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+.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.