EFFECTS OF BA++ ON THE DELAYED OUTWARD CURRENT IN AN IDENTIFIED NEURON OF HELIX POMATIA

Some nerve cells maintain excitability substituting Ba++ f'or Ca++ and in the absence of Na+ in the external solution. The present research is concerned with the study of the effect of' the replacement of Ca by Ba on the ionic currents in some neurons of the Helix pomatia subesophageal ganglia. The results of' measurements on an identified cell (diameter ~130 microm; M.P. = -35 ÷ -40 mV) in the visceral ganglion are summarized here. The cell was impaled with two microelectrodes and voltage-clamped by means of a conventional type electronic circuit. The action of barium was voltage dependent, that is replacement of the external Ca with an equimolar concentration of Ba resulted in a decrease of the total delayed outward current as the membrane potential was made more positive. The net outward current during repeated voltage-clamp pulses was also studied in normal saline and when Ca++ was replaced by Ba++. Responses to repetitive activation with 100 ms pulses and 100 ms repolarization interval were recorded. In these two experimental conditions, the deficit in net outward charge transfer under voltage-clamp was compared. In normal saline this deficit in net charge transfer has been related to the inward flow of Ca ions occurring at high depolarizations and obscured by the outward current of potassium. Divalent cation Ba++ produced a reversible depression of delayed outward current and it seems that both the voltage-dependent and Ca-dependent components of the delayed K+ current are affected by this ion.