PARATHYROID HORMONE-INDUCED CALCIUM-RELEASE FROM INTRACELLULAR STORESIN A HUMAN KIDNEY-CELL LINE IN THE ABSENCE OF STIMULATION OF CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE PRODUCTION

PTH-induced mobilization of cytosolic Ca2+ in a human kidney cell line (HEK/W) occurring in the absence of cAMP stimulation was characterize d and compared with that obtained in the same cells stably transfected by the PTH/PTH-related peptide (PTHrp) receptor (HEK/T). In both cell lines, N-terminal fragments of PTH and PTHrp induced a concentration- dependent biphasic stimulation in [Ca2+](i): a transient peak followed by a slow linear increase. These increases in [Ca2+](i) were inhibite d by the PTH antagonist [Nle(8,18),Tyr(34)]bPTH(3-34). The transient p eaks were due to calcium release from intracellular stores, as they re sisted quenching of calcium in the extracellular buffer and were aboli shed by prior emptying of intracellular stores. These peaks differed, however, both in latency period and in magnitude, in the two cell line s. The phospholipase C inhibitor U73122 inhibited the PTH-induced incr ease in [Ca2+](i) in HEK/T cells, but not in HEK/W. Similarly, PTH-ind uced inositol phosphate (InsP(s)) production was detected in HEK/T but not in HEK/W cells. PTH-induced calcium release in HEK/W cells was in hibited by the simultaneous presence of ryanodine and U73122. Low leve l PTH/PTHrp receptor messenger RNA expression was demonstrated by ribo nuclease protection in HEK/W cells, although no specific binding of [I -125]PTHrP(1-34) could be detected. Amplification products for the PTH /PTHrp receptor 1, but no other isoforms, were detected by RT-PCR in H EK/W cells. As expected, HEK/T cells responded to PTH by a 500-fold st imulation in cAMP production and expressed large numbers of PTH PTHrp receptors, as shown by [I-125]PTHrp binding. These results demonstrate that the signal transduction pathways activated by PTH in HEK/W and H EK/T cells are different. Because the major difference in these cell l ines is the number of PTH/PTHrp receptors expressed, these results sug gest that the transduction of signals by the PTH/PTHrp receptor is con trolled by receptor number in such a way that PTH stimulates an increa se in intracellular calcium in the absence of stimulation of InsP(s) a nd cAMP production in cells expressing low levels of PTH/PTHrp recepto r, but stimulates calcium release through an InsP(s) pathway and induc es cAMP production in cells expressing large numbers of PTH/PTHrp rece ptors. The control of receptor number may be one of the mechanisms thr ough which PTH effects are regulated.