Electrogenic action of calcium on crayfish gill

When intact crayfish are in an ion-poor medium (KCl, 0.1 mmol·l^-1+KHCO₃, 0.1 mmol·l^-1) there is a large potential difference (transepithelial potential difference),-20 to-40 mV (hemolymph negative), across the gills. Addition of Ca²⁺ to the medium is followed by a rapid change in transepithelial potential difference to near 0 mV. The transepithelial potential difference showed a non-linear dependence on Ca²⁺]_out with a limiting value of+2 to+10 mV at>1 mmol·l^-1. The concentration generating a half-maximum transepithelial potential difference change (15–20 mV) was 0.1 to 0.2 mmol·l^-1. Three other alkaline earth ions were also electrogenic; Ba²⁺ caused slightly larger transepithelial potential difference changes, Sr²⁺ and Mg²⁺ were a little less effective. It has been suggested that the transepithelial potential difference in ion-poor medium (in fish) is due to the diffusive efflux of NaCl across the gills, with a Cl^-/Na⁺ permeability ratio of <1. Evidence is presented that this might be the case in crayfish. The electrogenic effect of Ca²⁺ might then be due to its effect on gill permeability to Na⁺ and Cl^- such that the permeability ratio increased and approached unity as the transepithelial potential difference approached 0. However, this was shown to be unlikely. An alternative explanation for Ca²⁺ dependence of the transepithelial potential difference is that active inward Ca²⁺ transport is electrogenic.Abbreviations FW fresh water - I _out ion efflux - IP ion-poor solution - P _c Cl-permeability - P _Na Na⁺ permeability - R electrical resistance - SW sea water - TEP transepithelial potential difference