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L Simchowitz and EJ Cragoe
The ability of a number of analogues of the diuretic, amiloride, to inhibit chemotactic factor-stimulated Na+/H+ exchange in human neutrophils was investigated. Intracellular pH (pHi) changes were measured from the equilibrium distribution of 14C-labeled 5,5- dimethyloxazolidine-2,4-dione (DMO). Exposure of cells to 10 nm N- formyl-methionyl-leucyl-phenylalanine (FMLP) caused activation of Na+/H+ exchange: in 140 mM Na+ medium (extracellular pH 7.40), the pHi rose from a resting value of approximately 7.25 to reach a new steady state of approximately 7.75 by 10-15 min. This intracellular alkalinization was sensitive to amiloride (apparent Ki approximately 75 microM), a known inhibitor of Na+/H+ countertransport. The structure- activity relationships in the amiloride series were characterized by testing the effect of these compounds on the DMO-derived pHi changes and on the FMLP-stimulated rate of 22Na+ efflux from the cells. Substitutions of the guanidino group of amiloride resulted in relatively inactive products (Ki greater than or equal to 1 mM). Replacement of the 6-Cl group of amiloride by other halogen atoms had only modest effects on drug efficacy. However, replacement of one or both H atoms of the 5-amino group by short alkyl groups led to a 10-500- fold increase in potency for inhibition of Na+/H+ exchange. Amiloride and three of its more potent derivatives (compounds I, O, and MM, the 5- N,N-dimethyl, 5-N,N-diethyl, and 5-N,N-hexamethylene analogues, respectively) caused parallel inhibition of FMLP-activated 22Na+ efflux and the rate of intracellular alkalinization, with apparent Ki values of approximately 75, 8, 1, and 0.2 microM, respectively. In each instance, the inhibitory effects of the drugs were readily reversible on washing the cells. None of the compounds altered the binding of 3H- labeled FMLP to its cell surface receptors. The development of potent derivatives of amiloride should provide powerful tools for assessing the role of FMLP-activated Na+/H+ exchange and the resultant pHi transients on stimulated neutrophil functions.
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