Potassium transport in the freshwater bivalve Dreissena polymorpha
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&plusmn;0.05 mmol l-1 (mean &plusmn; 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&plusmn;0.06 &micro;equiv g-1 dry tissue h-1, significantly different from the value of 0.50&plusmn;0.08 &micro;equiv g-1 dry tissue h-1 for animals transferred to 0.30 mmol l-1 K+ APW. A transepithelial membrane potential of -3.6&plusmn;0.7 mV (blood negative compared with the bathing medium) was measured in APW-acclimated mussels. Potassium influx was measured with 42K and displayed Michaelis&shy;Menten saturation kinetics at dilute K+ concentrations. The Km was 0.084&plusmn;0.054 mmol l-1 and the Jmax was 1.74&plusmn;0.39 &micro;equiv 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.
Wilcox
S
Dietz
T