Modulation By K+ Plus NH4+ of Microsomal (Na+, K+)ATPase Activity in Selected Ontogenetic Stages of the Diadromous River Shrimp Macrobrachium amazonicum (Decapoda, Palaemonidae) Francisco A. Leone1*, Thais M. S. Bezerra1, Daniela P. Garc¸on3, Malson N. Lucena4, Marcelo R. Pinto1, Carlos F. L. Fontes5, John C. McNamara2 1 Departamento de Quı´mica, Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Ribeira˜o Preto, SP, Brasil, 2 Departamento de Biologia, Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Ribeira˜o Preto, SP, Brasil, 3 Departamento de Biologia Molecular, Centro de Cieˆncias Exatas e da Natureza, Universidade Federal da Paraı´ba, Joa˜o Pessoa, PB, Brasil, 4 Departamento de Bioquı´mica e Imunologia, Faculdade de Medicina de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Ribeira˜o Preto, SP, Brasil, 5 Instituto de Bioquı´mica Me´dica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brasil

Abstract We investigate the synergistic stimulation by K+ plus NH4+ of (Na+, K+)-ATPase activity in microsomal preparations of whole zoea I and decapodid III, and in juvenile and adult river shrimp gills. Modulation of (Na+, K+)-ATPase activity is ontogenetic stage-specific, and particularly distinct between juveniles and adults. Although both gill enzymes exhibit two different sites for K+ and NH4+ binding, in the juvenile enzyme, these two sites are equivalent: binding by both ions results in slightly stimulated activity compared to that of a single ionic species. In the adult enzyme, the sites are not equivalent: when one ion occupies its specific binding site, (Na+, K+)-ATPase activity is stimulated synergistically by