Calcium Conductance

Figure 7.2 G protein-mediated mechanisms of opioid cellular actions. Activation of the p receptor results in inhibition ofadenylyl cyclase (AC), the enzyme responsible for the formation ofcAMP, via the Gi protein, and increased potassium conductance and decreased calcium conductance, mediated via Go proteins.

Macdonald, R. L., Skerritt, J. H., Werz, M. A. Adenosine agonists reduce voltage-dependent calcium conductance of mouse sensory neurones in cell culture, J. Physiol. 1986, 370, 75-90. 

In two children with cranial diabetes insipidus, desmopressin requirements fell while they were taking lamotrigine (590). Lamotrigine may act at voltage-sensitive sodium channels and reduce calcium conductance. Both of these mechanisms of action are shared by carbamaze-pine, which can cause hyponatremia secondary to inappropriate secretion of antidiuretic hormone. 

Tsakiridou E, Bertolhni L, de Curtis M, Avanzini G, Pape HC (1995) Selective increase in T-type calcium conductance of reticular thalamic neurons in a rat model of absence epilepsy. J Neurosci 15 3110-3117. 

Heaton, G.M. and Nicholls, D. G., 1976, The calcium conductance of the inner membrane of rat liver mitochondria and the determination of the calcium electrochemical gradient, Biochem. J. 156, pp. 635-646... 

The barbiturates were used extensively in the past as hypnotic sedatives, but have been replaced by the much safer benzodiazepine derivatives. They do continue to be used as anesthetics and as anticonvulsants. The primary mechanism of action of barbiturates is to increase inhibition through the GABA system. Anesthetic barbiturates also decrease excitation via a decrease in calcium conductance. 

Dunlap K, Fischbach GD (1981) Neurotransmitters decrease the calcium conductance activated by depolarization of embryonic chick sensory neurones. J Physiol 317 519-35 Ellinor PT, Zhang JF, Home WA, Tsien RW (1994) Structural determinants of the blockade of N-type calcium channels by a peptide neurotoxin. Nature 372 272-5... 

Fig. 2.9 Two-dimensional bifurcation diagram outlining the main bifurcation structure for the /3-cell model in the (Vs, ks) parameter plane. Notice the squid-shaped black region with chaotic dynamics. Vs characterizes the voltage dependence of the gating variable for the calcium conductance, and k is a measure of the ratio of the fast and the slow time scales.

Carbone E, Lux HD (1984) A low voltage-activated calcium conductance in embryonic chick sensory neurons. Biophys J 46 413-418... 

Cepeda C, Colwell CS, Itri JN, Chandler SH, Levine MS (1998) Dopaminergic modulation of NMDA-induced whole cell currents in neostriatal neurons in slices contribution of calcium conductances. J Neurophysiol 79 82-94. 

Baufreton J, Garret M, Rivera A, de la Calle A, Gonon F, Dufy B, Bioulac B, Taupignon A (2003) D5 (not Dl) dopamine receptors potentiate burst-firing in neurons of the subthalamic nucleus by modulating an L-type calcium conductance. J Neurosci 23 816-825. 

Combining flocculation and coagulation in a pretreatment process has also been smdied. In a key paper by Lopez-Ramirez et al. [64], the secondary effluent from an activated sludge unit was pretreated, before RO, with three levels intense (coagulation-flocculation with ferric chloride and polyelectrolite and high pH sedimentation), moderate (coagulation-flocculation with ferric chloride and polyelectrohte and sedimentation), and minimum (only sedimentation). The optimum for membrane protection, in terms of calcium, conductivity, and bicarbonates reduction, was the intense treatment. Membrane performance varied with pretreatment but not reclaimed water quahty. The study recommended intense pretreatment to protect the membrane. 

Phenytoin is a hydantoin derivative like dantrolene and the oldest non-sedative anticonvulsant drug known. It alters sodium, potassium and calcium conductance across cell membranes thereby altering membrane potentials and amino acid and neurotransmitter concentrations (i.e. norepinephrine (noradrenaline), acetylcholine and GABA). Its major mode of action appears to be the blockade of sodium channels and e inhibition of the generation of repetitive action potentials (membrane stabilization) (see Chs 9 and 12). 

Stafstrom CE, Schwindt PC, Chubb MC, Crill WE 1985 Properties of persistent sodium conductance and calcium conductance of layer V neurons from cat sensorimotor cortex in vitro. J Neurophysiol 53 153-170... 

Tritthart, H., Volkmann, R., Weiss, R., Heckenstein, A. Inhibition of calcium-dependent action potentials in mammalian myocardium by specific inhibitors of the transmembrane calcium conductivity (verapamil, D 600).. Recent. Adv. Stud. Cardiac Struct. Metab. 1975, 5, 27-33. 

In the experiments described in this report calcium conductance was measured in nerve terminals (synaptosomes) isolated from rat cerebral cortex. Calcium conductances in these subcellular particles have been characterized and described extensively by Blaustein and colleagues and it appears that synaptosomes possess many of the properties of in situ nerve terminals (4 -J7). 

Amobarbital, a barbiturate, is used as a sedative to treat insomnia and as a preanesthetic medication. The barbiturates were used extensively in the past as hypnotic-sedatives but have been replaced by the much safer benzodiazepine derivatives (see Table 9). They do continue to be used as anesthetics (e.g., thiopental) and anticonvulsants (e.g., phenobarbital). The primary mechanism of action of barbiturates is to increase inhibition through the gamma-aminobutyric acid (GABA) system (see Figure 50). Anesthetic barbiturates also decrease excitation via a decrease in calcium conductance. The most commonly used barbiturates are thiopental (Pentothal), methohexital (Brevital), secobarbital (Seconal), pentobarbital (Nembutal), amobarbital (Amytal), and phenobarbital (Luminal). 

These drugs are cardiac calcium channel blockers. By decreasing calcium conduction in the calcium-dependent portions of the AV node, His bundle and in the myocardium itself, ectopic foci can be suppressed. 

Magnesium has been shown to affect the conduction of both sodium and potassium in the heart, which may be due, at least in part, to the dependence of cardiac adenosinetriphosphatase (ATPase) on magnesium. It also competes with calcium and thus decreases calcium conductance across the cardiac membranes, resulting in a prolongation of phase II of the cardiac action potential. Given intravenously, it has been shown to decrease digitalis-induced arrhythmias and torsades de pointes in susceptible patients, as well as arrhythmias produced by myocardial ischemia (e.g., due to infarction). 

Negative chronotropic and dromotropic effects. ACh acts to decrease the rate of spontaneous depolarization of the cells of the SA node, decreasing heart rate. It also acts to decrease the rate of conduction through the atrioventricular (AV) node, through changes in sodium and calcium conductance. 

Fish, R.D., Sperti, G., Colucci, W.S., and Clapham, D.E. (1988) Phorbol ester increases the dihydropyridine-sensitive calcium conductance in a vascular smooth muscle cell line. Circulation Research, 62 1049-1054. 

Class IV anti-arrhythmic drugs usually interfere with calcium conductance such as verapamil hydrochloride. Verapamil inhibits the aetion potential of the upper and middle nodal regions of the heart where the slow inward ealeium-ion-mediated current contributes to depolarisation. This is responsible for the bloekade of slow-ehannel eonduetion in the atrioventricular node. It has been found to inhibit one limb of the re-entry circuit which is assumed to underlie most paroxysmal supraventricular tachycardias, thereby causing the reduction of ventricular rate in atrial flutter and fibrillation. 

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