Iron-56 Irradiation Diminishes Muscarinic but Not α₁-Adrenergic-Stimulated$\text{Low-}{\rm K}_{{\rm m}}$ GTPase in Rat Brain

Initial findings from our laboratory have indicated that muscarinic enhancement of K⁺-evoked release of dopamine from perifused striatal slices is reduced after exposure to${}^{56}{\rm Fe}\text{-particle}$ irradiation. This finding suggested that there is a radiation-induced deficit in muscarinic receptor sensitivity. Subsequent findings have indicated that at least part of the loss in sensitivity may occur as a result of alterations in the initial steps of the signal transduction process and involve muscarinic receptor-G protein coupling/uncoupling. The present study was carried out to localize this deficit further by determining carbachol-stimulated low- K _m guanosine triphosphatase (GTPase) activity in striatal and hippocampal tissue obtained from rats exposed to 0, 0.1 or 1.0 Gy of${}^{56}{\rm Fe}\text{-particle}$ irradiation. In addition, to examine the specificity of the effect of${}^{56}{\rm Fe}\text{-particle}$ irradiation, α₁-adrenergic-stimulated$\text{low-}{\rm K}_{{\rm m}}$ GTPase activity was also examined in these tissues. The results showed that there was a high degree of specificity in the effects of${}^{56}{\rm Fe}$ particles. Decrements were observed in muscarinic-stimulated$\text{low-}{\rm K}_{{\rm m}}$ GTPase in striatum but not in hippocampus, and${}^{56}{\rm Fe}\text{-particle}$ irradiation did not affect α₁-adrenergic$\text{low-}{\rm K}_{{\rm m}}$ GTPase activity in either brain tissue.