Apical membrane vesicles of dogfish and bovine lens epithelium were prepared by differential centrifugation and Mg2+ precipitation. Enrichment of the apical enzyme markers, for Na+,K(+)-ATPase was achieved. Na+,K(+)-ATPase was enriched 25.9-fold and 23.6-fold for dogfish and bovine lens epithelia, respectively, and acid phosphatase was enriched 10.4-fold and 12.6-fold, respectively. There was no enrichment of the cytoplasmic marker, NADH reductase. The majority of the apical membrane vesicles isolated from both dogfish and bovine lens epithelia were oriented right-side out. Electron micrographs of the purified vesicles show the presence of circular sealed vesicles with an average size of approximately 0.2-0.5 microns. Incubation of the vesicles with either the acetoxymethyl ester of SBFI, a new indicator for sodium, or with Fura-2, the calcium-specific indicator, leads to a large accumulation of the de-esterified SBFI or Fura-2 in the lens epithelial apical vesicles as determined by fluorescence measurements. When an outwardly direct Ca2+ gradient is formed across the vesicular membranes, the influx of Na+ is stimulated 77.8 and 63.0% for dogfish and bovine lens epithelia, respectively. When an outwardly directed Na+ gradient is formed across the vesicular membranes, the Ca2+ influx is also greatly enhanced. Both cases indicate that there is a bidirectional Ca2+/Na+ exchanger present in the apical side of the lens epithelial cell. The exchanger is inhibited by 50 microM bepridil or by 200 microM La3+. The stimulatory effects are also observed in membrane vesicles that are 'short-circuited' with valinomycin and high concentrations of K+.(ABSTRACT TRUNCATED AT 250 WORDS)