To determine whether newly described bombesin (BN) receptor antagonists distinguish subtypes of BN receptors, we investigated their abilities to interact with BN receptors on esophageal muscle or pancreatic acinar tissue. For inhibition of binding of 125I-[Tyr4]BN to rat pancreatic tissue, the relative potencies were [D-Phe6]BN-(6-13)ethyl ester (5 nM) greater than Ac-gastrin-releasing peptide (GRP)-(20-26)ethyl ester (17 nM) greater than [D-Phe6,Cpa,14, psi 13-14]BN-(6-14) (40 nM) greater than [Leu14, psi 13-14]BN (0.43 microM) greater than [Tyr4,D-Phe12]BN = [D-Pro4, D-Trp7,9,10]substance P (SP)-4-11 (13 microM) greater than [Leu14, psi 9,10]BN (32 microM) greater than [D-Arg1,D-Trp7,9,Leu11]SP (70 microM). Each antagonist also inhibited binding of 125I-[Tyr4]BN or 125I-Bolton-Hunter-neuromedin B to rat esophageal tissue, and the potency of each antagonist for each tracer was similar. In comparison to rat pancreas, [D-Phe6]BN-(6-13)ethyl ester, Ac-GRP-(20-26)ethyl ester, [D-Phe6,Cpa14, psi 13-14]BN-(6-14), [Leu14, psi 13-14]BN, and [Leu14, psi 9,10]BN had a 10,000-, 2,940-, 1,425-, 122-, and 4-fold, respectively, weaker affinity for BN receptors. In contrast [Tyr4,D-Phe12]BN, [D-Pro4,D-Trp7,9,10]SP-4-11, and [D-Arg1,D-Trp7,9,Leu11]SP had a 4-, 4-, and 9-fold, respectively, higher affinity compared with pancreatic tissue. Comparison of the activity of each peptide at inhibiting the ability of equipotent concentrations of BN or neuromedin B to stimulate contraction of rat esophageal muscle demonstrated that each peptide had the same relative potencies as for inhibiting binding.(ABSTRACT TRUNCATED AT 250 WORDS)

No abstract

Recent studies suggest that different subtypes of receptors may mediate the action of various bombesin-related peptides in different tissues. In the present study the ability of bombesin and its structurally related peptides [litorin, gastrin-releasing peptide (GRP), GRP18-27, neuromedin B, [Leu8]litorin, and bombesin nonapeptide BN(6-14)] to interact with smooth muscle cells isolated from guinea pig stomach was investigated. Each peptide induced a specific contractile response with potencies (D50 in pM) of [Leu8]litorin (0.7) greater than bombesin (1.2) greater than litorin (3) greater than neuromedin B (3.5) = GRP (3.8) = GRP18-27 (3.9) greater than BN(6-14) (70.9). The specific bombesin receptor antagonist psi 13,14-bombesin differed in its potency for inhibiting equipotent concentrations of bombesin, GRP, or neuromedin B, was equipotent for bombesin or GRP (IC50 12.7 and 22.1 nM), and was 11 times less potent for neuromedin B (IC50 234.5 nM), suggesting the presence of subtypes of receptors mediating the action of bombesin-related peptides. To further investigate this possibility, a technique of receptor protection that enables selective preservation of one receptor type was used. GRP or bombesin protected completely the response to GRP or bombesin but abolished the subsequent contractile response to neuromedin B. Neuromedin B, instead, protected only the response to neuromedin B. These results demonstrate that gastric smooth muscle cells possess specific receptors that interact with bombesin-related peptides and that two receptor subtypes mediate the contractile response to these peptides: one subtype is selective for bombesin or GRP, the other for neuromedin B.

Previous attempts to develop analogues of bombesin that function as specific receptor antagonists have been unsuccessful. Alteration of the histidine in luteinizing hormone releasing factor has resulted in analogues that function as competitive antagonists. In the present study we have used a similar strategy and altered the histidine in bombesin. [D-Phe12]bombesin, [D-Phe12,Leu14]bombesin, and [Tyr4,D-Phe12]bombesin did not stimulate amylase release from guinea pig pancreatic acini when present alone, but each analogue inhibited bombesin-stimulated secretion. For each analogue, detectable inhibition occurred at 1 microM and half-maximal inhibition at 4 microM. Each analogue inhibited amylase release by bombesin and other agonists that stimulate secretion by interacting with bombesin receptors. The analogues of bombesin did not alter stimulation by substance P or other agonists that interact with other receptors. The inhibition of the action of bombesin was competitive with Schild plots having slopes of 1.0. Each analogue also inhibited binding of 125I-labeled [Tyr4]bombesin but not 125I-labeled substance P. These results demonstrate that [D-Phe12] analogues of bombesin function as bombesin receptor antagonists and are the only bombesin receptor antagonists that interact only with the bombesin receptor. Because of their specificity, these analogues may prove useful for defining the role of bombesin in various physiological or pathological processes.

Injection of male rats with 40-200 ng of [D-Leu6, des-Gly-NH2(10)]LHRH ethylamide for 7 days caused a maximal 80% reduction of testicular LH/hCG receptor level with one injection per day being as efficient as 3 daily injections. A similar inhibitory effect was observed on testicular PRL receptors. Testis and seminal vesicle weight as well as plasma testosterone levels were also significantly reduced by this treatment. These data indicate that a LHRH agonist, when given at a relatively low dose, is capable of reducing testicular LH/hCG and PRL receptor levels as well as testicular function, the effect being probably mediated by increased endogenous gonadotropin secretion.

Intraventricular administration of the enkephalin analog, [D-Ala2]-metenkephalin, induces profound and long-lasting analgesia, as well as other opiate-like behavioral effects in the rat. This analgesia was highly dose dependent, of much greater magnitude, and about 30 times longer lasting than that produced by the naturally occurring peptide, methionine-enkephalin. The behavioral effects of the [D-Ala2] analog could be completely reversed by the opiate antagonist, naloxone, suggesting that these effects were mediated by opiate receptors. Systemic administration of naloxone alone resulted in a significant increase in pain sensitivity. These findings support the view that endogenous opiate systems may play an important role in modulating pain sensitivity.

A specific binding protein for human corticotrophin-releasing hormone (hCRH), which does not bind to the ovine hormone (oCRH), has recently been demonstrated in human plasma. No such binding protein has been found in sheep plasma. We have investigated the half-life of human and ovine CRH in man and in sheep. Peptides were measured directly in plasma with two-site immunoradiometric assays, as these assays are unaffected by the presence of inactivated peptide fragments. In man, the half-life of hCRH (30.5 +/- 3.3 min; mean +/- S.E.M.) was significantly (P less than 0.001) less than that of oCRH (42.8 +/- 6.4 min). In sheep, there was no significant difference between the half-life of hCRH (46.5 +/- 7.2 min) and that of oCRH (39.8 +/- 10.1 min); these half-lives were also significantly (P less than 0.001) longer than that of hCRH in man. One possible explanation for the shorter half-life of hCRH in man is that the clearance of hCRH is enhanced by CRH-binding protein, although other binding proteins often have the opposite effect. Peak ACTH and cortisol responses occurred earlier in sheep than in man, although no differences were found in the response times to oCRH or hCRH within either species. The responses were more sustained in sheep than in man, and the previously reported biphasic response was only seen in some of the sheep and not in man. Absolute responses to either peptide were greater in sheep than in man; however, in man an 8.1-fold rise in ACTH was measured in response to oCRH, while hCRH gave a significantly (P = 0.043) smaller 4.4-fold response.(ABSTRACT TRUNCATED AT 250 WORDS)

A synthetic 29-amino acid analogue of human pancreatic GH-releasing hormone (GHRH(1-29)NH2) has recently been shown to stimulate the release of GH in normal subjects. We have studied the GH response to GHRH(1-29)NH2 in nine children irradiated for brain and nasopharyngeal tumours, who were not growing and were deficient in GH as assessed by insulin-induced hypoglycaemia. Serum GH rose in response to GHRH(1-29)NH2 in all the children, and in five the peak serum GH response was greater than 20 mu./l. The data suggest that when hypothalamo-pituitary irradiation results in GH deficiency, this is due to a failure of the synthesis or delivery of endogenous GHRH from the hypothalamus to the pituitary cells. It also suggests that it may be possible to treat such children using synthetic GHRH in place of exogenous GH.