An analysis of the variations in potency of grayanotoxin analogs in modifying frog sodium channels of differing subtypes.
Responses of tetrodotoxin-sensitive (TTX-s) and insensitive (TTX-i) Na(+) channels, in frog dorsal root ganglion (DRG) cells and frog heart Na(+) channels, to two grayanotoxin (GTX) analogs, GTX-I and alpha-dihydro-GTX-II, were examined using the patch clamp method. GTX-evoked modification occurred only when repetitive depolarizing pulses preceded a single test depolarization; modification, during the test pulse, was manifested by a decrease in peak Na(+) current accompanied by a sustained Na(+) current. GTX-evoked modification of whole-cell Na(+) currents was quantified by normalizing the conductance for sustained currents through GTX-modified Na(+) channels to that for the peak current through unmodified Na(+) channels. The dose-response relation for GTX-modified Na(+) channels was constructed by plotting the normalized slope conductance against GTX concentration. With respect to DRG TTX-i Na(+) channels, the EC(50) and maximal normalized slope conductance were estimated to be 31 microM and 0.23, respectively, for GTX-I, and 54 microM and 0.37, respectively, for alpha-dihydro-GTX-II. By contrast, TTX-s Na(+) channels in DRG cells and Na(+) channels in ventricular myocytes were found to have a much lower sensitivity to both GTX analogs. In single-channel recording on DRG cells and ventricular myocytes, Na(+) channels modified by the two GTX analogs (both at 100 microM), had similar relative conductances (range, 0.25-0.42) and open channel probabilities (range, 0.5-0.71). From these observations, we conclude that the differences in responsiveness of DRG TTX-i, and ventricular whole cell Na(+) currents to the GTX analogs studied are related to the number of Na(+) channels modified.
Yakehiro
M
M
Division of Physiology, Department of Clinical Engineering, Hiroshima International University, Faculty of Health Sciences, Hiroshima, Japan.
Yuki
T
T
Yamaoka
K
K
Furue
T
T
Mori
Y
Y
Imoto
K
K
Seyama
I
I
D000818:Animals; D004224:Diterpenes / Q000737:chemistry / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005727:Ganglia, Spinal / Q000187:drug effects* / Q000378:metabolism; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D020033:Protein Isoforms / Q000378:metabolism; D011892:Rana catesbeiana; D015222:Sodium Channels / Q000145:classification / Q000187:drug effects / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
D004224:Diterpenes; D020033:Protein Isoforms; D015222:Sodium Channels; D013779:Tetrodotoxin; C001311:grayanotoxin I