Time dependence of the calcium-activated potassium current

133

SUMMARY1. The patch-clamp method was applied to single atrioventricular (a.v.) node cells of the rabbit heart to study the characteristics of the K+ channel.2. When the electrode contained 5-4 mM-K+, depolarizations of the cell-attached patch membrane induced outward single channel currents characterized by burst-like openings; the open-state probability increased from 0-005-0-01 at -40 mV to 0-07-01 at +20 mV of membrane potential. The reversal potentials of the current at K+ concentrations of 5 4, 20 and 130 mm in the electrode agreed with those given by the Nernst equation, indicating that this channel is selective for K+ ions.3. The slope conductance of the channel decreased beyond 60-90 mV positive to the reversal potential (inward-going rectification). The conductance near the reversal potential increased with increasing K+ concentrations on either side ofthe membrane: from 31-32 pS at 5-4 mM-K+ to 41-42 pS at 20 mM-K+ on the outside, and from 19 pS at 90 mM-K+ to 29-3 pS at 130 mM-K+ on the inside.4. Superfusion ofthe cell with 5-4 mM-CN-, glucose-free Tyrode markedly increased the number of channel openings in the cell-attached patch. In the inside-out patch, application of 1 mM-adenosine-5'-triphosphate (ATP) at the inner surface of the patch membrane blocked reversibly the channel activity, while 1 mM-adenosine-5'-diphosphate (ADP) failed to block it.5. The conductance and kinetics of the channel were not modified by increasing the Ca2+ concentration from lo-8 M to 5 x 10-6 M on the inner side of the membrane, while a further increase in Ca2+ to 10-4 M decreased the open-state probability.6. The probability density for the open time fitted well with an exponential distribution (time constant of 5-4 ms at 60 mV positive to the resting potential), while that for the closed time was separated into a fast and a slow component (time constants of4 0 and 132-0 ms, respectively). The time constant of the slow component decreased significantly with depolarization in some preparations. However, neither the time constant of the fast component of the closed-time histogram nor that of the open-time histogram was voltage-dependent.

Section: Discussionmentioning

confidence: 53%

Adenosine‐5'‐triphosphate‐sensitive single potassium channel in the atrioventricular node cell of the rabbit heart.

Kakei¹,

Noma²

1984

133

“…Currents were recorded in 12 mM-K solution, and the cell was The records shown in Fig. 4 suggest that the K current activated is a voltage-gated delayed K current similar to that described by Hodgkin & Huxley (1952 a, b) (Marty, 1981;Woolum & Gorman, 1981;Hermann & Hartung, 1982;Trautmann & Marty, 1984).…”

Section: Differentiation Of Ik(v)mentioning

confidence: 52%

Differentiation of voltage‐gated potassium current and modulation of excitability in cultured amphibian spinal neurones.

Barish¹

1986

123

SUMMARY1. Gigaohm-seal whole-cell voltage-clamp techniques were used to study the development of ionic currents in the membrane ofembryonic amphibian (Ambystoma) spinal neurones during in vitro differentiation.2. Dissociated neural plate cells, some of which are neuronal precursor cells, were placed into culture. Cells became excitable at the time of neurite outgrowth, 2-3 days later, and over the next 2-10 days the duration of the action potential shortened from about 100 ms to about 1 ms.3. Voltage-clamp recordings demonstrated that at the time of appearance of neurites, activatable Na, Ca and voltage-gated K channels were present in the membrane (Ca-dependent K channels were not studied). Over succeeding days in culture, records of total membrane current indicated that the amplitudes of peak inward and steady-state outward currents both increased. As a result of these increases, the pattern of total membrane current came to be increasingly dominated by outward currents.4. With inward Na and Ca currents blocked, a voltage-gated K current (IK(V)) could be studied in isolation. The reversal potential of this current varied in good agreement with the equilibrium potential for K ions predicted by the Nernst relation.The wave form of IK(V) activation was sigmoidal. Activation was more rapid at more positive voltages (relative to the usual holding potential of -70 mV), and deactivation was more rapid at more negative voltages.5. The amplitude of 'K(V) increased during neural development, while cell size remained approximately constant. Increases in rates of activation and deactivation were observed in parallel with the increase in current density. When measured at 0 mV, cells studied on day 4 of culture or earlier showed steady-state chord conductances (gK(V)) of less than 20 nS, and one-half activation times (ti) of 2 5-10 ms. Older cells showed gK(v)s of 10-80 nS, and tis of 0-8-2-5 ms.6. As Na, and to a lesser extent Ca, current amplitudes were also increasing during differentiation, these observations concerning IK(V) suggested that its amplitude and

“…It is highly voltage dependent, blocked by external TEA (Hermann & Gorman, 1981) and by charybdotoxin (Hermann & Erxleben, 1987), insensitive to apamin and it requires relatively large increase in [Ca2+]i for its opening. The macroscopic C-current may have a fast activation rate (Woolum & Gorman, 1981;Lux & Hofmeier, 1982b), which contrasts with the slow, apamin-sensitive Ca2+-activated K+ current involved in long-lasting after potentials (see Rudy, 1988).…”

Section: Discussionmentioning

confidence: 99%

Ca2(+)‐activated K+ current involvement in neuronal function revealed by in situ single‐channel analysis in Helix neurones.

Gola

Ducreux

Chagneux

1990

SUMMARY1. The properties of single calcium-activated potassium channels (or C-channels) were studied in cell-attached patches using the patch-clamp technique. Experiments were performed on identified Ca2+-dependent U cells in juvenile specimens (1-2 months old) of Helix aspersa.2. The criteria used to identify C-channels were based on comparison between macroscopic C-currents and currents reconstructed from unitary recordings. Both currents had a slow activation rate at large positive potentials which turned into fast activation after large Ca2+ entries. Both currents were blocked by intracellularly injected EGTA. 4. C-channels in U cells were distributed in clusters of three to ten channels (mean 5-05 channels in seventy-five patches). Calcium channels were present in patches containing clustered C-channels. C-channels within clusters behaved independently. 5. With patch electrode containing 8 mM-calcium, C-channels opened transiently upon patch depolarization. Reopenings in quiescent depolarized patches were induced by whole-cell spikes triggered by current pulses applied to an intracellular electrode. Apparent inactivation of C-channels in depolarized patches was in fact due to a decrease in [Ca2+]i resulting from inactivation of Ca2+ channels.6. Calcium-free saline solutions in the patch electrodes prevented C-channels from opening upon patch depolarization. Entry of calcium through the surrounding membrane induced delayed openings in the patch. Peak opening probability PO occurred 330 + 30 ms after a brief Ca2+ entry with a lag period of 50-80 ms. With patch electrodes filled with Ca2+-containing saline solutions and under conditions which maximized C-channel opening, peak P1 was reached in 20-50 ms. The same value was observed for the whole-cell C-current.7. The peak P. at a given patch potential and in response to a whole-cell spike was not altered by a previous long-lasting patch depolarization, or by producing several * To whom correspondence should be addressed. MS 7550M. GOLA, C. DUCREUX AND H. CHAGNEUX successive Ca2+ entries. Thus, C-channels did not appear to be inactivated by depolarization or increase in [Ca2+]i.8. C-channels were found to be relatively highly voltage dependent, with an e-fold increase in PO per 14-9 mV increase in potential. The location of the PO-V curve on the voltage axis, but not its shape, was variable along a 60 mV span. Openings were never observed at potentials below -30 mV.9. Increasing Ca2+ influxes shifted the PO-V relationship towards negative potentials. Reverse shifts were observed during the PO decay following a brief Ca2+ entry.10. The latency to first opening was of 80-100 ms when patch depolarizations were applied immediately after a brief Ca2+ entry. The latency decreased to 10-20 ms when the depolarization was applied 300-600 ms after the Ca2+ entry. These data would indicate that the C-current activation was rate limited by calcium while the voltage-dependent step had fast kinetics.11. The distributions of open time, closed time and burst duration were con...