Immunological and physicochemical evidence for tissue specific prolactin receptors in the rabbit

Late pregnant and midlactating rabbit tissues (adrenal gland, kidney cortex, liver, mammary gland, and ovary) were examined for evidence of tissue specific differences in PRL receptors by several criteria. These included analysis of association constants, gel filtration, analytical isoelectric focusing, and reactivity with specific anti-PRL receptor antibodies in both the direct competition and immunoprecipitation assays, using membrane bound and Triton X-100 solubilized receptors. Association constants of membrane bound PRL receptors were not significantly different (adrenal, 1.50 +/- 0.26 X 10(10) M-1; kidney cortex, 0.90 +/- 0.22 X 10(10) M-1; liver, 1.31 +/- 0.50 X 10(10) M-1; mammary, 0.95 +/- 0.36 X 10(10) M-1) except the ovarian receptor, which had a significantly higher affinity (2.05 +/- 0.04 X 10(10) M-1) than other tissue receptors. This difference did not result from a difference in tracer degradation. Higher affinity for the ovarian receptor may be a consequence of phospholipid modulation of receptor affinity, since Triton X-100 solubilization abolished this difference in affinity (ovary, 3.63 +/- 0.15 X 10(10) M-1; adrenal, 3.48 +/- 0.44 X 10(10) M-1; mammary, 3.61 +/- 0.41 X 10(10) M-1). However, both the solubilized kidney cortex receptor (2.68 +/- 0.52 X 10(10) M-1) and the solubilized liver receptor (2.40 +/- 0.38 X 10(10) M-1) were significantly different from PRL receptors in the other tissues, implying a possible structural distinction. In addition, clear differences in isoelectric point and immunoreactivity were evident between the kidney cortex receptor and PRL receptors in other tissues. Immunologic evidence of differences between mammary gland, adrenal, ovary, and liver PRL receptors was also obtained, although the degree of difference in the liver receptor was obscured by binding of 125I-ovine PRL to the GH receptor. No differences were seen in elution position of solubilized receptors by gel filtration on Sepharose CL6B. These differences support the notion of structurally distinct PRL isoreceptors in different tissues, although it is also possible that they result from differing proportions of two receptor forms (e.g. plasma membrane and golgi complex forms). It is suggested that the kidney cortex receptor is derived from a more primitive form of the PRL receptor concerned with the regulation of electrolyte balance, whereas PRL receptors in other tissues arose at later times to subserve other specialized functions.