Effect of calcium and calcium channel blockers on transient outward current of F76 and D1 neuronal soma membranes in the subesophageal ganglia of Helix aspersa

Twin-electrode voltage-clamp techniques were used to study the effect of ca lcium and calcium channel blockers on the transient outward current in isol ated F76 and D1 neurones of Helix aspersa subesophageal ganglia in vitro (s oma only preparation with no cell processes). On lowering extracellular Ca2 + concentration from 10 to 2 mM or removing extracellular calcium from the bathing medium, the threshold for this current shifted in a negative direct ion by 11.5 and 20 mV, respectively. On the other hand, increasing the extr acellular Ca2+ concentration from 10 to 20 and to 40 mM shifted the steady- state inactivation curves in positive directions on the voltage axis by 7 a nd 15 mV, respectively. Upon application of calcium channel blockers, Co2+, La3+, Ni2+ and Cd2+, transient potassium current amplitude was reduced in a voltage-dependent manner, being more effective at voltages close to the t hreshold. The current was elicited even at a holding potential of -34 mV. T he specific calcium channel blockers, amiloride and nifedipine did not shif t the activation and steady-stare inactivation curves and did not reduce th e transient outward current amplitude. It was concluded that the transient outward current is not dependent on intracellular Ca2+ but that it is modul ated by Ca2+ and di- and trivalent ions extracellularly. The effects of the se ions are very unlikely to be due to a surface charge effect because the addition of La3+ (200 mu M) completely reverses the shift in a hyperpolariz ing direction when the extracellular Ca2+ concentration was reduced from 10 to 1 mM and additionally shifts the kinetics further still in a depolarizi ng direction. The responses seen here are consistent with a specific effect of di- and trivalent ions on the transient outward current channels leadin g to a modification of gating.