Late after-depolarization

It is always best to understand a biological problem before trying to find ways to avoid it. The reason that action potential duration is important is that repolarization is a fragile process. It can fail, and when it does so, the action potential is followed by one or many oscillations. These are called early after-depolarizations (EADs). We will also encounter late after-depolarizations (DADs) later in this chapter. 

ICD implantable cardioverter-defibrillator LADs late after-depolarizations LQTS long-QT syndrome MI myocardial infarction... 

Administration of magnesium sulfate is currently recommended as immediate first-line treatment for torsades (Zipes et al. 2006). While the mechanism of action is uncertain, magnesium may reduce the amplitude of early after-depolarizations by inhibiting the late influx of calcium ions via L-type calcium channels that are associated with delayed ventricular repolarization. Consequently, early afterdepolarizations are less likely to reach threshold potential and provoke or sustain torsades (Kaye and O Sullivan 2002). 

Fig. 4. Electrophysiological traces from a prefrontal layer V showing the response to nearby electrical stimulation of corticocortical afferents. Stimulus artifact appears as a vertical line. (1) The fast evoked excitatory postsynaptic current (evEPSC) follows immediately, as depicted by the arrow. Under normal conditions, stimulation at 0.1 Hz evokes only a fast evEPSC. (2) However, after the application of a psychedelic hallucinogen (3 pMDOI, 15 min), stimulation at this frequency almost always evokes both a fast evEPSC and a late evEPSC, as depicted by the arrows. The neuron is voltage-clamped close to its resting potential and was not directly depolarized by DOI. It is not known what type of glutamate release accounts for the late evEPSC. Traces are averages of 10 sweeps taken during each of the conditions.

Fig. 120. ZF and LF spectra of CeRuj. (a) Spectral shape at temperatures above and below the claimed magnetic transition at 40 K. (b) Temperature variation of ZF relaxation rate, (c) Comparison of ZF and Lf data at low temperatures (i.e., in the magnetic state). The fits are based on Gaussian depolarization. Note the extension of the asymmetry spectra to late times (pulsed muons). After Huxley et al. (1996).

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