The regulation of intracellular calcium

Sequential determinations of glucose outflow and inflow, and rates of gluconeogenesis from alanine, before, during and after insulin-induced hypoglycemia were obtained in relation to alterations in circulating epinephrine, norepinephrine, glucagon, cortisol, and growth hormone in six normal subjects. Insulin decreased the mean (+/-SEM) plasma glucose from 89+/-3 to 39+/-2 mg/dl 25 min after injection, but this decline ceased despite serum insulin levels of 153+/-22 mul/ml. Before insulin, glucose inflow and outflow were constant averaging 125.3+/-7.1 mg/kg per h. 15 min after insulin, mean glucose outflow increased threefold, but then decreased at 25 min, reaching a rate 15% less than the preinsulin rate. Glucose inflow decreased 80% 15 min after insulin, but increased at 25 min, reaching a maximum of twice the basal rate. Gluconeogenesis from alanine decreased 68% 15 min after insulin, but returned to preinsulin rates at 25 min, and remained constant for the next 25 min, after which it increased linearly. A fourfold increase in mean plasma epinephrine was found 20 min after insulin, with maximal levels 50 times basal. Plasma norepinephrine concentrations first increased significantly at 25 min after insulin, whereas significantly increased levels of cortisol and glucagon occurred at 30 min, and growth hormone at 40 min after insulin. Thus, insulin-induced hypoglycemia in man results from both a decrease in glucose production and an increase in glucose utilization. Accelerated glycogenolysis produced much of the initial, posthypoglycemic increment in glucose production. The contribution of glycogenolysis decreased with time, while that of gluconeogenesis from alanine increased. Of the hormones studied, only the increments in plasma catecholamines preceded or coincided with the measured increase in glucose production after hypoglycemia. It therefore seems probable that adrenergic mechanisms play a major role in the initiation of counter-regulatory responses to insulin-induced hypoglycemia in man.

/pdf/the-role-of-adrenergic-mechanisms-in-the-substrate-and-2orzz5zvw0.pdf

The role of adrenergic mechanisms in the substrate and hormonal response to insulin-induced hypoglycemia in man.

Calcium in biological systems

Control, Modulation, and regulation of cell calcium.

Activation of pigeon erythrocyte membrane adenylate cyclase by guanylnucleotide analogues and separation of a nucleotide binding protein.

Role of GTP in epinephrine and glucagon activation of adenyl cyclase of liver plasma membrane

The adenylate-cyclase activity from a rat-liver plasma-membrane preparation purified according to the Neville's procedure was increased 10 to 12-fold by glucagon, but the addition of epinephrine, at any concentration, failed to activate the enzyme. When the membranes were isolated from adrenalectomized animals, the responses of the enzyme to glucagon and NaF were enhanced and mainly the adenylate-cyclase activity became sensitive to epinephrine with an optimal effect at 0.05 mM epinephrine. Intraperitoneal administration of cortisol for 2 days to the adrenalectomized animals reversed the effect of the hormonal deprivation. But addition of cortisol in vitro was without effect.



The use of liver plasma membranes from adrenalectomized rats is therefore proposed as a tool to study the adenylate-cyclase activation by both glucagon and epinephrine. The epinephrine-sensitive adenylate-cyclase activity was completely inhibited by propranolol but not by phentolamine, thus confirming that the metabolic effect of epinephrine is linked to a β-like receptor. Epinephrine and glucagon effects upon adenylate-cyclase activity were both inhibited by concentration of calcium above 0.01 mM but differed with respect to the Mg-ATP concentration. The two hormonally stimulated adenylate-cyclase activities were found to be additive.



The effect of epinephrine upon adenylate-cyclase activity was more sensitive to stimulation by low concentration of GTP and other nucleotides than the effect of glucagon. Addition of insulin in vitro at any concentration failed to alter the adenylate-cyclase activity regardless of the concentration of magnesium, the presence of glucose or GTP.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1973.tb03049.x

Adenylate-Cyclase Activity of Rat-Liver Plasma Membranes

Plasma membranes from rat liver purified according to the pro- cedure of Neville bind calcium ions by a concentration-dependent, saturable process with at least two classes of binding sites. The higher affinity sites bind 45 nmol calcium/mg membrane protein with a KD of 3 ptM. Adrenalectomy increases the number of the higher affinity sites and the corresponding KD. Plasma membranes exhibit a (Na+-K+)-independent-Mg2+-ATPase activity which is not activated by calcium between 0.1 /&M and 10 mM CaC12. Calcium can, with less efficiency, substitute for magnesium as a cofactor for the (Na+ - K+)-independent ATPase. Both Mg 2 + - and Ca 2 +-ATPase activities are identical with respect to pH dependence, nucleotide specificity and sensitivity to in- hibitors. But when calcium is substituted for magnesium, there is no detectable membrane phosphorylation from (y_3 2 P) ATP as it is found in the presence of magnesium. The existence of high affinity binding sites for calcium in liver plasma membranes is compatible with a regulatory role of this ion in membrane enzymic mechanisms or in hormone actions. Plasma membranes obtained by the procedure of Neville are devoid of any Ca2+-activated-Mg2+-ATPase ac- tivity indicating the absence of the classical energy-dependent calcium ion transport. These results would suggest that the overall calcium-extruding activity of the liver cell is mediated by a mechanism involving no direct ATP hydrolysis at the membrane level.

/pdf/calcium-binding-properties-and-atpase-activities-of-rat-486cdh4vsc.pdf

Anne-Marie Chambaut

Papers

Role of GTP in epinephrine and glucagon activation of adenyl cyclase of liver plasma membrane

Adenylate-Cyclase Activity of Rat-Liver Plasma Membranes

Calcium-Binding Properties and ATPase Activities of Rat Liver Plasma Membranes

Antagonistic actions between dibutyryl adenosine-3',5'-cyclic monophosphate and insulin on the metabolism of the surviving rat diaphragm.

The glucose effect and cortisone action upon rat liver and muscle protein metabolism.