Na+-dependent Ca2+ uptake in isolated opercular epithelium of Fundulus heteroclitus

It is concluded that Ca²⁺ transport across the basolateral membranes of the ionocytes in killifish skin is mediated for the major part by a Na⁺/Ca²⁺-exchange mechanism that is driven by the (transmembrane) Na⁺ gradient established by Na⁺/K⁺-ATPase. The conclusion is based, firstly, on the biochemical evidence for the presence of a Na⁺/Ca²⁺-exchanger next to the Ca²⁺-ATPase in the basolateral membranes of killifish gill cells. Secondly, the transcellular Ca²⁺ uptake measured in an Ussing chamber setup was 85% and 80% reduced in freshwater (FW) and SW (SW) opercular membranes, respectively, as the Na⁺ gradient across the basolateral membrane was directly or indirectly (by ouabain) reduced. Thapsigargin or dibutyryl-cAMP/IBMX in SW opercular membranes reduced Ca²⁺ influx to 46%, comparable to the effects seen in FW membranes [reduction to 56%; Marshall et al. 1995a]. Basal Ca²⁺ influx across the opercular membrane was 48% lower in membranes from fish adapted to SW than in membranes from fish adaptated to FW. Branchial Na⁺/K⁺-ATPase activity was two times higher in SW adapted fish. Accepted: 29 October 1996