As is also the case with crustacean branchial and renal Na, K-ATPases (reported K, values of 80-210 PM in the references just cited), the branchial enzyme in H. n&s is much less sensitive to ouabain (Is, of 95 PM) than that in vertebrate tissues ( Iso of 0.1-10 PM) (3 1). This higher
Is, for ouabain has made it difficult to use autoradiographic methods with ‘H- or “C-ouabain to demonstrate that the Na, K-ATPase is located in the basolateral cell membranes of crustacean gills. However, Towle and Kays (36) have used histochemical methods to show that the Na, KATPases
of Callinectes sapidus and Carcinus ~naemrs gills have a basolateral location.