Potassium Channel Types in Arterial Chemoreceptor Cells and Their Selective Modulation by Oxygen

Single K+ channel currents were recorded in excised membrane patches from dispersed chemoreceptor cells of the rabbit carotid body under conditions that abolish current flow through Na+ and Ca2+ channels. We have found three classes of voltage-gated K+ channels that differ in their single-channel conductance (gamma), dependence on internal Ca2+ (Ca2+i), and sensitivity to changes in O2 tension (PO2). Ca(2+)-activated K+ channels (KCa channels) with gamma approximately 210 pS in symmetrical K+ solutions were observed when [Ca2+]i was greater than 0.1 microM. Small conductance channels with gamma = 16 pS were not affected by [Ca2+]i and they exhibited slow activation and inactivation time courses. In these two channel types open probability (P(open)) was unaffected when exposed to normoxic (PO2 = 140 mmHg) or hypoxic (PO2 approximately 5-10 mmHg) external solutions. A third channel type (referred to as KO2 channel), having an intermediate gamma(approximately 40 pS), was the most frequently recorded. KO2 channels are steeply voltage dependent and not affected by [Ca2+]i, they inactivate almost completely in less than 500 ms, and their P(open) reversibly decreases upon exposure to low PO2. The effect of low PO2 is voltage dependent, being more pronounced at moderately depolarized voltages. At 0 mV, for example, P(open) diminishes to approximately 40% of the control value. The time course of ensemble current averages of KO2 channels is remarkably similar to that of the O2-sensitive K+ current. In addition, ensemble average and macroscopic K+ currents are affected similarly by low PO2. These observations strongly suggest that KO2 channels are the main contributors to the macroscopic K+ current of glomus cells. The reversible inhibition of KO2 channel activity by low PO2 does not desensitize and is not related to the presence of F-, ATP, and GTP-gamma-S at the internal face of the membrane. These results indicate that KO2 channels confer upon glomus cells their unique chemoreceptor properties and that the O2-K+ channel interaction occurs either directly or through an O2 sensor intrinsic to the plasma membrane closely associated with the channel molecule

Low pO2 selectively inhibits K channel activity in chemoreceptor cells of the mammalian carotid body

Producción CientíficaThe hypothesis that changes in environmental 02 tension (pOi) could affect the ionic conductances of dissociated type I cells of the carotid body was tested. Cells were subjected to whole-cell patch clamp and ionic currents were recorded in a control solution with normal pO 2 (pO~ = 150 mmHg) and 3-5 min after exposure to the same solution with a lower pO,. Na and Ca currents were unaffected by lowering pO, to 10 mmHg, however, in all cells studied (n = 42) exposure to hypoxia produced a reversible reduction of the K current. In 14 cells exposed to a pO 2 of 10 mmHg peak K current amplitude decreased to 35 +_ 8% of the control value. The effect of low pO2 was independent of the internal Ca 2+ concentration and was observed in the absence of internal exogenous nucleotides. Inhibition of K channel activity by hypoxia is a graded phenomenon and in the range between 70 and 120 mmHg, which includes normal pO, values in arterial blood, it is directly correlated with pO 2 levels. Low pO2 appeared to slow down the activation time course of the K current but deactivation kinetics seemed to be unaltered. Type I cells subjected to current clamp generate large Na- and Cadependent action potentials repetitively. Exposure to low pO~ produces a 4-10 mV increase in the action potential amplitude and a faster depolarization rate of pacemaker potentials, which leads to an increase in the firing frequency. Repolarization rate of individual action potentials is, however, unaffected, or slightly increased. The selective inhibition of K channel activity by low pO, is a phenomenon without precedents in the literature that explains the chemoreceptive properties of type I cells. The nature of the interaction of molecular O, with the K channel protein is unknown, however, it is argued that a hemoglobin-like O, sensor, perhaps coupled to a G protein, could be involved

Gating of O2-sensitive K + Channels of Arterial Chemoreceptor Cells and Kinetic Modifications Induced by Low PO2

We have studied the kinetic properties of the O2-sensitive K+ channels (KO2 channels) of dissociated glomus cells from rabbit carotid bodies exposed to variable O2 tension (PO2). Experiments were done using single-channel and whole-cell recording techniques. The major gating properties of KO2 channels in excised membrane patches can be explained by a minimal kinetic scheme that includes several closed states (C0 to C4), an open state (O), and two inactivated states (I0 and I1). At negative membrane potentials most channels are distributed between the left-most closed states (C0 and C1), but membrane depolarization displaces the equilibrium toward the open state. After opening, channels undergo reversible transitions to a short-living closed state (C4). These transitions configure a burst, which terminates by channels either returning to a closed state in the activation pathway (C3) or entering a reversible inactivated conformation (I0). Burst duration increases with membrane depolarization. During a maintained depolarization, KO2 channels make several bursts before ending at a nonreversible, absorbing, inactivated state (I1). On moderate depolarizations, KO2 channels inactivate very often from a closed state. Exposure to low PO2 reversibly induces an increase in the first latency, a decrease in the number of bursts per trace, and a higher occurrence of closed-state inactivation. The open state and the transitions to adjacent closed or inactivated states seem to be unaltered by hypoxia. Thus, at low PO2 the number of channels that open in response to a depolarization decreases, and those channels that follow the activation pathway open more slowly and inactivate faster. At the macroscopic level, these changes are paralleled by a reduction in the peak current amplitude, slowing down of the activation kinetics, and acceleration of the inactivation time course. The effects of low PO2 can be explained by assuming that under this condition the closed state C0 is stabilized and the transitions to the absorbing inactivated state I1 are favored. The fact that hypoxia modifies kinetically defined conformational states of the channels suggests that O2 levels determine the structure of specific domains of the KO2 channel molecule. These results help to understand the molecular mechanisms underlying the enhancement of the excitability of glomus cells in response to hypoxia

Depolarizing response of rat parathyroid cells to divalent cations

Membrane potentials were recorded from rat parathyroid glands continuously perfused in vitro. At 1 .5 mM external Ca", the resting potential averages -73 ± 5 mV (mean ± SD, n = 66). On exposure to 2.5 mM Ca", the cells depolarize reversibly to a potential of -34 ± 8 mV (mean ± SD). Depolarization to this value is complete in ^-2-4 min, and repolarization on return to 1 .5 mM Ca" takes about the same time. The depolarizing action of high Ca" is mimicked by all divalent cations tested, with the following order of effectiveness: Ca" > Sr" > Mg" > Ba++ for alkali-earth metals, and Ca" > Cd++ > Mn++ > Co' > Zn++ for transition metals . Input resistance in 1 .5 mM Ca" was 24 .35 ± 14 MQ (mean ± SD) and increased by an average factor of 2.43 ± 0.8 after switching to 2.5 mM Ca++. The low value of input resistance suggests that cells are coupled by low-resistance junctions. Theresting potential in low Ca' is quite insensitive to removal of external Na+ or Cl-, but very sensitive to changes in external K+. Cells depolarize by 61 mV for a 10=fold increase in external K+. In high Ca++, membrane potential is less sensitive to an increase in external K+ and is unchanged by increasing K+ from 5 to 25 mM. Depolarization evoked by high Ca' may be slowed, but is unchanged in amplitude by removal of external Na+ or Cl-. Organic (13600) and inorganic (Co++, Cd++, and Mn++) blockers of the Ca' channels do not interfere with the electrical response to Ca" changes. Our results show remarkable parallels to previous observations on the control ofparathormone (PTH) release by Ca". They suggest an association between membrane voltage and secretion that is very unusual: parathyroid cells secrete when fully polarized, and secrete less when depolarized. The extraordinary sensitivity of parathyroid cells to divalent cations leads us to hypothesize the existence in their membranes ofa divalent cation receptor that controls membrane permeability (possibly to K+) and PTH secretion

Ionic currents in dispersed chemoreceptor cells of the mammalian carotid body

Producción CientíficaIonic currents of enzymatically dispersed type 1 and type 11 cells of the carotid body have been studied using the whole cell variant of the patch-clamp technique. Type 11 cells only have a tiny, slowly activating outward potassium cur­ rent. By contrast, in every type 1 chemoreceptor cell studied we found (a) sodium, (b) calcium, and (e) potassium currents. (a) The sodium current has a fast activation time course and an activation threshold at --40 mV. At ali voltages inactivation follows a single exponential time course. The time constant of inactivation is 0.67 ms at O mV. Half steady state inactivation occurs at a membrane potential of --50 mV. (b) The calcium current is almost totally abolished when most of the extemal calcium is replaced by magnesium. The activation threshold of this cur­ rent is at --40 mV and at O mV it reaches a peak amplitude in 6-8 ms. The calcium current inactivates very slowly and only decreases to 27% of the maximal value at the end of 300-ms pulses to 40 mV. The calcium current was about two times larger when barium ions were used as charge carriers instead of calcium ions. Barium ions also shifted 15-20 mV toward negative voltages the conductance vs. voltage curve. Deactivation kinetics of the calcium current follows a biphasic time course well fitted by the sum of two exponentials. At -80 mV the slow com­ ponent has a time constant of 1.3 ± 0.4 ms whereas the fast component, with an amplitude about 20 times larger than the slow component, has a time constant of 0.16 ± 0.03 ms. These results suggest that type 1 cells have predominantly fast deactivating calcium channels. The slow component of the tails may represent the activity of a small population of slowly deactivating calcium channels, although other possibilities are considered. (e) Potassium current seems to be mainly due to the activity of voltage-dependent potassium channels, but a small percentage of calcium-activated channels may also exist. This current activates slowly, reaches a peak amplitude in 5-1O ms, and thereafter slowly inactivates. Inactivation is almost complete in 250-300 ms. The potassium current is reversibly blocked by tetraeth­ ylammonium. Under current-clamp conditions type I cells can spontaneously fire large action potentials