Noradrenaline and cyclic AMP—independent growth stimulation in newt limb blastemata

Adult newts regenerate functional limbs after amputation1,2. This process normally depends on the trophic influence of nerves on the regenerating limbs, particularly in the early stages before differentiation of the regeneration blastema3, when it stimulates growth by maintaining high rates of macromolecular synthesis4. The sequence of biochemical events involved is unknown, but it has been suggested that intracellular cyclic AMP may be a second messenger within the blastema5–10. Many studies have indicated that the neural agent(s) involved might be protein4,11–16. The recent finding that blastemata contain high levels of catecholamines17, however, has implicated noradrenaline (NA) as the neurotrophic agent, and suggested that it works via stimulation of β-adrenergic receptors on the blastemal cells, thereby raising the intracellular concentrations of cyclic AMP10. To test this hypothesis we studied the ability of NA alone and in combination with α-and β-adrenergic antagonists to increase cyclic AMP levels and to mimic the effects of nerves by maintaining high rates of protein synthesis13,14 and high mitotic indices (MI)15,16 in denervated blastemata in organ culture. We find that although NA raises cyclic AMP levels through a β-adrenergic effect, it does not maintain high rates of protein synthesis or high MI in cultured blastemata. It is unlikely therefore, that this hypothesis applies.