Rapidly inactivating and non‐inactivating calcium‐activated potassium currents in frog saccular hair cells

Abstract

Using a semi‐intact epithelial preparation we examined the Ca2+‐activated K+ (KCa) currents of frog (Rana pipiens) saccular hair cells. After blocking voltage‐dependent K+ (KV) currents with 4‐aminopyridine (4‐AP) an outward current containing inactivating (Itransient) and non‐inactivating (Isteady) components remained. The contribution of each varied greatly from cell to cell, with Itransient contributing from 14 to 90 % of the total outward current. Inactivation of Itransient was rapid (τ≈ 2–3 ms) and occurred within the physiological range of membrane potentials (V1/2=−63 mV). Recovery from inactivation was also rapid (τ≈ 10 ms). Suppression of both Itransient and Isteady by depolarizations that approached the Ca2+ equilibrium potential and by treatments that blocked Ca2+ influx (application Ca2+‐free saline or Cd2+), suggest both are Ca2+ dependent. Both were blocked by iberiotoxin, a specific blocker of large‐conductance KCa channels (BK), but not by apamin, a specific blocker of small‐conductance KCa channels. Ensemble‐variance analysis showed that Itransient and Isteady flow through two distinct populations of channels, both of which have a large single‐channel conductance (∼100 pS in non‐symmetrical conditions). Together, these data indicate that both Itransient and Isteady are carried through BK channels, one of which undergoes rapid inactivation while the other does not. Inactivation of Itransient could be removed by extracellular papain and could later be restored by intracellular application of the ‘ball’ domain of the auxiliary subunit (β2) thought to mediate BK channel inactivation in rat chromaffin cells. We hypothesize that Itransient results from the association of a similar β subunit with some of the BK channels and that papain removes inactivation by cleaving extracellular sites required for this association.

Role of BK Potassium Channels Shaping Action Potentials and the Associated [Ca²⁺]_i Oscillations in GH₃ Rat Anterior Pituitary Cells

“…The molecular nature of the channel carrying this A-type component of current and the reason for its sensitivity to the putatively specific inhibitor of BK channels remains to be established. The possibility that in some GH 3 cells a rapidly inactivating fraction of BK channels co-exists with non-inactivating ones [26, 28]cannot be excluded. Nevertheless, the relatively depolarized potentials from which firing takes place in GH 3 cells and the strong inactivating properties of this current probably determine its reduced contribution to shape the AP and to significantly modify the firing properties of the cell.…”

“…It is possible that different functional co-localization of BK and voltage-dependent Ca 2+ channels [33, 35, 36], the presence of different subsets of alternatively spliced BKα- or BKβ-subunits able to modify the Ca 2+ sensitivity or the activation and inactivation properties of the BK channels [26, 28], or subtle differences in the ability to handle intracellular Ca 2+ loading [21], contribute to these discrepancies. Thus both BKβ2- and BKβ3-subunits causing BK channel inactivation have been identified in the pituitary [26], although their cellular distribution is not known.…”

Extrasynaptic Localization of Inactivating Calcium-Activated Potassium Channels in Mouse Inner Hair Cells

“…Marcotti et al (2004) also examined currents over a more negative voltage range, a range in which inactivation is less apparent. Inactivation of the BK current has been observed in both nonmammalian hair cells (Armstrong and Roberts, 2001) and mammalian IHCs (Kimitsuki et al, 2003;Skinner et al, 2003).…”

Ionic Composition of Endolymph and Perilymph in the Inner Ear of the Oyster Toadfish,Opsanus tau

“…Perhaps most significant is the fact that the temporal responses of hair-cell MET currents are known to be sensitive to endolymph Ca 2ϩ concentration, as manifested in MET adaptation kinetics displaying Ca 2ϩ -dependent components (Assad et al, 1989;Fettiplace and Ricci, 2003). In addition, hair-cell resonance, thought to keep the hair cell maximally sensitive over a specific and narrow frequency band, is mediated by basolateral voltage-gated Ca 2ϩ channels and Ca 2ϩ -gated K ϩ channels and therefore would also be sensitive to the specific ionic milieu (Art et al, 1987;Armstrong and Roberts, 2001). [Ca 2ϩ ] was found to be significantly higher in fish endolymph than in mammalian endolymph (Fig.…”

“…Ions mediate a number of processes in hair cells, including mechanical hair bundle movements , fast and slow adaptation of MET currents Wu et al, 1999;Eatock, 2000;Hudspeth, 2005), and hair-cell electrical resonance (Art et al, 1987;Steinacker, 1996;Armstrong and Roberts, 2001).…”