Rectification of the olfactory cyclic nucleotide-gated channel by intracellular polyamines.
Polyamine-induced inward rectification of cyclic nucleotide-gated channels was studied in inside-out patches from rat olfactory neurons. The polyamines, spermine, spermidine and putrescine, induced an 'instantaneous' voltage-dependent inhibition with Kd values at 0 mV of 39, 121 micron and 2.7 mM, respectively. Hill coefficients for inhibition were significantly < 1, suggesting an allosteric inhibitory mechanism. The Woodhull model for voltage-dependent block predicted that all 3 polyamines bound to a site 1/3 of the electrical distance through the membrane from the internal side. Instantaneous inhibition was relieved at positive potentials, implying significant polyamine permeation. Spermine also induced exponential current relaxations to a 'steady-state' impermeant level. This inhibition was also mediated by a binding site 1/3 of the electrical distance through the pore, but with a Kd of 2.6 mM. Spermine inhibition was explained by postulating two spermine binding sites at a similar depth. Occupation of the first site occurs rapidly and with high affinity, but once a spermine molecule has bound, it inhibits spermine occupation of the second binding site via electrostatic repulsion. This repulsion is overcome at higher membrane potentials, but results in a lower apparent binding affinity for the second spermine molecule. The on-rate constant for the second spermine binding saturated at a low rate (approximately 200 sec(-1) at +120 mV), providing further evidence for an allosteric mechanism. Polyamine-induced inward rectification was significant at physiological concentrations.
Lynch
J W
JW
Department of Physiology and Pharmacology, University of Queensland, Brisbane, QLD 4072, Australia.
D000818:Animals; D001665:Binding Sites / Q000502:physiology; D000242:Cyclic AMP / Q000037:antagonists & inhibitors / Q000494:pharmacology; D004553:Electric Conductivity; D015640:Ion Channel Gating; D007473:Ion Channels / Q000037:antagonists & inhibitors*; D008564:Membrane Potentials; D018034:Olfactory Receptor Neurons / Q000502:physiology*; D011073:Polyamines / Q000494:pharmacology*; D011700:Putrescine / Q000494:pharmacology; D051381:Rats; D013095:Spermidine / Q000494:pharmacology; D013096:Spermine / Q000494:pharmacology
D007473:Ion Channels; D011073:Polyamines; D013096:Spermine; D000242:Cyclic AMP; D013095:Spermidine; D011700:Putrescine