The effects of Mn2+ and Ca2+ on the prolonged depolarising after-potential in barnacle photoreceptor

We have studied the effects on the PDA of modifying intracellular and extracellular concentrations of Ca2+ and Mn2+. The effect of decreased Ca2+ concentration or addition of EGTA is mainly an increase in the PDA amplitude and length. Raising Ca2+ concentration using ruthenium red or high external Ca2+ has the opposite effect. The effect of Mn2+ is much more striking: In the presence of 50-100 mM Mn2+ the PDA is initially greatly depressed but can rise slowly for up to 20 or 30 s (in the dark) until it approaches its original amplitude and time course. Bridge measurements showed that the depression of the PDA corresponds to a depressed conductance and so is not due to an increase in K+ conductance. The Mn2+ effect is potentiated by decreased Ca2+ Appropriate stimulation suppresses the rising PDA as promptly as it does a normal PDA, suggesting that if lateral diffusion is the source of the slow rise, the PDA and PDA-depressing processes must be spatially linked. The action of the anti-PDA is apparently prolonged by both Ca2+ and Mn2+.