Modulation of inwardly rectifying Na(+)-K+ channels by serotonin and cyclic nucleotides in salivary gland cells of the leech, Haementeria.
The electrically excitable salivary cells of the giant Amazon leech, Haementeria, display a time-dependent inward rectification. Under voltage clamp, hyperpolarizing steps to membrane potentials negative to about -70 mV were associated with the activation of a slow inward current (Ih) which showed no inactivation with time. The time course of activation of Ih was described by a single-exponential function and was strongly voltage dependent. The activation curve of Ih ranged from -72 to -118 mV, with half-activation occurring at -100 mV. Ion-substitution experiments indicated that Ih is carried by both Na+ and K+ ions. 5-Hydroxytryptamine (5-HT) increased the amplitude of Ih and its rate of activation. It also produced a positive shift of the activation curve of the conductance underlying Ih (Gh) without altering the slope factor, thus indicating that the voltage dependence of Ih was modulated by 5-HT. Cs+ blocked both Ih and the 5-HT-potentiated current in a voltage-independent manner, whereas Ba2+ had little effect. It is concluded that 5-HT increases Ih by modulating the inwardly rectifying Na(+)-K+ channels in the salivary cells. The effect of 5-HT may be mediated by an increase in adenylate cyclase activity since Ih was increased by 8-bromo-cyclic AMP and by the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. In contrast, Ih was reduced by 8-bromo-cyclic GMP and by zaprinast (an inhibitor of cyclic GMP-sensitive phosphodiesterase). Cyclic GMP itself also reduced Ih, and the effect was specific to the 3',5' form; 2',3'-cyclic GMP was inactive. The results suggest that the inward-rectifier channel may be modulated in opposite directions by cyclic AMP and cyclic GMP.
Wuttke
W A
WA
Biomedical and Physiological Research Group, School of Biological Sciences, University College of Swansea, Singleton Park, Wales.
Berry
M S
MS
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology; D002712:Chlorides / Q000378:metabolism; D000242:Cyclic AMP / Q000502:physiology*; D006152:Cyclic GMP / Q000502:physiology*; D066298:In Vitro Techniques; D007700:Kinetics; D007865:Leeches; D008564:Membrane Potentials; D012469:Salivary Glands / Q000166:cytology / Q000378:metabolism; D012701:Serotonin / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects / Q000502:physiology*
D002712:Chlorides; D002586:Cesium; D001464:Barium; D012701:Serotonin; D000242:Cyclic AMP; D000254:Sodium-Potassium-Exchanging ATPase; D006152:Cyclic GMP