Regulation of adenosine-sensitive adenylate cyclase from rat brain striatum

An adenosine-sensitive adenylate cyclase has been characterized from rat brain striatum. In whole homogenates as well as in particulate fractions, N6-phenylisopropyl adenosine (PIA), 2-chloroadenosine, and adenosine N'-oxide were equipotent in stimulating adenylate cyclase. Although GTP inhibited basal as well as PIA-stimulated activity of whole homogenates, the enzyme showed an absolute dependency on GTP for stimulation by PIA, dopamine, epinephrine, and norepinephrine in a particulate fraction derived from discontinuous sucrose gradient centrifugation. Adenosine exerts two effects on this adenylate cyclase, stimulation at low concentrations and inhibition at high concentrations, suggesting the presence of two adenosine binding sites. The stimulation of adenylate cyclase by PIA was dependent on the concentration of Mg2+. The degree of stimulation by PIA was greater at a low concentration of MG2+, which suggests that stimulation by PIA was accompanied by increasing the apparent affinity for Mg2+. Activation of adenylate cyclase by PIA was blocked by theophylline or 3-isobutyl-1-methylxanthine (IBMX). The pH optimum for basal or (PIA + GTP)-stimulated activities was broad, with a peak between 8.5 and 9.5. In the presence of GTP, stimulation by an optimal concentration of PIA was additive, with maximal stimulation by the catecholamines. Phospholipase A2 treatment at a concentration of 1 U/ml for 5 min completely abolished the stimulatory effect of dopamine, whereas PIA-stimulated activity remained unaltered. These data suggest that rat brain striatum either has a single adenylate cyclase, which is stimulted by catecholamines and adenosine by distinct mechanisms, or has different cyclase populations, stimulated by either adenosine or catecholamines.