Electrogenic action of calcium on crayfish gill

Abstract

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.