Action potential, potassium

III Block of repolarizing potassium channels, prolongation of action potential Amiodarone, Dronedarone, Sotalol, Dofetilide, Ibutilide... 

Inward Rectifier Potassium Channels or Kir Channels are a class of potassium channels generated by tetra-meiic arrangement of one-pore/two-transmembrane helix (1P/2TM) protein subunits, often associated with additional beta-subunits. Kir channels modulate cell excitability, being involved in repolarization of action potentials (see Fig. 1), setting the resting potential (see Fig. 1) of the cell, and contributing to potassium homeostasis. 

K Channels belong to a class of membrane proteins that form highly K-selective pores in membranes. All known K Channels are composed of several (usually four) pore forming alpha subunits and auxiliary beta subunits. K Channels play an essential role in cellular excitability, being involved in repolarization of Action Potentials and setting the cell resting potential as well as contributing to potassium homeostasis. 

Figure 21.5 Mechanisms of opioid analgesia at the spinal level. Action potentials in nociceptive afferent fibres invade the terminal and by opening calcium channels (L, N and P-type) cause the release of glutamate and peptides that further transmit pain subsequent to activation of their postsynaptic receptors. Presynaptic opioid receptor activation (mu- and delta-mediated effects have been most clearly shown) opens potassium channels which hyperpolarise the terminal, so reducing transmitter release and inhibiting the postsynaptic neuron...

As with the effects of oxidant stress on the calcium channel, part of the change in the steady-state background current could also be attributed to an indirect effect secondary to the elevation of intracellular calcium (Matsuura and Shattock, 1991b). However, oxidant stress also exerted a direct effect on the inward rectifying potassium current (7ki). The combination of an inhibition of 7ki and the activation of a calcium-dependent current are likely to contribute to the prolongation of the action potential duration and the increased susceptibility... 

Fromm and Spanswick [79] found that electrical stimulation of a plant is followed by ion shifts which are most striking in the phloem cells. While their content of potassium and chloride was diminished after stimulation, the amount of cytoplasmic calcium increased slightly (Table 1). These displacements lead to the conclusion that Ca + influx as well as and CP efflux are involved in the propagation of action potentials. The main difference between propagation of action potentials in animals and plants is that in an axon there is the K /Na transmembrane transport but in phloem cells the K /Ca channels are involved in this process [Fig. 22(b)]. 

The ventricular action potential is depicted in Fig. 6-2.2 Myocyte resting membrane potential is usually -70 to -90 mV, due to the action of the sodium-potassium adenosine triphosphatase (ATPase) pump, which maintains relatively high extracellular sodium concentrations and relatively low extracellular potassium concentrations. During each action potential cycle, the potential of the membrane increases to a threshold potential, usually -60 to -80 mV. When the membrane potential reaches this threshold, the fast sodium channels open, allowing sodium ions to rapidly enter the cell. This rapid influx of positive ions... 

Potassium is the second most abundant cation in the body and is found primarily in the intracellular fluid. Potassium has many important physiologic functions, including regulation of cell membrane electrical action potential (especially in the myocardium), muscular function, cellular metabolism, and glycogen and protein synthesis. Potassium in PN can be provided as chloride, acetate, and phosphate salts. One millimole of potassium phosphate provides 1.47 mEq of elemental potassium. Generally, the concentration of potassium in peripheral PN (PPN) admixtures should not exceed 80 mEq/L (80 mmol/L). While it is safer to also stick to the 80 mEq/L (80 mmol/L) limit for administration through a central vein, the maximum recommended potassium concentration for infusion via a central vein is 150 mEq/L (150 mmol/L).14 Patients with abnormal potassium losses (e.g., loop or thiazide diuretic therapy) may have higher requirements, and patients with renal failure may require potassium restriction. 

Parasympathetic stimulation causes a decrease in heart rate. Acetylcholine, which stimulates muscarinic receptors, increases the permeability to potassium. Enhanced K+ ion efflux has a twofold effect. First, the cells become hyperpolarized and therefore the membrane potential is farther away from threshold. Second, the rate of pacemaker depolarization is decreased because the outward movement of K+ ions opposes the effect of the inward movement of Na+ and Ca++ ions. The result of these two effects of potassium efflux is that it takes longer for the SA node to reach threshold and generate an action potential. If the heart beat is generated more slowly, then fewer beats per minute are elicited. 

The mechanism of these effects involves enhanced depolarization of these cells due to decreased potassium permeability and increased sodium and calcium permeability. With fewer K+ ions leaving the cell and with more Na+ and Ca++ ions entering the cell, the inside of the cell becomes less negative and approaches threshold more rapidly. In this way, action potentials are generated faster and travel through the conduction pathway more quickly so that the heart can generate more heartbeats per minute (see Figure 14.1). 

As metabolism increases, oxygen consumption and carbon dioxide production are enhanced. The concentration of hydrogen ions is also enhanced as more carbonic acid (formed from carbon dioxide) and lactic acid are produced by the working tissue. Furthermore, the concentration of potassium ions in the interstitial fluid is increased. The rate of potassium release from the cells due to repeated action potentials exceeds the rate of potassium... 

Cardiac Action Potential In Vitro Purkinje Fibers. Intracellular recording of action potentials from cardiac Purkinje fibers isolated from dog or sheep ventricle. Measurement of maximum rate of depolarization and action potential duration to detect sodium and potassium channel interactions, respectively, according to recommendations in EM A CPMP Points to Consider document, CPMP 986/96 (1998). 

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