Different effects of PTH on erythrocyte calcium influx

Since Ca ions mediate cellular response to parathyroid hormone (PTH), the effect of PTH on cell membrane Ca influx was studied by measuring erythrocyte passive Sr influx. Two different experiments have been performed: 1) Sr influx was determined in erythrocytes from 10 patients with primary hyperparathyroidism and 14 controls either in the absence and in the presence of nitrendipine, a dihydropiridine (DHP) Ca channel blocker; the results were compared in the two patient groups; 2) Sr influx was measured in erythrocytes from 4 normal subjects after incubation with and without PTH and the results in the presence of the hormone were compared with those in its absence. In the first experiment erythrocyte Sr influx was lower in patients with primary hyperparathyroidism when measured without nitrendipine (total Sr influx), but did not differ with controls when measured in presence of the inhibitor (DHP- independent Sr influx), Erythrocyte Sr uptake inhibited by nitrendipine (DHP- dependent Sr influx) was calculated by the difference between total and DHP- independent Sr influx values and was decreased in hyperparathyroid patients. In the second experiment Sr influx was increased after incubation of erythrocytes with PTH. We hypothesize that in primary hyperparathyroidism the high plasma levels of PTH may induce a downregulation of the erythrocyte Ca influx, mediated by the increase in cellular Ca content. These events may sustain the desensitization to physiological effects of PTH that has been reported in patients with primary hyperparathyroidism and after prolonged PTH infusion.