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 Cai levels was observed in
lymphocytes from diabetic subjects. Cai levels were
positively correlated with fasting, plasma glucose
and HbAlc. 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
fliorescence and C45a assays) as a consequence of
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.
