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

Effect of Prozac on whole cell ionic currents in lens and corneal epithelia

Prozac (fluoxetine), a compound used therapeutically in humans to combat depression, has substantial effects on ionic conductances in rabbit corneal epithelial cells and in cultured human lens epithelium. In corneal epithelium, it reduces the current due to the large-conductance potassium channels that dominate this preparation. Its effects seem largely to decrease the open probability while leaving the single-channel current amplitude unaltered. In cultured human epithelium, currents from calcium-activated potassium channels and inward rectifiers are unaffected by Prozac. Delayed-rectifier potassium currents are reduced by Prozac in a complicated way that involves both gating and single-channel current amplitude. Fast tetrodotoxin-blockable sodium currents are also decreased by Prozac in this preparation. For all of these ion conductance effects, Prozac concentrations of 10(-5) to 10(-4) M are required. Whereas these levels are 10- to 100-fold higher than the plasma levels achieved in therapeutic use in humans, they are comparable to or less than levels needed for many other blockers of the ionic conductances studied here.

Sodium channels in ocular epithelia

Voltage-gated, tetrodotoxin(TTX)-blockable sodium channels are found in most excitable cells and are the primary contributors to action potentials generated by many of these cells. To date, there has only been one report of a non-cultured vertebrate epithelial cell type containing TTX-blockable Na+ channels: rabbit non-pigmented ciliary body epithelial cells [Cilluffo MC et al. (1991) Invest Opthalmol Vis Sci 32: 1619-1629], and three reports of cultured epithelial cells containing TTX-blockable Na+ channels: rabbit non-pigmented and pigmented ciliary body epithelium [Ciluffo MC et al. (1991) Invest Opthalmol Vis Sci 32: 1619-1629; Fain GL, Farahbakhsh (1989) J Physiol (Lond) 417: 83-103] and human lens epithelium [Cooper K et al. (1990) J Membr Biol 117: 285-298]. We report here the presence of sodium currents in two different non-cultured, freshly dissociated transporting epithelial cell types: the rabbit corneal endothelium and the frog lens epithelium. We also report the occurrence of sodium currents in six additional cultured ocular epithelial cell types from three different species. These currents have a current/voltage (I/V) relationship consistent with traditional voltage-gated Na+ currents, are quinidine- and TTX-blockable (of the low-affinity TTX-sensitive type), and disappear following bath substitution of Na+ with Cs+ or K+.

Response of the intracellular potentials of cultured bovine lens cells to ions and inhibitors

Micropuncture of bovine lens epithelial cells cultured on plastic culture dishes gave values for the plasma membrane voltage (V) which remained stable for periods of up to several hours. The value of V was mainly in the range -30 to -45 mV, mean value -36.9 +/- 0.5 mV (S.E.M., n = 188). Raising extracellular [K+] from 5 to 40 mM depolarized V by 10 +/- 3 mV. The extent of this depolarization increased with increasing steady-state V. Barium (2 mM) caused a rapid, reversible depolarization of 7.9 +/- 1.2 mV. In the presence of Ba2+, the response to 40 mM K was reduced to 3.6 +/- 1.1 mV. Ouabain (10(-5) M) caused a fast depolarization by 5.3 +/- 1.2 mV. Exposure to calcium-free EGTA-Ringer's depolarized V reversibly by 19.5 +/- 5.0 mV. In Ca-free medium, the depolarization induced by 40 mM K was reduced to 3.2 +/- 2.4 mV. Whereas in control Ringer's sodium conductance (as measured by exposure to a 10 mM [Na]-Ringer's) is small as compared to potassium conductance, it increased markedly in Ca-depleted medium. Amiloride (10(-4) and 10(-3) M) had no effect on this Na conductance. An increase in the relative conductance for sodium was also elicited by Ba2+ (2 mM). Extracellular acidification led to a depolarization, alkalinization to a hyperpolarization. The extent of this effect was virtually equal in the absence or presence of HCO3-, excluding a significant pathway for bicarbonate. No evidence for a significant chloride conductance could be obtained.

Origins of the transient anterior-posterior asymmetry in the frog lens fiber potential

The origin of the transient asymmetry of intracellular resting potentials between the anterior and posterior lens fibers was investigated in the isolated American bullfrog lens by a conventional microelectrode technique. In high K+, Rb+, Cs+, or NH+4 test solution applied only to the lens anterior or posterior side, anterior fibers depolarized at a slower rate than posterior ones. After a long exposure, however, the transient potential difference disappeared. The magnitude of the depolarizations of the lens fibers was in the order of K+ greater than Rb+ greater than Cs+ greater than NH+4. The resting potentials plotted as a function of external K+ concentrations ([K]0) were in agreement with Nernst equation predictions with a slope of 58 mV/decade ion concentration change. A small Na+ permeability is unmasked at a [K]0 less than 10 mM. It was concluded that the transient difference measured in potentials of anterior and posterior lens fibers on increasing external K+, Rb+, Cs+ or NH+4 depends on the anterior epithelial cell layer, which is a diffusional barrier for ions penetrating into the lens interior.