AJP - Heart and Circulatory Physiology, Vol 245, Issue 6 1024-H1030, Copyright © 1983 by American Physiological Society
Calcium channels resistant to organic calcium entry blockers in a rabbit vein
R. J. Winquist and E. P. Baskin
We compared the effects of calcium entry blockers on myogenic tone with the
effects of these compounds on agonist- and depolarization-induced
mechanical responses in isolated rings of the rabbit facial vein. The
development of intrinsic tone in response to distension of the vessel wall
and its potentiation by increasing the ambient temperature were antagonized
by MnCl2 (0.5 mM) but not verapamil, diltiazem, or nifedipine. Similarly,
the contractile response to histamine was blocked by MnCl2 but was
unaffected by moderate concentrations of the organic calcium entry
blockers. The histamine contractile response elicited at 34 degrees C (a
condition which abolishes intrinsic tone) was also refractory to verapamil
(10(-6) M). Verapamil, diltiazem, and nifedipine antagonized the
contractile response to calcium in depolarized facial vein rings. Thus both
extrinsic and intrinsic contractile responses in the rabbit facial vein
depend on an extracellular source of calcium. The distension-operated
channels are unique in exhibiting a marked temperature sensitivity.
However, receptor- and distension-operated calcium channels can be
distinguished from the voltage-operated channels in their insensitivity to
the organic calcium entry blockers.
