Further characterization of the slow muscarinic responses in Xenopus oocytes

In immature follicular ocytes of the frog Xenopus laevis, application of muscarinic agonists evokes a complex response consisting of a fast and a slow Cl currents (the dominant responses), Cl current fluctuations, and a less prominent slow K current. The characteristics of the slow ACh-evoked potassium current were studied using the two-electrode voltage clamp method, and compared to those of the ACh-evoked Cl currents. In experiments designed to study the K current response separately, without the interference of ACh-evoked Cl currents, the holding potential was set close or equal to Cl equilibrium potential (measured as the reversal potential of the ACh-evoked Cl current). The Cl current responses were studied in cells that had negligible K current response. The dose-response curve of the potassium response followed classical Michaelis-Menten kinetics. The dose-response characteristics of the slow ACh-evoked Cl current displayed a positive cooperativity of at least 3. In spite of this difference, kinetic analysis revealed that these two responses, as well as the fast Cl current response that was characterized earlier (Dascal and Landau 1982), had almost identical apparent equilibrium dissociation constants (0.29-0.39 μM), suggesting involvement of a single receptor class. Both K and Cl currents were reduced (to 32-56% of control) by millimolar concentrations of phosphodiesterase (PDE) inhibitors, theophylline and isobutylmethylxanthine. Elevation of extracellular Ca concentration from 1 to 10 mM doubled the K current; depletion of external Ca caused a partial inhibition of this response. The K current was potentiated by 0.1 μM 4-phorbol 12,13-dibutyrate (PDBu). Ca-dependence of the ACh-evoked K current resembles that of ACh-evoked Cl currents, described earlier, and suggests mediation by a similar mechanism, i.e. mobilization of Ca from intracellular stores. On the other hand, most of the features described here are in a sharp contrast to those reported for adenosine-evoked, cAMP-mediated slow K current. Thus, we suggest that purinergic and muscarinic receptors in Xenopus follicular oocytes are coupled to potassium channels through different molecular mechanisms.