Hormonal regulation of gluconeogenic gene transcription in the liver.

Glucose homeostasis in mammals is achieved by the actions of counterregulatory hormones, namely insulin, glucagon and glucocorticoids. Glucose levels in the circulation are regulated by the liver, the metabolic centre which produces glucose when it is scarce in the blood. This process is catalysed by two rate-limiting enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) whose gene expression is regulated by hormones.

Down-regulation of mu-opioid receptor by full but not partial agonists is independent of G protein coupling.

In C6 glial cells stably expressing rat mu-opioid receptor, opioid agonist activation is negatively coupled to adenylyl cyclase through pertussis toxin-sensitive G proteins. In membranes, [D-Ala2, N-MePhe4,Gly-ol5]enkephalin (DAMGO) increases guanosine-5'-O-(3-[35S]thio)triphosphate (GTP[gamma-35S]) binding by 367% with an EC50 value of 28 nM. Prolonged exposure to agonists induced desensitization of the receptor as estimated by a reduction in the maximal stimulation of GTP[gamma-35S] binding by DAMGO and rightward shifts in the dose-response curves.

Regulation of mu-opioid receptor in neural cells by extracellular sodium.

SH-SY5Y neural cells expressing mu- and delta-opioid receptors were maintained viable in isotonic, sodium-free buffer in vitro. Intracellular sodium levels were manipulated by various methods, and ligand binding to intact cells was studied. In physiological buffer containing 118 mM sodium, [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol ([3H]-DAMGO) and [3H]naltrexone bound to mu receptor with KD values of 3.

Reversible modulation of opioid receptor binding in intact neural cells by endogenous guanosine triphosphate.

Incubation of SH-SY5Y neural cells with mycophenolic acid (MPA), an inhibitor of inosine monophosphate dehydrogenase (the key enzyme in purine nucleotide biosynthesis), reduced the cellular content of GTP by 94% relative to its concentration in control cells (43 nmol/mg protein) without altering the level of GDP. Although in GTP-depleted intact cells the receptor binding parameters (Kd and Bmax) of the opioid antagonist [3H]naltrexone were unchanged from those in untreated cells, the binding affinity of the mu-selective opioid agonist [3H]Tyr-D-Ala-Gly-(Me)- Phe-Gly-ol ([3H]DAMGO) was enhanced 2-fold. Furthermore, the kinetics of ligand/receptor interaction revealed that in the nucleotide-depleted cells, the dissociation rate constant for [3H]DAMGO was reduced by 44%.