1. From the Laboratory of Kidney and Electrolyte Metabolism, NHLBI, National Institutes of Health (K.T.B., S.M., S.T., J.N., H.L.B., S.A., R.A.F., M.A.K.), Bethesda, Md; and Department of Biological Sciences, George Washington University (K.T.B., R.K.P.), Washington, DC.