(Im)munosuppressants such as cyclosporinA and FK506 (tacrolimus) are widely prescribed to treat numerous disorders and to treat organ transplant recipients. However, cyclosporine A and FK506 are both known to produce hypomagnesaemia. The mechanism of this effect is still unclear. The present study determined the effects of immunosuppressant treatment on the parathyroid hormone (PTH) mediated Mg2+ uptake and the mitogen-activated protein kinase (MAPK) activation in mouse distal convoluted tubule (MDCT) cells. The intracellular Ca2+ and Mg2+ concentrations in a single MDCT cell were measured by using the fluorescentdye Fura-2 AM and Mag-fura-2 AM, respectively. Cyclosporine A and FK506 illicited a transient increase of intracellular Ca2+ from a basal level of 99 16 nM to 685 +/- 105 and 608 +/- 96 nM, respectively. In order to determine the Mg2+ transport, the MDCT cells were Mg2+-depleted by culturing them in Mg2+-free media for 16 h, and the Mg2+ uptake was measured by microfluorescence after placing the depleted cells in 1.5 mM MgCl2. The mean rate of Mg2+ uptake, d([Mg2+](i))/dt was 140 +/- 16 nM/s in the control MDCT cells. PTH increased the Mg2+ uptake more than 2 times in this cell. Cyclosporine A (10 mu M) and FK506 (0.1 pM) did not affect the basal Mg(2+)uptake (140 +/- 16 and 142 +/- 14 nM/s, respectively), but they inhibited the PTH-stimulated Mg2+ entry, decreasing it from 248 +/- 12 to 147 +/- 7 and 148 +/- 14 nM/s, respectively. These effects were inhibited by L685818, which is a potent competitive antagonist of FK506. PTH stimulated the extracellular signal-regulated kinase1/2 (ERK1/2) protein synthesis. This PTH-stimulated ERK1/2 activation was inhibited by cyclosporine A and FK506. In the present study, the role of ERK1/2 activation on the PTH-dependent magnesium uptake was examined in MDCT cells, and we showed that immunosuppressants inhibit the hormone-stimulated Mg2+ uptake into the MDCT cells by inhibiting the MAPK pathway. (c) 2006 Elsevier Inc. All rights reserved.