AJP - Cell Physiology, Vol 257, Issue 6 C1171-C1176, Copyright © 1989 by American Physiological Society
Catecholamine modulation of calcium currents in clonal pancreatic beta-cells
H. H. Keahey, A. E. Boyd 3rd and D. L. Kunze
Department of Molecular Physiology, Baylor College of Medicine, Houston, Texas 77030.
The mechanisms by which norepinephrine and epinephrine activate alpha
2-adrenergic receptors and inhibit insulin release from the pancreatic
beta-cell (19, 21, 23) are not yet clear but may involve modulation at
several sites. Because intracellular calcium has been implicated in the
secretory process, it has been suggested that catecholamines may inhibit
secretion by blocking calcium influx, thus reducing the free cytosolic
calcium concentration (23). The present study examines the effects of
epinephrine, norepinephrine, and clonidine on calcium current in an
SV40-transformed hamster beta-cell line (HIT cells). Under voltage-clamp
conditions, calcium currents were reversibly inhibited by norepinephrine,
epinephrine, and clonidine in the low nanomolar range. The effects were
blocked by 1) the alpha 2-antagonist yohimbine, 2) preincubation of the
cells with pertussis toxin (PTX), and 3) guanosine 5'-O-(2-thiodiphosphate)
(GDP beta S), the nonhydrolyzable GDP analogue that competitively inhibits
the interaction of GTP with G proteins. In contrast, guanosine
5'-O-(3-thiotriphosphate) (GTP gamma S) caused irreversible blockade by
catecholamines. These effects could not be overcome by adenosine
3',5'-cyclic monophosphate (cAMP), suggesting that the adenylate cyclase
pathway is not involved in the G protein coupling with the channels. These
studies show that catecholamines inhibit calcium currents in beta-cells
through an alpha 2-adrenoreceptor PTX-sensitive G protein pathway and could
inhibit insulin secretion by this mechanism.
