Multiple receptor activation elicits synergistic IP formation in nonpigmented ciliary body epithelial cells

We have examined the interaction between muscarinic and alpha(2)-adrenergic receptor activation on inositol phosphate (IP) formation in the nonpigmented cells of the ciliary body epithelium (NPE cells) of the rabbit. We have compared these changes with those previously observed in the intracellular free Ca2+ concentration. Whereas muscarinic receptor activation causes an increase in intracellular Ca2+ and IP formation, activation of alpha(2)-receptors does not significantly increase either intracellular Ca2+ or IPs over basal levels. However, simultaneous activation of muscarinic and alpha(2)-adrenergic receptors with the specific agonists carbachol and UK-14304 produces massive Ca2+ increases and results in a synergistic increase in IP formation. This synergistic IP formation is inhibited by both muscarinic and alpha(2)-adrenergic receptor antagonists as well as by pertussis toxin and an inhibitor of phospholipase C. IP formation is predominantly independent of intracellular Ca2+, because it is decreased but not prevented by blocking the entry of Ca2+ with LaCl3 or chelating intracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Thus synergistic IP formation underlies, at least in part, the synergistic increase in intracellular Ca2+ resulting from simultaneous activation of muscarinic and alpha(2)-adrenergic receptors.