Oxytocin induces a transient increase in cytosolic free [Ca2+] in renal tubular epithelial cells: evidence for oxytocin receptors on LLC-PK1 cells

We examined the effects of oxytocin on renal tubular epithelial LLC-PK1 cells. In cells loaded with Fura 2, we found that 1 microM oxytocin induced a rapid increase in cytosolic free [Ca2+]i from 120 nM to 250 nM within 12 sec. [Ca2+]i then decreased and leveled at 148 nM. Calcium was mobilized from intra- and extra-cellular sources. Oxytocin-induced calcium mobilization was dose dependent (EC50 between 5 and 30 nM). Oxytocin also stimulated calcium efflux which was blocked by the selective oxytocin antagonist KB-5-21. Calcium mobilization was a likely consequence of enhanced phosphatidylinositol turnover, because oxytocin rapidly increased the formation of inositol phosphates including Ins1,4,5P3. Calcium transients were induced by oxytocin and the oxytocin selective analog AM-2-40 and blocked by the oxytocin-selective antagonist KB-5-21. Lysine vasopressin, the selective V2 agonist dDAVP, and the V1-selective agonist SK&F 105349 were at least 10- to 100-fold less potent than oxytocin and exhibited only partial agonist activity. Using peptide analogs, a poor correlation was found between antagonism of oxytocin-induced calcium transients of LLC-PK1 cells and pig kidney V2 and rat liver V1 receptor affinity. These data indicate that oxytocin-induced calcium transients in LLC-PK1 cells were not mediated by V1 or V2 vasopressin receptors, but by oxytocin receptors. However, the poor correlation between antagonism at the LLC-PK1 receptors and the rat uterus oxytocin receptors suggests marked differences in antagonist recognition. We have also identified specific, saturable, high affinity oxytocin-binding sites of low density on intact LLC-PK1 cells (KD = 1.9 nM; Bmax = 3.2 fmol/10(6) cells). The relative analog affinities for these binding sites correlated well with their effects on oxytocin-induced calcium transients. We conclude that in LLC-PK1 cells, oxytocin stimulates a transient rise in cytosolic free [Ca2+]i and the formation of inositol phosphates, including Ins1,4,5P3. The effects on [Ca2+]i probably are not mediated by V1 and V2 vasopressin receptors, but by putative oxytocin receptors.