Membrane‐Bound Form of ADP‐Ribosyl Cyclase in Rat Cortical Astrocytes in Culture

ADP‐ribosyl cyclase activities in cultured rat astrocytes were examined by using TLC for separation of enzymatic products. A relatively high rate of [ 3 H]cyclic ADP‐ribose production converted from [ 3 H]NAD + by ADP‐ribosyl cyclase (2.015 ± 0.554 nmol/min/mg of protein) was detected in the crude membrane fraction of astrocytes, which contained ∼50% of the total cyclase activity in astrocytes. The formation rate of [ 3 H]ADP‐ribose from cyclic ADP‐ribose by cyclic ADP‐ribose hydrolase and/or from NAD + by NAD glycohydrolase was low and enriched in the cytosolic fraction. Although NAD + in the extracellular medium was metabolized to cyclic ADP‐ribose by incubating cultures of intact astrocytes, the presence of Triton X‐100 in the medium for permeabilizing cells increased cyclic ADP‐ribose production three times as much. Isoproterenol and GTP increased [ 3 H]cyclic ADP‐ribose formation in crude membrane‐associated cyclase activity. This isoproterenol‐induced stimulation of membrane‐associated ADP‐ribosyl cyclase activity was confirmed by cyclic GDP‐ribose formation fluorometrically. This stimulatory action was blocked by prior treatment of cells with cholera toxin but not with pertussis toxin. These results suggest that ADP‐ribosyl cyclase in astrocytes has both extracellular and intracellular actions and that signals of β‐adrenergic stimulation are transduced to membrane‐bound ADP‐ribosyl cyclase via G proteins within cell surface membranes of astrocytes.