Plateau phase

Another way to increase the entry of Ca2+ across the plasma membrane is to hyperpolarize the plasma membrane by elevating active ion transport. Fischer et al. [130] demonstrated that hyperpolarization of colonic epithelial cells (HT-29) with carbachol elevates the intracellular levels of Ca2+, [Ca2+]i while depolarization with gramicidin D or elevation of K+ in the bathing fluid reverses it. Treatment with 0.1 mM carbachol produced a spontaneous increase in [Ca2+]i from 63 nM to 901 nM. This lasted for about 3 min, beyond which a plateau level of 309 nM was maintained. While the initial Ca2+ transient was present in Ca2+-free medium containing 0.1 mM EGTA, the plateau phase was suppressed to baseline levels, suggesting that carbachol initially releases Ca2+ from the intracellular stores and subsequently increases the Ca2+ entry across the plasma membrane. In cells hyperpolarized with carbachol, induction of depolarization by ele-... 

The action potential generated in the ventricular muscle is very different from that originating in the SA node. The resting membrane potential is not only stable it is much more negative than that of the SA node. Second, the slope of the depolarization phase of the action potential is much steeper. Finally, there is a lengthy plateau phase of the action potential in which the muscle cells remain depolarized for approximately 300 msec. The physiological significance of this sustained depolarization is that it leads to sustained contraction (also about 300 msec), which facilitates ejection of blood. These disparities in the action potentials are explained by differences in ion channel activity in ventricular muscle compared to the SA node. 

Pig producers mainly try to approach maximal rates of lean tissue deposition and carcass index values by providing diets formulated to meet all of the known requirements. In the growing period, protein accretion increases as the supply of limiting amino acids increases (Heger et al., 2002). The dose-effect ratio can be subdivided into the nutrition-dependent phase, which is substantially linear, and the plateau phase, which is independent of nutrition supply and whose maximum depends on features of the animals, primarily characterised by the genotype (Susenbeth, 2002). 

The bulk of the curve appears identical to the normal curve. However, during the plateau phase, a large cleft is seen as the patient makes a transient respiratory effort and draws fresh gas over the sensor. 

Fig. 2. Potentiation of rat H4 hepatoma by 2.5 uM cisplatin present during the postirradiation incubation prior to assay for viability demonstrating inhibition of PLD recovery. Relative survival of plateau phase cells irradiated with 7.5 Gy after incubation for different time periods or in the presence of cisplatin. Results have been corrected for drug toxicity. Adapted from Carde and Laval (154).

In most studies where a supraadditive interaction was seen, cells were incubated with the drug prior to irradiation (38). In general the use of moderate concentrations (5-100 nmol/L) of paclitaxel in the culture medium for over 24 h lead to maximum sensitization. Most studies used actively proliferating cells in order to conduct their experiments and most investigators reached the conclusion that maximal radiosensitization occurred when cells were arrested in G2 and M, as this part of the cell cycle is the most sensitive to radiation damage. This assumption does, however, presuppose that the majority of the G2/M arrested cells will not die unless they are exposed to ionizing XRT. Plateau phase, i.e., nonproliferating, cells were found to be sensitized to radiation... 

Topotecan has radiation-sensitizing properties demonstrated in log- and plateau-phase cell cultures (23,36) and in murine fibrosarcomas (37,38). Trials have begun in patients with nonsmall-cell lung cancer (NSCLC) and in those with central nervous system (CNS) tumors. 

A next-level assay is usually an isolated heart/cardiac tissue preparation. The canine Purkinje fiber assay (GLP) measures several action potential parameters, like resting membrane potential, upstroke velocity, action potential duration and shape, but also if a drug acts reverse-use dependently [72]. Based on changes of the action potential shape it is possible to conclude which ion channels are modulated (e.g., L-type calcium channel block would abolish the plateau phase). The papillary muscle assay (e.g., guinea pigs) determines similar parameters [73]. 

Fig. 2. Effect of calcium antagonists (CA) on a cardiac cell. Top typical cardiac action potential. The calcium (slow) inward current flows during the characteristic plateau phase (phase 2) of the action potential. This calcium influx is selectively inhibited by CA. Activation of the sarcoplasmic reticulum (SR) and other cellular calcium pools occurs via Ca + and Na+ ions which flow into the cell. The SR and other pools donate activator Ca + ions which stimulate the contractile proteins. The presence of tubular systems (invaginations), which are characteristic of cardiac tissues, results in considerable enlargement of the cellular surface, thus enabling an effective influx of Na+ and Ca + ions. Inhibition of the calcium inward flux by a CA causes diminished activation of the contractile proteins.

Autonomic receptors further regulate calcium influx through the sarcolemma (Fig. 15.1). (3-Adrenergic stimulation results in the association of a catalytic subunit of a G protein coupled to the (3-receptor. This stimulates the enzyme adenylyl cyclase to convert ATP to cyclic adenosine monophosphate (cAMP). Increasing cAMP production results in a cAMP-dependent phosphorylation of the L-type calcium channel and a subsequent increase in the probability of the open state of the channel. This translates to an increase in transsarcolemmal calcium influx during phase 2 (the plateau phase) of the cardiac muscle action potential. The effects of transient increases in intracellular levels of cAMP are tightly con-... 

The action potential has been divided into five phases, rapid depolarization (phase 0), early repolarization (phase 1), plateau (phase 2), rapid repolarization (phase 3) and finally the resting phase in myocytes or slow diastolic depolarization (phase 4). The last is a property in cells with the potential for automaticity (defined later). A brief outline of each of these phases in the normal myocyte is given next. 

Outward repolarizing currents oppose the effect of the inward Ica on the plateau phase. This current is carried predominantly through delayed rectifier potassium channels (Ik).These channels are voltage sensitive, with slow inactivation kinetics. Three distinct subpopulations of Ik with differing activation and inactivation kinetics have been described. A rapidly activating subset (Ikf), a slowly inactivating subset (Iks), and an ul-tra-rapidly activating subset to date are identified only in atrial tissue (Ikui)-... 

Dofetihde prolongs the plateau phase of the action potential, thereby lengthening the refractory period of the myocardium. The effects on atrial tissue appear to be more profound than those observed in the ventricle. The reason for this is unclear. There is no effect on the voltage-gated sodium channel and as such no effect on the conduction velocity. 

One application was determining the time course for of the batch methods. The example (Figure 13) shows the time course of removal can be fairly rapid, and an estimate of the capacity is indicated by the "plateau phase."... 

Most calcium channels become activated and inactivated in what appears to be the same way as sodium channels, but in the case of the most common type of cardiac calcium channel (the "L" type), the transitions occur more slowly and at more positive potentials. The action potential plateau (phases 1 and 2) reflects the turning off of most of the sodium current, the waxing and waning of calcium current, and the slow development of a repolarizing potassium current. 

The effects of digitalis on the electrical properties of the heart are a mixture of direct and autonomic actions. Direct actions on the membranes of cardiac cells follow a well-defined progression an early, brief prolongation of the action potential, followed by shortening (especially the plateau phase). The decrease in action potential duration is probably the result of increased potassium conductance that is caused by increased intracellular calcium (see Chapter 14). All these effects can be observed at therapeutic concentrations in the absence of overt toxicity (Table 13-2). 

Ion channel modulation represents another approach to positive inotropy [13]. Sodium channel modulators increase Na+ influx and prolong the plateau phase of the action potential sodium/calcium exchange then leads to an increase in the level of calcium available to the contractile elements, thus increasing the force of cardiac contraction [13,14]. Synthetic compounds such as DPI 201-106 and BDF 9148 (Figure 1) increase the mean open time of the sodium channel by inhibiting channel inactivation [15]. Importantly, BDF 9148 remains an effective positive inotropic compound even in severely failing human myocardium [16] and in rat models of cardiovascular disease [17]. Modulators of calcium and potassium channel activities also function as positive inotropes [13], but in the remainder of this article we shall focus on sodium channel modulators. 

Gradient HPLC system identical to that used for urinary porphyrins. Protoporphyrin present only in feces but not in urine elutes last. If it is not eluted, the plateau phase can be prolonged for some minutes. 

The maximum corresponds to the "plateau" phase of of normal Type II, provided that Ho 10o km/s/Mpc. 

---