Effects of growth hormone-releasing factor and(or) thyrotropin-releasing hormone on growth, feed efficiency, carcass characteristics, and blood hormones and metabolites in beef heifers

The objective of this study was to determine the effect of long-term administration of a growth hormone (GH)-releasing factor analog (GRFa) and(or) thyrotropin-releasing hormone (TRH) on growth, feed efficiency, carcass characteristics, and blood hormones and metabolites in beef heifers. Crossbred heifers (n = 48; 345.9 +/- 2.8 kg) were divided into four equal groups: control (vehicle), 1 microgram of GRFa (human GRF 1-29 analog).kg BW-1.d-1, 1 microgram of TRH.kg BW-1.d-1, or GRFa + TRH. Daily s.c. injections continued for 86 d. Blood samples were collected from half of the heifers after injection on d 1, 36, and 78. On d 89, all heifers were slaughtered. Treatments did not affect (P > .05) ADG but GRFa + TRH decreased (P < .05) ADFI relative to all other treatments. Feed conversion efficiency tended (P < .10) to be improved in the groups given GRFa alone or TRH alone. Treatment with GRFa and(or) TRH did not affect carcass weight, dressing percentage, conformation score, backfat thickness, or weights of liver, kidneys, pituitary, and ovaries. The GRFa + TRH treatment reduced (P < .05) fat score and increased (P < .05) longissimus muscle area relative to other treatments. The GRFa treatments reduced (P < .05) the weight and fat percentage of the mammary gland and increased (P < .05) heart weight. Treatment with TRH alone failed to stimulate GH on d 1, 36, and 78. Treatment with GRFa alone increased (P < .05) GH above controls on d 36, whereas GRFa + TRH increased (P < .05) GH on d 1, 36, and 78. Treatment with GRFa alone increased (P < .05) IGF-I only on d 1, whereas GRFa + TRH was without effect on all days. Across sampling days, treatments had little effect on blood concentrations of insulin, triiodothyronine, nonesterified fatty acids, urea nitrogen, and glucose. The GRFa alone and GRFa + TRH decreased (P < .05) and TRH alone increased (P < .05) thyroxine concentrations. In conclusion, with the dose and administration regimen used, GRFa and(or) TRH yielded small but positive improvements in animal performance.

Synthesis and biological activity of novel C-terminal-extended and biotinylated growth hormone-releasing factor (GRF) analogs

A series of novel hGRF(1-29)-NH2 analogs were synthesized and biotinylated. The immunological and biological activities of these analogs were then characterized. To distance the biotin moiety from the putative bioactive core, a C-terminal spacer arm consisting of -Gly-Gly-Cys-NH2 (-GGC) was added to hGRF(1-29)-NH2 (hGRF29) and analogs, with subsequent biotinylation performed at the cysteine residue. Neither addition of the C-terminal spacer arm nor biotinylation affected affinity of these analogs for GRF antibody. Relative to hGRF(1-44)-NH2 (hGRF44: potency = 1.0), the biotinylated analogs were equipotent in vitro to their nonbiotinylated, parent compounds: [desNH2Tyr1,D-Ala2,Ala15]hGRF29-GGC-(tpBiocyt in)-NH2 (4.7) = [Ala15]hGRF29-GGC-(tpBiocytin)-NH2 (3.9) greater than hGRF29-GGC-(tpBiocytin)-NH2 (0.8). Based upon cumulative GH release data in vivo (0-60 min postinjection), [desNH2Tyr1,D-Ala2,Ala15]hGRF29-GGC-(tpBiocyt in)-NH2, [Ala15]hGRF29-GGC-(tpBiocytin)-NH2, and hGRF29-GGC-(tpBiocytin)-NH2 displayed 8.6, 5.5, and 0.8 times, respectively, the potency of hGRF44. These in vivo potency values were not significantly different from the corresponding parent compounds (i.e., with or without the C-terminal spacer arm). In summary, biotinylated hGRF analogs have been developed that retain full immunoreactivity and potent bioactivity (in vitro and in vivo), thus permitting their use in GRF receptor isolation, ELISA, and histochemical procedures.

Solution structures of cyclic and dicyclic analogues of growth hormone releasing factor as determined by two-dimensional NMR and CD spectroscopies and constrained molecular dynamics

Solution structures were determined for a linear analogue of growth hormone releasing factor (GRF), and cyclic and dicyclic analogues in which the side chains of aspartyl and lysyl residues spaced at positions i-(i + 4) were joined to form a lactam. The four analogues were [Ala15]-GRF-(1-29)-NH2 and its cyclo8-12, cyclo21-25, and dicyclo8-12;21-25 derivatives. The peptides were studied in two solvent systems: 75% methanol/25% water at pH 6.0; and 100% water at pH 3.0. CD spectroscopy was used to assess the overall alpha-helical content. Nuclear magnetic resonance spectroscopy was used to determine the structures in more detail. Nearly complete proton resonance assignments were made for each of the peptides, in both solvents. Nuclear Overhauser effects were converted into distance constraints and applied in the molecular dynamics program CHARMM to evaluate the range of low-energy structures that satisfied the nmr data. In 75% methanol, all of the peptides are comprised of a single alpha-helical segment with fraying of one to three residues at each end. The linear analogue has a tendency to kink. In water, the analogues have two helical segments with flexible regions between them and at the termini of the peptides. The linear analogue is helical at residues 7-14 and 21-28. In the cyclo8-12 analogue, the N-terminal helical region extends to include residues 7-19, while the other helical region is slightly shortened. In the cyclo21-25 analogue, the C-terminal helical region is extended to include residues 19-28, while the N-terminal helical region is destabilized. The dicyclic analogue has the largest N-terminal helix, spanning residues 7-20, but its helical segment at residues 21-28 is not well ordered. All of the analogues exhibit substantial biological activity. The cyclic and dicyclic analogues show dramatically increased resistance to degradation during incubation with human plasma. The i-(i + 4) lactam, therefore, appears to be a synthetic means of stabilizing a local alpha-helical conformation, which may be of general use in the design of active, stable peptides.

Effects of growth hormone-releasing factor and vasoactive intestinal peptide on proliferation and steroidogenesis of bovine granulosa cells

Recent studies have suggested that growth hormone-releasing factor (GRF), like vasoactive intestinal peptide (VIP), may enhance follicle-stimulating hormone (FSH)-stimulated steroidogenesis in cultured rat granulosa cells (GC). Because effects of GRF or VIP on GC proliferation have not been reported, we evaluated and compared the effect of GRF to that of VIP using cultured bovine GC. Undifferentiated GC from 1-5 mm bovine follicles were established for 2 days in medium containing 10% fetal calf serum, washed and then cultured in chemically defined medium for an additional 2 days. Two-day treatment with 2.5-1000 ng/ml of VIP had no effect (P greater than 0.05) on proliferation or progesterone production of bovine GC in the presence or absence of 200 ng/ml FSH. In comparison, 100, 250, 500, 1000 or 2000 pg/ml of human [desNH2Tyr1,D-Ala2,Ala15]-GRF(1-29)-NH2 analog caused a dose-dependent stimulation (P less than 0.05) of GC proliferation in the absence and presence of 5 micrograms/ml insulin. However, the GRF analog had no effect (P greater than 0.05) on GC progesterone production (expressed as ng/10(5) cells/24 h) in the absence or presence of 5 micrograms/ml insulin. The effects of GRF analog on progesterone production and cell proliferation were not influenced by co-culture with 200 ng/ml FSH. GRF(1-44)-NH2 also stimulated cell proliferation but had no effect on basal or FSH-induced progesterone production. These results suggest that GRF may play a role in GC proliferation during follicular development in the bovine.

Effect of human growth hormone-releasing factor and a potent analog on antibody formation in African green monkeys

African Green monkeys were injected (2 x daily subcutaneously for six months) with human GRF(1-44)-NH2 (10 micrograms/kg BW) or a more potent analog, [desNH2Tyr1,Ala15]-hGRF(1-29)-NH2 (2 micrograms/kg BW) to determine the potential of each peptide to induce antibody formation. Blood samples were taken every two weeks, diluted 1:100 and tested for ability to bind radioiodinated hGRF. One animal in the hGRF(1-44)-NH2 group [N = 6] produced low-titer GRF antibodies by 6 weeks (19% binding) and continued throughout the 24 weeks of treatment (average = 50-60% binding). Similarly, one animal in the hGRF analog group [N = 6] displayed low-titer GRF antibodies by 18 weeks (14% binding), with the highest binding observed at 24 weeks (51% binding). Subsequent dilutions (1:1,000 and 1:3,000) of these bleedings confirmed that higher GRF antibody titers were not masked by antibody excess. Dialyzed sera from these two animals did not affect the abilities of hGRF(1-44)-NH2 or [desNH2Tyr1,Ala15]-hGRF(1-29)-NH2 to stimulate GH secretion by rat pituitary cells in vitro. After 20 weeks of treatment, significant GH responses (increased mean GH area under the curve 2.3-2.5 fold and GH peak 3.5-3.7 fold, that of control) were observed following hGRF or hGRF analog injection. Therefore, the low titer GRF antibodies detected in monkey sera during six months of treatment with hGRF or a potent analog were biologically non-neutralizing.

Characterization of growth hormone releasing factor analog expression in Saccharomyces cerevisiae

An analog of growth hormone releasing factor (GRF), [Leu27]GRF(1-40)-OH, has been expressed and secreted in Saccharomyces cerevisiae under the control of the alpha-factor gene promoter and prepro sequence. A single pair of consecutive basic residues served as a processing site between the alpha-factor sequences and the GRF sequences. [Leu27]GRF(1-40)-OH from fermentor broth containing 20-30 mg/L of immunoreactive peptides was shown to be correctly processed and to possess biological activity as measured in vitro and in vivo. Additional peptides purified from broth appear to result from proteolytic degradation of the original translation product. Analysis of the amino acid compositions and sequences of these peptides suggests that processing enzymes may be responsible for some of the degradation.

Serum glucose and free fatty acids modulate growth hormone and luteinizing hormone secretion in the pig

Two experiments were conducted to determine the effect of free fatty acids (FFA) and glucose treatment on growth hormone (GH) and luteinizing hormone secretion in the pig. In Experiment (Exp) 1, 15 prepuberal gilts received an intravenous infusion of FFA (n = 5; 3 ml of 10% Liposyn II/kg), glucose (n = 5; 1 g/kg), or saline (n = 5; 3 ml of 0.9%/kg). Jugular blood samples were collected every 15 min for 2 hr before and 3 hr after intravenous infusion of saline, FFA, and glucose. Synthetic [Ala15]-h growth hormone-releasing factor-(1-29)NH2 (1 microgram/kg) and gonadotropin-releasing hormone (0.2 micrograms/kg) were administered 30 min after infusion (Time 0 = infusion). In Exp 2, eight prepuberal gilts received either FFA (n = 4) or saline (n = 4) as described in Exp 1, except that treatments were given every hour ove a 10-hr period. Blood samples were collected every 15 min from 1 hr before to 10 hr after the start of FFA or saline infusion. In Exp 1, the peak GH response to growth hormone-releasing factor was delayed by 45 min (P less than 0.01) by glucose treatment and suppressed (P less than 0.01) by FFA treatment. The luteinizing hormone response to gonadotroph-releasing hormone was suppressed (P less than 0.03) by glucose and enhanced (P less than 0.03) by FFA. In Exp 2, the number of GH pulses was increased (P less than 0.05) by FFA infusion and GH concentrations were positively correlated (r = 0.58, P less than 0.0003) with FFA concentrations, while luteinizing hormone pulse amplitude was greater (P less than 0.01) in FFA gilts than in saline gilts. These results indicate that FFA are more effective modulators of GH secretion than acute hyperglycemia, while metabolic status can alter pituitary responsiveness to gonadotropin-releasing hormone.

GRF analogs and fragments: correlation between receptor binding, activity and structure

GH-releasing activity in vitro was directly correlated with GRF receptor binding affinity for all hGRF analogs examined. hGRF(1-29)-NH2 analogs with Ala15-substitution (for Gly15) displayed 4-5 times higher affinity for the GRF receptor relative to hGRF(1-44)-NH2. Replacement of Gly15 with Sar15 resulted in a dramatic loss of activity and receptor binding. The present data supports the proposal that Ala15-substitution increases receptor affinity, and hence potency, due to increased amphiphilic alpha-helical interactions. Fragments of hGRF, representative of DPP-IV and trypsin-like cleavage, are inactive as a consequence of greatly diminished GRF receptor binding. These results provide a comprehensive analysis of the structural features required for both GRF receptor binding and activation.

In vitro stability of growth hormone releasing factor (GRF) analogs in porcine plasma

The stability of growth-hormone releasing factor (growth regulating factor; GRF) analogs in porcine plasma was examined. GRF analogs were incubated in porcine plasma at 37 degrees C, extracted and subsequently analyzed using high performance liquid chromatography (HPLC). GRF(1-29)-NH2 was rapidly broken down in the plasma with a degradation rate of t1/2 = 13 min. The primary degradation product was identified as GRF(3-29)-NH2. Substitution of Gly15 by Ala15 slightly prolonged the plasma half-life (t1/2 = 17 min) and the major degradative fragment was found to be [Ala15]GRF(3-29)-NH2. The cleavage between the 2 and 3 position of the peptide was not inhibited by trasylol at a concentration of 1,000 KIU/ml but was dramatically reduced by the combined use of diprotin A and trasylol. Absence of the free amino group at the N-terminus and/or substitution of a D-amino acid residue at the penultimate position completely prevented cleavage between the 2 and 3 position in the structural linear GRF analogs. Side-chain to side-chain cyclization between Asp8 and Lys12 amino acid residues significantly improved the stability of GRF in plasma with t1/2 greater than 2 hr. An additional stability was provided by substitution of D-Ala2 for Ala2 in the structural cyclic analog. Cyclization between Lys21 and Asp25 also improved the stability of the GRF peptide in the plasma. Stability was further enhanced by the presence of D-Ala2 or by forming a dicyclic analog through an additional linkage between Asp8 and Lys12.

Growth performance and carcass composition of lambs infused for 28 days with a growth hormone-releasing factor analogue

A human growth hormone-releasing factor analogue, [DesNH2Tyr1,D-Ala2,Ala15]hGRF(1-29)NH2 (GRF-A), was infused s.c. into lambs for 28 d to determine its effects on growth performance and carcass composition. Twenty crossbred wethers weighing 47.0 +/- .5 kg were implanted with 7-d osmotic minipumps at weekly intervals. Minipumps contained either vehicle (dimethyl sulfoxide:H2O, 1:1) or GRF-A, released at a rate of 208 pmol (or .7 micrograms).h-1.kg-1. During the infusion period, plasma GH levels were increased (P less than .01) in GRF-A-treated wethers compared with control wethers (15.0 vs 9.3 ng/ml) and were higher on days that closely followed minipump implantation. Plasma IGF-I and hepatic IGF-I RNA concentrations were similar in lambs of both groups. Analogue treatment improved feed conversion (4.9 vs 5.8 kg dry matter/kg gain, P less than .05), increased average daily gain (.35 vs .30 kg, P = .05) and had no effect on feed intake, wool growth and body, carcass, selected organ and pituitary weights. Carcasses from GRF-A-infused lambs had less adjusted fat depth, a lower percentage of fat and a higher percentage of protein (P less than .05) than carcasses from control lambs. Magnitude of most effects of GRF-A on carcass measurements were correlated with the mean GH level that a lamb had during the infusion period. In conclusion, s.c. infusion of GRF-A improved feed utilization and altered carcass composition of feeder lambs in a relatively short period of time (28 d).

Effect of dose and frequency of administration of a potent analog of human growth hormone-releasing factor on hormone secretion and growth in pigs

Ninety-six pigs (49.5 +/- .5 kg BW) were allotted to six treatments and were injected once (SID) or three times daily (TID) s.c. with a [desamino-Tyr1, D-Ala2, Ala15] human growth hormone-releasing factor (1-29) NH2 analog (GRF-AN). Treatments were T1, noninjected control; T2, saline-injected control (TID); T3, GRF-AN (1.66 micrograms/kg BW, TID); T4, GRF-AN (3.33 micrograms/kg BW, TID); T5, GRF-AN (6.66 micrograms/kg BW, TID) and T6, GRF-AN (10 micrograms/kg BW, SID). Feed protein levels were 14% for T1 and 18.8% for T2 through T6. The GRF-AN increased serum growth hormone (GH) concentration for the entire growing period (about 56 d) in a dose-related manner and did not induce desensitization of the somatotroph cells; in fact, an increase (P less than .05) in the GH response to GRF-AN was observed in T4 and T5 after 1 mo of treatment. This GRF-AN produced (P less than .05) a dose-dependent effect on several variables in animals grown to 110 kg BW: in comparison to T2, T5 increased meat in carcass (6%), carcass length (3%), loin eye area (13%), liver weight (19%), kidney weight (30%), improved feed efficiency (20%) and decreased total feed intake by 50 kg (26%). Compared to T2, average daily gain was increased (P less than .05) by 13% by the 3.33 micrograms/kg TID dose. Blood parameters were measured on d 1, 29 and 57. Increased serum glucose and insulin levels were observed. Triiodothyronine and thyroxine concentrations were increased and decreased, respectively after 28 d of treatment but were unchanged on d 57. This potent GRF analog maintained high GH concentration for at least 56 d and affected several growth parameters and carcass characteristics in a dose-related manner similar in magnitude to that reported in studies using porcine GH.

Effect of a potent analog of human growth hormone-releasing factor on carcass composition and quality of crossbred market pigs

The effect of a human growth hormone-releasing factor (hGRF) analog ([desamino-Tyr1, D-Ala2, Ala15] hGRF(1-29)NH2) on the carcass composition of crossbred barrows was evaluated. pH, color and collagen content were measured on 74 animals distributed among the following five treatments started at about 50 kg BW: T1, control saline three times daily (TID); T2, hGRF analog (1.66 micrograms/kg, TID); T3, hGRF analog (3.33 micrograms/kg, TID); T4, hGRF analog (6.66 micrograms/kg, TID) and T5, hGRF analog (10 micrograms/kg, once daily). Animals were slaughtered at approximately 106 kg BW giving an average of 55 d on test. Carcass composition was determined on eight animals from T1 and eight animals from T4. The left side of each carcass was divided into four untrimmed commercial cuts: ham, loin, shoulder and belly, which then were dissected into muscle, separable fat, bone and skin. Ham, loin and belly weights were not affected by GRF treatment, but shoulder weight was increased (P less than .05; 10.11 vs 11.15 kg, SE = .21). There was an increase (P less than .0.05) in muscle content of all the cuts considered and a concomitant decrease (P less than .05) in fat content. The analog increased muscle weight by 16% and decreased fat weight by 25% in the pooled tissues of the shoulder, ham and loin. Overall, hGRF analog increased skin and bone weights by 39% and 19%, respectively. Chemical analysis demonstrated that the hGRF analog increased overall protein accretion in the carcass by 10.5% and decreased crude fat by 28.7%.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis, biological activity and conformational analysis of cyclic GRF analogs

A novel cyclic GRF analog, cyclo(Asp8-Lys12)-[Asp8,Ala15]-GRF(1-29)-NH2, i.e. cyclo8,12[Asp8,Ala15]-GRF(1-29)-NH2, was synthesized by the solid phase procedure and found to retain significant biological activity. Solid phase cyclization of Asp8 to Lys12 proceeded rapidly (approximately 2 h) using the BOP reagent. Substitution of Ala2 with D-Ala2 and/or NH2-terminal replacement (desNH2-Tyr1 or N-MeTyr1) in the cyclo8,12[Asp8,Ala15]-GRF(1-29)-NH2 system resulted in highly potent analogs that were also active in vivo. Conformational analysis (circular dichroism and molecular dynamics calculations based on NOE-derived distance constraints) demonstrated that cyclo8,12[Asp8,Ala15]-GRF(1-29)-NH2 contains a long alpha-helical segment even in aqueous solution. A series of cyclo8,12 stereoisomers containing D-Asp8 and/or D-Lys12 were prepared and also found to be highly potent and to retain significant alpha-helical conformation. The high biological activity of cyclo8,12[N-MeTyr1,D-Ala2,Asp8,Ala15]-GRF(1-29)- NH2 may be explained on the basis of retention of a preferred bioactive conformation.

Use of osmotic pumps for subcutaneous infusion of growth hormone-releasing factors in steers and wethers

Osmotic pumps were evaluated for 7-d delivery of growth hormone-releasing factor (GRF). In Exp. 1, 12 steers weighing 253 kg received hGRF(1-29)NH2 in H2O at rates of 0, 3, 30 and 300 pmol.h-1.kg-1. Pumps were implanted s.c. on d 0 and removed at 1200 on d 7. Blood samples were drawn at 20-min intervals from 0800 to 1200 on d -1, 1, 3, 5, 7 and 9. Growth hormone levels were not altered by GRF treatment (P greater than .05). Solubility and volume limitations render hGRF(1-29)NH2 delivery via osmotic pumps problematical. Flow rate and duration of release of dimethyl sulfoxide (DMSO):H2) (1:1) from osmotic pumps incubated in vivo and in vitro were found to be consistent with manufacturer's specifications. Two hGRF(1-29) analogues, Ro23-7863 and 4SG-29, were dissolved in DMSO:H2O. In Exp. 2, six 222-kg steers had pumps implanted and blood samples were taken as in Exp. 1. Three steers received each analogue at a rate of 300 pmol.h-1.kg-1. Analogues had similar GH-releasing ability and GH levels differed (P less than 0.001) among days, being approximately fourfold higher on d 3, 5 and 7 than on d -1, 1 and 9. Residual analogue solutions retained full bioactivity after 7-d implantation, and in vitro biopotencies of Ro23-7863 and 4SG-29 were similar (Exp. 3). In Exp. 4, 15 wethers (means = 31.3 kg) received osmotic pumps delivering 0, 3, 15, 75 and 300 pmol.h-1.kg-1 Ro23-7863 in DMSO:H2O for 7 d. Lambs were bled at 0800 and 1400 from d -1 to 8. The latter two doses increased (P less than .01) mean GH levels 2.7- and 4.3-fold over those in control animals during the treatment period. Results demonstrate that increased GH secretion can be elicited in steers and wethers for 1 wk by continuous s.c. infusion of GRF analogues utilizing osmotic pumps.

Pyrido[2,1-b]quinazolinecarboxamide derivatives as platelet activating factor antagonists

A series of N-[(heteroaryl)alkyl]pyrido[2,1-b]quinazolines were evaluated for their ability to inhibit the binding of radiolabeled platelet activating factor (PAF) to its receptor on dog platelets. The most potent compounds in this series were found to be pyrido[2,1-b]quinazoline-8-carboxamides possessing a four- or six-carbon chain between the carboxamide nitrogen atom and a 3-pyridinyl or 5-pyrimidinyl moiety. Since earlier metabolism studies with pyridoquinazolinecarboxamides suggest that the carboxamide moiety is labile to hydrolysis in vivo, attempts were made to find isosteric replacements for this group. The substitutions examined led to a loss of activity; however, insertion of a methyl group on the carbon atom alpha to the carboxamide nitrogen led to an enantioselective enhancement of potency. (R)-2-(1-Methylethyl)-N-[1-methyl-4-(3-pyridinyl)butyl]-11-oxo-11H- pyrido[2,1-b]quinazoline-8-carboxamide (34) was more potent than the corresponding S enantiomer in the PAF binding assay and was also shown to be more resistant to degradation by amidases present in whole liver homogenates obtained from guinea pig, dog, and squirrel monkey. The corresponding rac-2-(1-methylethyl)-N-[1-methyl-4-(3-pyridinyl)butyl]-11-oxo-11H- pyrido[2,1-b]quinazoline-8-carboxamide (33) was found to inhibit transient PAF-induced thrombocytopenia and decreases in blood pressure in guinea pigs after intravenous or oral administration and to have a duration of action of greater than 5 h after an oral dose of 200 mg/kg. Compound 33 thus represents the prototype of a new class of orally active PAF antagonists.

Isolation and characterization of growth hormone from blue fox pituitary glands

Growth hormone has been purified to homogeneity from blue fox pituitary glands. It has 191 amino acids with two disulfide bridges and a single tryptophan residue. The somatotropin activity is only 8% when compared with the bovine hormone in the receptor-binding assay. From radioimmunoassay data using baboon antisera to porcine or bovine growth hormone, the fox hormone has 14-17% immunoreactivity of bovine or porcine hormone.

Metabolites of an antipsychotic chiral pyrroloisoquinoline and their effect on prolactin secretion in the rat

Racemic-2, 6-dimethyl-3-ethyl- 4,4a,5,6,7,8,8a, 9-octahydro-4a,8a-trans-1H-pyrrolo[2, 3-g]isoquinoline-4-one hydrochloride (rac-l HCl) appeared to be equipotent to haloperidol in increasing serum prolactin levels in rats although only about one-twentieth as potent as haloperidol in reversing dopamine-inhibited prolactin release by rat anterior pituitary cells in vitro. The metabolism of 14C-labeled rac-l HCl was studied in rats and the activity of metabolites was evaluated in the in vitro prolactin release assay. Four metabolites, two C11-monohydroxy diastereomers and one C10-and one C12-monohydroxy metabolite, plus parent drug were isolated from the urine of rats administered [14C]rac-l HCl. These were shown to be optically active, indicating that the racemic drug was stereoselectively metabolized. All of the identified metabolites plus those in extracts of urine and feces proved inactive in the in vitro prolactin release assay. In addition, the brains of three rats removed 1 hr after single 3.6 mg/kg oral doses of [14C]rac-l HCl contained essentially only unchanged drug. We conclude that the potency of rac-l HCl in raising serum prolactin levels is not due to the formation of active metabolite(s) in vivo.