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 FK506 concentrations up to 10 μM (Km=0.47 μM), and 60% of total 45Ca2+ uptake was released at FK506 concentrations over 10 μM (Km=55 μM). Preincubation of the vesicles with cyclic ADP-ribose (cADPR, 0.5 μM) increased the FK506 (≤10 μM)-induced 45Ca2+ release (Ozawa T, Biochim Biophys Acta 1693: 159-166, 2004). Preincubation with heparin (200 μg/ml) resulted in significant inhibition of the FK506 (30 μM)-induced 45Ca2+ release. Subsequent addition of inositol 1,4,5-trisphosphate (IP3, 5 μM) after FK506 (100 μM)-induced 45Ca2+ release did not cause any release of 45Ca2+. These results indicate that two types of FK506-induced Ca2+ release mechanism operate in the endoplasmic reticulum of rat pancreatic acinar cells: a high-affinity mechanism of Ca2+ release, which involves activation of the ryanodine receptor, and a low-affinity mechanism of Ca2+ release, which involves activation of the IP3 receptor.

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