Stimulation of intracellular calcium concentration by adenosine triphosphate and uridine 5'-triphosphate in human term placental cells: evidence for purinergic receptors.

Recent data suggest an important role for calcium (Ca2+) in human placental endocrinology. Thus, the regulation of Ca2+ influx seems to be implicated in the modulation of human placental lactogen and hCG release. A possible mechanism of influx regulation is through receptor-operated channels. One of the most characterized receptor gating Ca2+ channels, the ATP receptor, stimulates the intracellular calcium concentration ([Ca2+]i) in various tissues. The aim of this study was to determine whether ATP receptors gating Ca2+ channels are also present in placental cells. We thus determined the effect of ATP on [Ca2+]i in human term trophoblastic cells loaded with the Ca(2+)-responsive fluorescent dye fura-2. ATP stimulated a 4.3 +/- 0.4 (+/- SE)-fold increase in [Ca2+]i, with a half-maximal effective concentration (EC50) of 1.5 mumol/L. The pharmacological activation profile suggests the presence of purinergic P2u receptors (nucleotide receptors), because uridine 5'-triphosphate (UTP) also stimulated [Ca2+]i (4.0-fold increase, with an EC50 of 10 mumol/L). The ATP-stimulated [Ca2+]i was partly sensitive to pertussis toxin; we observed a 58% inhibition of ATP-induced [Ca2+]i with the toxin without effect on basal [Ca2+]i. The ATP- and UTP-stimulated [Ca2+]i declined with time in the presence of ATP (or UTP). The rate of deactivation was rapid (t1/2, < 60 s with 10(-5) mol/L ATP) and concentration dependent. The deactivation occurring during one application of ATP or UTP resulted in a diminution of subsequent responses. The recovery was incomplete even with long waiting times (up to 30 min). ATP and UTP also stimulated inositol phosphate production with EC50 values of 11 and 15 mumol/L, respectively, but not human placental lactogen or hCG release in experiments in which known secretagogues were effective. The results suggest the presence in human term placental cells of P2u receptors pharmacologically similar to those observed in other tissues, especially in the pituitary and amnion. The physiological significance of this stimulation of [Ca2+]i by ATP and UTP in the human placenta remains to be investigated.