AJP - Cell Physiology, Vol 253, Issue 3 C369-C374, Copyright © 1987 by American Physiological Society
Beta-adrenergic relaxation of smooth muscle: differences between cells and tissues
C. R. Scheid
The present studies were carried out in an attempt to resolve the
controversy about the Na+ dependence of beta-adrenergic relaxation in
smooth muscle. Previous studies on isolated smooth muscle cells from the
toad stomach had suggested that at least some of the actions of
beta-adrenergic agents, including a stimulatory effect on 45Ca efflux, were
dependent on the presence of a normal transmembrane Na+ gradient. Studies
by other investigators using tissues derived from mammalian sources had
suggested that the relaxing effect of beta-adrenergic agents was Na+
independent. Uncertainty remained as to whether these discrepancies
reflected differences between cells and tissues or differences between
species. Thus, in the present studies, we utilized both tissues and cells
from the same source, the stomach muscle of the toad Bufo marinus, and
assessed the Na+ dependence of beta-adrenergic relaxation. We found that
elimination of a normal Na+ gradient abolished beta-adrenergic relaxation
of isolated cells. In tissues, however, similar manipulations had no effect
on relaxation. The reasons for this discrepancy are unclear but do not
appear to be attributable to changes in smooth muscle function following
enzymatic dispersion. Rather they may be attributable to unique properties
of tissues, such as the presence of pacemaker cells, nerves, and so on.
Thus the controversy concerning the mechanism of beta-adrenergic relaxation
may reflect inherent differences between tissues and cells.
