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

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 N a+/K +-ATPase. The conclu­ sion 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) re­ duced. Thapsigargin or dibutyryl-cAMP/IBMX in SW opercular membranes reduced Ca2+ influx to 46%, comparable to the effects seen in FW membranes [re­ duction 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.