Depolarization elicits two distinct calcium currents in vertebrate sensory neurones

The calcium currents of rat sensory neurones (of the IX and X cranial nerves) grown in culture were studied using whole cell recordings. In cells loaded with CsCl, and bathed in a solution where Na was replaced by choline or Tris, a step depolarization from -80 mV to 0 mV elicited the well-documented sustained Ca current (iCa,s). In contrast, depolarization from -80 mV to -60 mV and up to -20 mV evoked a distinct transient inward current (iCa,t) which could be isolated by imposing an internal pCa 7. It relaxed in about 100 ms and could possibly occur independently of the former current. The transient current was only affected by manipulation of the Ca concentration in the external medium and therefore was considered to be also a transfer of Ca. Ba was shown to act as a substitute with a lower affinity than Ca. The maximal amplitude of this current was in the order of a few hundred pA in Ca 5 mM and Mg 2 mM. Both activation and inactivation occurred in the same voltage span. The underlying event was studied using noise analysis and compared to the Ca transfer occurring during the sustained current as measured in chromaffin cells by Fenwick et al. (1982). We found them to be of similar amplitude.