Desensitization and resensitization of lutropin receptors expressed in transfected Y-1 adrenal cells

Stimulation of gonadal cells by lutropins such as human chorionic gonadotro pin (hCG) is often transient and followed by down-regulation and/or desensi tization of lutropin receptor; (LHR). Here we describe desensitization/rese nsitization of LHR in Y-1 adrenal cell lines (termed Y-1L) expressing a rat cDNA lacking most 5' and 3' LHR untranslated regions under the control of a metallothionein promoter. Using a simple morphological assay in which sti mulated cells are round and unstimulated cells are flat, we identified clon es that rounded and remained round and others that became insensitive to lu tropin stimulation and reverted to their flat appearance within 2-4 h. Flat tened cells were insensitive to further hormonal stimulation but rounded af ter treatments with cholera toxin, forskolin, or cyclic AMP, showing that l oss of responsiveness was associated with an early step in signal transduct ion, not loss of rounding potential. Removing the lutropin stimulus for at least 90-120 min reversed hormone insensitivity, even in the presence of th e protein synthesis inhibitor puromycin. The number of surface bound recept ors did not change during a cycle of rounding/flattening and hCG bound to r ounded or flattened cells was replaced equally by radioiodinated hCG during incubations at 4 degrees C. Thus, desensitization/resensitization of LHR i n Y-1L cells occurred in the absence of new receptor synthesis, receptor de gradation, or receptor recycling. These observations suggest that LHR desen sitization/resensitization in Y-1L cells was closely coupled to receptor oc cupancy and that this cell line may be useful for identifying factors that modulate the activities of occupied receptors.