Effect of calcium on uptake of α-aminoisobutyric acid by parathyroid glands

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Abstract
The uptake of α-aminoisobutyric acid (AIB) has been used as a model for the study of amino acid transport in the parathyroid gland. Freshly excised, whole rat parathyroids rapidly took up AIB when incubated in Krebs bicarbonate buffer under 5% CO2, 95% O2 at 37 C, with glucose as the only nutrient. Uptake was inversely proportional to calcium concentration in the medium over the range of 0.75 to 2.25 mm/ liter. Increasing magnesium concentration enhanced AIB uptake. Although AIB uptake was decreased in the absence of phosphate there was no stimulatory effect of increasing phosphate concentration from 0.5 to 3.0 mm/liter. Strontium had an inhibitory effect on uptake but was not as effective as calcium on an equimolar basis. High concentrations of leucine and glycine caused inhibition of uptake as did acidifying the medium to pH 7.1. Growth hormone, vitamin D, parathyroid extract, and hydrocortisone were without effect. AIB uptake of thyroid, kidney, muscle, and liver were not inhibited by calcium. The hypothesis that changes in calcium concentration control parathyroid hormone synthesis by inhibition of amino acid transport at the cell membrane is presented.