The Additional Efficacy of the Diuretic-Calcium Antagonist Combination Depends on the Potency of the Drug Administered First and Not on the Sequence of Administration. A Double Blind Study in Salt-Sensitive Black Hypertensives [A Eficacia Adicional da Associacao Nifedipina-Diuretico Depende da Potencia do Farmaco Administrado En Primeiro Lugar e Nao da Sequencia da Administracao. Estudo Em Hipertensos Negros Sensiveis Ao Sal]