摘要：

The presence of two classes of adenosine receptors linked to adenylate cyclase has been firmly established for a variety of cell types (see Daly, 1982; 1983). The A1 (Ri) class of receptor is inhibitory to adenylate cyclase while the A2 (Ra) class of receptor is stimulatory to the enzyme (Londos et al., 1980; Van Calker et al., 1979). The stimulatory A2-receptors may prove to consist of two subclasses, one with an affinity constant for adenosine of 1 M or less, and one with an affinity constant of about 10 M (see Londos et al., 1983). In addition, there is an internal P-site through which adenosine inhibits adenylate cyclase (Londos and Wolff, 1977). The external adenosine receptors and the internal adenosine P-site show marked differences in recognition domains for adenosine analogs and for xanthines. At the stimulatory A2-receptors the usual rank order of potency of adenosine analogs is as follows: N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > N6-R-phenylisopropyladenosine (R-PIA, usually L-PIA in the literature) > S-PIA. At the inhibitory A1-receptors the usual rank order of potency is as follows: R-PIA > NECA ≈ 2-chloroadenosine > S-PIA. S-PIA.

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