The FPA and FPB prostanoid receptor isoforms are G-protein-coupled receptors that are activated by prostaglandin F2α (PGF2α). 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]PGF2α. 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 PGF2α to FPA-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 FPB-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 PGF2α (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 PGF2α 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.
- Chloride channel
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