Potential dependent channels

Ca + mobilized in response to membrane stimuli is derived from either intra- or extracellular sources (Figure 2). Membrane Ca + channels mediating Ca2+ entry have been classified into two major types (18-20). Receptor-operated channels (ROC, Figure 2) are associated with membrane receptors and are activated by specific agonist-receptor interactions, whilst potential-dependent channels (PDC, Figure 2) are activated by membrane depolariza-... 

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. 

Similar amplification events occur during the opening or closing of ion channels mediated by chemical (ligandgated channels) or physical (potential-dependent channels). 

In the following, the cardiac action potential is explained (Fig. 1) An action potential is initiated by depolarization of the plasma membrane due to the pacemaker current (If) (carried by K+ and Na+, which can be modulated by acetylcholine and by adenosine) modulated by effects of sympathetic innervation and (3-adrenergic activation of Ca2+-influx as well as by acetylcholine- or adenosine-dependent K+-channels [in sinus nodal and atrioventricular nodal cells] or to dqjolarization of the neighbouring cell. Depolarization opens the fast Na+ channel resulting in a fast depolarization (phase 0 ofthe action potential). These channels then inactivate and can only be activated if the membrane is hyperpolarized... 

Verapamil (Class IV antiarrhythmic drug) is an effective agent for atrial or supraventricular tachycardia. A Ca++ channel blocker, it is most potent in tissues where the action potentials depend on calcium currents, including slow-response tissues such as the SA node and the AV node. The effects of verapamil include a decrease in heart rate and in conduction velocity of the electrical impulse through the AV node. The resulting increase in duration of the AV nodal delay, which is illustrated by a lengthening of the PR segment in the ECG, reduces the number of impulses permitted to penetrate to the ventricles to cause contraction. 

Taste cells have multiple types of ion channels. TRCs are electrically excitable and capable of generating action-potentials voltage-dependent channels for Na+, Ca2+ and K+, similar to those in neurons, have been detected in vertebrate TRCs. The surface distribution of these channels... 

Recent work using confocal microscopy has found localized increases of [Ca2+]j named Ca2+ sparks which are due to the release of Ca2+ from one or a small number of RyRs (Jaggar et al 2000). These localized releases of Ca2+ activate Ca2+-dependent channels in the surface membrane (Perez et al 2001). Activation of the Ca2+-activated K+ current will hyperpolarize the membrane potential (Herrera et al 2001) and thereby decrease Ca2+ entry into the cell on voltage-dependent Ca2+ channels. This provides a mechanism whereby Ca2+ release from the SR can decrease contraction. It is therefore important, in different smooth muscles, to consider to what extent SR Ca2+ release activates rather than decreases contraction. It is, of course, possible that, in the same smooth muscle, SR release may sometimes directly activate contraction and, at other times, decrease it by activating K+ channels. 

Nelson There are three separate roles for Ca2+ waves. First, delivering Ca2+ for contraction. Second, modulating Ca2+ dependent ion channels that control the membrane potentials. Depending on the tissue this can involve BK channels, SK channels or Ca2+-activated Cl- channels. Third, controlling Ca2+-dependent transcription factors. There is quite clear evidence that the frequency and the amplitude components of the Ca2+ signals can determine which Ca2+-dependent transcription factors are activated. This can encode both short and long-term information to control smooth muscle function. 

Due to the increase in the membrane potential, voltage-dependent channels open and IC ions flow out. In addition, Na" / IC ATPase (see A) pumps the Na+ ions that have entered back out again. This leads to repolarization of the membrane. 

A circuit that uses a differential amplifier to maintain constant membrane potential by electronically balancing the ion channel current. This method allows the experimenter to analyze action potentials of excitable membranes resulting from an initial transient rise in sodium ion permeability followed by a transient rise in potassium ion permeability The technique is especially valuable for studying kinetic properties of voltage-gated channels as well as voltage-dependent channels. See Membrane Potential Patch Clamp Methods... 

Figure 9(a) shows the response patterns to typical amino acids, each of which elicits different taste quality in humans [23]. Each channel responded to them in different ways depending on their tastes. L-Tryptophan, which elicits almost pure bitter taste, increased the potentials of channels 1, 2 and 3 greatly. This tendency was also observed for other amino acids which mainly exhibit bitter taste L-phenylalanine and L-isoleucine. L-Valine and L-methionine, which taste mainly bitter and slightly sweet, decreased the potential of channel 5 the responses of channels 1 and 2 were small. 

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