A low threshold calcium current recorded at physiological Ca concentrations in single frog atrial cells

The presence of a low threshold calcium current (I_Ca,LT) besides the classical (high threshold) calcium current (I_Ca,HT) is investigated using the whole-cell patch-clamp technique (10) on Cs-loaded frog atrial cells at physiological Ca concentrations. I_Ca,LT, which inactivates within tens of milliseconds, gives rise to a shoulder in the negative part of the peak current-voltage curve. It is completely abolished by 40 M Ni while I_Ca,HT is not modified. Cd (30 M) decreases I_Ca,LT by about 50% while I_Ca,HT is completely abolished.