Characterization of two components of the N-like, high-threshold-activated calcium channel current in differentiated SH-SY5Y cells.

Retinoic acid differentiated SH-SY5Y cells exhibit only a high-threshold-activated (-30 to -20 mV) whole cell calcium channel current. When barium was used as the charge carrier, the high-threshold-activated current showed bi-exponential inactivation kinetics during a 500 ms voltage step from -90 to +10 mV. The time constants of inactivation were approximately 75 and 750 ms. The fast inactivating component was more sensitive than the slow inactivating component to steady-state inactivation at depolarized holding potentials. The calcium channel current was inhibited by externally applied cadmium (10-300 microM) and gadolinium (10-30 microM) as well as by high concentrations of nickel and cobalt. omega Conus toxin (1 microM) irreversibly blocked the calcium channel current. However, the dihydropyridine agonist, BAY K 8644 (3-10 microM) and antagonists, nifedipine (3-10 microM) and nimodipine (10 microM) did not affect either component of the calcium channel current. Agents which blocked the calcium channel current did not exhibit any selectivity for the fast inactivating over the slow inactivating component of the current. These results indicate that whilst the calcium channel current recorded in differentiated SH-SY5Y cells can be classified on the basis of the blocking agents as being of the N type, the current shows more than one form of inactivation.