Catecholamine and Ca 2+ activation of adenylate cyclase systems in synaptosomal fractions from rat cerebral cortex.

Abstract

SEVERAL inconclusive attempts2–6 have been made to reproduce and extend the findings of Klainer et al.1 that certain catecholamines stimulate the formation of cyclic AMP in cell-free preparations from brain. In earlier investigations, we were able to demonstrate consistent stimulation of adenylate cyclase activity in homogenates and particulate fractions from rat cerebral cortex in the presence of norepinephrine and dopamine7. Although the percentage increases were relatively small, the absolute increments in cyclic AMP formation in these latter studies were comparable to those observed in other cell-free systems which were hormone dependent8–14. The data suggested that enhancement of basal adenylate cyclase activity during brain cell disruption was partly responsible for the low percentage activation by catecholamines and the difficulty in obtaining reproducible responses in earlier studies. We now find that an important factor may be exposure of the intracellular membrane-bound enzyme systems to relatively high levels of extracellular Ca2+ (ref. 15) during homogenization and fractionation, as well as to Ca2+ which is present as a contaminant in ATP and other constituents of the assay medium16. The data also reveal that, in the presence of the selective Ca2+ chelating agent, EGTA, adenylate cyclase activity in synaptosomal and synaptic membrane fractions from rat cerebrum is specifically activated by catecholamines in concentrations as low as 0.005 mM.