Amiloride and its analogs as tools in the study of ion transport

Abstract

Amiloride inhibits most plasma membrane Na+ transport systems. We have reviewed the pharmacology of inhibition of these transporters by amiloride and its analogs. Thorough studies of the Na+ channel, the Na+/H+ exchanger, and the Na+/Ca2+ exchanger, clearly show that appropriate modification of the structure of amiloride will generate analogs with increased affinity and specificity for a particular transport system. Introduction of hydrophobic substituents on the terminal nitrogen of the guanidino moiety enhances activity against the Na+ channel; whereas addition of hydrophobic (or hydrophilic) groups on the 5-amino moiety enhances activity against the Na+/H+ exchanger. Activity against the Na+/Ca2+ exchanger and Ca2+ channel is increased with hydrophobic substituents at either of these sites. Appropriate modification of amiloride has produced analogs that are several hundred-fold more active than amiloride against specific transporters. The availability of radioactive and photoactive amiloride analogs, anti-amiloride antibodies, and analogs coupled to support matrices should prove useful in future studies of amiloride-sensitive transport systems. The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, as well as effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. One must consider whether the effects seen on various cellular processes are direct or due to a cascade of events triggered by an effect on an ion transport system.

Na+/H+ Exchangers of Mammalian Cells

“…Drug Sensitivity and Binding Sites-The NHE is a known target for inhibition by the diuretic compound amiloride and its analogs (39) and by benzoyl guanidinium compounds (e.g. HOE694) (40,41).…”

Cytostatic potential of novel agents that inhibit the regulation of intracellular pH

“…Our laboratory and others have evaluated previously the efficacy of available NHE-1 inhibitors, such as amiloride and its analogue 5-N-ethyl-N-isopropyl amiloride (EIPA) with the goal of assessing their possible role in the treatment of solid tumours, where they might contribute to cellular toxicity under acidic conditions. These agents are quite effective inhibitors of Na + /H + exchange but lack specificity (Kleyman and Cragoe, 1988;Newell et al, 1992;Horvat et al, 1993;Murata et al, 1995). Despite their lack of specificity, experimental studies using amiloride and EIPA have suggested that inhibition of NHE-1 might be beneficial to patients with myocardial ischemia (Pierce et al, 1993;Baumgarth et al, 1996).…”

Reversal of intrinsic multidrug resistance in Chinese hamster ovary cells by amiloride analogs

“…These amiloride analogs (Table I) have been previously characterised for their effects on ion transport (Kleyman & Cragoe, 1988;Cragoe and coworkers, unpublished results). Of the many compounds tested, several had activity as sensitisers (Table II).…”

“…The activity of this ion transporter is elevated in drug resistant cell lines (Boscoboinik et al, 1990). Analogs of amiloride vary greatly in the potency and specificity by which they inhibit this antiporter (Kleyman & Cragoe, 1988 (Keizer & Joenje, 1989;Boscoboinik et al, 1990), it seemed possible that acidification of pH,, through inhibition of the Na+/H+ antiporter, could contribute to the sensitisation of drug resistant cell lines. …”