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 approximately 12 ms and tau h2 approximately 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 approximately 3-5 ms) than the other components (tau approximately 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 microM) 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.
Marchetti
C
C
Istituto di Cibernetica e Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy.
Amico
C
C
Podestà
D
D
Robello
M
M
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D002122:Calcium Chloride / Q000494:pharmacology; D006224:Cricetinae; D003470:Culture Media; D004594:Electrophysiology; D007340:Insulinoma / Q000378:metabolism*; D008647:Mesocricetus; D009543:Nifedipine / Q000494:pharmacology; D009761:Nystatin / Q000494:pharmacology; D010190:Pancreatic Neoplasms / Q000378:metabolism*; D012964:Sodium / Q000502:physiology; D014407:Tumor Cells, Cultured
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D003470:Culture Media; D009761:Nystatin; D012964:Sodium; D009543:Nifedipine; D002122:Calcium Chloride