NEUROBLASTOMA CELL ADENYLATE CYCLASE: DIRECT ACTIVATION BY ADENOSINE AND PROSTAGLANDINS1

Abstract

Abstract—Adenylate cyclase activity of permeabilized neuroblastoma cells was measured by the conversion of [α32P]ATP into labelled cyclic AMP. Adenosine (10−6 ‐ 10−4m) induced a dose‐dependent increase in cyclic AMP formation. This effect could not be accounted for either by an adenosine‐induced inhibition of the phosphodiesterase activity present in the enzyme preparation, or by a direct conversion of adenosine into cyclic AMP. This indicates that the observed increase in cyclic AMP accumulation reflected an activation of adenylate cyclase. Adenosine is partially metabolized during the course of incubation with the enzyme preparation. However, none of the identified non‐phosphorylated adenosine metabolites were able to induce an adenylate cyclase activation. This suggests that adenosine itself is the stimulatory agent. The apparent Km of the adenylate cyclase for adenosine was 5 ± 10−6‐10−5m. Maximal activation represented 3‐4 times the basal value (10‐100 pmol cyclic AMP formed/10 min/mg protein). The adenosine effect was stereospecific, since structural analogues of adenosine were inactive. Adenosine increased the maximal velocity of the adenylate cyclase reaction. The stimulatory effect of adenosine was inhibited by theophylline. Prostaglandin PGE1 had a stimulatory effect much more pronounced than that of adenosine (6‐10‐fold the basal value at 10−6m). Dopamine and norepinephrine induced a slight adenylate cyclase activation which was not potentiated by adenosine. It is concluded that adenosine is able to activate directly neuroblastoma cell adenylate cyclase. It seems very likely that such a direct activation is also present in intact nervous tissue and account, at least partly, for the observed cyclic AMP accumulation in response to adenosine.