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

Abstract

Adult newts regenerate functional limbs after amputation^1,2. 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 blastema^5–10. Many studies have indicated that the neural agent(s) involved might be protein^4,11–16. 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 synthesis^13,14 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.