Parathyroid hormone increases the sensitivity of inositol trisphosphate receptors by a mechanism that is independent of cyclic AMP

In fura 2‐loaded HEK‐293 cells stably expressing human type 1 parathyroid hormone (PTH) receptors, PTH potentiated the Ca2+ mobilization evoked by carbachol by >4 fold without itself increasing the intracellular [Ca2+]. PTH potentiated the Ca2+ release evoked by a cell‐permeant analogue of inositol 1,4,5‐trisphosphate (InsP3BM). Prolonged incubation with InsP3BM emptied the Ca2+ stores as effectively as PTH in combination with a maximal concentration of carbachol, indicating that PTH did not increase the size of the InsP3‐sensitive Ca2+ pool. Responses to PTH were unaffected by disruption of the cytoskeleton. The EC50 for carbachol‐evoked Ca2+ release and InsP3 formation were indistinguishable (∼40 μM), consistent with even the highest concentrations of carbachol generating insufficient InsP3 to release the entire InsP3‐sensitive Ca2+ pool. Inhibition of cyclic AMP‐dependent protein kinase A (PKA), using H89 or CMIQ, did not affect potentiation of carbachol‐evoked Ca2+ signals by PTH. SQ22536 or DDA, inhibitors of adenylyl cyclase, inhibited PTH‐evoked cyclic AMP formation and IBMX, an inhibitor of cyclic nucleotide phosphodiesterase, increased the amount of cyclic AMP detected after stimulation by PTH. None of these drugs affected the potentiation of Ca2+ signals by maximal or submaximal concentrations of PTH. We conclude that PTH potentiates the Ca2+ release evoked by receptors that stimulate InsP3 formation by sensitizing InsP3 receptors through a cyclic AMP‐independent mechanism. British Journal of Pharmacology (2003) 138, 81–90. doi:10.1038/sj.bjp.0705011