AJP - Cell Physiology, Vol 262, Issue 5 C1211-C1219, Copyright © 1992 by American Physiological Society
Activation of protein kinase C reduces L-type calcium channel activity of GH3 pituitary cells
A. A. Haymes, Y. W. Kwan, J. P. Arena, R. S. Kass and P. M. Hinkle
Department of Pharmacology, University of Rochester School of Medicine and Dentistry, New York 14642.
These studies describe the effect of protein kinase C (PKC) activation on
the activity of voltage-sensitive L-type Ca2+ channels of GH3 pituitary
cells. The rate of 45Ca2+ uptake was stimulated greater than 25-fold by
depolarization in the presence of BAY K 8644; the phorbol ester
12-O-tetradecanoylphorbol 13-acetate (TPA) reduced this response by 70% in
a concentration-dependent fashion. Phorbol 12,13-dibutyrate (PDBu)
inhibited depolarization-induced 45Ca2+ uptake within 1 min and caused a
nearly maximal reduction after 1 h; its effects were rapidly reversible.
TPA decreased the high K(+)-stimulated increase in intracellular free
calcium ion concentration ([Ca2+]i) from 8.5- to 3.2-fold by 5 min and to
2.0-fold after 18 h without altering the peak [Ca2+]i response to the
peptide hormone TRH. Ca2+ channel current, measured directly using the
whole cell configuration of the patch-clamp technique, declined an average
of 6.4% over 5 min for control cells and 28.9% when TPA was added to the
bathing medium for 5 min. Treatment with 100 nM TPA for 24 h dramatically
reduced peak current without shifting the peak of the current-voltage
relationship. The mean peak Ca2+ channel current was reduced from 423 to
128 pA, although a few cells seemed completely resistant. To determine
whether the effects of phorbol esters were due to the activation of PKC we
tested the potency of several drugs to inhibit L-channel activity and to
shift the affinity of the epidermal growth factor (EGF) receptor, an
established PKC response.(ABSTRACT TRUNCATED AT 250 WORDS)
