Abstract
Sarcoplasmic reticulum (SR) K+ channels are voltage-regulated channels that are thought to be actively gating when the membrane potential across the SR is close to zero as is expected physiologically. A characteristic of SR K+ channels is that they gate to subconductance open states but the relevance of the subconductance events and their contribution to the overall current flowing through the channels at physiological membrane potentials is not known. We have investigated the relationship between subconductance and full conductance openings and developed kinetic models to describe the voltage sensitivity of channel gating. Because there may be two subtypes of SR K+ channels (TRIC-A and TRIC-B) present in most tissues, to conduct our study on a homogeneous population of SR K+ channels, we incorporated SR vesicles derived from Tric-a knockout mice into artificial membranes to examine the remaining SR K+ channel (TRIC-B) function. The channels displayed very low open probability (Po) at negative potentials (≤0 mV) and opened predominantly to subconductance open states. Positive holding potentials primarily increased the frequency of subconductance state openings and thereby increased the number of subsequent transitions into the full open state, although a slowing of transitions back to the sublevels was also important. We investigated whether the subconductance gating could arise as an artifact of incomplete resolution of rapid transitions between full open and closed states; however, we were not able to produce a model that could fit the data as well as one that included multiple distinct current amplitudes. Our results suggest that the apparent subconductance openings will provide most of the K+ flux when the SR membrane potential is close to zero. The relative contribution played by openings to the full open state would increase if negative charge developed within the SR thus increasing the capacity of the channel to compensate for ionic imbalances.
Highlights
sarcoplasmic reticulum (SR) Kþ channels are selective for monovalent cations and their conduction properties are comprehensively described [1,2,3,4,5,6,7,8,9]
From the relationship between voltage and subconductance Po (Fig. 3 B), we cannot measure channel openings at 0 mV in symmetrical conditions, we can extrapolate the data to suggest that TRIC-B is normally gating to subconductance open states at potentials close to 0 mV with only rare openings to the full open state
As there is thought to be no large resting membrane potential across the SR [40], TRIC-B would be expected to be gating in a low Po mode and primarily to subconductance open states unless there was buildup of negative charge in the SR relative to the cytosol
Summary
SR Kþ channels are selective for monovalent cations and their conduction properties are comprehensively described [1,2,3,4,5,6,7,8,9]. There is evidence that there may be two subtypes of SR Kþ channels termed the trimeric intracellular cation channels (TRIC-A and TRIC-B) [10,11,12]. Mice devoid of both TRIC-A and B are not viable and die in cardiac arrest at embryonic day 10.5 [10] highlighting their necessity in the heart. Cardiac myocytes from TRIC double knockout (DKO) mice exhibit regions of swollen sarcoplasmic reticulum (SR) containing Ca2þ-deposits that are not present in control mice. At embryonic day 8.5 in DKO cardiac myocytes, spontaneous Ca2þ transients were reduced, whereas caffeine-induced release from intra-
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