Effects of calciseptine on unitary barium channel currents in guinea-pig portal vein

Abstract

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.

The Virally Encoded Fungal Toxin KP4 Specifically Blocks L-Type Voltage-Gated Calcium Channels

“…Calciseptine also shows a similar sensitivity to calcium as reported here for KP4 (De Weille et al, 1991;Kini et al, 1998). As with calciseptine block of Ca v 1.2, KP4 does not affect the voltage-dependence of activation but slightly shifts the voltage-dependence of inactivation of the channel (Teramoto et al, 1996). Therefore, the mode of Ca v 1.2 inhibition by KP4 seems very similar to that of calciceptine.…”

Differential Antifungal and Calcium Channel-Blocking Activity among Structurally Related Plant Defensins

“…6A), whereas KP4 blocks only 60% of the L-type Ca 21 channel activity (Gu et al, 1995;Gage et al, 2002). In addition, MsDef1 takes approximately 13 min to reach equilibrium and is very reminiscent of the timedependent effects of calciseptine (Teramoto et al, 1996). Like KP4, this block does not seem to change the voltage dependency of the channel appreciably (Fig.…”

Coexpression of Voltage-Dependent Calcium Channels Ca v 1.2, 2.1a, and 2.1b in Vascular Myocytes

“…On the other hand, 20 nmol/L calciseptine inhibited a depolarizationinduced increase in Ca i 2ϩ in glomerular arterioles, 4 and Ca 2ϩ currents and depolarization-induced contractions in smooth muscle preparations have been reported to be inhibited with IC 50 values in the range of 5 to 75 nmol/L. 14,15 Immunolabeling of A7r5 cells, freshly isolated renal VSMCs, and renal cryosections showed labeling of all of the preglomerular VSMCs regardless of whether anti-Ca V 1.2 or anti-Ca V 2.1 antibody was used, indicating that both channels were expressed in all of the cells. Similarly, our electrophysiological data showed Ca 2ϩ currents in all of the renal smooth muscle cells.…”