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Abstract
The effects of 3.0 mM Mn 2+ on parathyroid hormone (PTH) release, 45Ca fluxes and the cytoplasmic Ca 2+ concentration (Ca 2- i) were studied in vitro with use of parathyroid glands from rats and cattle. Mn 2+ inhibited PTH release by about 70 and 60% at Ca 2+ concentrations of < 10 nM or 0.5 mM, respectively, and like the inhibitory effect of Ca2+, that of Mn2+ was rapid and reversible. Mn 2+-induced inhibition of the PTH release was associated with reduction of 45Ca uptake by about 50% and a marked but temporary increase in 45Ca efflux. Mn 2+ induced a rapid transient increase in Ca 2+ i provided that the intracellular release of Ca 2+ had not already been triggered by previous exposure to 3.0 mM Ca 2+. This initial action was apparently followed by influx of Mn 2+, since pronounced quenching of the fluorescence from the Ca 2+ indicator fura-2 was observed. The results indicate that Mn 2+ behaves in several respects like a calcium analogue in parathyroid cells and may inhibit PTH release by interfering directly with the secretory machinery.
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