The Effect of In Vivo Glucose Administration on Human Erythrocyte Ca2+-ATPase Activity and on Enzyme Responsiveness In Vitro to Thyroid Hormone and Calmodulin

Abstract

To characterize endogenous control mechanisms for human erythrocyte membrane Ca²⁺-ATPase (“calcium pump”) activity, we studied the effect of changes in blood glucose concentration in vivo within the physiologic range on Ca²⁺-ATPase activity in red cells. Red cells obtained in the course of induced hyperglycemia were also studied to determine susceptibility of membrane Ca²⁺-ATPase to stimulation in vitro by thyroid hormone and calmodulin, both of which have been shown previously to enhance Ca²⁺-ATPase activity. Oral glucose administration (75 g) to eight healthy, adult subjects induced predictable increases in concentrations of blood glucose and immunoreactive insulin. Basal levels of activity of Ca²⁺-ATPase in red cells obtained after glucose ingestion fell 55% (P < 0.025) by 30 min after glucose with recovery of enzyme activity to levels not significantly different from basal by 60 min. Activity of red cell Ca²⁺-ATPase at time zero was significantly stimulated in vitro by thyroxine (T4, 10⁻¹⁰ M), triiodo-L-thyronine (T₃, 10⁻¹⁰ M), and calmodulin (100 ng/mg membrane protein). In vivo glucose administration led to depression of red cell enzyme responsiveness in vitro to T₄ and T₃; recovery from this effect did not occur by 120 min after oral administration of glucose. Calmodulin responsiveness of the enzyme in vitro was less significantly reduced in red cells obtained after glucose ingestion. Intravenous (i.v.) glucose administration (20 g) to five subjects also led to decreased basal enzyme activity (61% of fasting level at 20 min). A significant decrease in response of enzyme to T4 was achieved by 8 min after glucose administration (P < 0.02), with recovery by 60 min. Calmodulin responsiveness showed an insignificant downward trend. Exposure of red cell membranes to glucose (200 mg/ dl) or insulin (100 nil/ml) in vitro resulted in 26% and 17% reductions, respectively, in basal Ca²⁺-ATPase activity (P < 0.001), together with a reduction in thyroid hormone stimulation of the enzyme. Tunicamycin, an inhibitor of enzymatic protein glycosylatioh, reversed these in vitro effects of glucose, but not of insulin, on erythrocyte Ca²⁺-ATPase activity. These observations are relevant to the interpretation of clinical studies of Ca²⁺-ATPase activity and to the physiology of the enzyme. Glucose administration predictably decreases Ca²⁺-ATPase in the human red cell, an alteration in calcium pump activity that would allow pulsing Of Ca²⁺ into the cell.