POTASSIUM CURRENT OSCILLATIONS ACROSS THE RABBIT LENS EPITHELIUM

Rabbit lenses expressing spontaneous oscillations in translens short-c ircuit current (I-sc) are obtained somewhat frequently, with this phen omenon observed in approximate to 30% of isolated lenses as described earlier (Exp. Eye Res. 61, 129-140, 1995). Since pharmacological proto cols to consistently elicit I-sc oscillations were not found, characte rizations of the underlying transport processes have been limited to t he application of various inhibitors on the spontaneous phenomenon. Th e present report extends the initial observations by confirming that o scillations are immediately inhibited upon the anterior addition of th e Ca2+ channel blocker nifedipine (10 mu M), and by demonstrating that other treatments which should affect epithelial Ca2+ homeostasis are also inhibitory (e.g., Bay K 8644 (10 mu M), diltiazem (10 mu M), EGTA (2 mM), and Ca2+-free media). Furthermore, I-sc oscillations are imme diately inhibited by the K+ channel blocker, Ba2+, but not by the Na+- K+ pump inhibitor, ouabain. The intracellular Ca2+ mobilizing agents t hapsigargin (0.1 mu M) or acetylcholine (1 mu M) modified but did not permanently inhibit the oscillations, confirming earlier observations, At 50 mu M, however, acetylcholine addition was inhibitory, but rever sible, for oscillations restarted upon its subsequent removal, In addi tion, lens oscillations were also characterized under open-circuit con ditions with microelectrodes inserted in the superficial cells near th e equator of lenses isolated in a divided chamber, The potential diffe rence (PD) across each lens face was recorded, as was the translens PD (PDt), which equals the difference between the PDs across each lens s urface. Oscillations in PDt were obtained in 7 of 26 lenses. The oscil lations arose only from an oscillation in the PD across the anterior f ace (PDa). While PDa and PDt oscillated with the same amplitude (appro ximate to 12 mV) and period (approximate to 70 sec), the PD across the posterior surface remained stable. During these oscillations the cond uctance of the anterior surface was maximal at the most positive volta ge of the anterior bath with respect to the lens interior (46 mV), whe reas, minimal conductance occurred at the least positive PDa (34 mV). Overall, these observations are consistent with the likely presence of voltage-operated Ca2+ channels in parallel with various Ca2+-sensitiv e K+ channels in the epithelial basolateral membrane. A model to expla in the oscillatory pattern across the anterior face while the PD acros s the posterior face remains unaltered is presented. (C) 1997 Academic Press Limited.