. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 358 B. H. TOOMEV AND D. EPEL damine dye. a substrate of the mammalian multidrug transporter (Neyfakh, 1988). In the absence of an inhibitor or competitive substrate of the transport activity (, verapamil), there is little rhodamine accumulation (little fluorescence increase), but when the transporter is inhib- ited, dye accumulates in the cells (fluorescence increase). This phenomenon is seen with Urechis embryos. Figure 1 shows the difference in intensity of dye in Urechis em- bryos at the 2-cell stage incubated in rhod
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 358 B. H. TOOMEV AND D. EPEL damine dye. a substrate of the mammalian multidrug transporter (Neyfakh, 1988). In the absence of an inhibitor or competitive substrate of the transport activity (, verapamil), there is little rhodamine accumulation (little fluorescence increase), but when the transporter is inhib- ited, dye accumulates in the cells (fluorescence increase). This phenomenon is seen with Urechis embryos. Figure 1 shows the difference in intensity of dye in Urechis em- bryos at the 2-cell stage incubated in rhodamine ± vera- pamil for 1 h. Embryos in rhodamine + verapamil ac- cumulate much more dye than embryos in rhodamine alone. To quantify this difference, measurements were made on single embryos over a 3-h period. In rhodamine alone the cells accumulate little dye. In the presence of rhodamine + verapamil. however, the fluorescence of Urechis embryos increases several-fold over the 3-h in- cubation. As shown in Figure 2A. uptake of dye in the presence of verapamil comes to an apparent equilibrium in about 1 h. Experiments with different batches of eggs showed slight variability in the fluorescence values, but the shape of the curves was the same. In mammalian cells containing the multidrug transport protein, the accumulation of dye in the presence of ver- apamil is through the inhibition of this efflux pump (Fojo ct a/., 1985), with verapamil acting as a competitive sub- strate or as an inhibitor. This is also true for Urechis. Embryos preloaded with rhodamine and washed into sea- water show a rapid efflux of dye from the cells (Fig. 2B). However, in embryos washed into seawater + verapamil. this efflux of dye is retarded, as seen in the higher fluo- rescence of the embryos (Fig. 2B). The Urechis transporter is effective at exporting low concentrations of substrate (1 < 10 8 to 5 X 1CT6A/); there is increased dye accumulation in the presence of verapamil at concentrations rang
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