Meeting Abstract
This paper describes ion transport mechanisms present in the plasma membranes of lobster (Homarus americanus) branchiostegite epithelial cells and the effects of pH and heavy metals on the uptake of 45Ca by these processes. Partially purified membrane vesicles (PPMV) of branchiostegite cells were produced by a homogenization/centrifugation method. 45Ca (1 mM) uptake was highest at pH 8.5 and decreased in a step-wise fashion at pH 8.0, 7.5, and 7.0 with lowest uptake occurring at pH between 6.0 and 7.0. 45Ca uptake was a biphasic process consisting of a saturable mechanism at low [Ca] and a linear process at higher [Ca]. 45Ca influxes (15 sec uptakes) at both pH 8.5 and 7.5, in the presence of 100 μM verapamil (Ca channel blocker), were both hyperbolic functions of [Ca], with the Michaelis-Menten constant (Km) significantly greater at pH 7.5, suggesting competitive inhibition between protons and 45Ca during uptake. Increasing zinc concentrations (5 to 500 μM) reduced apparent carrier-mediated 1 mM 45Ca uptake at pH 8.5 and 7.5 completely, the remaining uptake being accounted for by channel uptake and non-specific isotope binding. Uptake of 1 mM 45Ca at pH 8.5, 7.5, 7.5 + Zn, and 7.5 + Zn + Cu in the presence of 100 μM verapamil displayed a step-wise reduction of 45Ca uptake with addition of each treatment until only non-specific isotope binding occurred with all cation inhibitors. Results suggest that branchiostegite uptake of 45Ca takes place by carrier-mediated and channel proteins. Carrier-mediated 45Ca uptake is synergistically abolished by the combination of increased acidity and heavy metals.
