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Ion channels in the plasma membrane are important for the apoptotic process. Different types of voltage-gated ion channels are up-regulated early in the apoptotic process and block of these channels prevents or delays apoptosis. In the present investigation we examined whether ion channels are up-regulated in oocytes from the frog Xenopus laevis during apoptosis. The two-electrode voltage-clamp technique was used to record endogenous ion currents in the oocytes. During staurosporine-induced apoptosis a voltage-dependent Na(+) current increased three-fold. This current was activated at voltages more positive than 0 mV (midpoint of the open-probability curve was +55 mV) and showed almost no sign of inactivation during a 1-s pulse. The current was resistant to the Na(+)-channel blockers tetrodotoxin (1 µM) and amiloride (10 µM), while the Ca(2+)-channel blocker verapamil (50 µM) in the bath solution completely blocked the current. The intracellular Na(+) concentration increased in staurosporine-treated oocytes, but could be prevented by replacing extracellular Na(+) with either K(+) or Choline(+). Prevention of this influx of Na(+) also prevented the STS-induced up-regulation of the caspase-3 activity, suggesting that the intracellular Na(+) increase is required to induce apoptosis. Taken together, we have found that a voltage dependent Na(+) channel is up-regulated during apoptosis and that influx of Na(+) is a crucial step in the apoptotic process in Xenopus oocytes

Intracellular potassium (K+) concentration decrease is not obligatory for apoptosis

K+ efflux is observed as an early event in the apoptotic process in various cell types. Loss of intracellular K+ and subsequent reduction in ionic strength is suggested to release the inhibition of proapoptotic caspases. In this work, a new K+-specific microelectrode was used to study possible alterations in intracellular K+ in Xenopus laevis oocytes during chemically induced apoptosis. The accuracy of the microelectrode to detect changes in intracellular K+ was verified with parallel electrophysiological measurements within the same cells. In concordance with previous studies on other cell types, apoptotic stimuli reduced the intracellular K+ concentration in Xenopus oocytes and increased caspase-3 activity. The reduction in intracellular K+ was prevented by dense expression of voltage-gated K (Kv) channels. Despite this, the caspase-3 activity was increased similarly in Kv channel expressing oocytes as in oocytes not expressing Kv channels. Thus, in Xenopus oocytes caspase-3 activity is not dependent on the intracellular concentration of K+.Funding agencies|Swedish Research Council||Swedish Heart-Lung Foundation||Swedish Brain Foundation||County Council of Ostergotland, King Gustaf V and Queen Victorias Freemasons Foundation||Swedish Society for Medical Research|

Properties of the STS-induced current.

<p><b>A</b>. The voltage pulse protocol. <b>B</b>. Typical corresponding currents, showing inward current at intermediate voltages (see red traces). The recording has been corrected for leak conductance of 1.4 µS. <b>C</b>. Magnification from <b>B</b>. The inward currents between +10 and +40 mV are shown in red. <b>D</b>. Steady-state current vs. voltage from <b>B</b>. <b>E</b>. Typical <i>G</i>(<i>V</i>) curve for the STS-induced currents. <b>F</b>. Time constants for the fast activation component from a double exponential fit (open symbols, <i>n = </i>5), and the single exponential closing of the studied current (closed symbols, <i>n = </i>8).</p

The voltage-gated ion channel is a Na⁺ channel.

<p><b>A</b>. Tail currents measured at 0 mV after a prepulse to +100 mV demonstrating that the inward tail current in 100Na solution (black, <i>n = </i>4) is abolished in 0Na solution (blue) in STS-treated oocytes. Switching back to 100Na recovered the inward current (grey). <b>B</b>. The same recording but at +30 mV showing inward current in control solution (black) and outward in 0Na (blue).<b>C</b>. The inward tail currents measured in control oocytes are also abolished when 100Na (black) is replaced with 0Na (blue, <i>n = </i>3) in the extracellular solution.</p

Comparison of Na⁺ currents in STS-treated and control oocytes.

<p><b>A</b>. Typical Steady-state current versus voltage curve for the fast activated verapamil-sensitive Na⁺ current after subtraction of the slow activated verapamil-insensitive current in STS –treated oocytes (red) and control oocytes (black). <b>B</b> Mean steady-state current at +100 mV (left) after subtraction of the slow activated current for STS-treated oocytes (red, 3.55±0.3 µA, <i>n</i> = 3) and control oocytes (black, 0.8±0.1 µA, <i>n</i> = 3). Mean reversal potential (V_rev, right) after subtraction of the slow activated current for STS-treated oocytes (red, +55±7 mV, <i>n</i> = 3) and control oocytes (black, +76±6 mV, <i>n</i> = 3). Statistical analyses are mean±SEM and unpaired t-test (* P<0.05 and ** P<0.01) <b>C</b>. Normalized <i>G</i>(<i>V</i>) curves for the fast activated Na⁺ current after subtraction of the slow activated current in STS-treated (red) and control oocytes (black). <b>D</b> Mean V₅₀ (left) after subtraction of the slow activated current for STS-treated oocytes (red, +68±4 mV, <i>n</i> = 3) and control oocytes (black, +74±4 mV µA, <i>n</i> = 3). Mean slope (Slope, right) after subtraction of the slow activated current for STS-treated oocytes (red, 11.2±1.1 mV, <i>n</i> = 3) and control oocytes (black, 11.4±1.5 mV, <i>n</i> = 3). Statistical analyses are mean±SEM and unpaired t-test.</p

Voltage-gated ion channel activated during apoptosis.

<p><b>A</b>: Electrophysiological properties of an outward current in control oocytes (black) and staurosporine (1 µM) treated oocytes (red) in <i>Xenopus</i> oocytes at +100 mV. The difference between the current in control and STS-treated oocytes is also plotted (dashed line). <b>B</b>. Mean steady-state current at +100 mV in control (1.8 µA±0.5 µA, <i>n</i> = 17) and apoptotic oocytes treated with 1 µM STS (4.8 µA±0.3 µA, <i>n</i> = 13). Steady state currents at +100 mV are significantly larger in the apoptotic oocytes than in controls (**** P<0.0001). Statistical analyses are mean ± SEM and unpaired t-test. Recordings were done in 100Na solution.</p

Apoptosis in <i>Xenopus laevis</i> oocytes.

<p><b>A</b>. Morphological changes of <i>Xenopus laevis</i> oocytes before (Control) and after 6 hours of treatment with 20 µM staurosporine (STS-treated). <b>B</b>. Measurement of caspase-3 activity in control oocytes (black, 120,000±30,000, <i>n = </i>9) and staurosporine (1 µM) treated oocytes (red, 240,000±25,000, <i>n = </i>23). Statistical analysis is unpaired t-test (* P<0.05).</p