Evidence for suppression of parathyroid gland activity by hypermagnesemia

Abstract

The effect of hypermagnesemia, produced by MgCl₂ infusion, on the activity of parathyroid glands, as assessed by changes in levels of serum calcium (S_Ca) and in the fraction of filtered phosphate excreted (C_P/C_Cr), was studied in 11 intact and 4 thyroparathyroidectomized (T-PTX) dogs. To exclude the effect of diurnal variation in C_P/C_Cr on the results, studies were initiated in both morning and afternoon hours and each study with MgCl₂ infusion was paired with a control experiment in the same dog not receiving MgCl₂. During MgCl₂ infusion, serum phosphorus rose progressively. Despite this rise, the levels of C_P/C_Cr fell in all experiments and were significantly different from values observed at the same time of the day in the paired control experiments. The concentrations of total S_Ca fell by 1.0-2.4 mg/100 ml with a proportional decrease in the levels of the diffusible and ionized fractions. The pattern of the fall in C_P/C_Cr during MgCl₂ resembled that observed after CaCl₂ infusion (seven dogs) and that which acutely followed thyroparathyroidectomy (seven dogs). When parathyroid extract was given to dogs receiving MgCl₂ infusion both C_P/C_Cr and S_Ca rose, and MgCl₂ infusion did not affect C_P/C_Cr and S_Ca in T-PTX dogs. These results indicate that hypermagnesemia suppresses the activity of the parathyroid glands, probably, by inhibiting production and (or) release of the hormone, without interfering with end-organ response. An increase in serum magnesium of 1.7-2.0 mg/100 ml was capable of producing the suppressive effect. Evaluation of the effect of simultaneous modest hypocalcemia and hypermagnesemia suggests that a decrease in the level of serum calcium is more potent than an increase in the concentration of serum magnesium in the regulation of parathyroid activity.