Ideally, agonists and antagonists should differ in potency by at least two orders of magnitude at different receptor subtypes in order to be really useful in receptor classification. This is rarely the case for the compounds often used in classifying adenosine receptors. Nevertheless, with a judicious use of agonists and antagonists at A1, A2a, and A3 receptors in in vitro experiments, strong conclusions can be drawn (see Table 11.1). The situation is less fortunate in vivo as the pharmacokinetics of these compounds has not been studied extensively, and because differences in potency at the different receptors may be offset by huge differences in receptor abundance.

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