Effects of calciseptine on unitary barium channel currents in guinea-pig portal vein.
Effects of synthesized calciseptine (CaS), found naturally in the venom of the black mamba, on voltage-dependent Ca2+ channels in smooth muscle cells of the guinea-pig portal vein were investigated. In the whole-cell voltage-clamp configuration, extracellular application of CaS (>/= 10 nM) inhibited the inward current in a concentration- and voltage-dependent manner at a holding potential of -90 mV. The Ca2+ current recorded at a high holding potential (-50 mV) was approximately 8 times more sensitive to CaS than that at a more negative holding potential (-90 mV). CaS (50 nM) shifted to the left the steady-state inactivation curve obtained by using single 8-s conditioning pulses of various amplitudes. When CaS (>/= 200 nM) was present in the pipette, the Ca2+ current remained for the duration of the experiments (more than 60 min) in the whole-cell configuration. Two different Ca2+ channel conductances are present in this tissue (25-pS and 12-pS channels). Both channels are blocked by dihydropyridine (DHP) derivatives, but have different sensitivities. In the cell-attached condition, CaS hardly changed the activity of either unitary Ca2+ channel current. To prevent the "run down" of the Ca2+ channels in cell-free conditions, we added cardiac cytosol, a supernatant from homogenized cardiac cells and an endogenous Ca2+ channel activating factor, in the pipette. The unitary Ca2+ channel currents were then recorded using the outside-out membrane patch configuration. Application of CaS (1 microM) in the bath completely blocked the open events of the 25-pS Ca2+ channel. CaS (10 nM) in the bath reduced the mean open time and channel availability, resulting in a decrease in the open probability of the 25-pS channel currents without affecting the amplitude of the single-channel conductance. CaS also reduced the open probability (though less potently) and channel availability of the 12-pS Ca2+ channel without a change in its amplitude. From these results, we conclude that CaS has inhibitory effects on the voltage-dependent Ca2+ current that are similar to those of DHP derivatives and that it acts from the outside of the membrane.
Teramoto
N
N
Department of Pharmacology, Faculty of Medicine, Kyushu University, Fukuoka 812-82, Japan.
Ogata
R
R
Okabe
K
K
Kameyama
A
A
Kameyama
M
M
Watanabe
T X
TX
Kuriyama
H
H
Kitamura
K
K
D000818:Animals; D001464:Barium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D004546:Elapid Venoms / Q000494:pharmacology*; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011169:Portal Vein / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*
D015220:Calcium Channels; D004546:Elapid Venoms; D007473:Ion Channels; D001464:Barium; C067971:calciseptine