readdition of Ca2+ to Ca2+-deprived slices restored the actions of media osmolality and carbamylcholine on cGMP. Ionophore A23187 enhanced the effects of Ca2+ to increase slice cGMP content. Analogous to the effects of Ca2+ alone, increases in cGMP accumulation in response to Ca2+ plus A23187 were significantly suppressed at high osmolality. In slices prelabeled with [14C]-arachidonate, the stimulatory effects of Ca2+ plus A23187 on cGMP were correlated with enhanced release of [14C]-arachidonate into the media and with increased accumulation of prostaglandin E in the media, all of which were depressed in slices incubated at 1650 compared to 750 mosmole/liter. Exogenous arachidonate increased cGMP 50% to twofold in the absence of Ca2+. However, the effects of exogenous arachidonate on cGMP were clearly less than those of either Ca2+ or carbamylcholine. Addition of indomethacin or exclusion of O2 abolished effects of exogenous arachidonate, Ca2+ plus A23187, reduced osmolality, and carbamylcholine to increase cGMP. In the presence of indomethacin or in the absence of O2, Ca2+, and arachidonate-induced prostaglandin E accumulation was also not detectable. By contrast, addition of indomethacin or exclusion of O2 had no effect on Ca2+-induced [14C]-arachidonate release. Changes in cGMP accumulation in inner medulla in response to changes in media osmolality, Ca2+ plus A23187, and exogenous arachidonate were accompanied by directionally similar alterations in cAMP. Moreover, increases in cAMP induced by Ca2+ or arachidonate were inhibited by indomethacin and O2 deprivation. These data suggest that Ca2+-dependent fatty acid release and oxygenation by fatty acid release and oxygenation by cyclooxygenase may participate in the control of the metabolism of both cGMP and cAMP in inner medulla. However, PGE2 and PGI2 increased cAMP but not cGMP, whereas carbamylcholine increased cGMP but not cAMP. Thus, to the extent that arachidonate oxygenation products mediate changes in cyclic nucleotide metabolism in inner medulla, it is likely that more than one product is involved or that changes in cGMP and cAMP occur in separate compartments of inner medulla.     

Microcirculatory effects of arachidonic acid and a prostaglandin endoperoxide (PGH2)

We have examined the vascular effects of arachidonic acid and a prostaglandin endoperoxide, precursors of prostaglandins (PG), in the rat cremaster (skeletal muscle) microcirculation. Five-week-old male, Wistar-strain rats were anesthetized with pentobarbital (30 mg/kg) and the cremaster muscle was prepared for direct in vivo observation and quantitation of changes in vascular diameters in response to the topical application of arachidonic acid (AA), PGH₂, or PGE₂. All three agents elicited dose-dependent arteriolar dilator responses. However, threshold doses were different; for AA they were between 0.5 × 10^?7 and 0.5 × 10^?6M, for PGH₂ between 0.5 × 10^?8 and 0.5 × 10^?7M, and for PGE₂ between 0.5 × 10^?9 and 0.5 × 10^?8M. At the high dose studied (0.5 × 10^?3M) both AA and PGH₂ required approximately 60 sec for the development of a maximum dilator response, whereas PGE₂ required only about 30 sec. Indomethacin, an inhibitor of prostaglandin synthesis, did not significantly alter control arteriolar diameters but inhibited completely the responses to AA. We conclude that the rat cremaster skeletal muscle microcirculatory compartment does possess the necessary enzymatic machinery for the conversion of AA and PGH₂ into vasodilator prostaglandin metabolites.     

The role of cyclic AMP in the induction of estrogen and progestin synthesis in cultured granulosa cells

The role of cyclic AMP in the induction of enzymes involved in estrogen and progestin biosynthesis in undifferentiated granulosa cells was investigated. When granulosa cells from immature hypophysectomized, DES-treated rats were cultured for 2 days in serum-free medium with aromatase substrate (10^?7M androstenedione) together with graded doses of FSH, prostaglandin E₂ (PGE₂), cholera toxin (CT), or dibutyryl cyclic AMP (Bu₂cAMP), there was a dose-related increase in estrogen (E) production. The induction of E production by saturating doses of FSH, PGE₂, CT and Bu₂cAMP required a lag phase of ～24 h, after which the E response increased sharply to maximum levels at day 3 and then declined gradually to day 5. Treatment for 24 h (day 0–1) with FSH, together with 1 μg/ml of either actinomycin D or cycloheximide, completely abolished the stimulatory action of FSH on E production. When the inhibitors were removed, the FSH-induced increases in E returned to near normal levels after a 24-h lag period. Similar effects of the inhibitors upon E production by CT, PGE₂ and Bu₂cAMP were observed. As with E, the production of progesterone and 20 α-dihydroprogesterone was markedly stimulated by FSH, PGE₂, CT and Bu₂cAMP and the results of the time course, dose response and inhibitor experiments were similar to those for E production.These results indicate that FSH induces the de novo synthesis of enzymes required for both estrogen and progestin biosynthesis by undifferentiated granulosa cells and suggest that this action is mediated by cyclic AMP.     

Effect of acute and chronic alcohol feeding on prostaglandin E2 biosynthesis in rat stomach

The effect of acute and chronic alcohol ingestion on gastric prostaglandin E₂ synthesis and the PGE₂ content in the stomach was studied in rats. Up to 8 hr following a single oral load of 20% alcohol (v/v; 4 g/kg body weight), the PGE₂ synthesis in isolated microsomes from rat stomach remained unchanged as compared with control values. Feeding a liquid alcohol-containing diet (37% of total Joules) for 1,6, or 12 weeks significantly decreased the rate of PGE₂ synthesis (percentage inhibition as compared with control values 39, 27, and 57, respectively). In addition, chronic alcohol feeding led to a drop in the tissue content of PGE₂, the decrease being more pronounced after 6 (–49%) and 12 (–58%) weeks than after 1 week (–24%). The results suggest that the inhibition of endogenous PGE₂ synthesis in the stomach following ingestion of appreciable quantities of alcohol might play a role in the pathogenesis of alcohol-induced injury of the gastric mucosa.     

Tumour necrosis factor alpha-induced oxidative burst in neutrophils adherent to fibronectin: effects of cyclic AMP-elevating agents

Summary. Human neutrophils, plated on flbronectin-coated polystyrene wells, were found to exhibit a prolonged production of superoxide anion (O^?₂) in response to tumour necrosis factor-alpha (TNF). The TNF-triggered O^?₂ production was significantly reduced by 10μM prostaglandin E₂ (PGE₂), which was ineffective at lower doses. Moreover, the O^?₂ production was slightly reduced by the phosphodiesterase type IV (PDEIV) inhibitor RO 20-1724. When PGE₂ and RO 20-1724 were added together to TNF-triggered neutrophils they caused a marked synergistic inhibition of O^?₂ production. The action of PGE₂ could be mimicked by forskolin (FK), a well-known direct activator of adenylate cyclase. These results suggest that cyclic AMP (cAMP)-elevating agents (PGE₂, FK, RO 20-1724) down-regulate the capacity of adherent neutrophils to mount the respiratory burst in response to TNF. Consistent with this interpretation, PGE₂ and RO 20-1724 increased the intracellular levels of cAMP displaying synergistic activity. Moreover, the membrane-permeable analogue of cAMP, dibutyryl cAMP, was found to inhibit the TNF-induced O^?₂ production in a dose-dependent manner. As all the aforementioned cAMP-elevating agents did not affect the O^?₂ production in response to phorbol myristate acetate, they appear to act by interfering with the assembly of the O^?₂-generating NADPH oxidase complex rather than by directly inhibiting the activity of already working oxidase complex. In conclusion, taking into account the TNF capacity to promote PGE₂ formation at sites of inflammation, our observations suggest the existence of a negative PGE₂-dependent feed-back, potentially capable of controlling the neutrophil response to TNF and susceptible to amplification by PDE IV-inhibiting compounds.     

Neuroprotection by selective allosteric potentiators of the EP2 prostaglandin receptor

Activation of the Gαs-coupled EP2 receptor for prostaglandin E2 (PGE₂) promotes cell survival in several models of tissue damage. To advance understanding of EP2 functions, we designed experiments to develop allosteric potentiators of this key prostaglandin receptor. Screens of 292,000 compounds identified 93 that at 20 μM (i) potentiated the cAMP response to a low concentration of PGE₂ by > 50%; (ii) had no effect on EP4 or β2 adrenergic receptors, the cAMP assay itself, or the parent cell line; and (iii) increased the potency of PGE₂ on EP2 receptors at least 3-fold. In aqueous solution, the active compounds are largely present as nanoparticles that appear to serve as active reservoirs for bioactive monomer. From 94 compounds synthesized or purchased, based on the modification of one hit compound, the most active increased the potency of PGE₂ on EP2 receptors 4- to 5-fold at 10 to 20 μM and showed substantial neuroprotection in an excitotoxicity model. These small molecules represent previously undescribed allosteric modulators of a PGE₂ receptor. Our results strongly reinforce the notion that activation of EP2 receptors by endogenous PGE₂ released in a cell-injury setting is neuroprotective.     

The mode of action of carp gonadotropin on the stimulation of androgen production by carp testis in vitro

Carp gonadotropin (cGTH) can stimulate carp testis to produce androgen in vitro. This action can be mimicked by dibutyryl cAMP (Bt₂ cAMP) and prostaglandins (PG). Bt₂ cAMP can enhance the androgen production when submaximal doses of cGTH are used but cannot do so when maximal dose of cGTH is used. PG F_2α is more effective than PG E₁ and E₂. Both the inhibitors of RNA and protein synthesis can interfere the stimulatory effect of cGTH. Actinomycin D has 87% while cycloheximide has 100% inhibition. The stimulatory effect of cGTH is much more dependent on de novo protein synthesis than on RNA synthesis. In addition, steroidogenesis inhibitors, glutethemide, metyrapone, and spirocyanoketone, can also inhibit the stimulatory effect of cGTH on androgen production.     

Neuronal prostaglandin E2 receptor subtype EP3 mediates antinociception during inflammation

The pain mediator prostaglandin E₂ (PGE₂) sensitizes nociceptive pathways through EP2 and EP4 receptors, which are coupled to G_s proteins and increase cAMP. However, PGE₂ also activates EP3 receptors, and the major signaling pathway of the EP3 receptor splice variants uses inhibition of cAMP synthesis via G_i proteins. This opposite effect raises the intriguing question of whether the G_i-protein–coupled EP3 receptor may counteract the EP2 and EP4 receptor-mediated pronociceptive effects of PGE₂. We found extensive localization of the EP3 receptor in primary sensory neurons and the spinal cord. The selective activation of the EP3 receptor at these sites did not sensitize nociceptive neurons in healthy animals. In contrast, it produced profound analgesia and reduced responses of peripheral and spinal nociceptive neurons to noxious stimuli but only when the joint was inflamed. In isolated dorsal root ganglion neurons, EP3 receptor activation counteracted the sensitizing effect of PGE₂, and stimulation of excitatory EP receptors promoted the expression of membrane-associated inhibitory EP3 receptor. We propose, therefore, that the EP3 receptor provides endogenous pain control and that selective activation of EP3 receptors may be a unique approach to reverse inflammatory pain. Importantly, we identified the EP3 receptor in the joint nerves of patients with painful osteoarthritis.     

A role for endogenous prostaglandins in defective glucose potentiation of nonglucose insulin secretagogues in diabetics

Noninsulin dependent diabetics have insulin responses to nonglucose secretagogues that are subnormal for their plasma glucose levels. Since endogenous prostaglandins have been implicated in the abnormal insulin responses to glucose in diabetics, the present study was performed to explore whether prostaglandins might also play a role in the defective insulin responses to nonglucose stimuli. We examined the effects of infusions of either prostaglandin E₂ (PGE₂) or sodium salicylate (SS), a PG synthesis inhibitor, on the acute insulin responses (AIR's) to arginine and isoproterenol and on the glucose potentiation of the insulin response to arginine in both normal and diabetic subjects. The AIR to arginine was augmented by SS in diabetics (SS = 61 ± 12 μU/ml, control = 37 ± 5 μU/ml, n = 11, p < .01). SS, however, had no effect on the AIR to arginine in normal subjects (SS = 39 ± 4 μU/ml, control = 34 ± 4 μU/ml, n = 6, p = ns). Similarly, SS augmented the AIR to an isoproterenol pulse in diabetics (SS = 38 ± 9 μU/ml, control = 18 ± 3, n = 9, p < .05) but not in normal subjects (SS = 19 ± 4 μU/ml, control = 21 ± 4 μU/ml, n = 8, p = ns), suggesting a SS-sensitive defect in the insulin response to these nonglucose stimuli in diabetics. Conversely, PGE₂ inhibited the AIR to arginine in diabetics (PGE = 28 ± 5 μU/ml, control = 39 ± 7 μU/ml, n = 7, p < .05), but not in normal subjects (PGE = 74 ± 7 μU/ml, control = 80 ± 14 μU/ml, n = 5, p = ns). The effect of SS on glucose potentiation of the AIR to arginine was studied by measuring the AIR to arginine at two different levels of plasma glucose, one before and one after an insulin infusion, with glucose potentiation defined as the ratio ΔAIR/Δprestimulus glucose. Glucose potentiation was significantly less in diabetics than in normals and SS significantly improved glucose potentiation toward normal values in diabetics but did not change glucose potentiation in normals. These findings suggest that endogenous PG's may play a role in the defective glucose potentiation of the AIR to nonglucose secretagogues in diabetics resulting in impaired insulin responses to these stimuli. This defect is partially reversible by an inhibitor of PG synthesis.     

Prostaglandins of the one,two, and three series affect blood pressure in the American bullfrog, Rana catesbeiana

The effects of prostagandins (PGs), and three potential prostaglandin precursors, were studied on blood pressure and heart rate of the American bullfrog, Rana catesbeiana. Bullfrogs were chronically cannulated with a T cannula in the right sciatic artery. The mean systemic arterial blood pressure (SAP) prior to infusion was 20.4 ± 1.1 mm Hg. Mean preinfusion systolic and diastolic pressures were 23.9 ± 1.4 and 17.1 ± 0.9 mm Hg, respectively. Mean preinfusion heart rate was 41.1 ± 0.5 beats/min. Of the PGs tested, PGI₂ was a potent hypotensive agent, with effects at 0.03 μg/kg bw. PGE₂ was more potent than PGE₃, and PGE₁. PGA₁ and PGA₂ were the least potent, and were ineffective at doses below 100 μg/kg bw. PGF_2α was the most potent hypertensive agent tested, with thromboxane B₂ less potent. All compounds tested elevated heart rate, with PGE₂ the most effective. The prostaglandin precursors, eicosatrienoic acid, arachidonic acid, and eicosapentaenoic acid (2000 μg/kg bw) all decreased blood pressure by approximately 25%. The decrease was attenuated by indomethacin (4 mg/kg bw). These results indicate that the bullfrog utilizes all three hypotensive activity. The ability of the bullfrog to utilize several substrates makes it a good choice for comparative studies on prostaglandin synthesis.     

Prostacyclin analogs stimulate receptor-mediated cAMP synthesis and ATP release from rabbit and human erythrocytes

Objectives: The purpose of this study was to establish that the prostacyclin (PGI₂) receptor (IP receptor) is present on rabbit and human erythrocytes and that its activation stimulates cyclic adenosine monophosphate (cAMP) synthesis and adenosine triphosphate (ATP) release. Methods: The effect of incubation of erythrocytes with the active PGI₂ analogs, iloprost or UT‐15C, on cAMP levels and ATP release was determined in the absence and presence of the IP receptor antagonist, CAY10441. Western analysis was used to determine the presence of the IP receptor on isolated membranes. To establish that effects of PGI₂ analogs were not due to prostaglandin E₂(PGE₂) receptor activation, the effect of PGE₂ on cAMP levels and ATP release was determined. Results: Rabbit and human erythrocytes possess IP receptors. Iloprost and UT‐15C stimulated increases in cAMP and ATP release that were prevented by the IP receptor antagonist, CAY10441. PGE₂ did not stimulate cAMP accumulation or ATP release and did not inhibit iloprost‐induced increases in cAMP. Conclusions: This study establishes that the IP receptor is present on rabbit and human erythrocytes and that its activation results in increases in cAMP and ATP release. These results suggest a novel mechanism by which PGI₂ and its active analogs, when administered pharmacologically, could produce vasodilation.     

Renal prostaglandin E2 biosynthesis: Dependence on angiotensin II

To study the interaction of the prohypertensive renin-angiotensin axis with the antihypertensive renal prostaglandins, angiotensin-converting enzyme inhibition (captopril and teprotide) and angiotensin blockade (Sar¹-lle⁸-All) were produced in rabbits in vivo and renal slice prostaglandin E₂ was measured in vitro after 30 minutes of incubation in Krebs-Ringer HCO^?₃ buffer. In rabbits with angiotensin-converting enzyme inhibition, de novo prostaglandin E₂ synthesis decreased in cortical, outer medullary and papillary slices by 85, 52 and 47 percent, respectively. Similar degrees of inhibition were observed with angiotensin blockade. It is concluded that renal prostaglandin E₂ synthesis is angiotensin II dependent under these conditions. This finding suggests that any increase in the prohypertensive antinatriuretic renin-angiotensin axis may be associated with a secondary increase in renal prostaglandin E₂ which may be acting in an antihypertensive natriuretic compensatory fashion.