Stimulation of specific GTPase activity by vasopressin in isolated membranes from cultured rat hepatocytes

AbstractMembranes were isolated by isotonic homogenization and differential centrifugation from rat hepatocytes cultured overnight. The specific GTPase activity of the membranes was 1–1.3 pmol γ-labelled GTP hydrolysed/mg protein per min in the presence of 1.2 mM Na+, 2 mM EGTA, 1 mM ATP and 0.2 mM 5-adenylyl imidodiphosphate. Under these conditions there was a stimulation of specific GTPase activity of no more than 20% by 11–115 nM vasopressin. No effect of vasopressin was seen in the presence of 1.7 μM free Ca2+ or 100 mM Na+. The findings indicate that vasopressin is able to influence GTPase activity as well as accelerate phosphoinositide breakdown in rat hepatocytes

Spermine antagonises the effects of dexamethasone, glucagon and cyclic AMP in increasing the activity of phosphatidate phosphohydrolase in isolated rat hepatocytes

AbstractRat hepatocytes were incubated in monolayer culture, under serum free conditions, for 8 h. Glucagon (10 nM), 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (100 μM) and dexamethasone (100 nM) increased the activity of phosphatidate phosphohydrolase by approx. 2-, 3.6- and 3.3-fold, respectively. Spermine alone had no significant effect. Spermine (2.5 mM) almost completely inhibited the glucagon induced increase in phosphohydrolase activity. It only partially inhibited the dexamethasone and cyclic AMP mediated inductions. Spermidine had no significant effect in this respect. The results are discussed in relation to the known effects of polyamines on glycerolipid synthesis, in particular, and on intermediary metabolism

Amylin and epinephrine have no direct effect on glucose transport in isolated rat soleus muscle

AbstractAmylin and epinephrine did not significantly affect insulin stimulated, or basal, 3-O-methylglucose transport in isolated rat soleus muscle, as measured by the release of 3-O-methylglucose from pre-loaded tissue. Both amylin and epinephrine inhibited insulin-stimulated 2-deoxyglucose uptake (by 25% and 38%, respectively) in soleus muscle from fed rats but not from fasted rats. The latter results are consistent with amylin and epinephrine stimulating glycogenolysis and inhibiting hexokinase activity by intracellular accumulation of glucose 6-phosphate. We conclude that amylin, like epinephrine, does not specifically inhibit glucose transporters in skeletal muscle