Mn ions pass through calcium channels. A possible explanation.

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Abstract
The divalent transition-metal cations Fe, Co and Ni were used to test the hypothesis that Mn ions pass through Ca 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 results obtained using deoxygenated solutions (required to stabilize Fe2+ ions) could be compared with those using solutions containing O2 and 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) < Ni (8.3 mM .+-. 0.7; n = 16). The test cations diminished Mn spikes by decreasing maximum rates of rise (Fe = Co < Ni) and overshoot amplitudes (Fe < Co < Ni). The test cations also increased the current intensity required for Ca (Fe = Co < Ni) or Mn spike initiation (Fe < Co < 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 .ltoreq. Co < Ni, these data support the hypothesis than 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.