The signals from many receptors for hormones and neurotransmitters are sent through the plasma membrane by a set of heterotrimeric GTP-binding proteins composed of a GTP-binding α subunit and a βγ subunit that is a nondissociable dimer made up of two different polypeptides (for recent reviews, see Gilman, 1987; Stryer and Bourne, 1986; Lochrie and Simon, 1988; Neer and Clapham, 1988). The receptors on the cell surface are extremely specific for their ligands, which have very low affinities for heterologous receptors. Thus, there is very little cross-talk due to agonist binding at an inappropriate receptor. However, the next steps in the cascade appear to be less precise. Within a single cell, several separate receptors can often regulate each final pathway. For example, in the heart, acetylcholine and adenosine receptors appear to regulate the same K+ channel (Kurachi et al., 1986).
KeywordsAdenylate Cyclase Adenylyl Cyclase Human Fetal Tissue Guanine Nucleoside Neonatal Cardiocytes
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