Differential regulation of Ca²⁺-dependent Cl⁻ currents by FP prostanoid receptor isoforms in Xenopus oocytes

The FP_A and FP_B prostanoid receptor isoforms are G-protein-coupled receptors that are activated by prostaglandin F_2α (PGF_2α). Differences in their carboxyl termini prompted us to examine the intracellular calcium (Ca²⁺) signaling of these receptor isoforms using the Xenopus oocyte expression system. Protein expression was determined by immunofluorescence microscopy and whole cell binding with [³H]PGF_2α. Positive immunolabeling was observed on the outer membranes of oocytes expressing FLAG-tagged FP receptor isoforms, but not on control (water-injected) oocytes. Intracellular signaling was examined using a two-electrode voltage clamp. Specific whole-cell binding was also detected for both receptor isoforms. Bath application of 10μM PGF_2α to FP_A-expressing oocytes produced a chloride (Cl^-) current response similar to that of an injection of inositol 1,4,5-trisphosphate (InsP₃) (5.76±0.6μA, peak current; N=23) that returned to control levels within 25min. In FP_B-expressing oocytes the activation of the Cl^- current was delayed or completely absent (1.38±0.2μA, peak current; N=18). Control oocytes were not responsive to the application of PGF_2α (0.87±0.1μA, peak current; N=10). Activation of Cl^- currents for both FP receptor isoforms was dependent upon intracellular Ca²⁺ stores as a 30-min pretreatment with thapsigargin (1μM; N=5) blocked the PGF_2α induction of the Cl^- current. These data indicate that the FP prostanoid receptor isoforms differ in their ability to activate Ca²⁺-dependent Cl^- channels when expressed in Xenopus oocytes. The difference appears to be in the ability of the two FP prostanoid receptor isoforms to mobilize intracellular calcium.