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.

Photoreactive neurohypophyseal hormone analogs: effects on transport processes in mammalian and amphibian epithelia.

The synthesis of the photoreactive neurohypophyseal hormone analog [3-(p-azidophenylalanine]arginine vasopressin [( Phe(p-N3)3]AVP is described. [Phe(p-N3)3]AVP exhibits a rat antidiuretic activity of 173 +/- 18 U/mg and a high binding affinity for the renal V2 vasopressin receptor in bovine kidney; an apparent dissociation constant KD of (1.3 +/- 0.2) X 10(-8) M was determined. In the toad bladder [Phe(p-N3)3]AVP was the most potent photoreactive vasopressin analog studied to date. It stimulated urea transport to the same maximal value as AVP with an ED50 of (3.1 +/- 0.7) X 10(-8) M. After irradiation with UV light, [Phe(p-N3)3]AVP bound irreversibly to toad bladder receptors and generated a persistent increase in bladder permeability to urea and to water. Analogs of 1-deamino-vasopressin with a photoreactive aryl azido group either in position 4 or 8 of the vasopressin sequence retain substantial rat antidiuretic activity. Compounds with a photoreactive group in position 8 showed a low activity in the isolated toad bladder in the dark; but on exposure to UV light, the activity of these analogs increased both with respect to urea and water transport, and the permeability response persisted during prolonged periods of washout. These studies provide evidence that analogs of vasopressin with an azido moiety in position 3 or 8 bind covalently to toad bladder receptors and produce an irreversible stimulation of transport processes.

Synthesis and biological activities of a photoaffinity probe for vasotocin receptors.

This study reports the synthesis and biological activities of 1-desamino, 7-lysine-(4-azidobenzoyl), 8-arginine vasotocin (d7-N3-AVT). This compound was found to be biologically active in the rat antidiuretic assay (20 U/mg), to behave as an antagonist of vasopressin in the rat pressor assay (pA2 = 6.6), and to yield a half-maximal hydroosmotic response in the isolated toad urinary bladder at a bath concentration of 2.4 X 10(-8) M. When toad bladders were exposed to d7-N3-AVT in the presence of long wavelength UV light, the hydroosmotic response persisted in spite of prolonged and repeated periods of washout. By contrast, the hydroosmotic response in control bladders after stimulation with d7-N3-AVT in the absence of UV irradiation was fully reversed within 15 min of washout. A membrane preparation derived from bladders that had been photolabeled with d7-N3-AVT and washed for 1 h specifically bound 325 fmol [3H]vasopressin/mg protein. Matched bladders exposed to the analog in the absence of UV irradiation and washed for 1 h specifically bound 591 fmol [3H]vasopressin per mg of protein. These studies indicate that d7-N3-AVT binds covalently to hydroosmotic receptors of toad urinary bladder and forms a complex that is functional in triggering an increase in the permeability to water of the epithelium. This analog may prove useful in the isolation and purification of vasotocin receptors in lower vertebrates.

The design of effective in vivo antagonists of rat uterus and milk ejection responses to oxytocin.

Several new synthetic analogs of the oxytocin antagonist [1-deaminopenicillamine]oxytocin have been prepared and tested for their abilities to inhibit responses to oxytocin by the isolated rat uterus in the absence and presence of Mg++, by the rat uterus in situ, and by the rat mammary gland in situ. Substituting 2-O-methyltyrosine in [1-deaminopenicillamine]oxytocin strikingly enhances antagonism of all uterin responses, and [1-deaminopenicillamine, 2-O-methyltyrosine]oxytocin and its 4-threonine analog are also potent inhibitors of the milk ejection response. Substituting 2-phenylalanine in [1-deaminopenicillamine]oxytocin also enhances antagonistic activities in all uterine assays, but [1-deaminopenicillamine, 2-phenylalanine]oxytocin retains agonistic activity on milk ejection assays. From these studies we can conclude that changes in the 1-position (1-deaminopenicillamine substitution) and the 2-position (2-O-methyltyrosine or 2-phenylalanine substitution) can have additive effects on antagonistic activities. Substitution of an 8-ornithine also enhances inhibitory potency in vivo, and this effect may also be additive to those of the substitutions in 1- and 2-positions. These findings provide many clues that may lead to the design of even more effective antagonists; several of the analogs reported here appear to the most effective antagonists of oxytocin in vivo yet reported and may be useful agents in further studies on the physiological functions of endogenous oxytocin.

Photoaffinity, biotinyl, and iodo analogues as probes for vasotocin receptors.

Vasotocin (AVT) analogues with tyrosine or phenylalanine in position 9, i.e., [9-tyrosine]AVT, [2-phenylalanine,-9-tyrosine]AVT, and 1-desamino[9-(p-aminophenylalanine)]AVT, were synthesized by the solid-phase method. These compounds showed a high biological activity in the hydroosmotic toad bladder assay. Using the chemically reactive functional groups on tyrosine and p-aminophenylalanine in position 9, we prepared iodinated, photoreactive, and affinity ligands, i.e., [2-phenylalanine,9-(iodotyrosine)]AVT, 1-desamino[9-(p-azidophenylalanine)]AVT, and 1-desamino[9-(biotinylphenylalanine)]AVT, respectively. Half-maximal hydroosmotic responses (ED-50 values) were obtained with 2.5 X 10(-9) M for the iodinated analogue, with 0.9 X 10(-10) M for the photoaffinity analogue, and with 1.2 X 10(-8) M for the biotinyl analogue. The hydroosmotic activity of the biotinyl analogue was reversed following addition of avidin, whereas the photoaffinity analogue induced a persistent response following UV irradiation that was not reversed upon repeated and prolonged periods of washout. These analogues of vasotocin are the most potent that have been synthesized to date, and they should serve as useful probes in the isolation and characterization of vasotocin receptors in toad bladders and tissues from other species that use vasotocin as an antidiuretic/pressor principle. The photoaffinity and biotinyl analogues had a rat antidiuretic activity of 66 and 40 units/mg, respectively. These compounds are, therefore, also suitable for the isolation of V-2 vasopressin receptors from mammalian tissues.

Synthesis and some pharmacological properties of [4-threonine,7-sarcosine]oxytocin, a peptide with high oxytocic potency, and of [4-threonine,7-N-methylalanine]oxytocin.

Two analogues of oxytocin, [Thr4,Sar7]- and [Thr4,MeAla7]oxytocin, were synthesized and their pharmacological properties investigated. [Thr4,Sar7]oxytocin was found to exhibit high biological activity (uterotonic activity of 1174 +/- 104 and milk ejection activity of 731 +/- 57 units/mg) and high selectivity for oxytocin-like relative to vasopressin-like activities (antidiuretic activity of 0.037 +/- 0.012 unit/mg, undetectable pressor activity). [Thr4,MeAla7]oxytocin was characterized by markedly lower biological activities. In both analogues, the additivity of the effects of the residues in positions 4 and 7 of oxytocin on the biological activity of the analogues was ascertained.

Synthesis and some pharmacological properties of oxytocin and vasopressin analogues with sarcosine or N-methyl-L-alanine in position 7.

Eight analogues of oxytocin and arginine-vasopressin were synthesized, in which the proline residue in position 7 was replaced by either sarcosine or N-methylalanine; some of the pharmacological properties of these analogues were evaluated. In peptides containing a beta-mercaptopropionic acid residue in position 1, the additivity of the effects of deletion of the amino group in position 1 and of the above-noted replacements in position 7 on biological properties of these analogues was ascertained. All of the analogues were found to be potent in either antidiuretic or uterine activity and also selective in action. From the point of view of pharmacological properties, substitution of sarcosine in position 7 of oxytocin gave analogues with higher oxytocic and milk-ejecting activities than did the substitution of N-methylalanine. The opposite structure-activity relationship was observed with arginine-vasopressin, where the N-methylalanine-containing analogues were more potent than the sarcosine-containing analogues with respect to pressor activity and also, if not deaminated, with respect to antidiuretic activity.

Synthesis and biological activities of arginine-vasopressin analogues with 4-hydroxyproline in position 7.

Three arginine-vasopressin (AVP) analogues in which the proline residue in position 7 was substituted with 4-hydroxyproline were synthesized by solid-phase techniques, and their biological activities were evaluated by antidiuretic, pressor, and uterotonic bioassays. The [7-trans-4-hydroxy-L-proline]AVP, the 1-desamino[7-trans-4-hydroxy-L-proline]AVP, and the 1-desamino[7-cis-4-hydroxy-L-proline]AVP analogues showed a high antidiuretic and strikingly high uterine activity, a sharp decrease in pressor activity, and a better antidiuretic and uterine to pressor selectivity than the parent compound, arginine-vasopressin. The uterine activities are the highest so far assayed in AVP analogues with replacements in position 7.

[1-Desamino, 7-lysine, 8-arginine]vasotocin: attachment of reporter groups and affinity ligands through the lysine side chain.

The chemically reactive groups of [8-arginine]vasotocin (AVT) are the alpha-amino group in position 1, the phenolic hydroxyl group of the tyrosyl residue in position 2, and the side-chain functional group of the basic amino acid residue in position 8. Acylation or alkylation of any of these chemically reactive groups yields hormone analogues with sharply diminished biological activities in most assay systems. Since none of the chemically reactive groups in the native AVT (or other neurohypophyseal hormone) sequences is a suitable chemical port for acylation with affinity ligands and reporter groups, we have undertaken the rational design of AVT analogues in which a residue capable of being acylated has been incorporated into points of the AVT structure where structural modifications are expected to have as little effect as possible on biological activity. Empirical structure-activity relationships among neurohypophyseal hormone analogues as well as conformational models in solution and in the crystalline state suggest that positions 4 and 7 are likely points for the introduction of acylation ports. We have previously synthesized an analogue of [1-desamino]AVT (dAVT) with a lysyl residue in position 4 ([4-lysine]dAVT) and demonstrated that acylation of the side chain of this residue yields useful reporter and photoaffinity analogues. We now report the synthesis of the corresponding analogue with a lysyl residue in position 7 ([7-lysine]dAVT), which also yields potent acyl derivatives suitable for affinity ligands and photoaffinity ligands.

Elimination of infused arginine-vasopressin and its long-acting deaminated analogue in rats.

Arginine-vasopressin (AVP) and deamino-arginine-vasopressin (dAVP) were infused into rats. When the concentrations of the two peptides were steady, the rate of clearance of AVP from the plasma was six times the rate of clearance of alphaAVF. Only 6% of the infused AVP was excreted unchanged in the urine, whereas approximately 100% of the dAVP was excreted. When the infusions were stopped, AVP disappeared from the plasma much more rapidly than dAVP. The plasma concentrations of the two peptides did not decay as simple exponential functions, suggesting that both AVP and dAVP entered a slowly exchanging compartment or compartments during prolonged infusion. These differences in the metabolic clearance of AVP and dAVP may well explain the prolonged antidiuretic effect of dAVP in rats.

Fluorescent, photoaffinity, and biotinyl analogs of oxytocin.

This study reports the solid phase synthesis and biological activities of two oxytocin analogs, [1-desamino, 4-lysine,7-(L-3,4,-dehydroproline)]oxytocin and [1-desamino, 4-threonine,7-(L-3,4-dehydroproline),8-lysine]oxytocin, and several fluorescent, photoaffinity, or biotinylated derivatives of these analogs and of oxytocin. The activities (in IU/mg) of the lysine-containing parent compounds, respectively, were as follows: uterus (without Mg++) 4.8 and 54; uterus (with Mg++) 19 and 440; milk ejection 65 and 414. The above analogs were coupled through the chemically reactive epsilon-amino group of lysine in position 4 or 8 or, in the case of oxytocin, through the N-terminal amino group of fluoresceine, photoaffinity, or biotinyl ligands. Fluoresceine coupled in position 1 of oxytocin gave an analog of low to moderate uterine (3.8 without Mg+ and 1.9 with Mg++) and milk ejection (7.9) activities. Analogs with biotin or fluoresceine coupled to lysine in position 4 had moderate uterine (11 and 23 without Mg++; 38 and 11 with Mg++) and milk ejection (33 and 13) activities. Analogs with fluoresceine, photoaffinity, or biotinyl labels coupled to lysine in position 8 retained good uterine (106, 62, and 147 without Mg++; 79, 78, and 509 with Mg++) and milk ejection (101, 181, and 247) activities and represent potentially useful experimental tools for studying hormone-receptor interactions and for receptor localization and isolation.

In vivo apparent peptide-receptor dissociation rate constants for arginine vasopressin analogs estimated from inhibition of rat pressor responses.

Apparent pressor receptor dissociation rate constants for arginine vasopressin, arginine vasotocin, oxytocin, oxypressin, and [1-deamino, 9-D-alanineamide]arginine vasopressin were estimated by the following method. Two minutes after injection of a moderate dose of agonist into urethane-anesthetized rats prepared for recording mean blood pressure, a large dose of inhibitor was injected. Under these conditions, in the first few moments after inhibitor injection, there should be no rebinding of the agonist after it dissociates, because vacant receptors should be immediately occupied by inhibitor. The rate of the blood pressure drop at the initiation of inhibition was calculated and used as an estimate of the dissociation rate of the agonist. Apparent dissociation rate constants thus estimated were 1.1, 1.1, 6.9, 5.8, and 13.9 min-1 for arginine vasopressin, arginine vasotocin, oxytocin, oxypressin, and [1-deamino, 9-D-alanineamide]arginine vasopressin, respectively. These rate constants were inversely related to the pressor potencies (435, 250, 5, 3, and 0.7 U/mg, respectively) of these five compounds. Such a relationship is to be expected if decreased potency is in part due to a faster "off" rate from pressor receptors.

Probes for vasopressin receptors. Attachment of affinity and fluorescent groups in vasopressin.

Fluorescent, photoreactive, and biotinylated analogs of vasopressin have been prepared in which one of these three groups has been attached to a reactive amino group in either position 4 or position 7. Using solid phase methodology, we have synthesized two active parent compounds, [1-desamino,4-lysine,7-hydroxyproline]arginine vasopressin and [1-desamino,7-aminoproline]arginine vasopressin, and acylated them to obtain biotinyl, azidobenzoyl, and fluoresceinyl derivatives. We have also prepared analogs in which a "spacer arm" was inserted between lysine in position 4 and the marker group. Some of these derivatives have good antidiuretic activity and could be valuable probes in studying hormone-receptor interaction and in receptor visualization and purification.

4-Proline and 4-hydroxyproline analogs of arginine vasopressin: role of the proline substitution in the two beta-turns of vasopressin.

[4-L-Proline]arginine vasopressin, [4-D-proline]arginine vasopressin, [4-hydroxyproline]arginine vasopressin and [4-proline, 7-hydroxyproline] arginine vasopressin were synthesized and found to have antidiuretic activities of 91 +/- 4, 1.7 +/- 0.2, 1.0 +/- 0.1 and 4.4 +/- 1.0 units/mg, respectively. None of these analogs exhibited a significant level of rat pressor activity. The observed activities of these and other analogs with substitutions at position 4 and/or 7 are discussed on the basis of hypotheses and data bearing on the solution conformation of vasopressins.

Verapamil blockade of pressor and uterine responses to AVP-OT analogue, oxypressin.

The effects of the calcium channel blocker verapamil on simultaneously recorded uterine and pressor responses to the equipotent (in eliciting these responses) oxytocin-vasopressin analogue, oxypressin, were studied in urethan-anesthetized and pentolinium- and indomethacin-treated rats during injections and infusions of this analogue. Doses of verapamil that almost completely blocked the pressor response to infused oxypressin had no effect on a pressor response to injected oxypressin of equal magnitude. Larger doses of verapamil blocked the pressor response to injected oxypressin somewhat. Uterine responses were only marginally affected by these doses of verapamil, and there was no significant difference between infusion and injection or between estrus and diestrus.

In vivo simultaneous comparison of pressor and uterine responses to a single agonist (oxypressin) in estrous rats.

To try to compare receptor compartment kinetics, receptor binding, and binding-response coupling for two smooth muscle types in vivo, pressor and uterine responses to oxypressin, an equipotent analog of oxytocin and vasopressin, were studied simultaneously in urethane-anesthetized, pentolinium-indomethacin treated rats. Access of peptide to the pressor and uterine receptor compartments and peptide-receptor dissociation rate had a negligible effect on the two responses. During both injections and infusions, the blood pressure response seemed to be determined largely by plasma levels of oxypressin. The uterine response to oxypressin, however, was paradoxical. The heights of individual uterine contractions seemed to be determined throughout by plasma oxypressin concentrations. The interval between contractions also seemed to be determined by plasma peptide concentrations during injections. However, as plasma peptide increased and reached steady state during infusion, contraction interval behaved as if plasma peptide concentrations were decreasing. This effect was more marked at the beginning of infusion. The implication of these results is that binding determines the pressor response to oxypressin and binding-response coupling does not change. In contrast, although binding determines the uterine response to oxypressin during injection and binding-response coupling appears constant, some factor in addition to binding affects the contraction interval portion of the uterine response and modifies the apparent binding-response coupling of this parameter during infusion.

Adrenergic interactions in uterus and vascular smooth muscle in rats in vivo.

Simultaneous blood pressure and uterine responses to norepinephrine infusions were recorded in urethane-anesthetized, pentolinium-indomethacin treated rats in natural estrus under conditions in which no blockers or blockers of alpha 1-, alpha 2-, and beta-adrenergic receptors or of "reuptake" of norepinephrine were present. The contributions of alpha 1- and alpha 2-adrenergic receptors to the blood pressure response were similar during the initial portion of the response. At later times, however, alpha 1-adrenergic receptors were responsible for the major portion of the response. The tachyphylaxis of the pressor response that occurs during norepinephrine infusion could be prevented by preventing norepinephrine "reuptake" with imipramine. In the uterus, the initial small alpha-adrenergic contractile response (seen only at the lowest infusion rate) was quickly overwhelmed by a beta-adrenergic relaxing component. Administration of the beta-adrenergic receptor blocker, propranolol, during norepinephrine infusion caused similar increases in blood pressure in control, yohimbine-, and prazosin-treated rats. Uterine contractions, in contrast, were only significantly elevated during beta-adrenergic receptor blockade when yohimbine or imipramine had also been administered.

Effects of the substitution of photoreactive groups in positions 4 and 8 of vasopressin.

In an effort to synthesize potent vasopressin analogs containing photoreactive groups, we prepared, by solid phase synthesis, three analogs with proline or hydroxyproline substitutions in positions 4 and/or 7, lysine in positions 4 or 8, and beta-mercaptopropionic acid in position 1. From these three parent analogs, 1-desamino[4-proline,8-lysine]VP, 1-desamino[4-hydroxyproline,8-lysine]VP, and 1-desamino[4-lysine,7-hydroxyproline]AVP, we then prepared the corresponding azido compounds using the epsilon-amino group of lysine as the attachment point. These six analogs were then assayed for antidiuretic and pressor activities in rats. One of the resulting analogs, 1-desamino[4-lysine(N epsilon-4-azidobenzoyl),7-hydroxyproline)]AVP has the highest antidiuretic activity of any photoreactive compound reported to date.