Two opposite effects of ATP on the apparent sensitivity of the cGMP-gated channel of the carp retinal cone.
Effects of ATP on the activity of cGMP-gated channels from carp cone photoreceptors were studied. In 29% of the patches examined (N = 45), ATP (1 mM) enhanced a current evoked by cGMP (20 microM, up to about 100%), in 33%. ATP suppressed it by up to about 90%, and in the remaining 38%, ATP had no effect. ATP showed similar effects on a current evoked by 8-bromoguanosine 3',5'-cyclic monophosphate (2 microM, enhancing in 42% of the patches, suppressing in 25%, no effect in 33%, N = 12), suggesting that the effects were not through modulation of the phosphodiesterase. Both of the effects, enhancement and suppression, were produced by a change in apparent affinity for cGMP, since (1) the maximum current evoked by cGMP of the saturating concentration (> or = 1 mM) was not affected, and (2) the K1/2 value decreased by approximately 45% (N = 2) or increased by approximately 25% (N = 2). A lower pH (approximately 6) facilitated the enhancing effect. ATP-gamma-S (1 mM) showed a suppressing effect in 80% of the patches and no effect in 20% of the patches (N = 10). However, ATP-gamma-S did not show an enhancing effect. Thus, ATP had two opposite effects through different mechanisms on the apparent sensitivity of the channel to cGMP; increasing and decreasing.
Watanabe
S
S
Department of Physiology, Keio University School of Medicine, Tokyo, Japan.
Shen
J
J
D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology*; D000345:Affinity Labels / Q000494:pharmacology; D000818:Animals; D002347:Carps / Q000502:physiology*; D006152:Cyclic GMP / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006151:Guanosine / Q000031:analogs & derivatives / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D017949:Retinal Cone Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D014785:Vision, Ocular / Q000187:drug effects / Q000502:physiology
D000345:Affinity Labels; D007473:Ion Channels; D006151:Guanosine; C022571:adenosine 5'-O-(3-thiotriphosphate); C035169:8-bromoguanosine; D000255:Adenosine Triphosphate; D006152:Cyclic GMP