Potassium transport in the freshwater bivalve Dreissena polymorpha

Abstract

Potassium transport and blood ion composition were examined in the freshwater bivalve Dreissena polymorpha. Animals acclimated to artificial pondwater (APW, [K+]=0.05 mmol l-1) for 4 weeks gradually lost Na+ and Cl-, but the blood K+ concentration remained constant near 0.5 mmol l-1. Blood [K+] in D. polymorpha declined by 41 % after 1 week of exposure to K+-free APW. Conversely, blood [K+] rose to 1.52±0.05 mmol l-1 (mean ± s.e.m.) 24 h after exposure to 0.5 mmol l-1 K+ APW. Total tissue K+ content remained stable in animals maintained in APW, but fell significantly in animals exposed to K+-free APW for 2 weeks. The net K+ flux (Jnet) for animals incubated in APW, with an average K+ concentration of 0.07 mmol l-1, was -0.27±0.06 µequiv g-1 dry tissue h-1, significantly different from the value of 0.50±0.08 µequiv g-1 dry tissue h-1 for animals transferred to 0.30 mmol l-1 K+ APW. A transepithelial membrane potential of -3.6±0.7 mV (blood negative compared with the bathing medium) was measured in APW-acclimated mussels. Potassium influx was measured with 42K and displayed Michaelis­Menten saturation kinetics at dilute K+ concentrations. The Km was 0.084±0.054 mmol l-1 and the Jmax was 1.74±0.39 µequiv g-1 dry tissue h-1. Both the Km and Jmax for animals exposed to K+-free APW for 7 days were unchanged. Using 86Rb, qualitatively similar transport characteristics were observed for animals incubated in K+-free, Rb+ APW, but the 22 day K+ depletion time significantly increased Jmax. D. polymorpha compensated for changes in the ionic composition of the acclimation medium by tolerating alterations in blood solute composition and adjusting ion transport rates.