FK506-induced Ca²⁺ release from microsomal vesicles of rat pancreatic acinar cells is biphasic

Abstract

The effect of the immunosuppressant drug FK506 on microsomal Ca²⁺ release was investigated in rat pancreatic acinar cells. When FK506 (0.1-200 μM) was added to the microsomal vesicles at a steady state of ATP-dependent ⁴⁵Ca²⁺ uptake, FK506 caused a dose-dependent and a biphasic release of ⁴⁵Ca²⁺. Almost 10% of total ⁴⁵Ca²⁺ uptake was released at concentrations of FK506 up to 10 μM (K_m = 0.47 μM), and 60% of total ⁴⁵Ca²⁺ uptake was released at concentrations of FK506 over 10 μM (K_m = 55 μM). Preincubation of the vesicles with cyclic ADP-ribose (cADPR: 0.5 μM), which is known to modulate the ryanodine receptor, increased the FK506 (< 10 μM)-induced ⁴⁵Ca²⁺ release (V_max value of the release: 8.1% without cADPR vs. 14.4% with cADPR). Preincubation with 200 μg/ml of heparin, an inhibitor of inositol 1,4,5-trisphosphate (IP₃) receptor, resulted in significant inhibition of the FK506 (30 μM)-induced ⁴⁵Ca²⁺ release. Subsequent addition of IP₃ (5 μM) after FK506 (100 μM)-induced ⁴⁵Ca²⁺ release did not cause any release of ⁴⁵Ca²⁺. These results indicate that there are two different types of FK506-induced Ca²⁺ release mechanisms in the endoplasmic reticulum of rat pancreatic acinar cells: a high-affinity mechanism of Ca²⁺ release, which is activation of the ryanodine receptor, and a low-affinity mechanism of Ca²⁺ release, which is activation of the IP₃ receptor. [J Physiol Sci. 2006;56 Suppl:S108]