Effect of prostaglandins on hepatic cyclic nucleotide concentration, carbohydrate and lipid metabolism.

The effects of exogenous prostaglandin E1 (PGE1) or prostaglandin E2 (PGE2) were studied in the isolated perfused rat liver and in the intact canine liver in order to determine the possible physiological role of prostaglandins on hepatic carbohydrate and lipid metabolism. The data indicate that PGE1 and PGE2 did not stimulate cyclic AMP (cAMP) and cyclic GMP (cGMP) concentrations in intact dog liver and PGE1 failed to stimulate cAMP or cGMP in fed or fasted perfused rat liver. PGE1 did not promote hyperglycemia, glycogenolysis, lipolysis, or prevent epinephrine-induced hyperglycemia in the isolated perfused rat liver. Other known glycogenolytic agents including glucagon and epinephrine increased cAMP and glycogenolysis in the same perfusion system. This study does not support a physiologic role for PGE1 on hepatic glycogenolysis or lipolysis. If PGE1 subsequently is found to influence other metabolic parameters such as lipogenesis, gluconeogenesis, ureogenesis or amino acid transport in isolated perfused liver, such alterations would probably occur independent of changes in cyclic nucleotide activity.


INTRODUCTION
Our prior studies in isolated perfused rat liver demonstrated the effect of cyclic AMP (cAMP), glucagon, epinephrine and serotonin or its endogenous precursor, 5hydroxytryptophan, on glycogen metabolism [ 1,2]. These agents produced glycogenolysis, hyperglycemia, and stimulation of hepatic phosphorylase activity in isolated perfused rat liver [1,2]. The glycemic effect of serotonin observed in vitro was also noted in vivo. Serotonin may produce these physiological actions by activation of hepatic cAMP concentrations [1].
There is increasing evidence to show that E-type prostaglandins regulate intracellular cAMP levels but their complex interrelationships have not been clarified. It remains unclear whether the prostaglandin effect is related solely to adenylate cyclase activity or alteration in phosphodiesterase, adenylate kinase [3], and ATPase.
Although prostaglandins are rapidly metabolized by the liver, there is conflicting data regarding their specific actions on hepatic carbohydrate and lipid metabolism or on the adenylate cyclase-cAMP system in vitro or in vivo. In vitro experiments in adipose tissue demonstrate that prostaglandin El (PGE1) stimulates glucose uptake, glucose oxidation, and glycogen and triglyceride synthesis from labeled glucose [4]. rabbit, and dog by PGE1 in the presence of purine nucleotides, including guanosine triphosphate [5,6]. PGE, alone also increased adenylate cyclase activity in liver homogenates and in isolated rat liver cells [7][8][9][10][11]. Curnow and Nuttall [12] observed that PGE, decreased hepatic glycogen synthetase without altering glycogen concentrations. This change is compatible with increased hepatic cAMP and suggested possible direct effects on hepatic glycogen levels. Nevertheless, the actions of prostaglandins on glucose metabolism are variable. PGE, has been reported to cause hyperglycemia [13][14][15] or to inhibit glucose metabolism in liver [16].
Prior studies have not clarified the possible in situ role of PGEI on hepatic glycogenolysis. Exton et al. [17] concluded that PGE, perfusion in rat liver failed to alter cAMP levels or glucose production but detailed results were never presented. Others have reported a glycogenolytic effect of PGE, in rat liver slices [18]. In perfused rat liver a variable effect was noted in fed and fasted animals by Wilson and Handewych [19]. Only in the fasted state could they observe PGE I -induced glycogenolysis. Zenser et al. have shown transient modest increases in cAMP in perfused rat liver 1 to 3 min after PGE, administration [10]. There is only a single published report concerning the effects of PGE, on gluconeogenesis in perfused rat liver [20].
This study showed that PGE, inhibited gluconeogenesis, possibly between the glycerol and glucose steps [20].
Since notable species differences have been found in the hepatic adenylate cyclase-cAMP response to prostaglandins, we have investigated the possible interaction of prostaglandins on hepatic cyclic nucleotide concentration in both rats and dogs.
There are no published data on the hepatic cyclic GMP (cGMP) concentration after treatment with prostaglandins. Thus we have studied the response of this nucleotide to 2 types of E prostaglandins, PGE, and prostaglandin E2 (PGE2). We also have evaluated the effects of PGE, on hepatic glycogenolysis, and lipolysis. These studies extend out preliminary report [21]. METHODS The effects of PGE, or PGE2 were studied in the isolated perfused rat liver (3.3 ,ug/ min) and in the intact canine liver (25,g/kg bolus then 20u jg/ min). Because PGE I and PGE2 are rapidly metabolized by the liver and most of it removed during a single passage, they were administered continuously [22].
The effects of PGE, in isolated perfused rat liver bn both liver and perfusate cAMP content were compared to that induced by dibutyryl cAMP (DBC), glucagon, epinephrine, and a control infusion of a Ringer solution containing an appropriate amount of ethanol added to the perfusion. Ethanol was used as a control since ethanol is used as a diluent of PGEI, since extremely small amounts of ethanol can influence carbohydrate and lipid metabolism in the perfused liver, and since certain short chain alcohols, including ethanol, have been shown to activate adenylate cyclase in vitro [23,24].
Isolated male fed or fasted donor Sprague-Dawley rat livers (mean body weight 297 ± (S.E.M.) 25 g, mean liver weight = 6.4 ± 1.8 g) were perfused with 85 ml Krebs-Ringer bicarbonate solution containing 3% albumin and 0.95% ethanol, as previously described [25]. Samples of perfusate glucose, cAMP, cGMP and free fatty acids (FFA), hepatic glycogen, triglyceride (TG), and cAMP, and bile were taken during a control period of 45 min and at 2, 3, 5, 10, 15, 30, or 60 min intervals and after exposure to PGEI, epinephrine, glucagon, or DBC (see Table 1   used). Since PGE, may influence some of the metabolic effects of catacholamines in certain other tissues, the possible interaction between epinephrine and PGE1 was also studied in the perfusion experiments. Studies in the 18 hour-fasted intact canine liver were carried out using direct intraarterial infusion of PGE,, PGE2, or indomethacin, a prostaglandin synthetase inhibitor (5 mg/kg for 5 min then 5 mg/ min), into the hepatic artery via a catheter placed in the celiac axis. All studies were done in the acute dog anesthetized with intravenous sodium pentobarbital, 30 mg/kg. Infusions were given after a 30 min control period with saline or indomethacin. Approximately 25% of the infused radiopaque agent went directly into the hepatic artery, the remainder into the arterial blood supply to the stomach. Samples of dog liver were obtained for cAMP and cGMP at frequent intervals during a 30 min exposure to exogenous PGE, or PGE2 or indomethacin.
cAMP and cGMP were measured by radioimmunoassay [26]. Other biochemical determinations were performed as described [25] or by standard techniques. All results are given as mean ± S.E.M. and were compared using the Student's t test for mean differences of paired samples.

RESULTS
During the 45 min period of equilibration there was a moderate uptake of fat as determined by, a fall in perfusate FFA. Heparinization during the removal of fed donor livers accounted for initially higher levels of FFA. However, during the study period, PGE1 failed to alter FFA or cAMP levels in the perfusate, bile flow or induce hyperglycemia compared to control perfusions (Fig. 1).
That this system responded to known glycogenolytic agents which increase hepatic cAMP content is shown in Fig. 2. Epinephrine and glucagon increased hepatic cAMP content an average of 36% and 770%, respectively, with maximal responses 15 min after their administration. PGE1 did not significantly influence hepatic cAMP levels compared to control infusions. DBC caused extraordinarily high cAMP levels in the few livers analyzed, as well as in the perfusate and bile. Figure 3 illustrates the failure of PGE 1 to significantly alter hepatic TG concentration after 60 min infusion. These studies were carried out in only 6 of the 9 experiments, respectively, in the control and PGE1-treated groups. Table 1 summarizes the increment changes observed in perfusate glucose and cAMP and hepatic glycogen, TG and cAMP content during the 60 min period of PGE (9) Table 1. From Levine [21]. infusion. Epinephrine, glucagon, and DBC caused significant glycemia. Glycogen content in the liver decreased after 60 min of perfusion in control livers. The decrease was significantly greater when epinephrine, glucagon, or DBC was added to the perfusate, but unchanged by PGE,.
Epinephrine-induced glycogenolysis and augmentation of hepatic cAMP were not affected by pretreatment or concomitant administration of PGE1, as shown in a representative example of three such studies (Fig. 4).
The effects of PGE, or Ringer solution on certain biochemical parameters in the fasted perfused rat liver are shown in Fig. 5. PGE, did not significantly alter perfusate glucose or cAMP, or hepatic cAMP, glycogen, or TG as shown in Fig. 5. Note that the perfusate cAMP concentration appeared to be greater in the fasted than in the fed perfused liver (Fig. 1).
The effects of PGE, or Ringer solution on hepatic cAMP and cGMP concentrations in fasted rat liver, as early as 2,3,5,10,15 and 30 min after the onset of perfusion, are shown in Fig. 6. There was no significant difference in the response of the hepatic cyclic nucleotides to either PGE, or Ringer solution. Moreover, hepatic glycogen levels did not change significantly before and 30 min after perfusion. Mean control liver glycogen for Ringer solution and PGEI were, respectively, 0.199 ± 0.62 and 0.170 i 0.039 mg/g before and 0.190 ± 0.073 and 0.142 ± 0.030 mg/g 30 min after infusion. Thus, in the fasted rat liver PGEI failed to significantly alter cyclic nucleotide, TG, and glycogen content.
The effects of both PGE, and PGE2 on canine hepatic cyclic nucleotide concentrations are shown in Fig. 7 [21].   The effects of either PGE, or PGE2 on canine hepatic cyclic AMP concentration are shown in Table 2. Neither PGE1 or PGE2 showed a differential effect on hepatic cAMP concentration during the 30 min perfusion. The effects of PGE2 on cGMP concentration in 12 fasted dog livers are shown in Table 3. There were no significant differences between the treatment periods compared to the basal period (zero time value). Moreover, PGE2 administered in conjunction with indomethacin failed to potentiate the action of PGE2 on liver cAMP ( Table 2) or cGMP (Table 3) content. Pretreatment with indomethacin for 30 min before the zero time showed values essentially similar to the basal level (789 ± 62 pmol/g and 2.4 ± 0.5 pmol/ g, respectively, for cAMP and cGMP).

DISCUSSION
These studies indicate that PGE1 does not stimulate the rat liver adenylate cyclase-cAMP system even though the same system responds to known agents which activate cAMP (glucagon and epinephrine) and responds to exogenous DBC. Moreover, cGMP is not stimulated by PGE, in the same perfusion model. In another species, the dog, PGE, and PGE2 failed to activate hepatic cAMP or cGMP. Thus, it appears that hepatic cyclic nucleotides fail to respond to intraportal infusion of PGEI and PGE2.
Our findings fail to confirm the report of Zenser et al. [10] who did note transient stimulation of rat liver cAMP after PGE1 administration. The apparently conflicting observations may be reconciled in part by differences between their assay techniques, which may account for spurious cAMP values, and our own. First, cross reactivity and nonspecific reactivity producing an artificial elevation of assayable cAMP is less common with the use of radioimmunoassay compared to. the protein binding technique [26], as was utilized by Zenser et al. [10]. Second, pigments, including hemoglobin, can be another source of falsely elevated assayable cyclic nucleotides. Blood pigments are acid extractable during purification procedures. Unless great effort is made to avoid any tissue contamination by blood prior to processing, such pigments may significantly alter cAMP values. The use of a neutral alumina column in sample preparation effectively removes hemoglobin from contaminated liver samples [26]. Zenser et al. [10] failed to employ such an alumina precolumn.
The possibility exists that continuous infusion of PGE, or PGE2 did not increase intracellular prostaglandin levels. However, in perfused rat liver PGEI (6.7u pg/ min) increased prostaglandin concentrations in portal vein, perfusate, and liver as measured by radioimmunoassay (D.E. Wilson, personal communication). The initial perfusate concentration was 4.9 x 10-9M before PGEl infusion, 1.9 x 10-7M during infusion, and 6.2 x 10-8M 30 min after discontinuing PGE, administration. Thus PGE1 infusion, which resulted in a portal vein concentration of 6.4 x 10-7M during prostaglandin infusion, markedly increased perfusate concentrations. PGE concentration in liver rose from 20.9 ± 0.8 to 1091 ± 89 pg/ mg tissue and decreased to 362 ± 30 pg/mg 30 min after discontinuing the infusion. From this data, it appears that prostaglandin infusion in rat liver is capable of increasing hepatic concentrations of prostaglandins even though approximately 70% of the infused PGE, is metabolized during a single passage through the liver (D.E. Wilson, personal communication).
The interaction of PGE, with catecholamines in liver was of particular interest since prostaglandins antagonize cAMP-induced lipolysis in fat cells caused by epinephrine [27] and inhibit hormonally-induced responses in several other tissues [28,29]. Although PGE, was reported not to affect glucagon-stimulated increases in hepatic cAMP in vitro, when injected intraportally PGE1 inhibited glucagon stimulation of cAMP and stimulated cAMP itself [1 1]. In contrast, our studies would support the concept that PGE1 fails to alter the effects of another hepatic glycogenolytic agent, epinephrine, in rat liver.
Our data do not support a physiologic role for PGE, on hepatic glycogenolysis or lipolysis. Preliminary studies in our laboratory, using lactate as a substrate for gluconeogenesis in the isolated perfused rat liver, have shown that PGE1 failed to induce a greater gluconeogenic effect compared to control infusion with Ringer solution. Glucagon, added at the same time as lactate, induced a further increased glucose level than either a control or PGE, infusion. However, it was of interest that when glucagon was given simultaneously with PGE,, there appeared to be inhibition of the glucagon-induced hepatic gluconeogenesis. This particular observation would support the findings of Imesch and Rous [20] and indicate that PGE, may have inhibitory effects on hormone-mediated hepatic gluconeogenesis.
Other preliminary observations in our laboratory concerning the effects of PGE1 on liver glycogen synthetase indicate that PGE1 and PGE2 failed to alter glycogen synthetase in either fed or fasted perfused rat or fasted intact dog liver. These findings contrast with the report of Curnow and Nuttall [12], who showed a decrease in glycogen synthetase in the fed and fasted intact rat liver 5 and 30 min, respectively, after PGE, administration.
Although we have not yet investigated the effects of PGE, on lipogenesis, ureogenesis, or amino acid transport, if effects are subsequently found on these metabolic parameters they probably would occur independent of a change in hepatic cAMP or cGMP activity. It should be pointed out that our conclusions are limited to the perfused rat liver and intact dog liver. Further studies are indicated before one can conclude that other types of prostaglandins fail to stimulate the hepatic cyclic nucleotides in other species.