Mn2+ Ions Pass Through Ca2+ Channels in Myoepithelial Cells

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Abstract
Intracellular recordings were made from the myoepithelial cells of the proventriculus of the marine polychaete worm Syllis spongiphila. Overshooting responses were elicited either by carbamylcholine added to the bathing medium or by directly applied intracellular current pulses. In control artificial sea water (ASW) directly applied current pulses elicited regenerative responses of 68–119 mV in amplitude and 70–1800 ms in duration; these responses were associated with contractions of the myoepithelial cells. Both pharmacologically and electrically elicited responses were reversibly abolished in Ca-free ASW and were unaffected by TTX or lowsodium solutions. Regenerative responses were elicited by direct intracellular stimulation in calcium-free ASW containing 1 mM-B2+ or 10 mM-Sr2+. Directly elicited responses were blocked reversibly in ASW containing calcium and 15-20 mM-Co2+ or 2.5-10 mM-Ni2+; they were blocked irreversibly in ASW containing calcium and 10 mM-La3+ or 100 μM-Zn2+. Regenerative responses were elicited in Ca-free solutions containing 10-50 mM-Mn2+; these responses were not associated with contractions, were consistently of longer duration than responses elicited in control ASW, and were blocked by 20 mM-Co2+ or 10 mM-La3+. The overshoots of Mn2+ responses elicited in both Na-free and Na-containing, Ca-free solutions increased as the external concentration of Mn2+ was increased, with a slope of about 27 mV per 10-fold change in concentration of Mn2+. In Cacontaining solutions the slope was reduced to about 15mV per 10-fold change. The results indicate that the myoepithelial cells generate Ca-spikes and that Mn2+ ions, in addition to Sr2+ and Ba2+ ions, pass through the Ca2+ channels of the myoepithelial cell membranes. Although Mn^ can replace Ca2+ in generating spikes, it apparently cannot replace Ca2+ in initiating contraction, and it many compete with Ca2+ in activating repolarization of the cell.