Calcium block of guinea‐pig heart sodium channels with and without modification by the piperazinylindole DPI 201‐106.

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Abstract
1. External Ca2+ block of Na+ channels was studied by a gigaohm-seal patch clamp technique in single cardiac ventricular cells from guinea-pig. Single-channel currents were recorded from cell-attached patches. 2. Increasing external Ca2+ concentrations in the patch pipette from 0.1 to 20 mM reduced the single-channel conductance of normal Na+ channels from 27 to 14 pS without causing flickering (obtained from linear regression, eight patches). 3. Exposed to external Ca2+ concentrations of 20 mM, the single-channel currents decreased at potentials negative to -60 mV in spite of an increased driving force for inward Na+ currents. 4. An external concentration of 35 mM-Mg2+, which is supposed to exert a screening of surface charges nearly equal to that of 20 mM-Ca2+ (Hille, Woodhull & Shapiro, 1975), reduced the single-Na+-channel conductance only from 26 (1 mM-Mg2+) to 20 pS (linear regression, eight patches). A weaker voltage-dependent block at potentials negative to -50 mV was observed in 35 mM-Mg2+ than in 20 mM-Ca2+. Therefore, surface charge effects cannot explain the obvious reduction of the conductance of single Na+ channels found when the external Ca2+ concentration was increased. 5. Single Na+-channel currents increased with an increase in the external Na+ concentration [( Na+]o) but showed saturation. The Na+o-single-channel current relationship could be described by i = imax/(1 + kd/[Na+]o) with imax = 5.4 pA and kd = 359 mM (seventeen patches). 6. The mean open time of Na+ channels varied between 0.18 and 0.59 ms (potentials between -80 and -20 mV). No significant changes in the mean open time could be obtained when Ca2+ was varied between 0.1 and 20 mM. 7. The piperazinylindole compound DPI 201-106 was used as a tool to prolong the open time of single Na+ channels. If the external Ca2+ concentration was increased from 0.1 to 20 mM the currents through the modified channels were reduced. The reduction of single-channel currents was accentuated at potentials negative to -60 mV (20 mM-Ca2+) similar to the control channels. 8. In contrast to non-modified Na+ channels, the mean open time of DPI 201-106-modified channels proved extremely voltage and Ca2+ dependent.(ABSTRACT TRUNCATED AT 400 WORDS)