Possible involvement of two mechanisms of signal transduction in alpha 1-adrenergic action. Selective effect of cycloheximide

Studies on glycogen autophagy: effects of phorbol myristate acetate, ionophore A23187, or phentolamine

The effects of agents that could manipulate the lysosomal calcium such as phorbol myristate acetate, ionophore A23187, and phentolamine on the lysosomal glycogen degradation were studied by electron microscopy, morphometric analysis, and biochemical assays in newborn rat hepatocytes. Phorbol myristate acetate, which promotes the input of calcium to lysosomes, increased the total volume of autophagic vacuoles and the activity of lysosomal glycogen-hydrolyzing acid alpha 1,4 glucosidase and decreased the fractional volume of undigested glycogen inside the autophagic vacuoles and also decreased the activity of acid mannose 6-phosphatase. Ionophore A23187, which releases lysosomal calcium, produced opposite results in these enzyme activities. Phentolamine, an alpha-adrenergic blocking agent which interferes with the generation of phosphoinositides and may activate the lysosomal calcium uptake pump, increased the total volume of autophagic vacuoles and the activity of lysosomal glycogen-hydrolyzing acid glucosidase and decreased the fractional volume of undigested glycogen inside the autophagic vacuoles. The results of this study constitute evidence that changes in lysosomal calcium may influence certain aspects of autophagy, including the degradation of glycogen inside the autophagic vacuoles. They also support our previous postulate [Kalamidas and Kotoulas (2000a,b) Histol Histopathol 15:29-35, 1011-1018] that stimulation of autophagic mechanisms in newborn rat hepatocytes may be associated with acid mannose 6-phosphatase activity-deficient lysosomes.

The Ca2+-dependent actions of the alpha-adrenergic agonist phenylephrine on hepatic glycogenolysis differ from those of vasopressin and angiotensin

The stimulation of hepatic glycogenolysis by the Ca2+-dependent hormones phenylephrine, vasopressin and angiotensin II was studied as a function of intracellular and extracellular Ca2+. In the isolated perfused rat liver the decline in glucose formation was monophasic ('half-life' approximately equal to 3 min) with vasopressin (1 nM) or angiotensin II (0.05 microM), but biphasic (half-life of 4.8 min and 17.6 min) in the presence of the alpha-agonist phenylephrine (0.01 mM), indicating either a different mode of mobilization or the mobilization of additional intracellular calcium stores. Under comparable conditions an elevated [Ca2+] level was maintained in the cytosol of hepatocytes for at least 10 min in the presence of phenylephrine, but not vasopressin. Titration experiments performed in the isolated perfused liver to restore cellular calcium revealed differences in the hormone-mediated uptake of Ca2+. The onset in glucose formation above that seen in the absence of exogenous calcium occurred at approximately 30 microM or 70-80 microM Ca2+ in the presence of phenylephrine or vasopressin respectively. The shape of the response curve was sigmoidal for vasopressin and angiotensin II, but showed a distinct plateau between 0.09 mM and 0.18 mM in the presence of phenylephrine. The plateau was also observed at phenylephrine concentrations as low as 0.5 microM. The formation of plateaus observed after treatment of the liver with A 23187, but not after EGTA, is taken as an indication that intracellular calcium stores are replenished. A participation of the mitochondrial compartment could be excluded by pretreatment of the liver with the uncoupler 2,4-dinitrophenol. Differences in the Ca2+ dependence of the glycogenolytic effects of these hormones were also revealed by kinetic analysis. It is concluded that phenylephrine differs from vasopressin and angiotensin II in that, in addition to a more common, non-mitochondrial pool, which is also responsive to the vasoactive peptides, the agonist mobilizes Ca2+ from a second, non-mitochondrial pool. The results are consistent with the proposal that Ca2+ transport across subcellular membranes may be subject to different hormonal control.

Insulin-like effect of epidermal growth factor in isolated rat hepatocytes. Modulation of the alpha-1-adrenergic stimulation of ureagenesis

Insulin and epidermal growth factor (EGF) inhibit the stimulation of ureagenesis induced by adrenaline (alpha 1-adrenergic effect) in hepatocytes from control rats incubated in medium without calcium and in cells from hypothyroid rats. In hepatocytes from euthyroid rats incubated in normal buffer neither insulin or EGF diminished the alpha 1-adrenergic stimulation of ureagenesis. No effect of EGF or insulin on the alpha 1-adrenergic stimulation of phosphatidylinositol labeling was observed under any conditions. It is suggested that EGF mimics the action of insulin on one of the pathways of the alpha 1-adrenergic action: the calcium-independent, insulin-sensitive pathway which predominates in hepatocytes from hypothyroid rats.

Phorbol esters, vasopressin and angiotensin II block alpha 1-adrenergic action in rat hepatocytes. Possible role of protein kinase C

The effect of vasopressin, angiotensin II and phorbol myristate acetate on the alpha 1-adrenergic action (induced by epinephrine + propranolol), was studied. We selected three conditions: (a) ureagenesis in medium without added calcium and containing 25 microM EGTA; (b) ureagenesis using cells from hypothyroid animals, and (c) gluconeogenesis from dihydroxyacetone. Under these conditions epinephrine + propranolol produces clear metabolic effects, whereas the vasopressor peptides do not (although they stimulate phosphoinositide turnover). It was observed that the vasopressor peptides and the active phorbol ester inhibited in a concentration-dependent fashion the effect of epinephrine + propranolol. It is suggested that activation of protein kinase C by phorbol esters or physiological stimuli (hormones that activate phosphoinositide turnover, such as vasopressin or angiotensin II) modulate the hepatocyte alpha 1-adrenergic responsiveness.

Homologous and heterologous desensitization of one of the pathways of the alpha 1-adrenergic action. Effects of epinephrine, vasopressin, angiotensin II and phorbol 12-myristate 13-acetate

Activation of protein kinase C blocks the alpha 1-adrenergic action in hepatocytes. Preincubation of hepatocytes (in buffer with or without calcium) with vasopressin, angiotensin II, phorbol myristate acetate (PMA) or epinephrine + propranolol markedly diminished the alpha 1-adrenergic responsiveness of the cells (stimulation of ureagenesis) assayed in buffer without calcium. On the contrary, when the alpha 1-adrenergic responsiveness was assayed in buffer containing calcium no effect of the preincubation with vasopressin, angiotensin II or PMA was observed. Preincubation with epinephrine diminished the alpha 1-adrenergic responsiveness of the cells. In hepatocytes from hypothyroid rats the preincubation with the activators of protein kinase C (vasopressin, angiotensin II, phorbol 12-myristate 13-acetate and epinephrine) reduced markedly the alpha 1-adrenergic responsiveness of the cells, whereas in identical experiments using cells from adrenalectomized rats only the preincubation with epinephrine diminished the responsiveness. It is concluded that activation of protein kinase C induces desensitization of the alpha 1-adrenergic action in hepatocytes and that the calcium-independent pathway of the alpha 1-adrenergic action (predominant in cells from hypothyroid animals) resensitizes more slowly than the calcium-dependent pathway (predominant in cells from adrenalectomized rats). Epinephrine in addition to inducing this type of desensitization (through protein kinase C) leads to a further refractoriness of the cells towards alpha 1-adrenergic agonists.

Possible existence of two mechanisms involved in alpha 1-adrenergic action: effect of Sgd 101/75

The effect of 2-(2-methyl-indazol-4-imino)-imidazoline (Sgd 101/75) on the rate of urea synthesis by isolated rat hepatocytes was studied. This agent was observed to stimulate ureagenesis through the activation of alpha 1-adrenoceptors (prazosin-sensitive). The effect of Sgd 101/75 was dependent on the presence of calcium and was not affected by insulin. The active phorbol ester, phorbol 12-myristate 13-acetate, blocked the effect of Sgd 101/75. It was also observed that this adrenoceptor agonist was unable to stimulate ureagenesis in hepatocytes obtained from hypothyroid rats but produced clear stimulation of this metabolic pathway in cells obtained from adrenalectomized rats. The data indicate that this agonist stimulates hepatic metabolism through a calcium-dependent, insulin-insensitive pathway for alpha 1-adrenergic action, modulated by the thyroid status of the animal.