AJP - Endocrinology and Metabolism, Vol 256, Issue 4 E488-E493, Copyright © 1989 by American Physiological Society
Characterization of calcium channels of a glucose-responsive rat insulinoma
M. Hoenig, L. L. Russo and D. C. Ferguson
Department of Physiology and Pharmacology, College of Veterinary Medicine, University of Georgia, Athens 30602.
Voltage-dependent calcium channels are important in the control of calcium
influx into excitable cells. With the use of cells isolated from a
glucose-responsive rat insulinoma, the effect of the calcium-channel
blocker nitrendipine on insulin release was examined. When 50 nM
nitrendipine was added with a stimulatory glucose concentration (30 mM),
first-phase insulin release was preserved, whereas the second phase was
inhibited by 39.4 +/- 5.3%. When 50 nM nitrendipine was also present during
basal and stimulated insulin release, the insulin release from fresh cells
was significantly less during both phases compared with cells not exposed
to nitrendipine (P less than 0.05). In cells cultured for 1 day in 30 mM
glucose, a diminished insulin response was seen on stimulation with 30 mM
glucose. Nitrendipine (50 nM) did not lead to a further decrease in insulin
release. Saturable binding sites in homogenate, whole cells, and purified
plasma membranes prepared from the insulinoma were characterized using
[3H]nitrendipine. The Kd (in pM) for homogenate from uncultured cells was
225 +/- 34 (n = 11), whereas the Bmax (in fmol/mg protein) was 52 +/- 5.
The number of apparent binding sites for [3H]nitrendipine was reduced by
approximately 50% in cultured cells (Bmax = 30 +/- 5 fmol/mg protein). The
reduction in insulin release from cultured cells correlated well with the
reduction in nitrendipine binding. It is concluded that the decrease in the
number of apparent binding sites for nitrendipine after culture represents
a reduction in functional calcium channels associated with influx of
calcium and insulin release.
