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Properties of an inwardly rectifying ATP-sensitive K+ channel in the basolateral membrane of renal proximal tubule.

Mauerer UR, Boulpaep EL, Segal AS - J. Gen. Physiol. (1998)

Bottom Line: The channel conducts Tl+ and K+, but there is no significant conductance for Na+, Rb+, Cs+, Li+, NH4+, or Cl-.The K+ channel opener diazoxide opens the channel in the presence of 0.2 mM ATP, but does not alleviate the inhibition of millimolar doses of ATP.We conclude that this K+ channel is the major ATP-sensitive basolateral K+ conductance in the proximal tubule.

View Article: PubMed Central - PubMed

Affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.

ABSTRACT
The potassium conductance of the basolateral membrane (BLM) of proximal tubule cells is a critical regulator of transport since it is the major determinant of the negative cell membrane potential and is necessary for pump-leak coupling to the Na+,K+-ATPase pump. Despite this pivotal physiological role, the properties of this conductance have been incompletely characterized, in part due to difficulty gaining access to the BLM. We have investigated the properties of this BLM K+ conductance in dissociated, polarized Ambystoma proximal tubule cells. Nearly all seals made on Ambystoma cells contained inward rectifier K+ channels (gammaslope, in = 24.5 +/- 0.6 pS, gammachord, out = 3.7 +/- 0.4 pS). The rectification is mediated in part by internal Mg2+. The open probability of the channel increases modestly with hyperpolarization. The inward conducting properties are described by a saturating binding-unbinding model. The channel conducts Tl+ and K+, but there is no significant conductance for Na+, Rb+, Cs+, Li+, NH4+, or Cl-. The channel is inhibited by barium and the sulfonylurea agent glibenclamide, but not by tetraethylammonium. Channel rundown typically occurs in the absence of ATP, but cytosolic addition of 0. 2 mM ATP (or any hydrolyzable nucleoside triphosphate) sustains channel activity indefinitely. Phosphorylation processes alone fail to sustain channel activity. Higher doses of ATP (or other nucleoside triphosphates) reversibly inhibit the channel. The K+ channel opener diazoxide opens the channel in the presence of 0.2 mM ATP, but does not alleviate the inhibition of millimolar doses of ATP. We conclude that this K+ channel is the major ATP-sensitive basolateral K+ conductance in the proximal tubule.

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Kinetics of the BLM K+ channel. (A) Open-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned (Sigworth and Sine, 1987). The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τo1 = 0.78 ms (78%) and τo2 = 4.7 ms (22%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (vertical dotted line). (B) Closed-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned. The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τc1 = 1.27 ms (74%) and τc2 = 397 ms (26%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (dotted vertical line).
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Figure 6: Kinetics of the BLM K+ channel. (A) Open-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned (Sigworth and Sine, 1987). The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τo1 = 0.78 ms (78%) and τo2 = 4.7 ms (22%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (vertical dotted line). (B) Closed-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned. The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τc1 = 1.27 ms (74%) and τc2 = 397 ms (26%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (dotted vertical line).

Mentions: Due to the high density of this K+ channel in the BLM, a patch apparently containing only one channel is extremely rare. In over 550 seals, only four membrane patches appeared to contain only one channel (0.7%). Since the open probability (Po) of the channel is only 0.05 ± 0.01 (n = 4), long recordings were required to accumulate enough transitions for meaningful analysis of the long closed state. Kinetic analyses from such patches show that under resting state conditions at −Vpip = −60 mV, the BLM K+ channel has two apparent open states and two apparent closed states. Parameters from one c/a patch and one i/o patch show that the open dwell lifetimes are (ms): τo1 = 0.78 (c/a), 1.21 (i/o), and τo2 = 4.7 (c/a), 6.6 (i/o). The closed dwell lifetimes are (ms): τc1 = 1.27 (c/a), 0.72 (i/o), and τc2 = 397 (c/a), 502 (i/o). Fig. 6 shows the open and closed time histograms for a c/a patch.


Properties of an inwardly rectifying ATP-sensitive K+ channel in the basolateral membrane of renal proximal tubule.

Mauerer UR, Boulpaep EL, Segal AS - J. Gen. Physiol. (1998)

Kinetics of the BLM K+ channel. (A) Open-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned (Sigworth and Sine, 1987). The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τo1 = 0.78 ms (78%) and τo2 = 4.7 ms (22%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (vertical dotted line). (B) Closed-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned. The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τc1 = 1.27 ms (74%) and τc2 = 397 ms (26%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (dotted vertical line).
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC1887768&req=5

Figure 6: Kinetics of the BLM K+ channel. (A) Open-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned (Sigworth and Sine, 1987). The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τo1 = 0.78 ms (78%) and τo2 = 4.7 ms (22%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (vertical dotted line). (B) Closed-time histogram for the BLM K+ channel in a cell-attached patch at −60 mV with the time intervals logarithmically binned. The data were fitted with two probability density functions (dashed lines) to give the overall fit (solid line), yielding time constants of τc1 = 1.27 ms (74%) and τc2 = 397 ms (26%). Based on the bandwidth of the recording system, the data and the fit were cutoff at 500 μs (dotted vertical line).
Mentions: Due to the high density of this K+ channel in the BLM, a patch apparently containing only one channel is extremely rare. In over 550 seals, only four membrane patches appeared to contain only one channel (0.7%). Since the open probability (Po) of the channel is only 0.05 ± 0.01 (n = 4), long recordings were required to accumulate enough transitions for meaningful analysis of the long closed state. Kinetic analyses from such patches show that under resting state conditions at −Vpip = −60 mV, the BLM K+ channel has two apparent open states and two apparent closed states. Parameters from one c/a patch and one i/o patch show that the open dwell lifetimes are (ms): τo1 = 0.78 (c/a), 1.21 (i/o), and τo2 = 4.7 (c/a), 6.6 (i/o). The closed dwell lifetimes are (ms): τc1 = 1.27 (c/a), 0.72 (i/o), and τc2 = 397 (c/a), 502 (i/o). Fig. 6 shows the open and closed time histograms for a c/a patch.

Bottom Line: The channel conducts Tl+ and K+, but there is no significant conductance for Na+, Rb+, Cs+, Li+, NH4+, or Cl-.The K+ channel opener diazoxide opens the channel in the presence of 0.2 mM ATP, but does not alleviate the inhibition of millimolar doses of ATP.We conclude that this K+ channel is the major ATP-sensitive basolateral K+ conductance in the proximal tubule.

View Article: PubMed Central - PubMed

Affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.

ABSTRACT
The potassium conductance of the basolateral membrane (BLM) of proximal tubule cells is a critical regulator of transport since it is the major determinant of the negative cell membrane potential and is necessary for pump-leak coupling to the Na+,K+-ATPase pump. Despite this pivotal physiological role, the properties of this conductance have been incompletely characterized, in part due to difficulty gaining access to the BLM. We have investigated the properties of this BLM K+ conductance in dissociated, polarized Ambystoma proximal tubule cells. Nearly all seals made on Ambystoma cells contained inward rectifier K+ channels (gammaslope, in = 24.5 +/- 0.6 pS, gammachord, out = 3.7 +/- 0.4 pS). The rectification is mediated in part by internal Mg2+. The open probability of the channel increases modestly with hyperpolarization. The inward conducting properties are described by a saturating binding-unbinding model. The channel conducts Tl+ and K+, but there is no significant conductance for Na+, Rb+, Cs+, Li+, NH4+, or Cl-. The channel is inhibited by barium and the sulfonylurea agent glibenclamide, but not by tetraethylammonium. Channel rundown typically occurs in the absence of ATP, but cytosolic addition of 0. 2 mM ATP (or any hydrolyzable nucleoside triphosphate) sustains channel activity indefinitely. Phosphorylation processes alone fail to sustain channel activity. Higher doses of ATP (or other nucleoside triphosphates) reversibly inhibit the channel. The K+ channel opener diazoxide opens the channel in the presence of 0.2 mM ATP, but does not alleviate the inhibition of millimolar doses of ATP. We conclude that this K+ channel is the major ATP-sensitive basolateral K+ conductance in the proximal tubule.

Show MeSH
Related in: MedlinePlus