Reabsorção de Sódio e Aldosterona
Autor: Corticóides e Hormônios da Tireóide | Publicado: 2011-07-10T09:47:00-07:00

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A ação básica da aldosteroa sobre as células tubulares é pouco compreendida atualmente, entretanto é bem conhecida a sequência de eventos na célula que levam ao aumento da reabsorção de sódio:1.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; As membranas celulares, por serem lipossolúveis, permitem fácil difusão da aldsterona para o interior das células epiteliais tubulares.


  2.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Ocorre a ligação, no citoplasma, da aldosterona com um receptor proteico citoplasmático altamente específico. Tal receptor é uma proteína cuja estrutura terciária só permite se combinar com a molécula de aldosterona ou compostos muito semelhantes.
  3.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; O complexo formado (aldosterona-receptor ou produto dessa ligação) difunde-se para o núcleo da célula, onde pode sofrer maiores mudanças. Tais mudanças podem induzir uma ou mais porções do DNA a formar um ou mais tipos de RNA mensageiro cuja função se relaciona com o processo de transporte dos íons sódio e potássio.
  4.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; O RNA mensageiro difunde-se de volta ao citoplasma, onde age juntamete com ribossomos para promover síntese proteica. As proteínas sintetizadas são uma mistura de uma ou mais enzimas e proteínas transportadoras de membrana. Essas proteínas agirão em conjunto para promover o transporte de sódio, potássio e hidrogênio através da membrana da célula. 

  Um dos principais exemplos de enzimas produzidas é a adenosina trifosfatase sódio-potássio, que atua como parte principal da bomba de troca dos íons sódio e potássio nas membranas basolaterais das células tubulares renais. Há também proteínas que permitem a rápida de difusão de sódio do lúmen para o meio intracelular.
A aldosterona não possui, portanto, ação direta sobre o transporte de sódio. Sua atuação se deve a uma sequência de eventos celulares que levam á síntese de substâncias intracelulares específicas que promoverão o processo de transporte.
Bruno FonsecaMED93
REFERÊNCIAS BIBLIOGRÁFICAS:
- GUYTON, A. C. et al. Tratado de Fisiologia Médica. ELSEVIER, 11ª ed. Rio de Janeiro, 2006.