doi: 10.1161/hh0901.091206
© 2001 American Heart Association, Inc.
Editorial |
KATP Channel Regulators
Balanced Diets Include Carbohydrates, Proteins, and Fats
From the Department of Cell Biology and Physiology, Washington University School of Medicine, St Louis, Mo.
Correspondence to C.G. Nichols, Department of Cell Biology and Physiology, Washington University School of Medicine, 660 S Euclid Ave, St Louis, MO 63110. E-mail cnichols{at}cellbio.wustl.edu
Key Words: KATP • fatty acids • acyl–coenzyme A esters • ATP • PIP2
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Introduction |
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 Top Introduction A Smorgasbord of Channel...The Major Metabolic Substrate......References |
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In addition to the steep voltage-dependent K+ currents that play a role in repolarization of the heart, two major K+ conductances control the resting potential. The classical strong inward rectifier (IK1) is constitutively active and acts to stabilize the resting potential. During depolarization, the strong rectification of IK1 significantly reduces conductance, permitting a long action-potential plateau. The ATP-sensitive potassium current (IK,ATP) is not usually active, but the density of underlying channels is so high that when activated in conditions of metabolic inhibition, such as ischemia, the action potential is massively shortened and the ventricle eventually becomes inexcitable. Although this channel was named for its defining property of inhibition by ATP, it is far from clear what signals actually lead to channel opening in physiological and pathophysiological conditions. A central paradox is the following: in excised membrane patches, KATP channels are half-maximally inhibited by
10 to 50 µmol/L ATP, but
intracellular [ATP] does not fall below millimolar levels except
under very extreme
conditions.1
It has long been recognized that MgADP and MgGDP act as
antagonists to ATP inhibition of the
channel,1 and the cloning and
expression of the relevant SUR and Kir6 subunits of the
channel2 have revealed
details of this antagonism. ATP inhibition occurs through a direct
interaction with the pore-forming Kir6 subunit; Mg-diphosphate
activation of the channel occurs through interactions with the
nucleotide-hydrolyzing domains of the SUR subunit
(Figure
).
Both experiments and computer modeling indicate that the degree of
activation of
IK,ATP
conductance that is observed in ischemia or
metabolic inhibition may be achieved by the stimulatory
effects of elevated Mg diphosphates in the maintained presence of
ATP.3 4 5 6
In addition, the disease-causing effects of channel mutations that
specifically abolish diphosphate stimulation of pancreatic
KATP channels support this
concept.7 8
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However, there have remained some intriguing and unexplained observations regarding native cardiac channels, and new, extremely potent modulators of channel activity have appeared to additionally complicate the picture and bring new possibilities to channel regulation. Findlay and Faivre9 tested the ATP sensitivity of ATP-sensitive KATP channels in rat ventricular myocytes and found that in 102 inside-out patches, the half-maximal inhibitory concentration of the channels varied as much as 60-fold. This variation in ATP sensitivity is not experimental error, indicating either that different molecular species must underlie the variation or that ATP sensitivity is dynamically variable. It is now widely accepted that the cardiac channel–forming isoforms are Kir6.2 and SUR2A,10 although there is evidence of expression of both Kir6.1 and SUR1 in ventricle.11 12 However, recent studies have demonstrated dramatic effects of both protein modification and of lipids in controlling channel activity.
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A Smorgasbord of Channel Regulators |
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TopIntroductionA Smorgasbord of Channel...The Major Metabolic Substrate......References |
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Recently, modulation of KATP channels has been extended to include direct modification of the Kir6.2 subunit by protein kinases. Although there remains some controversy as to the exact residues that are phosphorylated, various groups have reported the existence of protein kinase A (PKA)–mediated phosphorylation of Kir6.2 (at S372 and T224)13 14 and SUR1 (at S1571).13 The effect of PKA on Kir6.2 was to increase channel open probability (Po)13 14 and decrease ATP sensitivity,14 whereas the effect on SUR1 was to reduce channel activity.13 Neurotransmitters that couple to adenylate cyclase and, therefore, PKA are likely to affect Kir6.2 phosphorylation and hence channel activity. Similarly, protein kinase C (PKC) modulates Kir6.2 via phosphorylation of residue T180.15 Here, too, we can envision a route for dynamic modulation. Activation of M1 muscarinic receptors in the heart would lead eventually to phospholipase C activation, the generation of diacyl glycerol, and hence activation of PKC and phosphorylation of Kir6.2 on T180.
Hilgemann and Ball16 first reported the ability of phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate (PIP2), to activate ion transporters and channels. Removing PIP2 from the membrane by addition of phospholipase C caused channels or transporters to lose activity, or "rundown," whereas adding PIP2 to the inner leaflet of the membrane prevented and reversed this rundown. For the KATP channel, the addition of PIP2 to the inner membrane causes channel Po to increase and sensitivity to ATP inhibition to decrease.17 18 The effect on ATP sensitivity is so dramatic that the addition of PIP2 can shift the K1/2ATP of the channels from an initial basal level of 10 µmol/L to >3 mmol/L,17 more than 3 orders of magnitude. The data are consistent with an antagonistic effect of PIP2 on ATP inhibition, suggesting that the two ligands may compete functionally for interaction with the channel protein subunits, stabilizing the open and closed conformations, respectively. This effect has been shown to have a physiological relevance, because PIP2 levels are modulated by several different pathways in the cell, including strict regulation of kinases and phosphatases. Baukrowitz et al18 showed that PIP2 levels and hence KATP channel activity could be modulated in a cell by expression and activation of purinergic (P2Y) receptors.19
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The Major Metabolic Substrate of Cardiac Myocytes Is Apparently a Major Regulator of KATP Channels |
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TopIntroductionA Smorgasbord of Channel...The Major Metabolic Substrate......References |
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The major energetic substrate of cardiac myocytes is fatty acids, and in this issue of Circulation Research, Liu et al20 demonstrate dramatic effects of fatty acyl–coenzyme A (CoA) (to which these substrates are converted in the mitochondria) on sarcolemmal KATP channels. Long-chain acyl-CoA esters were previously found to modulate KATP channels in mouse ß-cells by increasing the time the channel spends in an open state.21 As with PIP2, the stimulatory effect of oleoyl-CoA is primarily on the Kir6.2 subunit and acts to both increase Po and decrease ATP sensitivity and to reduce channel rundown. In the ß-cell studies,21 22 however, the authors noted a 2-fold increase in Po and only a 3-fold decrease in ATP sensitivity. In the present study by Liu et al,20 a much more dramatic effect is seen on KATP channels in guinea pig myocytes, with acyl-CoA causing >200-fold reduction of ATP sensitivity, similar in magnitude to the effects of PIP2. Another recent study from a different group also indicates similar dramatic effects of acyl-CoA on KATP channel activity.23 Consistent with the finding of Liu et al20 that addition of free fatty acids outside the myocyte may lead to intracellular accumulation of acyl-CoA and activation of KATP channels, these authors additionally reported significant protection from damage attributable to chemically induced hypoxia when octanoate was present in the medium.
Unlike PIP2, which is likely to activate all Kir channels,24 25 26 27 oleoyl-CoA did not prevent rundown of IK1 channels.20 This result may indicate a different mechanistic process underlying oleoyl-CoA and PIP2 activation. However, this may alternatively reflect a difference in affinity of various Kir channels for oleoyl-CoA. Effects of various phospholipids on several Kir channels have recently been explored. Rohacs et al25 have shown that different Kir channels have different affinities for PIP2 and different specificities for various PI moieties. For example, Kir2.1 (the likely molecular basis of IK1) is activated by PI(4,5)P2 but not PI(3,4)P2, whereas Kir3 channels are activated equally by several of the PI moieties. Although Kir channels are highly homologous, they are not, of course, identical, and subtle differences in the structure of the cytoplasmic domains of each may give rise to unique PI (and perhaps acyl-CoA) specificity and affinity without fundamental differences in underlying mechanisms.
Another major distinguishing characteristic that Liu et al20 report between the effects of PIP2 on the channel and acyl-CoA is that the PIP2 effects are abolished on addition of Ca2+, whereas the effects of oleoyl-CoA are unaffected by Ca2+. This again may imply a different mechanistic basis of action of the two lipids. However, multiple effects of Ca2+ are possible that may not directly reflect the nature of the interaction between the lipid moiety and the channel. Ca2+ can activate numerous protein kinases, protein phosphatases, and phospholipases, which, by differential actions on oleoyl-CoA or PIP2, may appear to qualitatively affect the ability of the lipid to activate the channel.
The list of direct effectors of cardiac
KATP channel activity becomes more complete and
elaborated. The effects of nucleotide triphosphates and diphosphates on
channel function have long been recognized and clearly link the
activity of the channel to the metabolic state of the cell.
Phosphorylation by PKA and PKC has similarly been shown
to directly modulate channel activity. Addition of
PIP2 to the cytoplasmic face activates
the channels. Now another lipid moiety, oleoyl-CoA, is added to the
list, linking channel functioning to fatty acid metabolism.
However, the regulatory picture of KATP may be
even more complex. These modulatory agents do not work in isolated
pathways. For instance, increasing glucose levels in the cell will
inhibit the channel through production of
inhibitory [ATP] but will also increase the amount of
lipid
phosphorylation,28
providing a counter-effect. Increasing [ATP] will also provide more
substrate for protein kinases. Finally, whereas increasing fatty acids
in the cell might activate the channel by inserting into the
plasma membrane, as proposed by Liu et
al,20 increased fatty acids
may also increase ß oxidation, additionally adding to the
inhibitory [ATP], and oleoyl-CoA may also regulate
PKC,29 30
providing yet another indirect effect on the channel
(Figure
).
Clearly, multiple different pathways are active in modulating and regulating KATP channel function, providing multiple means of dynamic regulation. The elucidation of these processes, furthered by the studies of Liu et al20 and Light et al,23 provides much food for thought and avenues to consider in addressing the significant remaining challenge: to explain when KATP channels really become active in the sarcolemma and why.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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TopIntroductionA Smorgasbord of Channel...The Major Metabolic Substrate......References |
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