scholarly journals Evidence that guanosine 5′-[γ-thio]triphosphate stimulates plasma membrane Ca2+ inflow when introduced into hepatocytes

1989 ◽  
Vol 257 (2) ◽  
pp. 591-598 ◽  
Author(s):  
B P Hughes ◽  
G J Barritt
Keyword(s):  
Plasma Membrane ◽  
Liver Cell ◽  
Glycogen Phosphorylase ◽  
Initial Rate ◽  
Effective Concentration ◽  
Phosphorylase Activity ◽  
Glycogen Phosphorylase Activity ◽  
Rate Of Activation ◽  
In Cells

1. Slowly hydrolysable analogues of GTP were introduced into hepatocytes by incubating the cells in the absence of Mg2+ and in the presence of ATP4-. Experiments using guanosine 5′-[gamma-[35S]thio]triphosphate (GTP[35S])indicated that about 50% of the GTP[S] loaded into the cells was subsequently hydrolysed. 2. In cells loaded with GTP[S] and incubated in the absence of added extracellular Ca2+ (Ca2+o), the rate of activation of glycogen phosphorylase observed after addition of 1.3 mM-Ca2+o was 250% greater than the rate observed in unloaded cells. Smaller effects (130%) were observed in cells loaded with either guanyl-5′-yl imidodiphosphate or guanosine 5-[beta-thio]diphosphate (GDP[S]). Cells loaded with adenosine 5′-[gamma-thio]triphosphate showed no increase in glycogen phosphorylase activity on addition of Ca2+o. 3. The effect of a submaximal concentration of GTP[S] on the Ca2+-induced activation of glycogen phosphorylase was additive with that of a half-maximally effective concentration of vasopressin. GTP[S] did not increase the effect of a maximally effective concentration of the hormone. 4. Cells loaded with GTP[S] exhibited an increased initial rate of 45Ca2+ exchange measured at 1.3 mM-Ca2+o. 5. GTP[S] did not affect the amount of 45Ca2+ exchanged by cells incubated at 0.1 mM-Ca2+o or the ability of vasopressin to release 45Ca2+ from these cells. 6. It is concluded that the introduction of slowly hydrolysable analogues of GTP to the liver cell cytoplasmic space stimulates the inflow of Ca2+ across the plasma membrane through a channel similar to that activated by vasopressin.

scholarly journals Effects of vasopressin and La3+ on plasma-membrane Ca2+ inflow and Ca2+ disposition in isolated hepatocytes. Evidence that vasopressin inhibits Ca2+ disposition

1986 ◽  
Vol 238 (3) ◽  
pp. 793-800 ◽  
Author(s):  
B P Hughes ◽  
S E Milton ◽  
G J Barritt
Keyword(s):  
Plasma Membrane ◽  
Glycogen Phosphorylase ◽  
Subcellular Fractionation ◽  
Isolated Hepatocytes ◽  
Ionophore A23187 ◽  
Phosphorylase Activity ◽  
45Ca Uptake ◽  
Maximal Stimulation ◽  
Glycogen Phosphorylase Activity ◽  
Stimulation Of

Vasopressin caused a 40% inhibition of 45Ca uptake after the addition of 0.1 mM-45Ca2+ to Ca2+-deprived hepatocytes. At 1.3 mM-45Ca2+, vasopressin and ionophore A23187 each caused a 10% inhibition of 45Ca2+ uptake, whereas La3+ increased the rate of 45Ca2+ uptake by Ca2+-deprived cells. Under steady-state conditions at 1.3 mM extracellular Ca2+ (Ca2+o), vasopressin and La3+ each increased the rate of 45Ca2+ exchange. The concentrations of vasopressin that gave half-maximal stimulation of 45Ca2+ exchange and glycogen phosphorylase activity were similar. At 0.1 mM-Ca2+o, La3+ increased, but vasopressin did not alter, the rate of 45Ca2+ exchange. The results of experiments performed with EGTA or A23187 or by subcellular fractionation indicate that the Ca2+ taken up by hepatocytes in the presence of La3+ is located within the cell. The addition of 1.3 mM-Ca2+o to Ca2+-deprived cells caused increases of approx. 50% in the concentration of free Ca2+ in the cytoplasm [(Ca2+]i) and in glycogen phosphorylase activity. Much larger increases in these parameters were observed in the presence of vasopressin or ionophore A23187. In contrast with vasopressin, La3+ did not cause a detectable increase in glycogen phosphorylase activity or in [Ca2+]i. It is concluded that an increase in plasma membrane Ca2+ inflow does not by itself increase [Ca2+]i, and hence that the ability of vasopressin to maintain increased [Ca2+]i over a period of time is dependent on inhibition of the intracellular removal of Ca2+.

scholarly journals Evidence that a pertussis-toxin-sensitive substrate is involved in the stimulation by epidermal growth factor and vasopressin of plasma-membrane Ca2+ inflow in hepatocytes

1987 ◽  
Vol 248 (3) ◽  
pp. 911-918 ◽  
Author(s):  
B P Hughes ◽  
J N Crofts ◽  
A M Auld ◽  
L C Read ◽  
G J Barritt
Keyword(s):  
Plasma Membrane ◽  
Growth Factor ◽  
Epidermal Growth Factor ◽  
Pertussis Toxin ◽  
Glycogen Phosphorylase ◽  
Regulatory Protein ◽  
Phosphorylase Activity ◽  
Rate Of Increase ◽  
Epidermal Growth ◽  
Glycogen Phosphorylase Activity

1. In hepatocytes, epidermal growth factor (EFG) (a) increased the rate of 45Ca2+ exchange in cells incubated at 1.3 mM extracellular Ca2+, (b) increased the activity of glycogen phosphorylase a and the intracellular free Ca2+ concentration (measured with quin2) in a process dependent on the concentration of extracellular Ca2+, and (c) enhanced the increase in glycogen phosphorylase activity which follows the addition of Ca2+ to cells previously incubated in the absence of Ca2+. It is concluded that EGF stimulates plasma-membrane Ca2+ inflow. 2. The effects of the combination of EGF and vasopressin on the rate of 45Ca2+ exchange and on the rate of increase in glycogen phosphorylase activity were the same as those of vasopressin alone. 3. The amount of 45Ca2+ released by EGF from internal stores was about 30% of that released by vasopressin. No detectable increase in [3H]inositol mono-, bis- or tris-phosphate was observed after the addition of EGF to cells labelled with myo-[3H]inositol. 4. In hepatocytes isolated from rats treated with pertussis toxin, the effects of EGF and vasopressin on phosphorylase activity (measured at 1.3 mM-Ca2+) and on the rate of Ca2+ inflow (measured with quin2) were markedly decreased compared with those in normal cells. 5. Treatment with pertussis toxin did not impair the ability of vasopressin to release Ca2+ from internal stores, but decreased vasopressin-stimulated [3H]inositol polyphosphate formation by 50%. 6. It is concluded that the mechanism(s) by which vasopressin and EGF stimulate plasma-membrane Ca2+-inflow transporters in hepatocytes involves a GTP-binding regulatory protein sensitive to pertussis toxin, and does not require an increase in the concentration of inositol trisphosphate comparable with that which induces the release of Ca2+ from the endoplasmic reticulum.

Regulation of muscle glycogen phosphorylase activity following short-term endurance training

1996 ◽  
Vol 270 (2) ◽  
pp. E328-E335 ◽  
Author(s):  
A. Chesley ◽  
G. J. Heigenhauser ◽  
L. L. Spriet
Keyword(s):  
Endurance Training ◽  
Muscle Glycogen ◽  
Glycogen Phosphorylase ◽  
Citrate Synthase ◽  
Phosphorylase Activity ◽  
Short Term ◽  
Mitochondrial Potential ◽  
Before And After ◽  
Glycogen Phosphorylase Activity ◽  
Muscle Biopsies

The purpose of this study was to examine the regulation (hormonal, substrate, and allosteric) of muscle glycogen phosphorylase (Phos) activity and glycogenolysis after short-term endurance training. Eight untrained males completed 6 days of cycle exercise (2 h/day) at 65% of maximal O2 uptake (Vo2max). Before and after training subjects cycled for 15 min at 80% of Vo2max, and muscle biopsies and blood samples were obtained at 0 and 30 s, 7.5 and 15 min, and 0, 5, 10, and 15 min of exercise. Vo2max was unchanged with training but citrate synthase (CS) activity increased by 20%. Muscle glycogenolysis was reduced by 42% during the 15-min exercise challenge following training (198.8 +/- 36.9 vs. 115.4 +/- 25.1 mmol/kg dry muscle), and plasma epinephrine was blunted at 15 min of exercise. The Phos a mole fraction was unaffected by training. Muscle phosphocreatine utilization and free Pi and AMP accumulations were reduced with training at 7.5 and 15 min of exercise. It is concluded that posttransformational control of Phos, exerted by reductions in substrate (free Pi) and allosteric modulator (free AMP) contents, is responsible for a blunted muscle glycogenolysis after 6 days of endurance training. The increase in CS activity suggests that the reduction of muscle glycogenolysis was due in part to an enhanced mitochondrial potential.

Regulation of glycolytic enzymes during anoxia in the turtle Pseudemys scripta

1989 ◽  
Vol 257 (2) ◽  
pp. R278-R283 ◽  
Author(s):  
S. P. Brooks ◽  
K. B. Storey
Keyword(s):  
Heart Muscle ◽  
Glycogen Phosphorylase ◽  
White Muscle ◽  
Fold Increase ◽  
Glycolytic Enzymes ◽  
Phosphorylase Activity ◽  
Enzyme Form ◽  
Red Muscle ◽  
Km Value ◽  
Glycogen Phosphorylase Activity

The glycolytic enzymes glycogen phosphorylase, phosphofructokinase (PFK), and pyruvate kinase (PK) were assessed in liver, heart, red muscle, and white muscle of aerobic and 5-h anoxic turtles (Pseudemys scripta) for changes in total activity and kinetic parameters. Anoxia induced statistically significant changes in these glycolytic enzymes in each of the four organs assayed. Compared with normoxic controls, anoxic liver showed a 3.3-fold increase in glycogen phosphorylase activity, a 1.5-fold increase in the PFK I50 value for citrate (concentration that inhibits initial activity by 50%), a 1.5-fold increase in the PFK Michaelis constant (Km) value for fructose 6-phosphate (P), and an increased maximal activity of PK. Anoxic heart muscle showed a 2.6-fold decrease in glycogen phosphorylase activity and, for PFK, a 1.7-fold decrease in the Km value for ATP and a twofold increase in the I50 value for citrate. In anoxic white muscle, PFK showed a fivefold lower Km value for fructose-6-P and a threefold lower activator concentration producing half-maximal activation (A50) for potassium phosphate than the aerobic enzyme form. Changes in anoxic white muscle PK included a twofold increase in the Km value for ADP and a 1.7-fold decrease in the I50 value for alanine. In red muscle, anoxia affected only the Km value for ATP, which was 50% higher than the value for the aerobic enzyme form. Fructose 2,6-diphosphate (P2) levels also decreased in heart muscle and increased in red and white muscle during anoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Axotomy increases glycogen phosphorylase activity in motoneurones

Neuroscience ◽  
1984 ◽  
Vol 12 (4) ◽  
pp. 1261-1269 ◽  
Author(s):  
C.J. Woolf ◽  
M.S. Chong ◽  
A. Ainsworth
Keyword(s):  
Glycogen Phosphorylase ◽  
Phosphorylase Activity ◽  
Glycogen Phosphorylase Activity

Cell specific events occurring during development

Development ◽  
1976 ◽  
Vol 35 (2) ◽  
pp. 335-343
Author(s):  
Charles L. Rutherford
Keyword(s):  
Inorganic Phosphate ◽  
Glycogen Phosphorylase ◽  
Glycogen Content ◽  
Cell Types ◽  
Specific Cell ◽  
Phosphorylase Activity ◽  
A Cell ◽  
Specific Degradation ◽  
Glycogen Phosphorylase Activity ◽  
Spore Cell

Ultra-microfluorometric techniques were adapted to follow the time sequence of glycogen degradation during the differentiation of two cell types in Dictyostelium discoideum. Glycogen content, glycogen phosphorylase activity, and inorganic phosphate accumulation were localized in specific cell types during stalk and spore development. Glycogen levels in pre-stalk cells remained constant during the pseudoplasmodium and early culmination stages of development. However, as pre-stalk cells migrated into the position of stalk formation, a cell specific degradation of glycogen was observed. The loss of glycogen from pre-stalk cells was accompanied by an increase in the activity of glycogen phosphorylase. This increase in activity from 0·04 to 0·14 moles/h/kg dry wt. occurred as pre-stalk cells entered the position of stalk formation. An inverse relationship was found between glycogen levels and inorganic phosphate (Pi) levels in the developing stalk. During the process of stalk construction, a gradient of Pi levels occurred from the apex to the base of the developing stalk. Glycogen degradation from pre-spore cells lagged behind that of pre-stalk cells. No change in pre-spore cell glycogen levels was observed until stalk construction was nearly completed. The results emphasize the importance of the physical position of a cell with respect to its composition and fate during development.

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