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Ca 2+ influx via the L‐type Ca 2+ channel during tail current and above current reversal potential in ferret ventricular myocytes
Author(s) -
Zhou Zhuan,
Bers Donald M.
Publication year - 2000
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2000.t01-2-00057.x
Subject(s) - membrane potential , thapsigargin , endoplasmic reticulum , chemistry , biophysics , reversal potential , myocyte , selectivity , patch clamp , analytical chemistry (journal) , endocrinology , biology , receptor , biochemistry , chromatography , catalysis
Current through L‐type Ca 2+ channels ( I Ca ) was measured electrophysiologically at the same time as Ca 2+ influx was measured by trapping entering Ca 2+ with a high concentration of indo‐1 (> 1 m m ) in ferret ventricular myocytes. Na + ‐free conditions prevented Na + ‐Ca 2+ exchange and K + currents were blocked by Cs + and TEA. Thapsigargin (5 μM) prevented Ca 2+ uptake and release by the sarcoplasmic reticulum. I Ca was pre‐activated by brief pulses to +120 mV (the equilibrium potential for Ca 2+ , E Ca ), followed by steps to different membrane potentials ( E m , −80 to +100 mV), in some cases in the presence of the Ca 2+ channel agonist FPL‐64176. Integrated I Ca ( I Ca ) was linearly related to the change in the concentration of Ca 2+ bound to indo‐1, which was assessed by the fluorescence difference signal Δ F d ( F d = F 500 – F 400 ). This created an internal calibration of Δ F d as a measure of Ca 2+ influx. The Δ F d / I Ca d t 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 Ca 2+ and channel selectivity are unchanged over this E m range, and also that the selectivity for Ca 2+ is very high. Ca 2+ influx was readily detected by Δ F 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 Ca 2+ flux at the I Ca reversal potential is exactly balanced by outward Cs + current through the Ca 2+ channels and can be described by classic Goldman flux analysis with a Ca 2+ /Cs + selectivity of the order of 5000. This result also emphasizes that net Ca 2+ influx via Ca 2+ channels occurs over a voltage range where the net channel current is outward.