1,721,060 research outputs found
Inactivation of delayed rectifier K+ current in semicircular canal hair cells
No full textSome properties of the inactivation process of delayed rectifier K+ current (Ik) were investigated in vestibular hair cells of the central region of frog crista ampullaris. These cells were chosen since they exhibited a very large Ik. Experiments were performed on thin slices of sensory epithelium using the whole-cell variant of the patch-clamp technique. Ik showed clear time-dependent inactivation over a period of some seconds, but the current did not completely inactivate even after 30 s depolarizing pulses. Another interesting finding was that inactivation could be well fitted by the sum of two exponentials: at 20 mV depolarization the fast time constant was 291.3 ms and the slow time constant was 2662.3 ms. In addition, an analysis of the steady-state inactivation process of Ik revealed that the inactivation curve was incomplete showing a non-inactivating current at potentials more positive than -50 mV. These results suggested that Ik in hair cells of frog crista ampullaris is composed of more than one component: by at least one inactivating and one non-inactivating component. The possible role of these components in hair cell excitability is discussed
Position-dependent expression of inwardly rectifying K+ currents by hair cells of frog semicircular canals
No full textThe identity and the expression of inwardly rectifying ionic currents were studied using the whole-cell variant of the patch-clamp technique in frog semicircular canal hair cells in situ. The currents were examined in club-, cylindrical- and pear-shaped sensory cells located in three discrete regions of the crista. A unique current of IK1 type was distinguished based on its K+ selectivity, rapid monoexponential activation, dependence of activation on external K+ and blockade by Ba2+ and Cs+. IK1 was found in virtually all cylindrical hair cells of the central region and in club-shaped cells located in the halves of the peripheral regions closest to the centre of the crista. Pear-shaped cells of the intermediate regions showed no inward rectification. The IK1 density (pA/pF) varied along the crista depending on cell position. It was maximal in cells located in the middle of the central region and decreased towards its ends. In the peripheral regions, the gradient of IK1 increased towards the centre of the crista. Current clamp experiments showed that sensory cells having larger IK1 constantly exhibited more negative resting potentials and required more depolarizing current to elicit an active response than cells having small or no IK1
Potassium currents of pear-shaped hair cells in relation to their location in frog crista ampullaris
No full textVoltage-dependent K+ currents in pear-shaped hair cells of the frog crista ampullaris were investigated in thin slice preparations using the whole-cell variant of the patch-clamp technique. Microscopy observation revealed that pear-shaped cells are located in intermediate and peripheral regions of the crista, whereas they are absent in the central region. Voltage-clamp recordings in cells from the peripheral regions revealed that the total outward K+ current could be separated pharmacologically into three distinct components: a A-type K+ current (IA); an inactivating calcium-activated K+ current (IK(Ca)) and a delayed rectifier K+ current (IK). IK and IK(Ca) exhibited similar magnitude and accounted for most of the membrane cell conductance. The same experimental protocol applied to cells from the intermediate regions showed the presence of a large and sustained IK(Ca) which represented 95% of the total outward current. In this region IA was absent. The present results demonstrated that pear-shaped hair cells located in two discrete regions of frog crista ampullaris exhibit a different complement of voltage-dependent conductances, suggesting that they can play a different role in processing the natural stimulus
Inactivating and non-inactivating delayed rectifier potassium currents in hair cells of frog crista ampullaris
No full textThe possible presence of different types of delayed rectifier K+ current (IK) was studied in vestibular hair cells of frog semicircular
canals. Experiments were performed in thin slice preparations of the whole crista ampullaris and recordings were made using the
whole-cell patch-clamp technique. We found that an apparent homogeneous IK, isolated from the other K+ currents, could be
pharmacologically separated into two complementary components: a capsaicin-sensitive current (IK;c) and a barium-sensitive
current (IK;b). IK;c was recruited at potentials more positive than 360 mV and showed a slow activation having a time constant (da)
ranging on average from 12 ms at 40 mV to 32 ms at 320 mV. This current inactivated slowly with two voltage-independent time
constants (dd1 and dd2 were 300 ms and 4 s respectively) and more than 80% of the channels were in an inactivated state at the cell
resting potential. IK;b was also recruited at potentials more positive than 360 mV, but in contrast to IK;c, it activated more rapidly (da
ranged on average from 1 ms at 40 mV to 4.5 ms at 320 mV) and it did not exhibit any inactivation process. Current clamp
experiments revealed that IK;b, at variance with IK;c, contributes to the cell resting potential and represents the main repolarizing
current when sensory cells are depolarized from rest. IK;c could have a role in hair cells when they are depolarized after
hyperpolarizing stimuli, a condition that removes channel inactivation
Gradients of expression of calcium and potassium currents in frog crista ampullaris.
No full textIn the present work we studied the intraregional
expression of voltage-dependent Ca2+ and K+ currents
in hair cells of frog crista ampullaris. The currents
were recorded in situ from sensory cells of the peripheral region, the most populated region of the crista, by using the whole-cell variant of the patch-clamp technique. Voltage-clamp recordings revealed that the calcium current (ICa) and the outward potassium currents of IA, IK, IKCa types and the inward rectifier potassium current of IK1 type exhibited a significant gradient of density (pA/pF) along the region. IA density was maximal in cells located at the beginning of the peripheral region and decreased gradually becoming very small at the opposite end. All the other currents showed an opposite gradient of expression. Current-clamp experiments showed that the voltage behaviour of hair cells changed in relation to cell position. Cells located at the beginning of the peripheral region showed large depolarizations from the resting potential (close to –45 mV) which are consistent with the presence of small IK and IKCa, and an IA largely inactivated at rest. These cells also exhibited slowly developing and large hyperpolarizations that approached passive ones, due to the lack of IK1. In contrast, cells located at the opposite side of the region showed smaller depolarizations and hyperpolarizations from the resting potential (close to –65 mV), due to the presence of large IK and IKCa, and IK1, respectively. The possible role of the intraregional variation of Ca2+ and K+ currents in both hair cell function and afferent discharge properties is discussed
Localization of Ca-ATPase in frog crista ampullaris.
No full textThe distribution of Ca-ATPase in frog crista ampullaris was mapped ultracytochemically by using a one-step lead citrate reaction. Electron-dense precipitates, as an expression of Ca-ATPase activity, were observed on the surface of stereocilia and on the apical membrane surrounding the cuticular plate of hair cells. Sensory cells of the isthmus region showed more reactivity than those of the peripheral regions of the crista. No reaction products were detectable on the basolateral membranes and in cytoplasmatic organelles. Supporting cells of the crista showed a quite variable Ca-ATPase reaction on microvilli and on basolateral membranes. The presence of an evident reactivity on the stereocilia is consistent with the existence of an apical calcium microdomain involved in the mechano-transduction process and supports the current view that calcium ions enter the stereocilia during natural stimulation. On the other hand, the lack of an observable reactivity on the basolateral membrane of hair cells suggests that in semicircular canals other mechanisms of active transport of calcium ions across the plasma membrane, such as Na-Ca exchange, may be involved in homeostasis of the ion
Exocytosis in mouse vestibular Type II hair cells shows a high-order Ca2+ dependence that is independent of synaptotagmin-4
No full textMature hair cells transduce information over a wide range of stimulus intensities and frequencies for prolonged periods of time. The efficiency of such a demanding task is reflected in the characteristics of exocytosis at their specialized presynaptic ribbons. Ribbons are electron-dense structures able to tether a large number of releasable vesicles allowing them to maintain high rates of vesicle release. Calcium entry through rapidly activating, non-inactivating CaV1.3 (L-type) Ca2+ channels in response to cell depolarization causes a local increase in Ca2+ at the ribbon synapses, which is detected by the exocytotic Ca2+ sensors. The Ca2+ dependence of vesicle exocytosis at mammalian vestibular hair cell (VHC) ribbon synapses is believed to be linear, similar to that observed in mature cochlear inner hair cells (IHCs). The linear relation has been shown to correlate with the presence of the Ca2+ sensor synaptotagmin-4 (Syt-4). Therefore, we studied the exocytotic Ca2+ dependence, and the release kinetics of different vesicle pool populations, in Type II VHCs of control and Syt-4 knockout mice using patch-clamp capacitance measurements, under physiological recording conditions. We found that exocytosis in mature control and knockout Type II VHCs displayed a high-order dependence on Ca2+ entry, rather than the linear relation previously observed. Consistent with this finding, the Ca2+ dependence and release kinetics of the ready releasable pool (RRP) of vesicles were not affected by an absence of Syt-4. However, we did find that Syt-4 could play a role in regulating the release of the secondary releasable pool (SRP) in these cells. Our findings show that the coupling between Ca2+ influx and neurotransmitter release at mature Type II VHC ribbon synapses is faithfully described by a nonlinear relation that is likely to be more appropriate for the accurate encoding of low-frequency vestibular information, consistent with that observed at low-frequency mammalian auditory receptors
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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