ISSN:
1432-1424
Keywords:
cyclic nucleotides
;
5-HT
;
inward rectification
;
leech
;
salivary gland
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary 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 (I h) which showed no inactivation with time. The time course of activation of I hwas described by a single-exponential function and was strongly voltage dependent. The activation curve ofhranged from −72 to −118 mV, with half-activation occurring at −100 mV. Ion-substitution experiments indicated that I his carried by both Na+ and K+ ions. 5-Hydroxytryptamine (5-HT) increased the amplitude of I hand its rale of activation. It also produced a positive shift of the activation curve of the conductance underlying I h Ghwithout altering the slope factor, thus indicating that the voltage dependence of I hwas modulated by 5-HT. Cs+ blocked both I hand the 5-HT-polentiated current in a voltage-independent manner, whereas Ba2+ had little effect. It is concluded that 5-HT increases I hby 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 I hwas increased by 8-bromocyclic AMP and by the phosphodiesterase inhibitor, 3-isobutyl-l-methylxanthine. In contrast, I hwas reduced by 8-bromo-cyclic GMPand by zaprinast (an inhibitor of cyclic GMP-scnsitive phosphodieslerase). Cyclic GMP itself also reduced I h, 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
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00232758
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