Evidence for mechanistic alterations of Ca2+ homeostasis in Type 2 diabetes mellitus.
Altered cytosolic Ca2+ is implicated in the aetiology of many diseases including diabetes but there are few studies on the mechanism(s) of the altered Ca2+ regulation. Using human lymphocytes, we studied cytosolic calcium (Cai) and various Ca2+ transport mechanisms in subjects with Type 2 diabetes mellitus and control subjects. Ca2+-specific fluorescent probes (Fura-2 and Fluo-3) were used to monitor the Ca2+ signals. Thapsigargin, a potent and specific inhibitor of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA), was used to study Ca2+-store dependent Ca2+ fluxes. Significant (P<0.05) elevation of basal Ca, levels was observed in lymphocytes from diabetic subjects. Cai levels were positively correlated with fasting plasma glucose and HbA1c. There was also a significant (P<0.05) reduction in plasma membrane calcium (PMCA) ATPase activity in diabetic subjects compared to controls. Cells from Type 2 diabetics exhibited an increased Ca2+ influx (as measured both by Fluo-3 fluorescence and 45Ca assays) as a consequence of thapsigargin-mediated Ca2+ store depletion. Upon addition of Mn2+ (a surrogate of Ca2+), the fura-2 fluorescence decayed in an exponential fashion and the rate and extent of this decline was steeper and greater in cells from type 2 diabetic patients. There was also a significant (P<0.05) difference in the Na+/Ca2+ exchange activity in Type 2 diabetic patients, both under resting conditions and after challenging the cells with thapsigargin, when the internal store Ca2+ sequestration was circumvented. Pharmacological activation of protein kinase C (PKC) in cells from patients resulted in only partial inhibition of Ca2+ entry. We conclude that cellular Ca2+ accumulation in cells from Type 2 diabetes results from (a) reduction in PMCA ATPase activity, (b) modulation of Na+/Ca2+ exchange and (3) increased Ca2+ influx across the plasma membrane.
Balasubramanyam
M
M
Center for Biotechnology, Anna University, Chennai, India.
Balaji
R A
RA
Subashini
B
B
Mohan
V
V
D000328:Adult; D000818:Animals; D002118:Calcium / Q000097:blood*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000097:blood; D002462:Cell Membrane / Q000201:enzymology; D003600:Cytosol / Q000378:metabolism; D003924:Diabetes Mellitus, Type 2 / Q000097:blood*; D004721:Endoplasmic Reticulum / Q000201:enzymology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D006706:Homeostasis; D006801:Humans; D008214:Lymphocytes / Q000378:metabolism; D008875:Middle Aged; D012519:Sarcoplasmic Reticulum / Q000201:enzymology; D019831:Sodium-Calcium Exchanger / Q000097:blood; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D019831:Sodium-Calcium Exchanger; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium