AJP - Cell Physiology, Vol 260, Issue 2 C249-C258, Copyright © 1991 by American Physiological Society
Cadmium block of isometric contractions of isolated bullfrog atrial cells
N. Shepherd, F. Kavaler and W. Spielman
Department of Physiology, State University of New York, Brooklyn 11203.
We studied the effect of cadmium, verapamil, and quinacrine on the force of
contraction (Fp) of isolated, single, field-stimulated bullfrog atrial
cells. All agents were applied or removed rapidly (t1/2 approximately 15
ms) to minimize intracellular concentration changes other than
intracellular calcium concentration. Two components of twitch force were
observed, one blocked by micromolar Cd2+ and the other by millimolar Cd2+.
The two contributed about equally to the activation of the twitch. The
"cadmium-sensitive" portion of force (that affected by [Cd] less than or
equal to 100 microM) had a K1/2 approximately 1 microM, was identical in
magnitude to, and not additive with, a "verapamil-sensitive" (10 microM)
component of force, was most strongly affected by 50-ms pulses of Cd2+ when
they were applied in the mechanical latent period, and was potentiated by
catecholamines. The cadmium-insensitive portion of force was abolished by
the removal of extracellular calcium and was greatly potentiated by
quinacrine (3 or 10 microM), a blocker of Na-Ca exchange. The results are
consistent with the idea that activating calcium enters the cell via both
an inactivating cadmium-sensitive L-type channel and a noninactivating
cadmium-insensitive mechanism that is not Na-Ca exchange and leaves the
cell via Na-Ca exchange.
