Fisiologia dos adrenoceptores cardíacos
Palavras-chave:
colesterol, fisiologia, fisiologia cardiovascular, receptores adrenérgicosResumo
Adrenoceptores (receptores adrenérgicos ou AR. 's) são proteínas de sete alças ancoradas à membrana plasmática, que respondem a estímulos catecolaminér gicos. São classificados em a-adrenoceptores e -adrenoceptores, sendo que há dois subtipos principais de a receptores adrenérgicos (a e a2) e três subtipos de -adrenoceptores ( 1, 2 e ), embora se especule sobre a existência de um receptor 4. Os receptores adrenérgicos-a1 ativam a maquinaria da fosfolipase C, gerando inositol trifosfato (IP) e diacilglicerol como segundos mensageiros intracelulares, enquanto que os receptores adrenérgicos-a estão acoplados à adenilato ciclase e, no meio intracelular, reduzem a formação de adenosina monofosfato cíclico, inibindo o canal de cálcio, de maneira que a ativação dessa subclasse de receptores promove efeitos inibitórios na célula onde está presente. Em contrapartida, todos os receptores adrenérgicos aumentam a formação de adenosina monofosfato cíclico intracelular. Na vigência de estímulos prolongados, os receptores adrenérgicos sofrem dessensibilização, assim como vários outros sistemas biológicos de sinalização. A dessensibilização pode se dar pelos seguintes meios: fosforilação via cinases específicas; fosforilação por cinases efetoras; por endocitose do receptor; down-regu!ation, via fosforilação da subunidade a da proteína G; e, finalmente, por redução da transcrição intracelular de RNA-mensageiro. A integridade fisiológica dos receptores adrenérgicos é dependente de um meio ideal que pode sofrer alterações em função de diversos fatores, tais como a hipercolesterolemia e o estresse oxidativo.
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