DIVALENT-CATIONS AND ACTION POTENTIAL OF LEECH RETZIUS CELLS

The effects of Sr, Ba, Mn, La and Co on the action potential of the leech [Macrobdella decora] Retzius cell were examined using intracellular recording techniques. These cells have been shown to fire Ca-dependent action potentials in Na-free solution provided TEA [tetraethylammonium] was present (Kleinhaus and Prichard, 1975). Under the same conditions Sr 1.5-20 mM was capable of substituting as a current carrier. Ba 2-25 mM added to normal Ringer prolonged the duration and increased the amplitude of the action potential of the Retzius cell, and supported action potentials without requiring TEA in Na-free solutions. The overshoots of the Sr- and Ba-dependent action potentials varied with a slope of 40 mM and 75 mV, respectively, per 10-fold change in divalent cation concentration. Mn and La selectively blocked that portion of the action potential resulting from an inward movement of Ca, Sr or Ba without affecting the Na-dependent depolarization. The actions of Ca 1 mM on Sr-dependent action potentials were compatible with reversible competitive antagonism. Outward K current must be blocked in order for divalent cations to dominate the Retzius cell''s behavior during excitation. The divalent cation conductance channel is characterized as pharmacologically distinct from the Na conductance channel in the Retzius cell and similar to those described in several other excitable membranes. The current carrying divalent cations probably flows through the same channel.