Interaction of extracellular calcium and magnesium in the regulation of cytosolic calcium and PTH release in dispersed bovine parathyroid cells

Abstract

We examined the effects of varying the extracellular magnesium (Mg 2+) concentration at different extracellular calcium (Ca 2+) concentrations on cytosolic Ca 2+ and PTH release in dispersed bovine parathyroid cells using the fluorescent, Ca 2+-sensitive dye QUIN-2. Raising extracellular Mg 2+ from 0.5 to 5 mM produced significant ( p < 0.01) increases in cytosolic Ca 2+ from 228 ± 13 to 306 ± 15 nM at 0.5 mM extracellular Ca 2+ and from 199 ± 9 to 299 ± 19 nM at 1.0 mM extracellular Ca 2+. The concentration of extracellular Mg 2+ which produced half of the maximal increase in cytosolic Ca 2+ was significantly higher, however, at 0.5 mM than at 1.0 mM extracellular Ca 2+ [3.3 ± 0.16 ( n = 5) vs. 2.0 ± 0.08 mM Mg 2+( n = 6), p < 0.01]. High extracellular Mg 2+ (5 mM) was associated with a similar inhibition of PTH release at 0.5 and 1.0 mM Ca 2+ [67 ± 2% ( n = 4) and 59 ± 5% ( n = 7), respectively], but half-maximal inhibition of secretion occurred at a higher Mg 2+ concentration with 0.5 than with 1.0 mM Ca 2+ [3.5 ± 0.43 ( n = 4) vs. 2.0 ± 0.21 mM ( n = 5), respectively, p < 0.01]. In contrast, in cells exposed to subphysiological extracellular Ca 2+ concentrations (⩽ 10 −6 M), 5 mM Mg 2+ had little or no effect on either cytosolic Ca 2+ or PTH release. At 10 mM extracellular Mg 2+ with ⩽ 10 −6 M Ca 2+, PTH release was inhibited 55 ± 3% ( n = 6). There was only a transient increase in calculated cytosolic Ca 2+ at 10 mM Mg 2+ in 4 of 6 experiments, however, possibly due to quenching of the fluorescence of QUIN-2 by increased intracellular Mg 2+ concentrations. This latter possibility is supported by the observation that high extracellular Mg 2+ (5 mM) reduced the cytosolic Ca 2+ measured with high (2 mM) extracellular Ca 2+ concentrations from 646 ± 68 nM ( n = 13) at 0.5 mM Mg 2+ to 392 ± 38 nM ( n = 8) at 5 mM Mg 2+. Thus, at physiological levels of extracellular Ca 2+ (0.5 and 1 mM), the close correspondence between the effects of extracellular Mg 2+ on cytosolic Ca 2+ concentration and PTH release provides further evidence for a role for cytosolic Ca 2+ in the regulation of parathyroid function by Mg 2+ . Moreover, the marked decrease in the potency of Mg 2+ in modifying PTH release and cytosolic Ca 2+ at very low extracellular Ca 2+ concentrations suggests that Mg 2+ may increase cytosolic Ca 2+, at least in part, through uptake of extracellular Ca 2+.