The Journal of General Physiology, Vol 81, 805-827, Copyright © 1983 by The Rockefeller University Press
Mn ions pass through calcium channels. A possible explanation
M Anderson
The divalent transition-metal cations Fe, Co, and Ni were used to test the
hypothesis that Mn ions pass through calcium channels because Mn ions have
a relatively low energy of hydration. The test ions were applied to the
bath and comparisons were made of their effects on Ca or Mn spikes elicited
from myoepithelial cells of the proventriculus of the polychaete worm
Syllis spongiphila. Control experiments showed that (a) results obtained
using deoxygenated solutions (required to stabilize Fe2+ ions) could be
compared with those using solutions containing oxygen, and (b) the test
cations did not measurably affect the electrical coupling between cells. Ca
spikes were reversibly abolished by the test cations in the order of
effectiveness: Fe (16.1 mM +/- 1.0, SE; n = 15) = Co (14.6 mM +/- 0.8; n =
27) less than Ni (8.3 mM +/- 0.7; n = 16). The test cations diminished Mn
spikes by decreasing maximum rates of rise (Fe = Co less than Ni) and
overshoot amplitudes (Fe less than Co less than Ni). The test cations also
increased the current intensity required for Ca (Fe = Co less than Ni) or
Mn spike initiation (Fe less than Co less than Ni). Since the energies of
hydration of Fe, Co, and Ni increase stepwise from that of Mn, and the
effectiveness of these ions in diminishing Ca and Mn spikes increased in
the order Fe less than or equal to Co less than Ni, these data support the
hypothesis that Mn ions pass through Ca channels because they shed waters
of hydration relatively easily. An additional observation was that, at
below-blocking concentrations, the test cations caused decreased duration
of Mn spikes and increased duration of Ca spikes.
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