A ROLE FOR ADENOSINE IN METABOLIC DEPRESSION IN THE MARINE INVERTEBRATE SIPUNCULUS-NUDUS

Involvement of neurotransmitters in metabolic depression under hypoxia and hypercapnia was examined in Sipunculus nudus. Concentration chang es of several putative neurotransmitters in nervous tissue during anox ic or hypercapnic exposure or during combined anoxia and hypercapnia w ere determined. Among amino acids (gamma-aminobutyric acid, glutamate, glycine, taurine, serine, and aspartate) and monoamines (serotonin, d opamine, and norepinephrine), some changes were significant, but none were consistent with metabolic depression under all experimental condi tions applied. Only the neuromodulator adenosine displayed concentrati on changes in accordance with metabolic depression under all experimen tal conditions. Levels increased during anoxia, during hypercapnia, an d to an even greater extent during anoxic hypercapnia. Adenosine infus ions into coelomic fluid via an indwelling catheter induced a signific ant depression of the normocapnic rate of O-2 consumption from 0.36 +/ - 0.04 to a minimum of 0.24 +/- 0.02 (SE) mu mol . g(-1). h(-1) after 90 min (n = 6). Application of the adenosine antagonist theophylline c aused a transient rise in O-2 consumption 30 min after infusion during hypercapnia but not during normocapnia. Effects of adenosine and theo phylline were observed in intact individuals but not in isolated body wall musculature. The results provide evidence for a role of adenosine in inducing metabolic depression in S. nudus, probably through the es tablished effects of decreasing neuronal excitability and neurotransmi tter release. In consideration of our previous finding that metabolic depression in isolated body wall musculature was elicited by extracell ular acidosis, it is concluded that central and cellular mechanisms co mbine to contribute to the overall reduction in metabolic rate in S. n udus.