Effects of veratrine and tetrodotoxin on the frog lens potential in normal and calcium-free media

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Abstract
The membrane potentials of posterior fibers of the isolated American bullfrog lens were measured in Ringer solution containing various external Ca2+ concentrations ([Ca]0) by using a conventional glass microelectrode technique. The reduction or removal of [Ca]0 evoked a rapid depolarization of the lens potential whereas the increase of [Ca]0 had almost no effects on the membrane potential. The magnitude of depolarization in Ca2+-free medium was augmented by adding veratrine but reduced by either the addition of tetrodotoxin (TTX) or the reduction of external Na+ concentration. A slight depolarization still remained after the blockade of Na-channels by adding TTX and developed progressively during a successive exposure of lens to Ca2+-free media. It was concluded that veratrine-sensitive rapid an large depolarization in the frog lens fibers bathed in Ca2+-free medium results from a marked elevation of Na+ permeability but that the TTX-insensitive time-dependent depolarization may depend on the loss of K+ content in lens fibers.