Effects of reduced ATP concent on hepatic responses to glucagon

Characterization of hepatic hemodynamics in cirrhotics and non-cirrhotics. Effect of glucagon infusion

The effect of glucagon on hepatic regional hemodynamics was investigated in patients with chronic liver disease during peritoneoscopy with reflectance spectrophotometry. When glucagon was infused intravenously in patients with a non-cirrhotic liver, the regional hepatic tissue oxygen consumption, as estimated spectrophotometrically, increased significantly, whereas the index of hepatic tissue blood volume did not change appreciably, and consequently, the oxygen saturation of hemoglobin in the hepatic tissue blood decreased. In contrast, the administration of glucagon in patients with liver cirrhosis resulted in a significant increase in the index of hepatic tissue blood volume and produced a minor increase in hepatic tissue oxygen consumption. The oxygen saturation of hepatic blood hemoglobin tended to increase in the cirrhotics. The result suggests the presence of functional vasoconstriction at the presinusoidal and/or sinusoidal vessels in the cirrhotic liver, possibly due to a decreased vasomotor activity and/or an abnormal regulatory function of vasoactive substances, which are released by glucagon.

Hepatic adenylate cyclase and phosphodiesterase activity during acute ethionine intoxication

Four hours after the administration of ethionine and in the face of an 80-90% depletion of hepatic ATP there is a twofold increase in cAMP. This increase in cAMP is shown not to be effected by a reduction in phosphodiesterase activity. Both the low Km and high Km forms of the enzyme retain their activity. The purification of liver plasma membranes is not affected by ethionine. There is a significant 20% increase in plasma membrane adenylate cyclase activity following acute ethionine intoxication. This was demonstrable using either ATP or 5'-adenylyl-imidodiphosphate as substrates. The plasma membranes, isolated in the presence of Ca2+, have a 20-fold higher basal adenylate cyclase activity then previously reported and are not further stimulated by GTP or NaF. A modified protocol for isolating cAMP from the adenylate cyclase reaction is described.

Hyperglucagonemia and altered responsiveness of hepatic adenylate cyclase-adenosine 3',5'-monophosphate system to hormonal stimulation during chronic ingestion of DL-ethionine

Basal activity and hormonal responsiveness of the adenylate cyclase-adenosine 3',5'-monophosphate system were examined in premalignant liver from rats chronically fed the hepatic carcinogen DL-ethionine, and these data were correlated with endogenous levels of plasma glucagon. By 2 weeks basal hepatic cyclic AMP levels, determined in tissues quick-frozen in situ, were 2-fold higher in rats ingesting ethionine than in the pair-fed control. Enhanced tissue cyclic AM content was associated with an increase in the adenylate cyclase activity of whole homogenates of fresh liver from rats fed ethionine (68 +/- 5 pmol cyclic AMP/10 min per mg protein) compared to control (48 +/- 4). Cyclic AMP-dependent protein kinase activity ratios were also significantly higher (control, 0.38 +/- 0.04; ethionine 0.55 +/- 0.05) and the percent glycogen synthetase activity in the glucose 6-phosphate-independent form was markedly reduced (control, 52 +/- 7%; ethionine, 15 +/- 1.5%) in the livers of ethionine-fed rats compared to the controls, suggesting that the high total hepatic cyclic AMP which accompanied ethionine ingestion was bilogically effective. These changes persisted throughout the 38 weeks of drug ingestion. Immunoreactive glucagon levels, determined in portal venous plasma, were 8-fold higher than control after 2 weeks of the ethionine diet (control, 185 +/- 24 pg/ml; ethionine, 1532 +/- 195). Analogous to the changes in hepatic parameters, plasma glucagon levels remained elevated during the entire period of drug ingestion until the development of hepatomas. The hepatic cyclic AMP response to a maximal stimulatory dose of injected glucagon was blunted in vivo in ethionine-fed rats (control, 14 -fold increase over basal, to 8.63 +/- 1.1 pmol/mg wet weight; ethionine, 4.6-fold rise over basal, to 5.42 +/- 0.9). Reduced cyclic AMP responses to both maximal and submaximal glucagon stimulation were also evident in vitro in hepatic slices prepared from rats fed the drug, and the reduction was specific to glucagon. Absolute or relative hepatic cyclic AMP responses to maximally effective concentrations of protaglandin E1 or isoproterenol in hepatic slices from ethionine-fed rats were greater than or equal to those observed in control slices. Parallel alterations in hormonal responsiveness were observed in adenylate cyclase activity of whole homogenates of these livers, implying that the changes in cyclic AMP accumulation following hormone stimulation were related to an alteration in cyclic AMP generation in the premalignant tissue. In view of the recognized hepatic actions of glucagon and the desensitization of adenylate cyclase which can occur during sustained stimulation of the liver with this hormone, the endogenous hyperglucagonemia that accompanies ethionine ingestion could play a role in the pathogenesis of both the basal alterations in hepatic cyclic AMP metabolism and the reduced responsiveness to glucagon observed in liver from rats fed this carcinogen.

Sequential alterations in the hepatic content and metabolism of cyclic AMP and cyclic GMP induced by DL-ethionine: evidence for malignant transformation of liver with a sustained increase in cyclic AMP

There is evidence than adenosine 3',5'-monophosphate (cAMP) and guanosine 3',5'-monophosphate (cGMP) may have antagonistic actions on cell growth, with cAMP inhibiting and cGMP stimulating this process. However, reductions in cAMP and increases in cGMP are not charactersitic of all neoplastic tissues. Thus, benign and malignant tissues from hepatoma-bearing rats exposed to the hepatic carcinogen DL-ethionine have elevated rather than depressed cAMP, compared to control liver, and parenteral administration of this drug increases hepatic cAMP within hours. In the present study, the effects of ethionine ingestion on the hepatic content and metabolism of both cAMP and cGMP were examined sequentially in rats at 2 and then 6 wk intervals, from the initiation of drug administration until the development of hepatomas. After 2 wk, cAMP content of quick-frozen liver from rats receiving ethionine (E) was significantly increased (826 +/- 91 pmole/g wet weight) above that of liver from pair-fed controls (C, 415 +/- 44), whether calculated by tissue wet weight, protein, or DNA content. In benign tissue from E, higher cAMP was still evident after in vitro incubations of slices with 2 mM 1-methyl-3-iso-butylxanthine (MIX) and was associated with enhanced adenylate cyclase and unchanged high or low Km cAMP-phosphodiesterase activities. These findings are compatible with accelerated cAMP generation in liver from E. Protein kinase activity ratios were significantly increased in frozen liver from E (0.52 +/- 0.04 versus 0.36 +/- 0.03 in C), and the percent glycogen synthetase in the I form was clearly reduced (19% +/- 2% in E versus 47% +/- 5% in c). incubation of hepatic slices from E or C with MIX and/or 10 muM glucagon further increased cAMP and protein kinase activity ratios, data which imply higher effective, as well as total, cellular cAMP in E. Changes in cAMP metabolism and action observed at 2 wk persisted throughout the 38-wk period of drug ingestion. Adenylate cyclase activity, cAMP content, and protein kinase activity ratios of ethionine-induced hepatomas exceeded those of both the surrounding liver from tumor-bearing rats and that of control liver, but alterations in these parameters were qualitatively similar in both tissues from E. By contrast, while cGMP in quick-frozen surrounding liver from tumor-bearing rats (36 +/- 4 pmole/g wet weight) did not differ from that of control liver (30 +/- 3), cGMP in the hepatomas was increased. This change was evident in both frozen tumor (89 +/- 10) and in tumor slices incubated in vitro with MIX (C, 90 +/- 11; surrounding liver, 85 +/- 10; hepatoma 231 +/- 29). These results indicate that malignant conversion can occur in liver with a sustained elevation of both total and effective cAMP during the premalignant phase. The increase in cGMP detected in ethionine-induced hepatomas could also be a key determinant of malignant transformation in the model, although premalignant changes in cGMP were not apparent.