Vasopressin binding sites in toad bladder: studies with the photoaffinity analogue [Phe(p-N3)3]AVP

Toad bladders were exposed to [Phe(p-N3)3]AVP (N3-AVP), an analogue of vasopressin with a photoreactive p-azido group in position three, in the presence and absence of ultraviolet (UV) light. Bladders exposed to the analogue in the presence of UV light showed an increase in membrane permeability to water, which persisted in spite of repeated and prolonged washout of analogue. In contrast, the hydroosmotic response induced by the analogue in the absence of UV light was readily reversed on washout. Aliquots of a broken epithelial cell preparation, derived from bladders that had been exposed to the analogue in the presence of UV light, bound less tritium-labeled vasopressin ([3H]AVP) than control aliquots that had been exposed to the analogue in the absence of UV irradiation or irradiated in the absence of the analogue. Membrane preparations that had not been photolabeled had specific binding sites for [3H]AVP in excess of 1,800 fmol/mg protein without evidence of saturation at a [3H]AVP concentration of 250 nM. Conversely, photolabeled membranes were saturated at a [3H]AVP concentration of 100 nM. The present studies demonstrate that a high proportion of [3H]AVP binding sites can be covalently labeled with N3-AVP and that at least some of these N3-AVP-bound sites are functional in triggering an increase in membrane permeability to water.

Occurrence of donor Langerhans cells in mouse and rat chimeras and their replacement in skin grafts

Evidence is presented that some endogenous Langerhans cells (LCs) may persist indefinitely in skin grafts. This evidence is based on the observation that although 2 weeks after grafting F1 hybrid mice and rats with genetically compatible skin, most of the LCs in the grafts were replaced with those of the host, some LCs of graft origin persisted for as long as the grafts were followed (154 days in mice and 249 days in rats). It has also been demonstrated that the spleen may be as good a source of LCs as the marrow. Thus, 6 weeks after lethally irradiated mice were restored with F1 hybrid spleen cells, most of the LCs in the epidermis of their pinnae were of donor origin. LCs of donor origin also were found in the epidermis of the pinnae of animals that had been inoculated at birth with spleen and lymph node cells (mice) or bone marrow cells (rats). Hence the occurrence of these cells provides another means of confirming that tolerance (chimerism) has been induced.

Downregulation of vasopressin receptors in toad bladder

Binding of tritium-labeled vasopressin [( 3H]AVP) to a broken epithelial cell preparation of the toad's urinary bladder has been related to hormonal action on water and urea transport across the intact tissue. Hormone binding to receptor sites and permeability changes were initiated at the same concentration of hormone (0.4 nM). Half-maximal urea and water permeability responses were observed with 3.1 and 5.6 nM AVP, respectively, although half-maximal receptor saturation required considerably higher concentrations of hormone (less than 500 nM). Because maximal permeability responses were obtained with occupation of approximately 200 fmol/mg protein receptor sites and the total receptor density was in excess of 2,000 fmol/mg protein, there is apparently a receptor reserve in this tissue. The antidiuretic hormone employed by the toad is vasotocin (AVT). This compound was 60-fold more effective than AVP in displacing [3H]AVP from receptor sites. Preincubation of bladders with AVT resulted in downregulation of receptor sites. Although the magnitude of receptor loss was equivalent in the presence or absence of a transmembrane osmotic pressure gradient, the capacity of AVT to induce permeability changes was more markedly reduced in the presence of an osmotic gradient. This observation suggests that the negative-feedback signal initiated by water flow through the hormone target cell diminishes sensitivity to hormone by a mechanism other than by a reduction in the number of surface receptors or by a decrease in their affinity for the hormone.

Synthesis and biological activities of a fluorescent photoaffinity analog of vasopressin

The present study describes the synthesis and biological activities of a vasopressin (VP) analog which binds covalently to receptors via a photoreactive p-azido group in position 3 and which contains a rhodamine label in position 8 for localization of hormone-receptor complexes by image-intensified fluorescence microscopy. 1-Deamino[3-(p-azidophenylalanine)]-N epsilon-rhodamyllysine-VP (Rhod-N3-dLVP) was obtained in a two-step procedure from the precursor 1-deamino[3(p-aminophenylalanine)]-LVP which was synthesized by a solid phase technique. The rat antidiuretic activity of this compound was 0.34 +/- 0.3 U/mg. Although both Rhod-N3-dLVP and its congener without a rhodamine label, N3-dLVP, did not have any hydroosmotic activity in the isolated toad urinary bladder in the absence of UV light, after UV irradiation they increased both urea and water transport across the bladder wall. Moreover, these permeability effects of Rhod-N3-dLVP persisted during prolonged and repeated periods of washout, suggesting that the photoproducts of this analog had formed covalent complexes with toad bladder receptors. Binding of Rhod-N3-dLVP was inhibited when photolysis was carried out in the presence of 1-deamino-LVP. These studies suggest that Rhod-N3-dLVP has the requisite biological properties to serve as a tool for the localization by fluorescence microscopy of VP receptors in various target tissues.

Persistent stimulation of adenylate cyclase and urea transport by an AVP photolabel

The effects of a photoaffinity label for arginine vasopressin receptors, [Phe2, Phe(p-N3)3]AVP (N3-AVP), on urea permeability and adenylate cyclase activity have been investigated in the toad urinary bladder. This compound, when activated by ultraviolet light, induced a maximal and persistent increase in the urea permeability of the intact bladder and a persistent increase in the adenylate cyclase activity of toad bladder epithelial cell homogenates. Covalent attachment of the analogue to target tissue during photolysis was equivalent at 4 and 20 degrees C. Bladders exposed to N3-AVP in the presence of AVP during photolysis were substantially less permeable to urea than controls that had been exposed to N3-AVP alone. These findings constitute further evidence in support of our previous suggestion that N3-AVP binds covalently to AVP receptors and, in addition, demonstrates that N3-AVP evokes a persistent increase in adenylate cyclase activity which, in turn, triggers a persistent increase in bladder permeability to urea.

Action of photoreactive analogs of vasopressin in toad bladder

A series of analogs of vasopressin with photoreactive groups in positions 1, 2, 3, 4, 8 or 9 of the nonapeptide sequence have been studied for their effects on water and urea permeability of the isolated toad urinary bladder. Compounds with photoreactive groups in positions 3 or 8 bound covalently to receptors as judged by a persistent increase in water and urea permeability following UV irradiation, prevention of photolabeling by incubation in the presence of vasopressin, and a persistent increase in membrane-bound adenylate cyclase activity. Some analogs were inactive in the dark, but became active and bound covalently to receptors during photolysis. Other analogs were inhibitors or agonists in the dark, but did not bind to receptors following UV irradiation. Time course studies with photolabelled bladders showed a stable urea flux for 4 hr in the absence of osmotic water flow. However, in the presence of water flow urea flux was initially enhanced (solvent drag effect) and later retarded (diminished urea permeability). Binding of photoaffinity analogs to receptors was not diminished with acidification of the serosal bathing medium, lowering of the bath temperature from 21 degrees C to 4 degrees C or with addition of prostaglandin E1. However, the capacity of photoreactive analogs to effect an increase in transmural water flux, once the analog was bound covalently to receptors, was markedly diminished under these conditions.