Dr. Long Jun Dai

Dai LJ, Bapty BW, Ritchie G, Kerstan D, and Quamme GA: Insulin stimulates Mg²⁺ uptake in  mouse distal convoluted tubule cells. American Journal of Physiology 277:F907-F913, 1999.

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Abstract Insulin has been shown to be a magnesium-conserving hormone acting, in part, through stimulation of magnesium absorption within the thick ascending limb. Although the distal convoluted tubule possesses the most insulin receptors, it is unclear what, if any, actions insulin has in the distal tubule. The effects of insulin were studied on immortalized mouse distal convoluted tubule (MDCT) cells by measuring cellular cAMP formation with radioimmunoassays and Mg21 uptake with fluorescence techniques using mag-fura 2. To assess Mg21 uptake, MDCT cells were first Mg21 depleted to 0.22 6 0.01 mM by culturing in Mg21-free media for 16 h and then placed in 1.5 mM MgCl2, and the changes in intracellular Mg21 concentration ([Mg21]i) were measured with microfluorescence. [Mg21]i returned to basal levels, 0.53 6 0.02 mM, with a mean refill rate, d([Mg21]i)/dt, of 164 6 5 nM/s. Insulin stimulated Mg21 entry in a concentration-dependent manner with maximal response of 214 6 12 nM/s, which represented a 30 6 5% increase in the mean uptake rate above control values. This was associated with a 2.5-fold increase in insulin-mediated cAMP generation (52 6 3 pmol·mg protein21 ·5 min21). Genistein, a tyrosine kinase inhibitor, diminished insulin-stimulated Mg21 uptake (169 6 11 nM/s), but did not change insulin-mediated cAMP formation (47 6 5 pmol·mg protein21 ·5 min21). PTH stimulates Mg21 entry, in part, through increases in cAMP formation. Insulin and PTH increase Mg21 uptake in an additive fashion. In conclusion, insulin mediates Mg21 entry, in part, by a genistein-sensitive mechanism and by modifying hormone-responsive transport. These studies demonstrate that insulin stimulates Mg21 uptake in MDCT cells and suggest that insulin acts in concert with other peptide and steroid hormones to control magnesium conservation in the distal convoluted tubule.

Bapty BW, Dai LJ, Ritchie G, Jirik F, Canaff L, Hendy GN, and Quamme GA: Mg²⁺/Ca²⁺ sensing inhibits hormone-stimulated Mg²⁺ uptake in mouse distal convoluted tubule cells. American Journal of Physiology 275:F353-F360, 1998.

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Abstract The distal convoluted tubule plays a significant role in renal magnesium conservation. An immortalized mouse distal convoluted tubule (MDCT) cell line has been extensively used to study the cellular mechanisms of magnesium transport in this nephron segment. MDCT cells possess an extracellular polyvalent cation-sensing mechanism responsive to Mg21, Ca21, and neomycin. The present studies determined the effect of Mg21/ Ca21 sensing on hormone-mediated cAMP formation and Mg21 uptake in MDCT cells. MDCT cells were Mg21 depleted by culturing in Mg21-free media for 16 h, and Mg21 uptake was measured by microfluorescence after placing the depleted cells in 1.5 mM MgCl2. The mean rate of Mg21 uptake was 164 6 5 nM/s in control MDCT cells. Activation of Mg21/Ca21 sensing with neomycin did not affect basal Mg21 uptake (155 6 5 nM/s). We have previously reported that treatment of MDCT cells with either glucagon or arginine vasopressin (AVP) stimulated Mg21 entry. In the present studies, the addition of extracellular Mg21 or Ca21 inhibited glucagon- and AVP-stimulated cAMP formation and Mg21 uptake in concentration-dependent manner with half-maximal concentrations of ,1.5 and 3.0 mM, respectively. Exogenous cAMP or forskolin stimulated Mg21 uptake in the presence of Mg21/Ca21 sensing activation.We infer from these studies that Mg21/Ca21-sensing mechanisms located in the distal convoluted tubule may play a role in control of distal magnesium absorption.