Inwardly rectifying K+(Kir) channels allow K+to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K+channels (Kir6.x) are tightly linked to cellular metabolism, and K+transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg2+and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH2and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

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SUMMARY1. The patch-clamp method of single-channel recording was applied to K+ channels which are inhibited by intracellular adenosine 5'-triphosphate (ATP: K+-ATP channels) in membrane patches obtained from the insulin-secreting cloned cell line RINm5F.2. The magnitude of K+ currents flowing outwards through these K+-ATP channels was reduced by internal Mg2+ ions in a dose-dependent manner. Currents flowing inwards through the channels were not affected by Mg2+. Internal Na+ ions had similar effects.3. Divalent cations (Mg2+, Sr2+ and Ca2+) applied to the internal surface of the patch membrane inhibited the opening of K+-ATP channels in a dose-dependent manner. Internal Na+ ions had no effect.4. K+-ATP channel activity was stimulated by adenosine 5'-diphosphate (ADP), guanosine 5'-triphosphate (GTP), guanosine 5'-diphosphate (GDP), guanosine 5'-o-(3-thiotriphosphate) (GTPyS) and guanosine 5'-o-(2-thiodiphosphate) (GDP,^S) when millimolar Mg2+ bathed the internal surface of the patch membrane. In the virtual absence of internal Mg2+ ions ADP, GTP, and GTPyS inhibited K+-ATP channels and GDP and GDPflS were without effect. Adenosine 5'-o-(2-thiodiphosphate) (ADP,fS) inhibited K+-ATP channel activity in the presence and absence of Mg2+.5. K+-ATP channel openings could be evoked by either ADP or GDP in the presence of an inhibitory concentration of ATP. These openings were abolished in the absence of internal Mg2+.6. Run-down K+-ATP channels could be reactivated by ATP in the presence of internal Mg2+, but not in its absence. Analogues of ATP were unable to reactivate K+-ATP channels even in the presence of Mg2+.7. It is concluded that internal Mg2+ ions (i) cause the rectification of the K+-ATP channel current-voltage relationship, (ii) are required for K+-ATP channel activity to be maintained by a phosphorylation process and (iii) are required for K+-ATP channel activity evoked by ADP, GTP and GDP.

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The Ca2+-activated K+ channel in rat pancreatic islet cells has been studied using patch-clamp single-channel current recording in excised inside-out and outside-out membrane patches. In membrane patches exposed to quasi-physiological cation gradients (Na+ outside, K+ inside) large outward current steps were observed when the membrane was depolarized. The single-channel current voltage (I/V) relationship showed outward rectification and the null potential was more negative than -40 mV. In symmetrical K+-rich solutions the single-channel I/V relationship was linear, the null potential was 0 mV and the single-channel conductance was about 250 pS. Membrane depolarization evoked channel opening also when the inside of the membrane was exposed to a Ca2+-free solution containing 2mM EGTA, but large positive membrane potentials (70 to 80 mV) were required in order to obtain open-state probabilities (P) above 0.1. Raising the free Ca2+ concentration in contact with the membrane inside ( [Ca2+]i) to 1.5 X 10(-7) M had little effect on the relationship between membrane potential and P. When [Ca2+]i was increased to 3 X 10(-7) M and 6 X 10(-7) M smaller potential changes were required to open the channels. Increasing [Ca2+]i further to 8 X 10(-7) M again activated the channels, but the relationship between membrane potential and P was complex. Changing the membrane potential from -50 mV to +20 mV increased P from near 0 to 0.6 but further polarization to +50 mV decreased P to about 0.2. The pattern of voltage activation and inactivation was even more pronounced at [Ca2+]i = 1 and 2 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

K+ channels in cultured rat pancreatic islet cells have been studied using patch-clamp single-channel recording techniques in cell-attached and excised inside-out and outside-out membrane patches. Three different K+-selective channels have been found. Two inward rectifier K+ channels with slope conductances of about 4 and 17 pS recorded under quasi-physiological cation gradients (Na+ outside, K+ inside) and maximal conductances recorded in symmetrical K+-rich solutions of about 30 and 75 pS, respectively. A voltage- and calcium-activated K+ channel was recorded with a slope conductance of about 90 pS under the same conditions and a maximal conductance recorded in symmetrical K+-rich solutions of about 250 pS. Single-channel current recording in the cell-attached conformation revealed a continuous low level of activity in an apparently small number of both the inward rectifier K+ channels. But when membrane patches were excised from the intact cell a much larger number of inward rectifier K+ channels became transiently activated before showing an irreversible decline. In excised patches opening and closing of both the inward rectifier K+ channels were unaffected by voltage, internal Ca2+ or externally applied tetraethylammonium (TEA) but the probability of opening of both inward rectifier K+ channels was reduced by internally applied 1-5 mM adenosine-5'-triphosphate (ATP). The large K+ channel was not operational in cell-attached membrane patches, but in excised patches it could be activated at negative membrane potentials by 10(-7) to 10(-6) M internal Ca2+ and blocked by 5-10 mM external TEA.

K+ currents were recorded from ATP-sensitive channels in inside-out patches from isolated rat ventricular myocytes. In the absence of internal divalent cations the current voltage relationship could be described by constant-field assumptions with a permeability of 1.25 X 10(-13) cm2/s; outward currents saturated under a high driving force for K+ movement. Internal 0.1-5.0 mM Mg2+, 0.1 microM Ca2+ and 10 mM Na+ each depressed the flux of K+ ions moving outwards through open channels. Internal 0.1-5.0 mM Mg2+, 0.1-1.0 microM Ca2+ and 1-10 microM Ba2+ and Sr2+ blocked K+ channel activity in a dose- and voltage-dependent manner. Run-down channels could be reactivated by Mg-ATP, but not by AMP-PNP, ATP gamma S or Mg-free ATP which suggested that phosphorylation of the channels was involved in their activity. Ca2+ (greater than = 1 microM) and Sr2+ (1 mM) markedly inactivated K+ ATP channels, millimolar Ba2+ or Mg2+ were less effective. This suggested that the run down of the channels was a Ca2+-dependent dephosphorylation of the K+ channel protein.

Doisne N, Maupoil V, Cosnay P, Findlay I. Catecholaminergic automatic activity in the rat pulmonary vein: electrophysiological differences between cardiac muscle in the left atrium and pulmonary vein. Am J Physiol Heart Circ Physiol 297: H102-H108, 2009. First published May 8, 2009 doi:10.1152/ajpheart.00256.2009.-Ectopic activity in cardiac muscle within pulmonary veins (PVs) is associated with the onset and the maintenance of atrial fibrillation in humans. The mechanism underlying this ectopic activity is unknown. Here we investigate automatic activity generated by catecholaminergic stimulation in the rat PV. Intracellular microelectrodes were used to record electrical activity in isolated strips of rat PV and left atrium (LA). The resting cardiac muscle membrane potential was lower in PV [Ϫ70 Ϯ 1 (SE) mV, n ϭ 8] than in LA (Ϫ85 Ϯ 1 mV, n ϭ 8). No spontaneous activity was recorded in PV or LA under basal conditions. Norepinephrine (10 Ϫ5 M) induced first a hyperpolarization (Ϫ8 Ϯ 1 mV in PV, Ϫ3 Ϯ 1 mV in LA, n ϭ 8 for both) then a slowly developing depolarization (ϩ21 Ϯ 2 mV after 15 min in PV, ϩ1 Ϯ 2 mV in LA) of the resting membrane potential. Automatic activity occurred only in PV; it was triggered at approximately Ϫ50 mV, and it occurred as repetitive bursts of slow action potentials. The diastolic membrane potential increased during a burst and slowly depolarized between bursts. Automatic activity in the PV was blocked by either atenolol or prazosine, and it could be generated with a mixture of cirazoline and isoprenaline. In both tissues, cirazoline (10 Ϫ6 M) induced a depolarization (ϩ37 Ϯ 2 mV in PV, n ϭ 5; ϩ5 Ϯ 1 mV in LA, n ϭ 5), and isoprenaline (10 Ϫ7 M) evoked a hyperpolarization (Ϫ11 Ϯ 3 mV in PV, n ϭ 7; Ϫ3 Ϯ 1 mV in LA, n ϭ 6). The differences in membrane potential and reaction to adrenergic stimulation lead to automatic electrical activity occurring specifically in cardiac muscle in the PV. atrium; norepinephrine; electrophysiology; atrial fibrillation; thoracic veins THE CAUSE OF FOCAL ELECTRICAL activity in myocardial sleeves within the pulmonary veins (6, 14) which can trigger and sustain atrial fibrillation in humans is unknown.A number of themes have been developed to address possible causes of ectopic activity in the pulmonary veins. An attractive but disputed hypothesis is that cardiac muscle in pulmonary veins contains pacemaker cells that would generate spontaneous activity (5, 24). Quantitative surveys of ion channel currents in isolated cardiac myocytes have looked for differences that might account for an increased tendency toward automatic activity in the pulmonary vein (1,12,21). Extensive studies have been made to determine whether cardiac muscle in pulmonary veins is more prone than the left atrium to develop arrhythmogenic phenomena such as early and delayed afterdepolarizations (EADs and DADs) due to differences in Ca 2ϩ handling and/or tachycardia-triggered activity (5, 34). The irregular orientation of cardiac muscle fibers in the pulmonary vein has been suggested to lead t...