Effect of clonidine on second messenger systems in rat adrenal gland

Clonidine, an alpha 2-adrenergic agonist, also binds to non-adrenergic imidazole receptors in brain and peripheral tissues. In adrenal medulla, however, clonidine appears to bind only to imidazole receptors. To assess whether the signal transduction mechanism of imidazole receptors differs from alpha 2-adrenergic receptors, we studied the actions of clonidine on the turnover of phosphoinositide and the production of cAMP and cGMP in slices of rat adrenal gland. Clonidine did not modify basal or carbachol mediated increases in phosphoinositide turnover or production of cAMP, however it increased the production of cGMP. The increase in cGMP was slow and unaffected by the addition of the phosphodiesterase inhibitor, IBMX. We conclude that the second messenger response triggered by clonidine in adrenal differs from that usually coupled to alpha 2-adrenergic receptors. Whether the effect is mediated by cell surface imidazole receptors remains to be established.