Metabolic signals and the ATP- sensitive potassium channel in the
Av Robert Brännström, Institutionen för Molekylär Medicin vid Karolinska
Institutet,Stockholm. Handledare har varit docent
Larsson och professor Per-Olof Berggren.
Avhandlingen kommer presenteras av pristagaren vid Svensk Förenings
för Diabetologis vår- möte i Huddinge 13-14 april 2000.
ATP-sensitive K+(KATP) channels are widely distributed in a variety
of tissues and cell types. KATP channels are comprised of two subunits,a
pore forming sub- unit (Kir6.x)and a regulatory sulfonylurea receptor
(SURx).Both subunits are required for fully functional channels.
In the present thesis the role of the pancreatic [beta
]-cell KATP channel,with regard to oscillations in electrical activity
and modulation of channel activity by various compounds as well as the
functional organization of channel subunits,were investigated.
In the [beta ]-cell the KATP channel (comprised of
Kir6.2 and SUR1)couples metabolic changes to electrical activity.Although
electrophysiological studies during the last decade have provided clues
to the complex control of the KATP channel by various nucleotides and
pharmacological agents, the metabolic signals responsible for its physiological
regulation remain to be clarified.
The widely accepted model of glucose-induced insulin
secretion involves a number of events. Following glucose metabolism,the
ATP/ADP ratio increases,inducing closure of the KATP channel, depolarization
of the [beta ]-cell plasma membrane and thereby opening of voltage-gated
Ca2+ channels.The rise in cytoplasmic free Ca2+concentration ([Ca2+]i)
triggers insulin release.Oscillations in some or all of these signals
are thought to play an important role in the pulsatile nature of insulin
The molecular mechanism underlying these oscillations
is unknown but is an important issue since absence of normal oscillations
in plasma insulin levels is observed in noninsulin dependent diabetes
In this thesis,it is shown that KATP channel activity
is oscillating in intact [beta ]-cells under stimulatory concentrations
of glucose,thereby inducing fluctuations in electrical activity and
[Ca2+]i. This effect is likely to reflect oscillations in metabolism,since
blocking of glucose metabolism induced openings of the KATP channel
that correlated with periods of hyperpolarization and lowering in [Ca2+]i.Glucose
is metabolized through glycolysis in the cytosol and through the tricarboxylic
acid cycle in the mitochondria.
The relative role of mitochondrial metabolism has frequently been studied
using [alpha ]-ketoisocaproate as substrate.
In contrast to previous findings,assuming that [alpha ]-ketoisocaproate
needs to be metabolized in order to close the KATP channel,it is shown
that [alpha ]- ketoisocaproate reversibly and in a dose-dependent manner
inhibits the KATP channel directly.Besides glucose,free fatty acids
(FFA) have been demonstrated to play a critical role in insulin secretion.
It is shown that the metabolic active form of FFA,long-chain CoA (LC-CoA)esters,are
accumulated intracellularly in response to prolonged exposure to elevated
FFA,and are potent and chainlength specific activators of the channel.
Using a truncated version of Kir6.2 (Kir6.2 [delta
]C26),which generates channels in the absence of SUR1,it was shown that
LC-CoA esters interact directly with Kir6.2.
These findings verify that LC-CoA esters have a unique
bin- ding site on the KATP channel complex and may be of physio- logical
importance.Finally,KAT- Pchannels are organized as heterooctameric complexes
assembled with a 4:4 stoichiometry of Kir6.2 and SUR1.
It is shown that SUR1 plays a critical role in the
functional organization of Kir6.2 by a mechanism distinct from the ability
of SUR1 to recruit Kir6.2 to the membrane and the ability to convey
sensitivity to sulfonylurea and MgADP.
1. Oscillations in KATP channel Årets förtjänstfulla
diabetesavhandling 1999 activity promote oscillations in cyto- plasmic
free Ca2+concentration in the pancreatic beta cell Larsson O,Kindmark
H,Bränström R,Fredholm B,Berggren PO Proc Natl Acad Sci U.S.A,1996;
2. Direct inhibition of the pancre- atic beta-cell
ATP-regulated potas- sium channel by alpha-ketoisocapro- ate Bränström
R,Efendic S,Berggren PO,Larsson O J Biol Chem,1998; 273:14113-14118
3. Activation of the ATP-sensitive K+channel
by long chain acyl-CoA. A role in modulation of pancreatic beta-cell
glucose sensitivity Larsson O,Deeney JT,Bränström R,Berggren PO,Corkey
BE J Biol Chem,1996; 271:10623-10626
4. Evidence for a unique long chain acyl-CoA
ester binding site on the ATP-regulated potassium channel in mouse pancreatic
beta cells Bränström R,Corkey BE,Berggren PO,Larsson O J Biol Chem,1997;
5. Long chain coenzyme A esters activate the
pore-forming subunit (Kir6.2)of the ATP-regulated potassium channel
Bränström R,Leibiger IB,Leibiger B,Corkey BE,Berggren PO,Larsson O J
Biol Chem,1998; 273:31395-31400
6. The sulfonylurea receptor pro- motes functional
orientation of the KATP channel pore forming subunit Kir6.2 Bränström
R,Leibiger B,Moede T, Lindqvist P,Leibiger I,Berggren PO,Larsson O Submitted