The suppression of Ca(2+)- and voltage-dependent outward K+ current during mAChR activation in rat adrenal chromaffin cells (original) (raw)
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The Journal of neuroscience : the official journal of the Society for Neuroscience, 1995
The properties of Ca(2+)- and voltage-dependent K+ currents and their role in defining membrane potential were studied in cultured rat chromaffin cells. Two variants of large-conductance, Ca2+ and voltage-dependent BK channels, one noninactivating and one inactivating, were largely segregated among patches. Whole-cell noninactivating and inactivating currents resulting from each of these channels were segregated among different chromaffin cells. Cell-to-cell variation in the rate and extent of whole-cell current decay was not explained by differences in cytosolic [Ca2+] regulation among cells; rather, variation was due to differences in the intrinsic properties of the underlying BK channels. About 75% of rat chromaffin cells and patches express inactivating BK current (termed BKi) while the remainder express noninactivating BK current (termed BKs). The activation time course of both currents is similar, as is the dependence of activation on [Ca2+] and membrane potential. However, de...
Journal of neurophysiology, 1999
BK channel activation by brief depolarizations requires Ca2+ influx through L- and Q-type Ca2+ channels in rat chromaffin cells. Ca2+- and voltage-dependent BK-type K+ channels contribute to action potential repolarization in rat adrenal chromaffin cells. Here we characterize the Ca2+ currents expressed in these cells and identify the Ca2+ channel subtypes that gate the activation of BK channels during Ca2+ influx. Selective Ca2+ channel antagonists indicate the presence of at least four types of high-voltage-gated Ca2+ channels: L-, N-, P, and Q type. Mean amplitudes of the L-, N-, P-, and Q-type Ca2+ currents were 33, 21, 12, and 24% of the total Ca2+ current, respectively. Five-millisecond Ca2+ influx steps to 0 mV were employed to assay the contribution of Ca2+ influx through these Ca2+ channels to the activation of BK current. Blockade of L-type Ca2+ channels by 5 microM nifedipine or Q-type Ca2+ channels by 2 microM Aga IVA reduced BK current activation by 77 and 42%, respecti...
The Journal of Neuroscience, 1996
Submembrane [Ca 2ϩ ] i changes were examined in rat chromaffin cells by monitoring the activity of an endogenous Ca 2ϩdependent protein: the large conductance Ca 2ϩ-and voltageactivated K ϩ channel (also known as the BK channel). The Ca 2ϩ and voltage dependence of BK current inactivation and conductance were calibrated first by using defined [Ca 2ϩ ] i salines. This information was used to examine submembrane [Ca 2ϩ ] i elevations arising out of Ca 2ϩ influx and muscarine-mediated release of Ca 2ϩ from intracellular stores. During Ca 2ϩ influx, some BK channels are exposed to [Ca 2ϩ ] i of at least 60 M. However, the distribution of this [Ca 2ϩ ] i elevation is highly nonuniform so that the average [Ca 2ϩ ] i detected when all BK channels are activated is only ϳ10 M. Intracellular dialysis with 1 mM or higher EGTA spares only the BK channels activated by the highest [Ca 2ϩ ] i during influx, whereas dialysis with 1 mM or higher BAPTA blocks activation of all BK channels. Submembrane [Ca 2ϩ ] i elevations fall rapidly after termination of short (5 msec) Ca 2ϩ influx steps but persist above 1 M for several hundred milliseconds after termination of long (200 msec) influx steps. In contrast to influx, the submembrane [Ca 2ϩ ] i elevations produced by release of intracellular Ca 2ϩ by muscarinic actetylcholine receptor (mAChR) activation are much more uniform and reach peak levels of 3-5 M. Our results suggest that during normal action potential activity only 10-20% of BK channels in each chromaffin cell see sufficient [Ca 2ϩ ] i to be activated.
1995
The properties of Ca2+- and voltage-dependent K+ currents and their role in defining membrane potential were studied in cultured rat chromaffin cells. Two variants of large-con- ductance, Ca2+ and voltage-dependent BK channels, one noninactivating and one inactivating, were largely segre- gated among patches. Whole-cell noninactivating and in- activating currents resulting from each of these channels were segregated among different chromaffin
Two components of calcium-activated potassium current in rat adrenal chromaffin cells
The Journal of Physiology, 1992
1. The activation of calcium (Ca2")-dependent potassium (K+) currents in dissociated rat adrenal chromaffin cells was investigated using the dialysed cell recording technique. 2. Ca2+-dependent K+ current was the major component of outward current at command potentials from-30 mV to about + 50 mV. 3. Two components of Ca2+-dependent outward current could be distinguished based on the voltage dependence of activation, the properties of tail currents following repolarization, and pharmacological properties. 4. One Ca2+-dependent current was similar to an after-hyperpolarization current (often termed IAHP) observed in other cell types. This current was largely blocked by 200 nM-apamin or 200 #uM-curare, was associated with slow Ca2+-dependent tail current, and exhibited little dependence on voltage. In cells with cytosolic Ca2+ buffered to 500 nM-1 /Im, curare-sensitive current accounted for most of the membrane current at potentials negative to about-40 mV. 5. A second component of Ca2+-activated K+ current exhibited voltage-dependent activation, was completely blocked by 1 mm-TEA, and turned off rapidly following repolarization. An unusual aspect of the TEA-sensitive currents was that they appeared to inactivate under conditions of constant cytosolic Ca2. 6. A novel observation during these experiments was a slow hump of outward current which appears to result from a non-monotonic elevation in cytosolic Ca2+ during prolonged voltage jumps.
Small-conductance Ca2+-activated K+ channels in bovine chromaffin cells
Pfl�gers Archiv European Journal of Physiology, 1993
Simultaneous whole-cell patch-clamp and fura-2 fluorescence [Ca2+]~ measurements were used to characterize Ca2+-activated K + currents in cultured bovine chromaffin cells. Extracellular application of histamine (10 gM) induced a rise of [Ca2+]~ concomitantly with an outward current at holding potentials positive to -80 mV. The activation of the current reflected an increase in conductance, which did not depend on membrane potential in the range -80 mV to -40 mV. Increasing the extracellular K + concentration to 20 mM at the holding potential of -78 mV was associated with inwardly directed currents during the [Ca2+]i elevations induced either by histamine (10 gM) or short voltageclamp depolarizations. The current reversal potential was close to the K + equilibrium potential, being a function of external K + concentration. Current fluctuation analysis suggested a unit conductance of 3-5 pS for the channel that underlies this K + current. The current could be blocked by apamin (1 ~tM). Whole-cell current-clamp recordings showed that histamine (10 txM) application caused a transient hyperpolarization, which evolved in parallel with the [Ca2+]~ changes. It is proposed that a small-conductance Ca2+-activated K + channel is present in the membrane of bovine chromaffin cells and may be involved in regulating catecholamine secretion by the adrenal glands of various species.
The Journal of neuroscience : the official journal of the Society for Neuroscience, 1996
Submembrane [Ca2+]i changes were examined in rat chromaffin cells by monitoring the activity of an endogenous Ca(2+)-dependent protein: the large conductance Ca(2+)-and voltage-activated K+ channel (also known as the BK channel). The Ca2+ and voltage dependence of BK current inactivation and conductance were calibrated first by using defined [Ca2+]i salines. This information was used to examine submembrane [Ca2+]i elevations arising out of Ca2+ influx and muscarine-mediated release of Ca2+ from intracellular stores. During Ca2+ influx, some BK channels are exposed to [Ca2+]i of at least 60 microM. However, the distribution of this [Ca2+]i elevation is highly nonuniform so that the average [Ca2+]i detected when all BK channels are activated is only approximately 10 microM. Intracellular dialysis with 1 mM or higher EGTA spares only the BK channels activated by the highest [Ca2+]i during influx, whereas dialysis with 1 mM or higher BAPTA blocks activation of all BK channels. Submembra...
Pflugers Archiv-european Journal of Physiology
BK channels modulate cell firing in excitable cells in a voltage-dependent manner regulated by fluctuations in free cytosolic Ca2+ during action potentials. Indeed, Ca2+-independent BK channel activity has ordinarily been considered not relevant for the physiological behaviour of excitable cells. We employed the patch-clamp technique and selective BK channel blockers to record K+ currents from bovine chromaffin cells at minimal intracellular (about 10 nM) and extracellular (free Ca2+) Ca2+ concentrations. Despite their low open probability under these conditions (V50 of +146.8 mV), BK channels were responsible for more than 25% of the total K+ efflux during the first millisecond of a step depolarisation to +20 mV. Moreover, BK channels activated about 30% faster (τ = 0.55 ms) than the rest of available K+ channels. The other main source of fast voltage-dependent K+ efflux at such a low Ca2+ was a transient K+ (IA-type) current activating with V 50 = −14.2 mV. We also studied the activation of BK currents in response to action potential waveforms and their contribution to shaping action potentials both in the presence and the absence of extracellular Ca2+. Our results show that BK channels activate during action potentials and accelerate cell repolarisation even at minimal Ca2+ concentration, and suggest that they could do so also in the presence of extracellular Ca2+, before Ca2+ entering the cell facilitates their activity.