Synthesis, properties and biological activity of tritiated N-benzylamidino-3,5-diamino-6-chloro-pyrazine carboxamide -- a new ligand for epithelial sodium channels.

A method is described for the synthesis and purification of tritiated N-benzylamidino-3,5-diamino-6-chloro-pyrazine carboxamide (benzamil). The tritium was inserted at the meta position of the benzyl ring, from which it apparently does not exchange with solvent hydrogen. When stored in ethanol at -4 degrees C the radioligand remains stable for at least 15 months. The pharmacology of benzamil is very similar to that of amiloride in terms of its effects on sodium transporting epithelia except that it has a higher affinity. The affinity of benzamil for sodium channels in amphibian epithelia in the absence of sodium is approximately 10(9) M-1. The new ligand can be used to label sodium channels in epithelia, and may be useful in channel isolation procedures.