Distribution of cyclic AMP phosphodiesterase in microdissected periportal and perivenous rat liver tissue with different dietary states

Effects of glucagon on glycogenolysis and gluconeogenesis are region-specific in periportal and perivenous hepatocytes

It has been established, mainly by histochemical and immunohistochemical studies, that liver cells are functionally heterogeneous, with periportal hepatocytes (PPHs) being predominantly gluconeogenic and perivenous hepatocytes (PVHs) being glycolytic. We therefore investigated the region-specific functional effects of glucagon on glycogenolysis and gluconeogenesis in isolated PPHs and PVHs prepared by the digitonin-collagenase method. BB rats, a model of insulin-dependent diabetes, were used to study the region-specific heterogeneity of gluconeogenesis in the diabetic state. Although glycogen content was not different between PVHs and PPHs in rats fed the normal diet, basal glucose release was 1.37 times greater in PVHs than in PPHs (P <.05). The increase in glucose release stimulated by 0.01 to 0.1 nmol/L glucagon was 1.52 times greater in PVHs than in PPHs (P < .05), whereas no differences were seen in response to 1 to 100 nmol/L glucagon. Glucose release from gluconeogenic substrates was 1.57 times greater in the PPHs than in the PVHs of fasted normal rats (P < .05), whereas the increase in gluconeogenesis produced by glucagon was not different between PPHs and PVHs. The glucagon-binding capacity, the cAMP release, and the increase in intracellular Ca2+ stimulated by glucagon were not different between PPHs and PVHs in the fed or fasted states. Gluconeogenesis from gluconeogenic substrates was 1.52 times greater in the PPHs than in the PVHs of fasted nondiabetic BB rats (P < .05). After the development of diabetes, the gluconeogenic capacity in PVHs increased to the level observed in PPHs, but that in PPHs did not change. Thus there was no difference in gluconeogenesis between the PPHs and PVHs of diabetic BB rats. In both the PPHs and PVHs of diabetic BB rats, the 0.01 to 100 nmol/L glucagon-induced increase in gluconeogenesis was greater than that in PPHs from nondiabetic BB rats (2.30 and 3.07 times, P < .01, respectively). We conclude that PPHs and PVHs of normal rat liver express region-specific differences in their glycogenolytic and gluconeogenic responses to glucagon. In diabetic BB rats, the difference in the gluconeogenic capacity between PPHs and PVHs disappeared, whereas glucagon-induced gluconeogenesis was enhanced.

Production, uptake, and metabolic effects of cyclic AMP in the bivascularly perfused rat liver

Production, uptake, and metabolic effects of cyclic AMP (cAMP) were measured in the bivascularly perfused rat liver in anterograde and retrograde perfusion. Glucagon, cAMP, N6,2'-O-dibutyryl cAMP and N6-monobutyryl cAMP were infused into the portal vein (anterograde perfusion), the hepatic vein (retrograde perfusion), or the hepatic artery (anterograde and retrograde perfusion) in order to reach different cell populations. The following results were obtained: (1) cAMP release caused by glucagon was directly proportional to the cell spaces that were accessible via the hepatic artery in anterograde and retrograde perfusion; since the metabolic effects of glucagon were not proportional to the accessible cell spaces, this observation also implies a disproportion between cAMP release and metabolic effects of the hormone; (2) when cAMP and N6,2'-O-dibutyryl cAMP were given to all liver cells (e.g. when infused into the portal vein), their metabolic effects were qualitatively and quantitatively the same and qualitatively equal to the effects of glucagon; (3) the changes caused by cAMP were a function of the cell spaces that can be reached via the hepatic artery in anterograde and retrograde perfusion; this behaviour contrasts markedly with that of glucagon, whose metabolic effects were practically independent of the accessible cell spaces; and (4) the effects of N6,2'-O-dibutyryl cAMP and N6-monobutyryl cAMP were independent of the cell spaces that were accessible via the hepatic artery in anterograde and retrograde perfusion; in this respect their behaviour was equal to that of glucagon. It is apparent that exogenously added cAMP mimicked the metabolic effects of glucagon in the liver only when it was supplied to all liver cells. Since glucagon, N6,2'-O-dibutyryl cAMP, and N6-monobutyryl cAMP were able to produce a full response even when given to only 30% of the liver parenchyma, it was concluded that cAMP production under the stimulus of glucagon or in consequence of the metabolic transformation of N6,2'-O-dibutyryl cAMP and N6-monobutyryl cAMP occurs in a compartment to which exogenous cAMP has no access. cAMP generated within this compartment is possibly able to diffuse from cell to cell.

The action of glucagon infused via the hepatic artery in anterograde and retrograde perfusion of the rat liver is not a function of the accessible cellular spaces

The metabolic action of glucagon in the different spaces that can be reached via the hepatic artery in the bivascularly perfused rat liver of fed rats was investigated. When perfusion was performed in the anterograde mode, glucagon (10 mM) was infused either into the portal vein (type 1 experiment) or into the hepatic artery (type 2); in the retrograde mode, the hormone was infused either into the hepatic vein (type 3) or into the hepatic artery (type 4). The aqueous cell spaces were measured by means of the multiple-indicator dilution technique. Glucose release, oxygen uptake and glycolysis (lactate plus pyruvate production) were measured as metabolic parameters. The following results were obtained. (1) The aqueous cell space accessible via the hepatic artery in the type 2 experiment was 0.63 ml/g; in the type 4 experiment this space was 0.18 ml/g (only periportal cells); glucagon up to 10 nM did not affect these cellular spaces nor did it affect the vascular spaces. (2) The effects of glucagon on glucose release, oxygen uptake and glycolysis were practically the same in all types of experiment (1 to 4), i.e., the action of glucagon was not a function of the accessible cell spaces. (3) When the respiratory chain of the liver cells accessible via the hepatic artery in the type 4 experiment was inhibited by cyanide, glucagon still increased oxygen uptake; oxygen uptake stimulation by glucagon was completely blocked only when cyanide was given to all liver cells. (4) Calcium depletion did not affect the action of glucagon on glucose release and oxygen uptake in the type 4 experiment. It was concluded that, in addition to the receptor-elicited response, the action of glucagon can also be propagated by cell-to-cell communication.

Enhancement of hepatic glucose release and bile flow by the phosphodiesterase-III-inhibitor enoximone in the perfused rat liver

The use of phosphodiesterase-III-inhibitors (PDI) as inotropic substances in the treatment of cardiac failure can be associated with hyperglycaemia. This phenomenon could be caused by hepatic events induced by PDI. The purpose of our study was to investigate the effects of the PDI enoximone on hepatic carbohydrate metabolism and bile flow. In the rat liver perfusion model, hepatic glucose and lactate production, portal flow and bile flow were determined. Administration of enoximone (1, 10, 100 microM) increased hepatic glucose output and bile acid-independent bile flow in a dose-dependent manner. The PDI enhanced the glycogenolytic effects of glucagon (from 15.7 to 38.6 mumol glucose/g/20 min), of epinephrine (from 7.1 to 38.7 mumol glucose/g/20 min), of norepinephrine (from 9.8 to 32 mumol/g/20 min) and of phenylephrine (from 25.5 to 40.8 mumol glucose/g/20 min). Furthermore, lactate production was significantly reduced by enoximone. The effect of epinephrine and phenylephrine on portal flow was blocked or diminished by enoximone administration. In summary, it was shown that the PDI enoximone is able to enhance hepatic glucose production. Bile acid-independent bile flow was increased by the inhibition of phosphodiesterase-III. The effects of enoximone and glycogenolytic hormones on glucose release were synergistic. The vasoconstrictive action of catecholamines was reduced or completely prevented by enoximone. In conclusion, enoximone has glycogenolytic, vasodilatory and choleretic properties in the liver.

Zonation of the action of glucagon on gluconeogenesis studied in the bivascularly perfused rat liver

We have measured the action of glucagon, infused into the hepatic artery, on gluconeogenesis from lactate in the rat liver, bivascularly perfused in both the anterograde and retrograde modes. Concerning glucose production and oxygen uptake per unit cell space, the response of the periportal cells reached via the hepatic artery in retrograde perfusion to glucagon is superior to the response of the cells reached via the same vessel in anterograde perfusion. This phenomenon, however, most probably reflects zonation of gluconeogenesis rather than zonation of the hormonal action. The latter conclusion is based on the observation that the fractional change caused by the hormone is the same for all liver cells.