nystatin-a1 and Insulinoma

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.

Topics: Animals; Calcium Channel Blockers; Calcium Channels; Calcium Chloride; Cricetinae; Culture Media; Electrophysiology; Insulinoma; Mesocricetus; Nifedipine; Nystatin; Pancreatic Neoplasms; Sodium; Tumor Cells, Cultured

The release of insulin from the pancreatic beta cell is dependent upon a complex interplay between stimulators and inhibitors. Recently, amylin, a peptide secreted by pancreatic beta cells, has been implicated in the development of type II (noninsulin dependent) diabetes through its modulation of the peripheral effects of insulin. However, the effect of amylin on insulin secretion from the beta cell has remained controversial. It is reported here that in single beta cells exhibiting normal glucose sensing, amylin causes membrane hyperpolarization, increases in net outward current, and reductions in insulin secretion. In contrast, in cells with abnormal glucose sensing (e.g., from db/db diabetic mice), amylin has no effect on electrical activity or secretion. Thus, amylin's effects on excitation-secretion coupling in the beta cell of the pancreas appear to be linked to the cell's capacity for normal glucose sensing.

Topics: Amyloid; Animals; Cell Membrane Permeability; Diabetes Mellitus, Type 1; Glucose; In Vitro Techniques; Insulin; Insulin Secretion; Insulinoma; Islet Amyloid Polypeptide; Islets of Langerhans; Membrane Potentials; Mice; Mice, Inbred C57BL; Nystatin; Pancreatic Neoplasms; Rats; Rats, Sprague-Dawley; Tumor Cells, Cultured