<p>This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.</p> <p>Altered cytosolic Ca<sup>2+</sup> is implicated in the aetiology
of many diseases including diabetes but there are
few studies on the mechanism(s) of the altered Ca<sup>2+</sup>
regulation. Using human lymphocytes, we studied
cytosolic calcium (Ca<sub>i</sub>) and various Ca<sup>2+</sup> transport
mechanisms in subjects with Type 2 diabetes
mellitus and control subjects. Ca<sup>2+</sup>-specific fluorescent
probes (Fura-2 and Fluo-3) were used to
monitor the Ca<sup>2+</sup> signals. Thapsigargin, a potent and
specific inhibitor of the sarco(endo)plasmic reticulum
Ca<sup>2+</sup>-ATPase (SERCA), was used to study Ca<sup>2+</sup>-
store dependent Ca<sup>2+</sup> fluxes. Significant (<italic>P</italic> &lt; 0.05)
elevation of basal Ca<sub>i</sub> levels was observed in
lymphocytes from diabetic subjects. Ca<sub>i</sub> levels were
positively correlated with fasting, plasma glucose
and HbAlc. There was also a significant (<italic>P</italic> &lt; 0.05)
reduction in plasma membrane calcium (PMCA)
ATPase activity in diabetic subjects compared to
controls. Cells from Type 2 diabetics exhibited an
increased Ca<sup>2+</sup> influx (as measured both by Fluo-3
fliorescence and C45a assays) as a consequence of
of thapsigargin-mediated Ca<sup>2+</sup> store depletion. Upon
addition of Mn<sup>2+</sup> (a surrogate of Ca<sup>2+</sup>), 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 (<italic>P</italic> &lt; 0.05) difference in the
Na<sup>+</sup>/Ca<sup>2+</sup> exchange activity in Type 2 diabetic
patients, both under resting conditions and after challenging the cells with thapsigargin, when the
internal store Ca<sup>2+</sup> sequestration was circumvented.
Pharmacological activation of protein kinase C
(PKC) in cells from patients resulted in only partial
inhibition of Ca<sup>2+</sup> entry. We conclude that cellular
Ca<sup>2+</sup> accumulation in cells from Type 2 diabetes
results from (a) reduction in PMCA ATPase activity,
(b) modulation of Na<sup>+</sup>/Ca<sup>2+</sup> exchange and (3)
increased Ca<sup>2+</sup> influx across the plasma membrane.</p>
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Int J Exp Diabetes Res. 2000; 1(4):275-287