Identification and properties of a novel type of Na+-permeable amiloride-sensitive channel in thyroid cells

Authors


Correspondence to M. Lazdunski, Centre de Biochimie du CNRS, Parc Valrose, F-06034 Nice Cedex, France

Abstract

Amiloride-sensitive cationic channels are present in the apical membrane of porcine thyroid cells in primary culture. An amiloride-sensitive (K0.5= 150 ± 28 nM where K0.5 is the concentration of unlabelled ligand which reduces the specific binding of the same labelled ligand by 50%) 22Na+-flux component (Km for Na+ at 18 mM) has been identified which was also blocked by the potent amiloride derivative phenamil (K0.5= 47 ± 21 nM). The most potent inhibitor of Na+/H+ exchange, ethylisopropyl-amiloride, hardly inhibited this 22Na+-influx component at a concentration of 21 μM. Amiloride binding sites were characterized using [3H]phenamil. The tritiated ligand binds to a single family of binding sites in thyroid membranes with a Kd value of 50 ± 10 nM and a maximal binding capacity of 5 ± 1 pmol/mg protein. Patch-clamp experiments have directly demonstrated the existence of a phenamil- and amiloride-sensitive cationic channel, with a conductance of 2.6 pS, which is permeable to sodium, but not very selective (PNa-/PK+= 1.2). This channel is an important element in the regulation of the resting membrane potential of thyroid cells.

Abbreviations
K0.5

the concentration of unlabelled ligand which reduces the specific binding of the same labelled ligand by 50%

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