Fig. 6.

The effect of external K⁺ on reverse-mode Na⁺/Ca²⁺ exchange.A, B, The Ca²⁺traces were recorded from two different axon terminals preloaded with fura-2 AM. The times when 0.5 mm ouabain was applied are indicated by arrows. The two bars underneath the Ca²⁺traces illustrate the sequence of changes in [Na⁺] and [K⁺] of the external bathing solutions. External Na⁺ was replaced with equimolar NMG⁺ in zero Na⁺ solutions. K⁺ was added or omitted to the normal Tyrode's solution to make 10 mm[K⁺]_o or zero [K⁺]_o. C, Average [Ca²⁺]_i increases during initial 30 sec after the solution change under various ionic conditions (n, number of individual nerve endings; error bars indicate SEMs). Numbers below theabscissa indicate external concentrations of K⁺. The major external cation was Na⁺ (left three bars, 145 mm Na⁺) or NMG⁺(right three bars, zero Na⁺).