α 2 ‐Adrenergic, k ‐Opiaie, and P r Purinergic Autoreceptors Have Mutually Antagonistic Effects: A New Regulatory Mechanism?

Rat cortical synaptosomes prepared on four‐step discontinuous Percoll density gradients were loaded with the fluorescent Ca 2+ ‐indicator fura‐2 to allow measurement of the intrasynaptosomal free calcium concentration ([Ca 2+ ] i ). When P 1 ‐purinergic, α 2 ‐adrenergic, or k ‐opiate agonists were incubated with these synaptosomes for 1 min, there was a highly significant, dose‐dependent reduction in [Ca 2+ ] i . The effects of these agonists were blocked by inclusion of appropriate specific antagonists. When α 2 ‐adrenergic and P 1 ‐puri‐nergic agonists were coincubated, a mutual antagonism of their effects was observed, and, in fact, an increase rather than a decrease in [Ca 2+ ] i was apparent. This mutual antagonism was reversed by addition of either a P 1 ‐purinergic or a α 2 ‐adrenergic antagonist. Parallel studies in which k ‐opiate and P 1 ‐purinergic agonists were coincubated also demon‐strated a mutual antagonism between the individual effects that was reversed by prior inclusion of either a k ‐opiate or P 1 ‐purinergic antagonist. As these mutually antagonistic effects have been observed between α 2 ‐adrenergic, k ‐opiate, and P 1 ‐purinergic receptor‐mediated events, we suggest that this may be a general phenomenon and may be a regulatory mechanism at nerve endings.