Plasma membrane channels

At the level of a single channel, addition of ACh is followed by transient openings of the channel. The current i flowing through an open channel is 4 pA at a membrane potential Voi-l 00 mV. Since one ampere (A) represents the flow of 6.24-1018 charges per second, 2.5-107 Na+ ions per second flow through an open channel. The conductance g of a plasma membrane channel is the measure of the ease of flow of cuirent between the extracellular space and the cytosol or vice... 

Plasma membrane channels. The most common mechanism for the movement of into smooth muscle cells Ifom the extracellular space is the electrodiffusion of Ca " ions through highly selective channels. This movement can be significant in two quite different ways. First, Ca ions carry two positive charges and, in fact, most of the inward charge movement across the plasma membrane of smooth muscle myocytes is carried by Ca. Most smooth muscle action potentials are known to be Ca " action potentials. And second, the concentration of intracellular free calcium, the second messenger, is increased by inward calcium movement. 

Ca2+ enters cells via a number of plasma membrane channels which belong to three families (Figure 11.2) (i) voltage-gated channels (ii) ligand-gated channels and (iii) capacitative, or store-operated channels (SOCs). 

Musil LS, Goodenough DA Multisubunit assembly of an integral plasma membrane channel protein, gap junction connexin43, occurs after exit from the ER. Cell 1993 74 1065-1077. 

In addition to mobilizing internal Ca2+, glucagon promotes the net entry of extracellular Ca2+ into hepatocytes [144,145]. The effect is apparently due to increased influx of Ca2+ through a plasma membrane channel(s) [144,145], but there is also inhibition of the plasma membrane Ca2+ ATPase-pump [150-153], The mechanism by which glucagon stimulates Ca2+ entry is unknown, but it almost certainly involves cAMP since the effect can be mimicked by forskolin, dibutyryl cAMP and /3-adrenergic agonists [144,145]. It may involve cAMP-dependent phosphorylation of a Ca2+ channel analogous to the situation in cardiac and skeletal muscle [154-156], but this is strictly speculative. 

J.C. Saez, V.M. Berthoud, M.C. Branes, A.D. Martinez, E.C. Beyer, Plasma membrane channels formed by connexins Their regulation and functions, Physiol Rev 83, 1359-1400 (2003). 

In those secretory tissues where extracellular calcium is necessary for secretion, calcium enters by way of plasma membrane channels. Therefore, the nature of membrane channels is obviously very important. Are the channels uniform on a given cell Do their characteristics vary from tissue to tissue Many questions remain unanswered, but several studies suggest that a cell may have more than one type of calcium channel. Although not a secretory tissue, smooth muscle has two types of calcium channel potential sensitive channels and receptor operated channels (30). So, in this tissue [and probably in secretory tissues as well (31)], the nature of the stimulus may determine which channels are opened, the extent of calcium entry and the extent of the response. A high potassium solution, which is commonly used to activate calcium mediated responses, would open potential dependent channels whereas drugs acting on their respective receptors would open a different set of channels, but cause the same overall response. 

The entry of into the cytoplasm is mediated by diverse channels Plasma membrane channels regulated by G proteins, membrane potential, K or Ca itself, and channels in specialized regions of endoplasmic reticulum that respond to IP or, in excitable cells, to membrane depolarization and the state of the release channel and its stores in the sarcoplasmic reticulum. Ccf is removed both by extrusion (Naf-Ccf exchanger and Ca ATPase) and by reuptake into the endoplasmic reticulum (SERCA pumps). propagates its signals through a... 

The physiological action is based on depolarization of neurons and muscie ceils by specific blockade of sodium ion transport through plasma membrane channels whereby muscular and finally respiratory paralysis results Antagonist of B. is tetrodotoxin. 

It is noteworthy that the inactivation of the Heliothis RyR at millimolar [Ca ] was prevented at all flubendiamide concentrations tested. This could plausibly explain the insecticidal mechanism since deactivation of calcium release channels at high [Ca ] would be essential to terminate the intracellular calcium transient (27). According to this hypothesis, ryanodine receptors would be fixed in the (sub)conductance conformation leading to calcium store depletion and, possibly, to subsequent activation of capacitative calcium entry through plasma membrane channels. This would override compensatory calcium removal mechanisms such as the sarcoplasmic Ca-ATPase (SERCA) activity and the sodium-calcium exchanger (NCX) in the plasma membrmie. The sustained high intracellular [Ca would finally lead to muscle contraction paralysis that is consistently observed in flubendiamide-affected lepidopteran larvae. 

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