PTH-induced mobilization of cytosolic Ca2+ in a human kidney cell line (HEK/W) occurring in the absence of cAMP stimulation was characterized 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 inhibited by the PTH antagonist [Nle(8,18),Tyr(34)]bPTH(3-34). The transient peaks were due to calcium release from intracellular stores, as they resisted quenching of calcium in the extracellular buffer and were abolished by prior emptying of intracellular stores. These peaks differed, however, both in latency period and in magnitude, in the two cell lines. The phospholipase C inhibitor U73122 inhibited the PTH-induced increase in [Ca2+]i in HEK/T cells, but not in HEK/W. Similarly, PTH-induced inositol phosphate (InsPs) production was detected in HEK/T but not in HEK/W cells. PTH-induced calcium release in HEK/W cells was inhibited by the simultaneous presence of ryanodine and U73122. Low level PTH/PTHrp receptor messenger RNA expression was demonstrated by ribonuclease protection in HEK/W cells, although no specific binding of [125I]PTHrP(1-34) could be detected. Amplification products for the PTH/PTHrp receptor 1, but no other isoforms, were detected by RT-PCR in HEK/W cells. As expected, HEK/T cells responded to PTH by a 500-fold stimulation in cAMP production and expressed large numbers of PTH/PTHrp receptors, as shown by [125I]PTHrp binding. These results demonstrate that the signal transduction pathways activated by PTH in HEK/W and HEK/T cells are different. Because the major difference in these cell lines is the number of PTH/PTHrp receptors expressed, these results suggest that the transduction of signals by the PTH/PTHrp receptor is controlled by receptor number in such a way that PTH stimulates an increase in intracellular calcium in the absence of stimulation of InsPs and cAMP production in cells expressing low levels of PTH/PTHrp receptor, but stimulates calcium release through an InsPs pathway and induces cAMP production in cells expressing large numbers of PTH/PTHrp receptors. The control of receptor number may be one of the mechanisms through which PTH effects are regulated.