Structure and function of A1 adenosine receptors.

The A1 adenosine receptor is the best characterized of the widely distributed purinergic receptor family. The purified brain A1 receptor is a monomeric 35- to 36-kDa glycoprotein. A1 receptors can be clearly distinguished from A2 adenosine receptors on the basis of structure activity relationships with selective ligands. Recent structure activity data suggest that subtypes of A1 (A1a, A1b, and A3) and A2 (A2a and A2b) receptors may exist. A1 receptor-mediated responses are coupled via multiple pertussis toxin-sensitive GTP binding proteins (G proteins) to many different effectors in various tissues: adenylate cyclase, phospholipase C, Na+- Ca2+ exchange, Ca2+ channels, Cl- channels, and K+ channels. The formation of calcium-mobilizing inositol phosphates can either be enhanced or inhibited. In general, adenosine has been found to act in concert with other hormones or neurotransmitters in either an inhibitory or a stimulatory way. The myriad modulatory actions of adenosine suggest that: 1) adenosine may simultaneously produce multiple effects within the same cell; and 2) activation of A1 receptors may lead to either a decrease or an increase in the coupling of other receptors to their G proteins.