There are five families of vertebrate Cys loop receptors as follows: the nicotinic acetylcholine receptor (nAChR), 3 the 5-hydroxytryptamine type 3 receptor (5-HT 3 R), the zinc-activated ion channel, the ␥-aminobutyric acid type A receptor, and the strychnine-sensitive glycine receptor (1, 2). Structural analysis by cryo-EM of tubular crystals prepared from the Torpedo marmorata electric organ revealed that five subunits combine in nAChRs, forming a rosette around the central ion channel (3). The second transmembrane (M2) domain of each subunit participates in lining the channel pore, and collectively they present a hydrophobic constriction adjacent to what is traditionally believed to be the rate-limiting portion of the ion conduction pathway that controls single channel conductance (␥) and ionic selectivity (2, 4).The homomeric 5-HT 3A receptor is unique among Cys loop receptors, having a ␥ below the resolution of single channel recording, estimated by variance analysis to be in the femtosiemen range. The incorporation of the 5-HT 3B subunit into human heteromeric 5-HT 3A/B receptors increases ␥ to 16 pS, enabling direct observation of events by single channel recording from outside-out patches (5). The use of chimeric 5-HT 3A -5-HT 3B constructs and site-directed mutagenesis revealed a critical role of three arginine residues within the MA helix of the M3-M4 cytoplasmic loop in determining ␥ (6, 7). This has prompted speculation that the MA helix may participate in the control of ␥ in other Cys loop receptors (4,7,8). In support of this interpretation, cryo-EM analysis revealed portals within the Torpedo nAChR formed, in part, by the MA helices of adjacent subunits that may participate in the ion conduction pathway (9).In this study we investigated the influence of the 5-HT 3A subunit's MA helix Arg-432 (Ϫ4Ј), Arg-436 (0Ј), and Arg-440 (4Ј) residues in the control of ␥. We investigated the effect of introducing arginine into the equivalent locations within the nAChR ␣ 4 and ␤ 2 subunits. Our data confirm the critical role of MA Ϫ4Ј, 0Ј, and 4Ј residues in controlling ␥ of 5-HT 3A receptors and support the hypothesis that the MA helix also forms part of the ion conduction pathway of nAChRs. Our functional data provide support for the existence of cytoplasmic portals depicted in the 4 Å structural model of the nAChR (10). EXPERIMENTAL PROCEDURES DNA Constructs and Transient Transfection of Subunit cDNAs-cDNAs encoding rat wild-type (WT) nAChR ␣ 4 and ␤ 2 subunits (Dr. J. M. Boulter, Department of Psychiatry and Biobehavioral Sciences, UCLA), human WT 5-HT 3A subunits, and mutant nAChR and 5-HT 3A subunits were cloned into pGW1 (11). Point mutations were introduced using standard molecular biological techniques (7). All cDNAs were sequenced to confirm fidelity. Transfection of tsA-201, or HEK-293 cells, with subunit cDNAs, at equimolar ratios when appropriate, was performed by either the calcium phosphate precipitation method or electroporation (400 V, infinite resistance, 125 microfarads) using a Bio-Rad gene ...

Kidney disease is an increasingly common comorbidity that alters the pharmacokinetics of many drugs. Prescribing to patients with kidney disease requires knowledge about the drug, the extent of the patient's altered physiology, and pharmacokinetic principles that influence the design of dosing regimens. There are multiple physiologic effects of impaired kidney function, and the extent to which they occur in an individual at any given time can be difficult to define. Although some guidelines are available for dosing in kidney disease, they may be on the basis of limited data or not widely applicable, and therefore, an understanding of pharmacokinetic principles and how to apply them is important to the practicing clinician. Whether kidney disease is acute or chronic, drug clearance decreases, and the volume of distribution may remain the same or increase. Although in CKD, these changes progress relatively slowly, they are dynamic in AKI, and recovery is possible depending on the etiology and treatments. This, and the use of kidney replacement therapies further complicate attempts to quantify drug clearance at the time of prescribing and dosing in AKI. The required change in the dosing regimen can be estimated or even quantitated in certain instances through the application of pharmacokinetic principles to guide rational drug dosing. This offers an opportunity to provide personalized medical care and minimizes adverse drug events from either under- or overdosing. We discuss the principles of pharmacokinetics that are fundamental for the design of an appropriate dosing regimen in this review.