Outward current

Inward Rectifier K+ Channels. Figure 2 High [K+] inside cells relative to outside results in normal rectification, whereby outward (positive by convention) potassium currents (/) when cells are depolarized (is positive relative to EK), are biggerthan inward (negative) currents at hyperpolarized (negative) voltages. Inward or anomalous rectifiers show strong or weak inward rectification whereby outward currents are smaller than inward currents. 

A particularly interesting example of Kv-channel inactivation is represented by HERG-channels. HERG-channels have faster inactivation than activation kinetics, and they very rapidly recover from inactivation at negative membrane potentials. This behavior may result in a situation where most of the current carried by HERG-channels occurs during their recovery from inactivation at negative potentials, that is, it represents an inward rather than outward current. 

Lue, W.M. and Boyden, P.A. (1992). Abnormal electrical properties of myocytes from chronically infarcted canine heart - alterations in V and the transient outward current. Circulation 85, 1175-1188. 

FIGURE 7.14 Time-course of G-protein-mediated activation of GIRK potassium channels in rabbit sinoatrial node cells, (a). Outward current evoked by a 33-msec, 50-nA iontophoretic pulse of acetylcholine (between arrows), (b). Response of the unclamped cell to an iontophoretic pulse of acetylcholine (ACh). (Record (a) is adapted with permission from Trautwein et al., in Drug Receptors and Their Effectors, Birdsall, N. J. M., Ed., Macmillan, New York, 1980, pp. 5-22 record (b) is adapted with permission from Noma, in Electrophysiology of Single Cardiac Cells, Noble, D. and Powell, T., Eds., Academic Press, San Diego, CA, 1987, pp. 223-246.)... 

ZhuGe R, Tuft RA, Fogarty KE, Bellve K, Fay FS, Walsh JV 1999 The influence of sarcoplasmic reticulum Ca2+ concentration on Ca2+ sparks and spontaneous transient outward currents in single smooth muscle cells. J Gen Physiol 113 215—228... 

McCarron We looked at the outward current activated by InsP3 in the presence and absence of tetracaine, and they were similar (McCarron et al 2002). 

Sanders So what you are saying is that the resting potential is sitting in a situation where the Ca2+ sparks are preferentially activating outward currents, not inward currents. 

During the action potential in vas deferens or urinary bladder the rise in [Ca2+] close under the cell membrane is responsible, in combination with the depolarization, for the repolarization phase as it causes the opening of Ca2+-activated K+ channels through which a large repolarizing outward current flows (Arnaudeau et al 1997, Imaizumi et al 1998, Ohi et al 2001). This may lead to a transient period of hyperpolarization (an afterhyperpolarization ) following the action potential (Imaizumi et al 1998). 

Benham CD, Bolton TB 1986 Spontaneous transient outward currents in single visceral and vascular smooth muscle cells of the rabbit. J Physiol 381 385—406 Boittin FX, Coussin F, Macrez N, Mironneau C, Mironneau J 1998 Inositol 1,4,5-trisphosphate-and ryanodine-sensitive Ca2+ release channel-dependent Ca2+ signalling in rat portal vein... 

To determine the contribution of BK channels to this mixed outward current, the portion of the current that was sensitive to the BK channel blocker tetraethylammonium (TEA+) was measured (see Nelson Quayle 1995). TEA+ was applied at a concentration (1 mM) which should have very little effect on Ky currents. BK current was found by subtracting the current in the presence of TEA"1" from the control current. Similar results were seen when BK channels were blocked with the highly selective peptide inhibitor of BK channels iberiotoxin (200 nM) (Galvez et al 1990). To ensure that Ky channels did not significantly contribute to the current attributed to BK channels, a subset of experiments were performed in which iberiotoxin (200 nM) was applied to block BK channels, followed by TEA+ (1 mM) in the continued presence of iberiotoxin. The mean current was reduced by iberiotoxin (200 nM). In the continued presence of iberiotoxin, TEA+ application was without further effect. Thus, 1 mM TEA+ appears to be a selective blocker of BK channels in UBSM. 

Bolton TB, Imaizumi Y 1996 Spontaneous transient outward currents in smooth muscle cells. Cell Calcium 20 141-152... 

The SR may contribute to excitation-contraction (EC) coupling in two ways firstly, by the release of Ca2+ for contraction as described above, but secondly by modulating membrane excitability. As will be described elsewhere in this book, the SR is an important mediator of surface membrane ion channel activity, and hence, excitability. Spontaneous Ca2+ release from the SR can activate Ca2+-sensitive ion channels. Both K+ (Kca) and Cl- (Clsmooth muscle cell membrane can be activated by SR Ca2+. If Kca channels are activated there will be a hyperpolarization, as K+ ions leave the cell and spontaneous transient outward currents (STOCs) can be recorded (Carl et al 1996, Nelson Quayle... 

Reversal potentials for LSD were also determined over a range of external K+ concentrations. According to the Nemst equation, reversal potentials should shift approximately 60 mV per 10-fold shift in K+ concentration if K+ were the ionic species involved in a conductance change. Reversal potentials of LSD were found to shift almost exactly to the extent predicted by the Nemst equation for a K+-dependent potential. Of course, there are several different types of K+ conductances that could be activated by LSD, including the Ca2+-dependent outward current. To evaluate the latter possibility, midbrain slices were exposed... 

Cardiac APD is controlled by a fine balance between inward and outward currents in the repolarization phase. Since outward K+ currents, especially the delayed rectifier repolarizing current, IK (which is the sum of two kinetically and pharmacologically distinct types of K+ currents a rapid, 1k and a slow, IKs, component), play an important role during repolarization and in determining the configuration of the action potential, small changes in conductance can significantly alter the effective refractory period, hence the action potential duration. 

Miledi R. 1982. A calcium-dependent transient outward current in Xenopus laevis oocytes. Proc R Soc Lond B Biol Sci 215 491. 

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