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Journal of Experimental Biology 150,395-405 (1990)
Published by Company of Biologists 1990

Potassium and Rubidium Uptake in Freshwater Bivalves

T. H. DIETZ ¹ and R. A. BYRNE ²

¹ Department of Zoology and Physiology, Louisiana State University Baton Rouge, LA 70803, USA
² Department of Zoology and Physiology, Louisiana State University Baton Rouge, LA 70803, USA; Department of Biological Sciences, University of Calgary, Calgary, Alta, Canada T2N 1N4

Potassium transport characteristics were investigated in three species of freshwater bivalves: a corbiculid, Corbiculafluminea, and two unionids, Carunculina texasensis and Ligumia subrostrata. Using ⁴²K, all three were found to take up potassium from dilute artificial pondwater ([K⁺] about 0.05 mmoll⁺¹). The influx (J_i) was 0.72µequiv g^-1dry tissue h^-1 in the corbiculid, significantly higher than the value of about 0.40µequiv g^-1dry tissuer g^-1 in the unionids. The K⁺ uptake displayed saturation kinetics in the range 0.05–0.36 mmoll^-1: in Co. fluminea, there was a J_max of 3.56µequiv g^-1 dry tissue h^-1 and the affinity coefficient (K_m) was 0.27mmoll^-1; in Ca. texasensis, J_max had a value of 1.8 µequiv g^-1dry tissue h^-1 and K_m was 0.16mmoll^-1. Using K⁺-free artificial pondwater containing 0.03–0.04 mmoll^-1 Rb⁺, the Rb⁺ influx was 0.41µequiv g^-1 dry tissue h^-1 in the corbiculid and 0.28 µequiv g^-1 dry tissue h^-1 in Ca. texasensis. All animals lost K⁺ during the rubidium flux studies, and since they contained no Rb⁺, the Rb⁺ efflux was zero and the net flux was equal to the influx. The J_max values for Rb⁺ were lower than the corresponding values for potassium: in Co. fluminea, J_max was 1.4µequiv g^-1 dry tissueh⁺¹, significantly higher than in Ca. texasensis, which had J_max of 0.84µequiv g^-1 dry tissue h^-1. The rubidium Km (approx. 0.05 mmoll^-1) values were significantly lower than corresponding values for potassium. Salt depletion increased the rubidium transport rate fourfold for both Co. fluminea and Ca. texasensis. High rates of net K⁺ uptake may account for the bivalves' inability to tolerate elevated environmental potassium.

Key words: rubidium transport, potassium transport, corbiculid, unionid, bivalve

Accepted on January 30, 1990