EXPRESSION OF FUNCTIONAL PITUITARY SOMATOSTATIN RECEPTORS IN XENOPUS OOCYTES

Functional somatostatin (SRIF, somatotropin release-inhibiting factor) receptors were expressed in Xenopus oocytes after injection of RNA isolated from the anterior pituitary tumor cell line AtT20. SRIF receptors were detected by measuring the ability of SRIF to inhibit cAMP formation stimulated by β2-adrenergic agonists in individual oocytes. β2-Adrenergic receptors (β2ARs) were expressed in oocytes by coinjecting RNA prepared by in vitro transcription of a β2AR cDNA clone with the pituitary cell RNA. Uninjected oocytes do not express detectable levels of either β2ARs or SRIF receptors. In oocytes coinjected with AtT20 and β2AR RNA, on the other hand, isoproterenol treatment led to a 2- to 3-fold increase in cAMP levels, whereas cotreatment with SRIF reduced this accumulation by 50-60%. The SRIF precursor somatostatin-28 and the cyclohexapeptide agonist MK678 also inhibited cAMP formation, whereas the biologically inactive N-terminal 14-amino acid fragment of somatostatin-28 was ineffective. The ability to detect changes in cAMP levels in individual oocytes may provide a simple procedure for the expression cloning of SRIF receptor cDNAs and other receptors functionally coupled to stimulation or inhibition of adenylate cyclase.