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Pflügers Archiv

, Volume 409, Issue 4–5, pp 512–520 | Cite as

Further characterization of the slow muscarinic responses inXenopus oocytes

  • Nathan Dascal
  • Sasson Cohen
Excitable Tissues and Central Nervous Physiology

Abstract

In immature follicular ocytes of the frogXenopus 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 inXenopus follicular oocytes are coupled to potassium channels through different molecular mechanisms.

Key words

Xenopus oocytes Muscarinic response Chloride current Potassium current Ca mobilization Phorbol esters Cyclic AMP 

Abbreviations

ACh

acetylcholine

cAMP

cyclic adenosine 3′,5′-monophosphate

cGMP

cyclic guanosine 3′,5′-monophosphate

EGTA

ethylenediaminetraacetic acid

Hepes

N-2-hydroxyethylpiperazine-N′-2-hydroxypropanesulphonic acid

IBMX

3-isobutyl-1-methylxanthine

IP3

inositol 1′,4′,5′-trisphosphate

PDBu

4β-phorbol 12,13-dibutyrate

PDE

phosphodiesterase

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Copyright information

© Springer-Verlag 1987

Authors and Affiliations

  • Nathan Dascal
    • 1
  • Sasson Cohen
    • 1
  1. 1.Department of Physiology and Pharmacology, Sackler School of MedicineTel Aviv UniversityRamat AvivIsrael

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