By S. Potros. University of Portland. 2018.
The expression of P-gp has been demonstrated by Western blot analysis and by polarized transport of P-gp substrates rizatriptan 10 mg on-line pain treatment clinic pune, such as cyclosporin A purchase 10 mg rizatriptan amex dna pain treatment center, which is reversed (i. Recently it has been shown that P-gp expression in Caco-2 cells is nearly identical to P-gp expression in normal adult intestinal tissue (356). The kinetics of P-gp-mediated efflux activity in Caco-2 cells are equivalent to those observed in rat intestine (based on P-gp-mediated efflux of digoxin) (357). The functional activity of P-gp in Caco-2 cells has been extensively evaluated with respect to various methodological factors such as culture time and passage number (355). Western blot analysis demonstrated that P-gp was expressed as early as day 7 of culturing. The absorptive transport of cyclosporin A was relatively constant from day 5 of culturing (treatment with the P-gp inhibitor verapamil significantly increased absorptive permeability, consistent with inhibition of polarized efflux mechanism). The secretory transport of cyclosporin A increased until day 17, at which time this permeability became constant. The reduced barrier function observed before day 17 is most likely due to incomplete monolayer differentiation or incomplete P-gp expression versus that observed at day 17. Caco-2 cells of lower passage numbers (*22) have been shown to have a shorter doubling time than those of higher passage number (*72), resulting in an increased number of cells per monolayer and thus an increased amount of membrane protein. However, several reports have stated that Caco-2 cells at higher passage numbers (>90) contain significantly more P-gp than those at lower passage numbers. P-gp expression in the Caco-2 cells has been shown to be stable, and this allows relatively accurate comparison of data from various monolayers as long as they represent a relatively narrow range of passage numbers. Expression of specific proteins can be induced in Caco-2 cells using simple culturing techniques. Overexpression of P-gp can also be achieved in the Caco-2 cell line by culturing with vinblastine, verapamil, and celiprolol (358,359). No morphological differences were noticed for vinblastine cultured cells with respect to appearance, formation of tight monolayers, and the corresponding transepithelial resistance (359). Both have been used to follow the passive diffusion of compounds across monolayers. The model’s considerable advantages have led to it being increas- ingly used as the model of choice to screen for P-gp efflux liability. These cultured cells have been shown to retain many morphological and biochemical properties of their in vivo counterparts, including distinguishable luminal and abluminal membrane domains that are functionally and biochemically distinct (371–381). The comparable leakiness of the system can also make it difficult to quantify differences in transport that may be mediated by transporter activity. Several examples have demonstrated the usefulness of this system to study polarized efflux via P-gp. For example, the influence of P-gp expressed in brain capillary endothelial cells on the transport of cyclosporin A (388,389), vincris- tine (381), protease inhibitors (amprenavir, saquinavir, and indinavir) (245,390), rhodamine 123 (211,383), opioid peptides (211,391,392), and the b-blocking agent bunitrolol (393) have all been determined using this system. Experimental Methods Used with Tissue Culture Transport Models to Study P-gp Efflux The use of appropriate experimental design can provide definitive evidence that P-gp-mediated efflux is altering the transport of a compound and can provide further mechanistic information regarding the transport of a compound. Recently it has been appreciated that P-gp efflux can be a potential source for drug interactions and in vitro experimentation can be very helpful to understand potential liability. The techniques described in this section can be used with any tissue culture transport model.
Part 1 is an introduction to basic principles—the mechanism of cardiac arrhythmias and how antiarrhythmic drugs work generic rizatriptan 10 mg line anterior knee pain treatment exercises. Part 2 discusses the clinically relevant features of the drugs themselves 10mg rizatriptan with mastercard pain tmj treatment, including emerging investigational drugs that appear to show promise. Part 3 draws on this basic infor- mation to explore the treatment of speciﬁc cardiac arrhythmias and emphasizes the current roll of antiarrhythmic drugs in managing these arrhythmias. Accord- ingly, when a choice had to be made between simplicity and Preface vii complexity, simplicity prevailed in almost every case. The author recognizes that some colleagues may not agree with an approach that risks oversimpliﬁcation of an inherently complex topic. It is an ap- proach, however, that reﬂects a deep-seated belief—by keeping the basics simple, the speciﬁcs (clinical cases and scientiﬁc reports) can be more readily weighed, categorized, absorbed, and implemented. Acknowledgments The author thanks Gina Almond, Publisher at Blackwell Publishing, for asking me to consider writing a second edition to this book, and Fiona Pattison, Senior Development Editor at Blackwell, for helping to shepherd me through the process of actually doing so. The author also thanks Anne, Emily, and Joe Fogoros for once again overlooking the temporary inattentiveness that always seems to accompany such endeavors. Indeed, it is nearly im- possible withoutaﬁrm understanding of the basic mechanismsof cardiac tachyarrhythmias and the basic concepts of how antiarrhyth- mic drugs work. Chapter 1 reviews the normal electrical system of the heart and the mecha- nismsand clinical features of the major cardiac tachyarrhythmias. Chapter 2 examines the principles of how antiarrhythmic drugs af- fect arrhythmias. The electrical system of the heart On a very fundamental level, the heart isan electrical organ. The electrical signals generated by the heart not only cause muscle con- traction (by controlling the ﬂuxofcalcium ionsacross the cardiac cell membrane) but also organize the sequenceofmuscle contrac- tionwith each heartbeat, thusoptimizing the pumping action of the heart. In addition,and especially pertinent to the subjectofthis book, the pattern and timing of the cardiac electrical signals deter- mine the heart rhythm. Thus, a well-functioning electrical systemis vital for adequate cardiacperformance. The ﬁbrous skeletonis electrically inert, and therefore stops the electrical impulse. Onceon the ventricular side, the electrical impulse follows the His-Purkinje system as it divides ﬁrst into the right and left bun- dle branches and theninto the Purkinje ﬁbers. The Purkinje ﬁbers speed the impulse to the furthermost reaches of the ventricular my- ocardium. In this way, the electrical impulse israpidly distributed throughout the ventricles. Mechanismsofcardiac tachyarrhythmias 5 The heart’s electrical system thusorganizes the sequenceofmy- ocardial contractionwith each heartbeat. Cardiac action potential The electrical impulse of the heart isactually the summation of thou- sandsoftiny electrical currents generated by thousandsofindivid- ual cardiaccells. The electrical activity of an individual cardiaccell is described by the cardiac actionpotential (Figure 1. Fortu- nately, for our purposes there are onlyafew thingsone needsto know about the actionpotential, and these are reasonably simple to understand. The voltage differenceacross the cell membrane(normally –80 to –90 mV) is called the transmembrane potential and is the result of an accumulation of negatively chargedmolecules within the cell. The magnitude of the transmembrane potential remains ﬁxed through- out the lives of most living cells.
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