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Calcified Tissue International

, Volume 37, Issue 3, pp 318–323 | Cite as

Skeletal adenylate cyclase: Effect of Mg2+, Ca2+, and PTH

  • Robert K. Rude
Laboratory Investigations

Summary

Plasma membranes were prepared from mineralized guinea pig bone in order to study Mg2+ and Ca2+ modulation of skeletal adenylate cyclase. Plasma membrane preparation was accomplished by crushing the bone in liquid N2 and subsequent multiple washings in buffer containing EGTA to remove all Ca2+ prior to adenylate cyclase assay. Skeletal adenylate cyclase was found to be dependent on GTP and Mg2+ and responsive to bovine 1-34 PTH. Ca2+ caused a competitive inhibition of Mg2+-activated skeletal adenylate cyclase. The apparent KaMg was 1.9±0.3 in the presence of 0.2 μM Ca2+ but increased to a mean of 7.2±1.3 in the presence of 5.0 μM Ca2+. Analysis of the Ca2+ inhibition curves at concentrations from .05 μM-1.0 mM were consistent with the presence of two Ca2+ inhibition sites, one with an apparent Ki of 1–2 μM and the other with an apparent Ki of approximately 500 μM. Lowering the Mg2+ concentration increased the contribution of the high affinity Ca2+ binding site to the overall Ca2+ inhibition, and raising the Mg2+ concentration had the opposite effect. While bPTH 1-34 enhanced adenylate cyclase activity, it did not increase the affinity of Mg2+ for skeletal adenylate cyclase nor did it alter the KiCa or the pattern of Ca2+ inhibition. These data may explain the skeletal resistance to PTH during Mg deficiency. A fall in the intracellular Mg would render the adenylate cyclase more susceptible to inhibition by the prevailing intracellular Ca2+ concentration. Since PTH does not appear to modulate either Mg2+ activation or Ca2+ inhibition of skeletal adenylate cyclase, cAMP-mediated PTH induction of bone resorption would be impaired, and hypocalcemia would occur.

Key words

Skeleton Magnesium Calcium Parathyroid hormone Adenylate cyclase 

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Copyright information

© Springer-Verlag 1985

Authors and Affiliations

  • Robert K. Rude
    • 1
    • 2
  1. 1.Department of MedicineUniversity of Southern California School of MedicineLos Angeles
  2. 2.Bone and Connective Tissue Research LaboratoriesOrthopaedic Hospital/University of Southern CaliforniaLos Angeles

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