AJP - Cell Physiology, Vol 253, Issue 1 C52-C59, Copyright © 1987 by American Physiological Society
Potentiation of contraction in bullfrog ventricle strips by manganese and nickel
F. Kavaler and G. Brommundt
Sodium-lack contractures by strips of bullfrog ventricle were found to be
increased in the presence of manganous ion (1-10 mM). In addition, peak
force development was usually attained earlier in the presence of manganous
ion and the rate of relaxation was decreased by nickel (0.7-2.0 mM),
although the latter cation did not potentiate contractural force. Both
manganese and nickel had only depressant effects on potassium-excess
contractures, as well as on electrically stimulated twitches. Depressant
effects of manganese and nickel on sodium-lack contractures were also
found. These were smaller, the lower the extracellular sodium concentration
and the higher the extracellular calcium concentration. When these
well-known negative inotropic effects of the divalent cations were
suppressed in sodium-free, calcium-rich media, their potentiating effects
were unmasked, resulting in marked augmentation by these cations of
potassium-excess contractures and of twitches, along with slowing of
relaxation. Experimental maneuvers that have been reported to bring about
entry of manganese into these cardiac cells did not increase the observed
positive inotropic effect. It thus seems probable that these ions act on
the membrane of the ventricle fiber. Also, in view of earlier evidence that
they neither affect myofibrillar function nor induce calcium release from
mitochondria, it is provisionally concluded that the mechanism of their
potentiating effect on mechanical activation is due to their inhibition of
calcium extrusion from the fibers, described in the accompanying paper.
