Effect of internal calcium concentration on calcium currents in rat sensory neurones.

Using the patch-clamp technique in whole-cell configuration we have investigated the effect of increasing the internal calcium concentration (Cai) from below 10(-8) M to 10(-6) M on the three calcium currents: ICa,T (T for transient), ICa,S (S for sustained), ICa,N (N for neither), recently described in rat sensory neurones. Increasing Cai led to a dose-dependent reduction of the amplitude of ICa,S and, as Cai reached 5 X 10(-7) M ICa,S was nearly abolished. ICa,N is well evidenced from 5 X 10(-10) M to 10(-7) M where its is a large current. Preliminary observations indicate an increase of its inactivation rate following, as expected for a possible Cai dependent-inactivation, the increase of Cai from 5 X 10(-10) M to 10(-7) M. With Ca = 5 X 10(-7) M, all the cells displayed ICa,T and half of the cells in addition ICa,N, but it was of small amplitude. At Cai = 10(-6) M, most of the recorded cells only exhibited ICa,T.