INACTIVATION OF VOLTAGE-DEPENDENT CALCIUM CURRENT IN AN INSULINOMA CELL-LINE

We have studied the mechanism of Ca current inactivation in the beta-cell line HIT-T15 by conventional and perforated patch recording techniques, using two pulse voltage protocols and a combination of current and tail current measurements. In 5 mM Ca, from a holding potential of -80 mV, the maximum current showed a complex time course of inactivation: a relatively fast, double exponential inactivation (tau(h1) approximate to 12 ms and tau(h2) approximate to 60 ms) and a very slowly inactivating component (tau > 1 s). The faster component (tau(h1)) was due to the voltage-dependent inactivation of a low-threshold-activated (LVA), T-type current, which deactivates more slowly (tau approximate to 3-5 ms) than the other components (tau approximate to 0.2-0.3 ms). The intermediate component (tau(h2)) was due to the Ca-dependent inactivation of a portion of the high-threshold-activated (HVA) current. A saturating dose of the dihydropyridine (DHP) nifedipine (10 mu M) did not affect the LVA current, but inhibited by 68 +/- 5% the transient, Ca-sensitive portion of the HVA current and by 33 +/- 12% the long lasting component. We suggest that three components of the calcium current can be resolved in HIT cells and the main target of DHPs is a HVA current, which inactivates faster than the DHP-resistant HVA component and does so primarily through calcium influx.