Mechanism of sodium uptake in PNA negative MR cells from rainbow trout, Oncorhynchus mykiss as revealed by silver and copper inhibition

The rate of acid-stimulated and phenamil-sensitive sodium (Na+) uptake was measured in three different cell lineages: pavement cells (PVC), total mitochondrion-rich (MR) cell populations, and peanut lectin agglutinin-negative mitochondrion-rich cells (PNA− MR) isolated from the rainbow trout gill epithelium. Despite the presence of basal levels of Na+ uptake in PVC, this transport was not enhanced by acidification, nor was it inhibited by independent treatment with bafilomycin (i.e., a V-type H+-ATPase inhibitor), phenamil (i.e., a specific inhibitor of ENaC), or Ag (a specific inhibitor of active Na+ transport in fish). In contrast, Na+ uptake in PNA− MR cells was increased by ~ 220% above basal levels following acidification of near 0.4 pH units in the presence of 1.0 mM external Na+. Acid-stimulated Na+ transport was entirely inhibited by both phenamil and bafilomycin. Silver (Ag) and copper (Cu), which are known to interfere with active Na+ transport in fish, were also responsible for inhibiting acid stimulated Na+ uptake in PNA− MR cells, but by themselves had no effect on basal Na+ transport. Thus, we demonstrate that Ag specifically prevented acid-stimulated Na+ uptake in PNA− MR cells in a dose-dependent manner. We also demonstrate rapid (< 1 min) and significant inhibition of carbonic anhydrase (CA) by Ag in PNA− MR cells, but not in PVC. These data lend further support to the idea of a PNA− MR cell type as the primary site for Na+ uptake in the freshwater (FW) gill phenotype of rainbow trout. Moreover, these findings provide support for the importance of intracellular protons in regulating the movement of Na+ across the apical surface of the fish gill.