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.