G proteins were quantitated by immunoblot in normal rat anterior pituitary, an estrone-induced pituitary adenoma, and in two transplantable pituitary tumors resistant to dopamine, 7315a and MtTW15. Antisera specific for the αo and β subunits or the αi subunit of G proteins were tested with all preparations. While the α and β subunits were found to be present in variable concentrations in all preparations, the αo subunit was very low or undetectable in the transplantable tumors. These findings suggest that tumor resistance to the dopamine inhibitory actions on prolactin release and on tumor growth may be due to the deficiency of a Go protein.
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