Binding of [3H]batrachotoxinin A benzoate to specific sites on rat cardiac sodium channels.

Radiolabeled neurotoxins have been used to study the structure and function of sodium channels. We studied the binding of [3H] batrachotoxinin A 20 alpha-benzoate [( 3H]BTX-B) to specific sites on sodium channels on rat cardiac myocytes. Calcium-tolerant myocytes were prepared by collagenase dispersion of adult rat hearts and were 75-83% viable. As with the nerve channel, specific binding of [3H]BTX-B to its receptor site was seen only in the presence of sea anemone toxin (ATX). The affinity of ATX for its binding sites may be estimated from its concentration-dependent stimulatory effect on [3H]BTX-B binding. These results suggest that, in the presence of 5.4 mM KCl, the myocytes have two affinities for ATX with estimated dissociation constants of 0.52 microM and 12.9 microM. Depolarization of the myocytes with either 65 mM KCl or 0.1 mM BaCl2 results in the loss of the 0.52 microM component, suggesting that it is voltage sensitive. The 0.52 microM and 12.9 microM components have maximal binding capacities corresponding to 4 and 11 sites/micron 2 of myocyte surface area, respectively. Scatchard analysis of [3H]BTX-B binding in the presence of ATX demonstrates a single class of sites with a KD of 25-35 nM. These results demonstrate that [3H]BTX-B can be used as a radioligand probe of the adult rat sodium channel and will facilitate a biochemical approach to the study of the interaction between antiarrhythmic drugs and the sodium channel.