1. Current through L-type Ca2+ channels (I(Ca)) was measured electrophysiologically at the same time as Ca2+ influx was measured by trapping entering Ca2+ with a high concentration of indo-1 (> 1 mM) in ferret ventricular myocytes. 2. Na+-free conditions prevented Na+-Ca2+ exchange and K+ currents were blocked by Cs+ and TEA. Thapsigargin (5 μM) prevented Ca2+ uptake and release by the sarcoplasmic reticulum. I(Ca) was pre-activated by brief pulses to +120 mV (the equilibrium potential for Ca2+, E(Ca)), followed by steps to different membrane potentials (E(m), -80 to +100 mV), in some cases in the presence of the Ca2+ channel agonist FPL-64176. 3. Integrated I(Ca) (∫I(Ca)) was linearly related to the change in the concentration of Ca2+ bound to indo-1, which was assessed by the fluorescence difference signal ΔF(d) (F(d) = F500-F400). This created an internal calibration of ΔF(d) as a measure of Ca2+ influx. 4. The ΔF(d)/∫I(Ca)dt relationship was virtually unchanged at all measurable inward I(Ca) (at E(m) from -80 to +50 mV). This indicates that the fractional current carried by Ca2+ and channel selectivity are unchanged over this E(m) range, and also that the selectivity for Ca2+ is very high. 5. Ca2+ influx was readily detected by ΔF(d) beyond the I(Ca) reversal potential (+65 to +100 mV) and was not abolished until E(m) was +120 mV (i.e. E(Ca)). This is explained by the fact that inward Ca2+ flux at the I(Ca) reversal potential is exactly balanced by outward Cs+ current through the Ca2+ channels and can be described by classic Goldman flux analysis with a Ca2+/Cs+ selectivity of the order of 5000. 6. This result also emphasizes that net Ca2+ influx via Ca2+ channels occurs over a voltage range where the net channel current is outward.
|Original language||English (US)|
|Number of pages||10|
|Journal||Journal of Physiology|
|State||Published - Feb 15 2000|
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