LOW-THRESHOLD CALCIUM CURRENT IN ISOLATED PURKINJE-CELL BODIES OF RAT CEREBELLUM

1. The low- and high-threshold Ca2+ currents were observed in Purkinje cell bodies isolated from the cerebellum of newborn (2 wk old) and adult (8 wk old) rats under whole-cell clamp. A transient Ca2+ current (low-threshold or 'T-type' I(Ca)) was elicited by depolarizing step pulses to -60 mV or more positive potentials from a holding potential (V(H)) of -100 mV. In cells dissociated from newborn rats, a long-lasting Ca2+ current (high-threshold or 'L-type' I(Ca)) was also elicited by depolarizing command pulses beyond -30 mV. 2. The low-threshold I(Ca) was resistant to the 'washout' effect during the internal perfusion, whereas the high-threshold I(Ca) faded gradually with time during the continuous internal perfusion. 3. In the current-voltage (I-V) relationship, the low-threshold I(Ca) had a threshold potential around -60 mV and reached the maximum inward current around -20 mV. The activation and inactivation kinetics of the current depended on membrane potential: for a test-potential change from -60 to +40 mV, the time to peak of the current (activation) decreased from 31.9 to 5.0 ms, and the time constant of current decay (inactivation) decreased from 78.5 to 22.9 ms. 4. Steady-state inactivation of low-threshold I(Ca) was membrane-potential dependent, and the inactivation of the 50% level was -79 mV. Recovery time constant from steady-state inactivation varied depending on the membrane potential. The time constants were 3.3 and 2.5 s at V(H)s of -100 and -120 mV, respectively. 5. The low-threshold I(Ca) was inhibited by various polyvalent metal cations in a concentration-dependent manner; the inhibition was in the order of La3+ >> Zn2+ > Cd2+ > Ni2+ > Co2+. 6. Various organic Ca2+ antagonists also inhibited the low-threshold I(Ca) in a concentration-dependent manner; the order of the inhibition was flunarizine > nicardipine > D-600 > diltiazem. The inhibitory potency of nicardipine varied depending on external Ca2+ concentration. At a concentration of 3 x 10-6 M nicardipine, the inhibitory effect on the low-threshold I(Ca) in the bathing medium with 2.5 mM Ca2+ was stronger than with 10 mM Ca2+. 7. We found that the pharmacological properties of low-threshold I(Ca) in cerebellar Purkinje cell bodies were similar to those in the isolated hypothalamic neurons, but differed from those in the peripheral ganglion cells.