Metformin regulates TRPM6, a potential explanation for magnesium imbalance in type 2 diabetes patients.

Metformin therapy is associated with lower serum magnesium (Mg2+) levels in type 2 diabetes patients. The TRPM6 channel determines the fine-tuning of Mg2+ (re)absorption in intestine and kidney. Therefore, we aimed to investigate the short- and long-term effects of metformin on TRPM6. Patch clamp recordings and biotinylation assays were performed upon 1 h of incubation with metformin in TRPM6-transfected HEK293 cells. Additionally, 24 h of treatment of mDCT15 kidney and hCaco-2 colon cells with metformin was applied to measure the effects on endogenous TRPM6 expression by quantitative real-time PCR. To assess Mg2+ absorption, 25Mg2+ uptake measurements were performed using inductively coupled plasma mass spectrometry. Short-term effects of metformin significantly increased TRPM6 activity and its cell surface trafficking. In contrast, long-term effects significantly decreased TRPM6 mRNA expression and 25Mg2+ uptake. Metformin lowered TRPM6 mRNA levels independently of insulin- and AMPK-mediated pathways. Moreover, in type 2 diabetes patients, metformin therapy was associated with lower plasma Mg2+ concentrations and fractional excretion of Mg2+. Thereby, short-term metformin treatment increases TRPM6 activity explained by enhanced cell surface expression. Conversely, long-term metformin treatment results in downregulation of TRPM6 gene expression in intestine and kidney cells. This long-term effect translated in an inverse correlation between metformin and plasma Mg2+ concentration in type 2 diabetes patients.

Diabetes-induced hypomagnesemia is not modulated by metformin treatment in mice.

Approximately 30% of patients with type 2 diabetes mellitus (T2D) have hypomagnesemia (blood magnesium (Mg2+) concentration <0.7 mmol/L). In T2D patients, treatment with metformin is associated with reduced blood Mg2+ levels. To investigate how T2D and metformin affect Mg2+ homeostasis db/m and db/db mice were treated with metformin or placebo. Mice were housed in metabolic cages to measure food and water intake, and to collect urine and feces. Serum and urinary Mg2+ concentrations were determined and mRNA expression of magnesiotropic genes was determined in kidney and distal colon using RT-qPCR. Db/db mice had significantly lower serum Mg2+ levels than db/m mice. Mild hypermagnesuria was observed in the db/db mice at two weeks, but not at four weeks. Metformin-treatment had no effect on the serum Mg2+ concentration and on the urinary Mg2+ excretion. Both in kidney and distal colon of db/db mice, there was a compensatory upregulation in the mRNA expression of magnesiotropic genes, such as transient receptor potential melastatin 6 (Trpm6), whereas metformin treatment did not affect gene expression levels. In conclusion, we show that T2D causes hypomagnesemia and that metformin treatment has no effect on Mg2+ homeostasis in mice.

Determinants of hypomagnesemia in patients with type 2 diabetes mellitus.

BACKGROUND: Hypomagnesemia (plasma magnesium (Mg2+) concentration <0.7 mmol/L) has been described in patients with type 2 diabetes. Polypharmacy is inevitable when treating a complex disease such as type 2 diabetes and could explain disturbances in the plasma Mg2+ concentration. In this study, we aimed to establish the extent of hypomagnesemia in a cohort of type 2 diabetes patients and to identify the determinants of plasma Mg2+ levels. METHODS: Patient data and samples of 395 type 2 diabetes patients were investigated. Plasma Mg2+ concentrations were measured using a spectrophotometric assay. Using Pearson correlation analyses, variables were correlated to plasma Mg2+ levels. After excluding confounding variables, all parameters correlating (P < 0.1) with plasma Mg2+ were included in a stepwise backward regression model. RESULTS: The mean plasma Mg2+ concentration in this cohort was 0.74 ± 0.10 mmol/L. In total, 121 patients (30.6%) suffered from hypomagnesemia. Both plasma triglyceride (r = -0.273, P < 0.001) and actual glucose levels (r = -0.231, P < 0.001) negatively correlated with the plasma Mg2+ concentration. Patients using metformin (n = 251, 62%), proton pump inhibitors (n = 179, 45%) or ß-adrenergic receptor agonists (n = 31, 8%) displayed reduced plasma Mg2+ levels. Insulin use (n = 299, 76%) positively correlated with plasma Mg2+ levels. The model predicted (R2) 20% of all variance in the plasma Mg2+ concentration. CONCLUSIONS: Hypomagnesemia is highly prevalent in type 2 diabetes patients. Plasma triglycerides and glucose levels are major determinants of the plasma Mg2+ concentration, whereas only a minor part (<10%) of hypomagnesemia can be explained by drug intake, excluding polypharmacy as a major cause for hypomagnesemia in type 2 diabetes.