Irradiation inactivation studies of the dopamine D1 receptor and dopamine-stimulated adenylate cyclase in rat striatum.

In frozen rat striatal tissue, exposed to 10 MeV electrons from a linear accelerator, the sizes of the dopamine (DA) D1 receptor and the DA-sensitive adenylate cyclase complex were determined using target size analysis. The number of D1 receptors (labelled by [3H]SCH 23390) declined monoexponentially with increasing radiation intensity, yielding a molecular weight (mol.wt.) of 80 kDa. Also the activity of the catalytic unit (C) of the adenylate cyclase (as measured by forskolin stimulation), decreased monoexponentially, however with a mol.wt. of 145 kDa. Both basal, DA- and fluoride (F-)-stimulated activity declined in a concave-downward fashion with a limiting mol.wt. of 134, 138 and 228 kDa, respectively. It was estimated that the basal and DA-stimulated activity originated from an enzyme complex with a mol.wt. of 325 kDa, a value close to the combined size of RGs and C. These data suggest that F- stimulation of the adenylate cyclase, which occurs by a Gs activation, does not cause dissociation of Gs into the alpha s and beta gamma subunits. Further, the DA-regulated adenylate cyclase apparently exists as a complex consisting of RGs and C; the mechanism of hormonal activation is a dissociation of C from this complex.