Modulation by galanin of growth hormone and gonadotropin secretion from perifused pituitary and median eminence of prepubertal male calves

Galanin is widely distributed in the peripheral and central nervous system and has been indicated as a putative hypothalamic-hypophysiotropic hormone. This study was performed to investigate the effects of galanin on both growth (GH) and luteinizing hormones (LH) from pituitaries of young male calves. Pituitary slices (P, 500 microm in thickness) were perfused alone or coincubated with median eminence terminals (ME) in DMEM-F12 plus BSA 0.1% and antibiotics. The perifusion chambers were kept in equilibrium for 150 min, and medium samples were collected every 10 min for 240 min and stored at -20 degrees C until the measurement of LH and GH levels. Basal GH release increased up to 60% after galanin infusion (p < 0.01 vs. baseline levels) for 60 min in P alone; in P + ME coincubation, galanin-stimulated GH secretion was further increased by up to 200%. Basal LH release in chambers with P was significantly increased (up to 25%; p < 0.05) for 70 min after galanin infusion; P + ME coincubation showed a galanin-mediated increase in LH release of up to 50%. GH and LH responsiveness to exogenous GH-releasing hormone and gonadotropin-releasing hormone was not significantly modulated by galanin in our experimental model. In conclusion, galanin is demonstrated to have a significant stimulatory role in the secretion of GH and LH, with a combined action at both the hypothalamic and pituitary levels. ......................

Adrenomedullin, amylin, calcitonin gene-related peptide and their fragments are potent inhibitors of gastric acid secretion in rats

Adrenomedullin, amylin and calcitonin gene-related peptides (CGRP) share close sequence homology and have overlapping spectra of biological activities, particularly with respect to cardiovascular and gastrointestinal functions. Comparisons of the effects of these three peptides on gastric acid release have been made by i.v. infusions in conscious rats equipped with gastric fistulae. All peptides were extremely potent inhibitors of basal, pentagastrin- and 2-deoxy-D-glucose-stimulated gastric acid secretion with IC50 values in the subnanomolar to nanomolar range. These effects were not inhibited by C-terminal extra-cyclic fragments of the peptides which often act as competitive receptor antagonists in other biological systems. At high concentrations C-terminal fragments of human adrenomedullin and rat alpha-calcitonin gene-related peptide displayed some receptor agonist activity. Furthermore, the N-terminally situated disulfide-bridged ring fragments, human adrenomedullin-(15-22), rat amylin-(1-8) and rat alpha-calcitonin gene-related peptide-(1-8), retained significant gastric acid inhibitory potencies thus suggesting involvement of receptor(s) with significantly differing ligand binding profiles than those characterized previously.

Beneficial hemodynamic and renal effects of adrenomedullin in an ovine model of heart failure

BACKGROUND

Adrenomedullin is a recently discovered endogenous peptide with hypotensive and natriuretic actions in normal animals. Circulating and ventricular adrenomedullin are elevated in congestive heart failure, suggesting a possible role in the pathophysiology of this disease. No studies have previously examined the effects of adrenomedullin in heart failure.

METHODS AND RESULTS

Eight sheep with pacing-induced heart failure received human adrenomedullin(1-52) at 10 and 100 ng x kg(-1) x min(-1) I.V. for 90 minutes each. Compared with vehicle control data, adrenomedullin increased plasma cAMP (high dose, P<.05) in association with dose-dependent falls in calculated peripheral resistance (13 mm Hg x L(-1) x min(-1), P<.001), mean arterial pressure (9 mm Hg, P<.001), and left atrial pressure (5 mm Hg, P<.001) and increases in cardiac output (0.5 L/min, P<.001). Adrenomedullin increased urine sodium (threefold, P<.05), creatinine (P<.05) and cAMP excretion (P<.01), creatinine clearance (P<.05), and renal production of cAMP (P<.05), whereas urine output was maintained during infusion and raised after infusion (P<.05). Adrenomedullin reduced plasma aldosterone levels (P<.05), whereas plasma atrial and brain natriuretic peptide concentrations were unchanged during infusion and rose after infusion (P<.01 and P<.05, respectively). Plasma catecholamine, cortisol, renin, calcium, and glucose concentrations were not significantly altered.

CONCLUSIONS

Adrenomedullin reduced ventricular preload and afterload and improved cardiac output in sheep with congestive heart failure. Despite the clear fall in arterial pressure, adrenomedullin increased creatinine clearance and sodium excretion and maintained urine output. These results imply an important pathophysiological role for adrenomedullin in the regulation of pressure and volume in heart failure and raise the possibility of a new therapeutic approach to this disease.

Bombesin-like peptides stimulate somatostatin release from rat fundic D cells in primary culture

In several species, bombesin-like neuropeptides stimulate somatostatin release in in vitro preparations of gastric mucosa. We sought to determine if this response is due to a direct effect on fundic D cells. Rat fundic mucosal cells were isolated by pronase E (1% D cells). D cells were separated by counterflow elutriation and subsequent density-gradient centrifugation (Nycodenz) (15% D cells) and grown in primary culture for 48 h (46% D cells). Cultured cells were double stained with affinity-purified rabbit-anti-gastrin-releasing peptide (GRP) receptor antibody and mouse monoclonal antibody to human somatostatin. After incubation with rhodamine-labeled anti-rabbit and fluorescein isothiocyanate-labeled anti-mouse antibodies, reactions were visualized by fluorescence microscopy. All cells positive for somatostatin had GRP receptors, whereas all non-D cells showed no expression in this G cell-free culture system. Somatostatin release from cultured cells was stimulated by sulfated cholecystokinin octapeptide (CCK-8; EC50 3 X 10(-10) M) and epinephrine (EC50 4 X 10(-8) M), which are established stimuli for canine fundic D cells. Bombesin (EC50 6 X 10(-11) M), its mammalian analog GRP-27, and neuromedin C (GRP-10) (EC50 1 X 10(-10) M, for both) were almost equally potent stimuli of somatostatin release, eliciting maximal response at 10(-9) M (400-550% above basal). Neuromedin B was less potent and effective (maximal response at 10(-8) M, 230% above basal). [D-Phe6]bombesin-(6-13)-OMe, a specific bombesin receptor antagonist, inhibited bombesin-stimulated somatostatin release in a competitive manner (IC50 9 X 10(-8) M). Potentiating interactions were observed between bombesin and dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP) or epinephrine, but not between bombesin and CCK-8. We conclude that bombesin-like peptides directly stimulate somatostatin release by interacting with specific receptors on rat fundic D cells. Bombesin-like peptides appear to induce Ca(2+)-phospholipid-dependent signal-response transduction, as is indirectly suggested by potentiating interactions with DBcAMP or epinephrine.

Comparison of responses to adrenomedullin and calcitonin gene-related peptide in the feline erection model

The purpose of the present study was to investigate the effects of intracavernosal injections of adrenomedullin (ADM) and calcitonin gene-related peptide (CGRP), two structurally similar peptides, on penile erection in the anesthetized cat. Erectile responses to ADM and CGRP were compared with responses to a standard drug combination (1.65 mg papaverine, 25 microg phentolamine, and 0.5 microg prostaglandin E1 [PGE1]). Intracavernosal injections of ADM (0.1-3 nmol) and CGRP (0.01-0.3 nmol) induced erection in a dose-dependent manner. The maximal increase in intracavernosal pressure in response to ADM was a 75% increase, while the maximal response to CGRP was comparable to that induced by the reference combination, and the maximal increase in penile length was comparable with ADM, CGRP, and the standard drug combination. The duration of the maximal pressure increase and the total duration of the response to ADM and CGRP were more abbreviated than with the control combination, and systemic blood pressure was reduced significantly after administration of CGRP, the control combination, and the higher doses of ADM. The nitric oxide synthase inhibitor, L-NAME, and the K+(ATP)-channel antagonist, glybenclamide, had no effect on the erectile response to CGRP or ADM. The CGRP receptor antagonist CGRP(8-37) attenuated the erectile response to CGRP but not to ADM. These data suggest that the erectile responses to ADM and CGRP are not mediated by nitric oxide release or the opening of K+(ATP) channels, two mechanisms reported to be involved in penile erection, and that CGRP and ADM induce penile erection by activating different receptors.

Localization and gene expression of the receptor for pituitary adenylate cyclase-activating polypeptide in the rat brain

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a recently identified member of the secretin/vasoactive intestinal polypeptide (VIP) family. There are at least two types of receptor for PACAP: type I (PACAPR), which specifically binds PACAP; and type II (VIP/PACAPR), which binds both PACAP and VIP. The localization of PACAPR in the rat brain was determined by in situ hybridization and immunocytochemistry. We raised antisera against a synthetic peptide that corresponds to the carboxy-terminal cytoplasmic domain which is found in all subtypes of PACAPR in order to localize PACAPR-like immunoreactivity (PACAPR-LI) in the rat brain. In general, the distribution of PACAPR-LI correlated well with the distribution of PACAPR transcripts. Particularly strong PACAPR mRNA expression was detected in the olfactory bulb, hippocampus, cerebellum and hypothalamus and moderate labeling was detected in other scattered regions. At the cellular level, PACAPR-LI appeared to be concentrated predominantly in neuronal perikarya and dendrites. At the ultrastructural level, strong immunostaining for the PACAPR was found in plasma membranes, rough endoplasmic reticulum, cytoplasmic matrix, and at synapses. This study provides the basis for a better understanding of the functions of PACAP in the rat brain.

Activation of human somatostatin receptor type 2 causes inhibition of cell growth in transfected HEK293 but not in transfected CHO cells

Somatostatin (SS) is known to have an antiproliferative effect on cell growth via somatostatin receptors (SSTR). The purpose of this study was to transfect cell lines with human SSTR2 and determine the subsequent effect on cell growth in response to SSTR agonist. Heterologous Chinese hamster ovary (CHO-K1) and human embryonic kidney 293 (HEK) cells were transfected with SSTR2 cDNA using lipofectin. Stable transformants were selected by G418 and confirmed by 125I-SS binding and RT-PCR. Binding studies were performed in the presence of 10(-6) to 10(-12) M SS-14, SS-28, SS analogue RC-160, SSTR2 agonist NC-9-74, and SSTR5 agonist DC-37-39. Cell growth was determined by counting cell numbers after 48 hr incubation in the presence of 10(-6) to 10(-12) M SSTR2 agonist NC-9-74. Binding of 125I-SS-14 to transfected CHO and transfected HEK293 cells showed that the cells had high affinity for SS-14, SS-28, NC-9-74, and RC-160 but low affinity for DC-37-39. Incubation with 10(-6) to 10(-12) M NC-9-74, showed that 1 nM to 1 microM NC-9-74 significantly inhibited transfected HEK293 cell growth but did not affect growth on transfected CHO cells (n = 4 for each dose, P < 0.01). The two cell lines transfected with the human SSTR2 showed similar high affinity for SS-14, SS-28, RC-160, and SSTR2 agonist but not SSTR5 agonist. The SSTR2 agonist NC-9-74 significantly inhibited transfected HEK293 cell growth but not CHO cells. These data suggest that activation of SSTR2 was more efficiently coupled to the signal transduction pathway of antiproliferation in the transfected HEK293 cells.

Lanthionine-somatostatin analogs: synthesis, characterization, biological activity, and enzymatic stability studies

A series of cyclic somatostatin analogs containing a lanthionine bridge have been subjected to studies of structure-activity relationships. A direct synthesis of the thioether bridged analog (1) of sandostatin (SMS 201,995) and several lanthionine hexa-, hepta-, and octapeptides was carried out by using the method of cyclization on an oxime resin (PCOR) followed by condensation reactions in solution. The structures of the target peptides were analyzed by liquid secondary ion mass spectrometry (LSIMS) and subjected to high-energy collision-induced dissociation (CID) studies after opening of the peptide ring by proteolytic cleavage. The biological activities of these compounds have been evaluated by assaying their inhibitory potencies for the release of growth hormone (GH) from primary cultures of rat anterior pituitary cells, as well as by their binding affinities to cloned somatostatin receptors (SSTR1-5). The structural modification of sandostatin by introducing a lanthionine bridge resulted in a significantly increased receptor binding selectivity. The lanthionine octapeptide with C-terminal Thr-ol (1) showed similar high affinity for rat SSTR5 compared to somatostatin[1-14] and sandostatin. However, it exhibits about 50 times weaker binding affinity for mSSTR2b than sandostatin. Similarly, the lanthionine octapeptide with the C-terminal Thr-NH2 residue (2) has higher affinity for rSSTR5 than for mSSTR2B. Both peptides (compounds 1 and 2) have much lower potencies for inhibition of growth hormone secretion than sandostatin. This is consistent with their affinities to SSTR2, the receptor which is believed to be linked to the inhibition of growth hormone release by somatostatin and its analogs. The metabolic stability of lanthionine-sandostatin and sandostatin have been studied in rat brain homogenates. Although both compounds have a high stability toward enzymatic degradation, the lanthionine analog has a 2.4 times longer half-life than sandostatin. The main metabolites of both compounds have been isolated and identified by using an in vivo technique (cerebral microdialysis) and mass spectrometry.

Comparison of responses to rat and human adrenomedullin in the hindlimb vascular bed of the cat

Responses to rat (r) adrenomedullin (ADM) and human (h) ADM were compared in the hindlimb vascular bed of the cat under conditions of controlled blood flow. Intra-arterial injections of rADM and hADM in doses of 0.03-1 nmol caused dose-related decreases in hindlimb perfusion pressure. In terms of relative vasodilator activity, rADM was similar to hADM. The time course of the vasodilator response and the recovery half times (T1/2) for the vasodilator response to rADM and hADM were not significantly different. Decreases in hindlimb perfusion pressure in response to rADM and hADM were not altered by the calcitonin gene-related peptide receptor antagonist, rCGRP(8-37), at the same time, vasodilator responses to calcitonin gene-related peptide (CGRP) were significantly reduced. The T1/2 of the vasodilator response to rADM and hADM were significantly greater after administration of the cAMP-selective, type IV phosphodiesterase inhibitor, rolipram. These data demonstrate that decreases in hindlimb perfusion pressure in response to rADM and hADM are similar and that vasodilator responses to rADM are not dependent on the activation of CGRP receptors in the hindlimb vascular bed of the cat. These data further suggest that decreases in hindlimb perfusion pressure in response to rADM are mediated by smooth muscle increases in cAMP levels.

Hemodynamic, hormonal, and renal effects of adrenomedullin in conscious sheep

Adrenomedullin is a recently discovered peptide that has been shown to reduce arterial pressure and induce natriuresis. However, few studies have examined the biological actions of adrenomedullin in conscious animals in an integrative manner. Accordingly, we have examined the hemodynamic, renal, and endocrine actions of adrenomedullin infused intravenously at 10 and 100 ng.kg-1.min-1 (each 90 min) in a vehicle-controlled study in eight normal conscious sheep. Adrenomedullin reduced right atrial pressure (P < 0.05) and diastolic (15 mmHg, P < 0.01) and mean arterial pressure (10 mmHg, P < 0.05) and increased cardiac output (3 l/min, P < 0.001). Total peripheral resistance was reduced 40% (P < 0.001). Urinary sodium was reduced to 35% of control during the 90-min clearance period immediately postinfusion (P < 0.05). Adrenomedullin increased plasma adenosine 3',5'-cyclic monophosphate levels (P < 0.001). Plasma renin activity was elevated during adrenomedullin (P < 0.001) coincident with the peak hypotensive effect, whereas plasma aldosterone was not affected and plasma norepinephrine levels fell (P < 0.05). In conclusion, adrenomedullin had clear blood pressure-lowering effects with increased cardiac output and stimulation of renin but suppressed sympathetic activation in conscious sheep. The physiological implications of these findings require further study.

Adrenomedullin: a hypotensive hormone in man

1. Adrenomedullin, a recently discovered 52-amino-acid peptide hormone, circulates in plasma at low picomolar levels in man. Animal studies and studies in vitro indicate that it has diverse biological actions, including vasodilatation, natriuresis and diuresis, and positive inotropism as well as anti-proliferative effects. We investigated the bioactivity of two doses of adrenomedullin in healthy human subjects. 2. Human adrenomedullin was given intravenously to eight male subjects at 2 and 8 ng min-1 kg-1, and haemodynamic, hormonal, renal and biochemical responses were recorded in a placebo (vehicle)-controlled, randomized study. 3. Compared with vehicle, adrenomedullin reduced mean arterial pressure (P = 0.05 for duration of infusion, mean difference at end of infusion 7.7 mmHg), systolic arterial pressure (P = 0.04 for duration of infusion, mean difference at end of infusion 10.7 mmHg) and at the lower dose reduced diastolic arterial pressure (P = 0.05 for lower dose, mean difference at end of infusion 6.3 mmHg) in the absence of compensatory responses in sympathetic activity or renin release. Urine volume and electrolyte excretion were unaffected. 4. The threshold for biological activity of adrenomedullin in man is lower, for arterial pressure than for renal or hormonal responses, and is evident at plasma concentrations seen in disorders of the circulation. Adrenomedullin may be an important hormone under pathophysiological circumstances.

GIP(6-30amide) contains the high affinity binding region of GIP and is a potent inhibitor of GIP1-42 action in vitro

GIP (Glucose-dependent Insulinotropic Polypeptide) is an important regulator of insulin secretion. The effects of truncated forms of the peptide, GIP(10-30), GIP(6-30amide) and GIP(7-30), on binding of 125I-GIP(1-42) to GIP receptors in transfected CHO-KI cells, and on cyclic AMP responses to GIP(1-42), have been studied with a view to defining further the receptor binding region of GIP, and to establish whether such truncated peptides exhibit agonist or antagonist activity. All three peptides were found to be receptor antagonists, however GIP(6-30amide) exhibited receptor binding affinity equivalent to that of GIP(1-42) in competitive binding studies (IC50 = 3.08+/-0.57 nM). GIP(6-30amide) inhibited GIP(1-42)-induced cAMP production by 58% at a concentration of 100 nM, whereas GIP(10-30) and GIP(7-30), inhibited only in the microM range. GIP(6-30amide) therefore contains the high affinity binding region of GIP and is a potent inhibitor of GIP(1-42) action in vitro.

Colonic smooth muscle cells possess a different subtype of somatostatin receptor from gastric smooth muscle cells

Somatostatin (SS) alters colonic motility. To investigate whether SS has a direct effect on colonic smooth muscle cells, we prepared isolated muscle cells from the descending guinea pig colon and compared the effects of SS with those on isolated gastric smooth muscle cells. In gastric cells, SS had no effect on carbachol-induced contraction, whereas in colonic cells it caused inhibition. In colonic muscle cells, SS-28 caused >85% inhibition of contraction by cholecystokinin octapeptide (CCK-8), bombesin, 12-O-tetradecanoylphorbol-13-acetate, and ionomycin, whereas it had no effect on contraction by these agents in gastric cells. In gastric cells, SS inhibited relaxation. Three synthetic SS analogs had different relative affinities for causing effects in gastric and colonic cells. Pertussis toxin inhibited the action of SS-28 in each muscle cell type by 50-75%. SS-28 alone had a small contractile effect on cells from the circular layer of the colon. SS-28 inhibited carbachol-induced contraction in colonic cells from both the longitudinal and circular layers. These results demonstrate that the action of SS differs in colonic and gastric smooth muscle cells. SS inhibits contractants in colonic cells and relaxants in gastric cells. In colonic cells, SS has a weak contractile effect due to an effect on circular muscle cells and an inhibitory effect on cells from both longitudinal and circular layers. A different SS receptor subtype mediates the actions of SS in colonic and gastric muscle cells. In both cell types, the actions of SS are mediated by pertussis toxin-sensitive and -insensitive G proteins.

Proadrenomedullin NH2-terminal 20 peptide has direct vasodilator activity in the cat

The mechanism by which proadrenomedullin NH2-terminal 20 peptide (PAMP) decreases vascular resistance was investigated in the hindlimb vascular bed in the cat. Injections of PAMP, a shortened form of the peptide PAMP-(12-20), and adrenomedullin (ADM) into the hindlimb perfusion circuit elicit dose-related decreases in perfusion pressure. The order of potency was ADM > PAMP > PAMP-(12-20), and the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP-(8-37) had no effect on vasodilator responses to PAMP or ADM. Vasodilator responses to PAMP were increased in duration by the adenosine 3',5'-cyclic monophosphate (cAMP) phosphodiesterase inhibitor Rolipram, whereas inhibitors of nitric oxide synthase and guanosine 3',5'-cyclic monophosphate phosphodiesterase had no effect. Vasodilator responses to PAMP were not altered by treatment with alpha-receptor or adrenergic nerve terminal blocking agents and were similar in innervated and denervated hindlimb preparations. Responses to PAMP were similar when vasoconstrictor tone was increased by stimulation of the sympathetic nerves or infusion of phenylephrine and were not altered by the passage of time. These data suggest that PAMP dilates the hindlimb vascular bed by a direct cAMP-dependent mechanism and that inhibition of norepinephrine release plays little if any role in mediating responses to the peptide in the regional vascular bed of the cat.

Role of vagal fibers and bombesin/gastrin-releasing peptide-neurons in distention-induced gastrin release in rats

In the rat the exact role of vagal fibers and the interaction between the extrinsic and intrinsic neural system in distention-induced gastrin release are still a matter of debate. Accordingly, the aim of the present study was to examine the contribution of afferent and efferent vagal fibers as well as intrinsic neurons on gastrin response to gastric distention. In anesthetized rats graded gastric distention by 5, 10 and 15 ml saline for 20 min caused a significant volume-dependent increase of plasma gastrin levels by 12+/-6 pg/ml (5 ml saline, n = 8, P =0.05), 26+/-7 pg/ml (10 ml saline, n = 10, P < 0.05) and 37+/-7 pg/ml (15 ml saline, n = 8, P < 0.01 ), respectively. To examine the role of the extrinsic vagal innervation, gastrin response to distention was studied in anesthetized rats after bilateral truncal vagotomy (n = 9) or selective afferent vagotomy following pretreatment with capsaicin (n = 6). Stimulation of gastrin release by 10 ml distention in sham-operated control rats was reversed to an inhibition after truncal vagotomy (26+/-7 vs. -11+/-4 pg/ml; P<0.05) and capsaicin-treatment (37+/-18 vs. -34+/-11 pg/ml; P<0.05). A contribution of cholinergic mechanisms to this vagovagal-mediated stimulation of distention-induced gastrin release was excluded, since atropine (100 microg/kg/h; n = 8) further augmented distention-stimulated gastrin release. Since bombesin/gastrin-releasing peptide (GRP)-neurons contribute to vagally stimulated gastrin secretion, we have examined gastrin response to distention in the presence of the specific bombesin-receptor antagonist D-Phe6-BN(6-13)OMe (400 microg/kg/h: n = 10). This bombesin-antagonist completely reduced distention-stimulated gastrin release in vivo. In contrast, distention of the isolated, extrinsically denervated stomach significantly decreased gastrin release by 13+/-5 pg/min (5 ml saline, n = 8, P < 0.05), 28+/-8 pg/min (10 ml saline, n = 11, P < 0.05) and 35+/-10 pg/min (15 ml saline, n = 8, P < 0.01), respectively, without changing the activity of bombesin/GRP-neurons. Distention-induced decrease of gastrin release was attenuated to 50 percent by atropine (10(-7) M: n = 10) or tetrodotoxin (TTX) (10(-6) M; n = 10), respectively. These data demonstrate, that in anesthetized rats distention-stimulated gastrin secretion depends on the activation of a vagovagal reflex and intrinsic bombesin/GRP-neurons. In contrast distention of the isolated rat stomach inhibits gastrin release in part via intrinsic cholinergic pathways and other as yet unknown mechanisms.

Somatostatin receptor subtype specificity in human fetal pituitary cultures. Differential role of SSTR2 and SSTR5 for growth hormone, thyroid-stimulating hormone, and prolactin regulation

Somatostatin (SRIF), a hypothalamic inhibitor of pituitary growth hormone (GH) and thyroid-stimulating hormone (TSH) secretion, binds to five distinct receptor (SSTR) subtypes. We therefore tested SSTR subtype-specific SRIF analogs in primary human fetal pituitary cultures (23-25-wk gestation) to elucidate their role in regulating human pituitary function. Using reverse transcription-PCR, mRNA expression of SSTR2 and SSTR5 were detected in fetal pituitary by 25 wk. SRIF analog affinities were determined by membrane radioligand binding in cells stably expressing the human SSTR forms. GH secretion was suppressed equally (40-60%, P < 0.005) by analogs preferential for either SSTR2 (IC50 for receptor binding affinity, 0.19-0.42 nM) or SSTR5 (IC50, 0.37 nM), and compounds with enhanced affinity for SSTR2 were more potent (EC50 for GH suppression, 0.05-0.09 nM) than Lanreotide (EC50, 2.30 nM) and SRIF (EC50, 0.19 nM). Similarly, analogs with high affinity for SSTR2 or SSTR5 decreased TSH secretion (30-40%, P < 0.005). However, prolactin was effectively inhibited only by compounds preferentially bound to SSTR2 (20-30%, P < 0.05). Luteinizing hormone was modestly decreased (15-20%) by SSTR2- or SSTR5-specific analogs. An SSTR5-specific analog also exclusively inhibited GH in acromegalic tumor cells. Thus, SRIF regulation of GH and TSH in primary human fetal pituitary cells is mediated by both SSTR2 and SSTR5, both of which are abundantly expressed by 25 wk. In contrast, suppression of prolactin is mediated mainly by SSTR2. These results indicate that SSTR5 is critical for physiologic regulation of GH and TSH. SRIF analogs with selective affinity for this receptor may therefore be more effective in the treatment of hormone-secreting pituitary adenomas.

Characterization of bombesin binding sites in the rat stomach

We characterized the bombesin receptor population in the rat stomach and determined the receptor subtype mediating the contractile effect of bombesin in the gastric fundus. Using in vitro receptor autoradiography, we evaluated the ability of the specific gastrin-releasing peptide-preferring receptor antagonist [D-F5,Phe6,D-Ala11]bombesin-(6-13) methyl ester to inhibit binding of 125I-[Tyr4]bombesin to the gastric fundus, corpus and antrum. Binding to these regions was completely inhibited by [D-F5,Phe6,D-Ala11]bombesin-(6-13) methyl ester suggesting that these receptors are the gastrin-releasing peptide-preferring subtype. We found that the rank order of potency for the contractile effect of bombesin, and the related mammalian peptides neuromedin C and neuromedin B, was bombesin > neuromedin C > neuromedin B. [D-F5,Phe6,D-Ala11]bombesin-(6-13) methyl ester was equipotent in antagonizing contractions produced by all three peptides. Furthermore, receptor tachyphylaxis to either neuromedin C or neuromedin B abolished the subsequent contractile response elicited by neuromedin C and neuromedin B, suggesting that one bombesin receptor subtype mediates rat gastric fundal contractions. Together, these results demonstrate that the bombesin receptor subtype in the rat stomach is gastrin-releasing peptide-preferring subtype and that this subtype is responsible for the effects of bombesin-like peptides on fundal smooth muscle contraction.

Adrenomedullin induces penile erection in the cat

The present study was undertaken to investigate the effects of intracavernosal injections of adrenomedullin, a novel hypotensive peptide, on penile erection in anesthetized cats. Responses to adrenomedullin were compared to those elicited by intracavernosal injection of the control triple-drug combination (1.65 mg papaverine, 25 micrograms phentolamine, and 0.5 microgram prostaglandin E1). Intracavernosal injections of adrenomedullin in doses of 0.1-1.0 nmol elicited dose-related increases in cavernosal pressure and penile length. The maximal effect of adrenomedullin injection on cavernosal pressure was an 8-fold increase in pressure, which was 74% of that induced by the triple-drug combination. The maximal effect on penile length was a 43% increase when compared to baseline value, which was comparable to that induced by the triple-drug combination. The duration of the peak pressure and total duration of the peptide effect were significantly shorter in response to the 1 nmol dose of adrenomedullin than was observed with the control triple-drug combination. Intracavernous injection of the control triple-drug combination resulted in a significantly greater decrease in systemic arterial blood pressure than did adrenomedullin. Erectile responses to adrenomedullin were not altered following administration of the nitric oxide synthase inhibitor. N omega-nitro-L-arginine, at a time when erectile responses to acetylcholine were significantly reduced. These data demonstrate that intracavernous injection of adrenomedullin induces a short-lived erection in cats that is not due to the release of nitric oxide.

Tone-dependent vasodilator responses to proadrenomedullin NH2-terminal 20 peptide in the hindquarters vascular bed of the rat

Responses to proadrenomedullin NH2-terminal 20 peptide (PAMP) were investigated in the systemic and hindquarters vascular bed of the rat. Intravenous injections of PAMP and adrenomedullin (ADM) produced dose-related decreases in systemic arterial and hindquarters perfusion pressure, which were not altered by alpha-receptor or adrenergic nerve terminal blocking agents. PAMP was 100-fold less potent than ADM, and hindquarters vasodilator responses to both peptides were similar in innervated and denervated preparations. When baseline tone was increased with phenylephrine and U46619 or decreased with sodium nitroprusside, vasodilator responses to PAMP and ADM were correlated with the basal level of tone, suggesting that responses to both peptides are dependent on the baseline level of vasoconstrictor tone in the hindquarters vascular bed of the rat.

Adrenomedullin (16-31) has pressor activity in the rat but not the cat

Responses to a newly synthesized human adrenomedullin (hADM) analog, hADM (16-31), were investigated in the rat and cat. Unlike the full-sequence peptide, which has potent hypotensive activity, hADM (16-31) had pressor activity in the rat but not in the cat. Injection of hADM (16-31) in doses of 10-300 nmol/kg i.v. induced dose-dependent increases in systemic arterial pressure in the rat, and the peptide was approximately 10-fold less potent than norepinephrine when doses are compared on a nanomole basis. In contrast, injection of hADM (16-31) in doses up to 1,000 nmol/kg i.v. had no significant effect on systemic arterial pressure in the cat. Increases in systemic arterial pressure in response to hADM (16-31) in the rat were significantly reduced after administration of phentolamine or reserpine. These data suggest that increases in systemic arterial pressure in response to hADM (16-31) are mediated by release of catecholamines and activation of alpha-adrenergic receptors in the rat. These data show that hADM (16-31) has significant pressor activity and that there are marked species differences in the response to hADM (16-31).

Caudal hindbrain neuromedin B-preferring receptors participate in the control of food intake

Recent studies have identified two subtypes of bombesin (BN) receptors in the rat central nervous system: gastrin releasing-peptide (GRP) preferring and neuromedin B (NMB) preferring. To investigate a role for the NMB-preferring receptor subtype in feeding suppression elicited by fourth ventricular (4V) BN administration, we evaluated the ability of a selective NMB-preferring receptor antagonist, BIM-23127, to block suppression of glucose intake produced by 4V BN (10 pmol). Our results showed that 4V administration of BIM-23127 dose dependently antagonized the suppression of glucose intake produced by 4V BN. In addition, 4V administration of BIM-23127 alone increased glucose intake above that observed in the baseline condition. These results support a role for the NMB-preferring BN receptor subtype in the suppression of intake produced by 4V BN administration and suggest that endogenously released NMB participates in ingestive control.

Proadrenomedullin NH2-terminal peptide (PAMP)(12-20) has vasodepressor activity in the rat and cat

Decreases in systemic arterial pressure in response to human proadrenomedullin NH2-terminal 20 peptide (hPAMP), a truncated analog, hPAMP(12-20), and human adrenomedullin (hADM) were compared in the rat and cat. The order of potency was hADM > hPAMP > hPAMP(12-20). hPAMP(12-20) was approximately 3-fold less potent than the full sequence peptide, hPAMP, and 10-fold less potent than the related peptide, hADM. The duration of the vasodepressor responses to hPAMP(12-20) and hPAMP were similar, and responses to both peptides were significantly shorter in duration than hADM. Vasodepressor responses to hPAMP(12-20), hPAMP, and hADM were greater in the rat when compared to responses to the peptides in the cat.

Adrenomedullin-(22-52) antagonizes vasodilator responses to CGRP but not adrenomedullin in the cat

The effects of adrenomedullin (ADM)-(22-52), a putative ADM receptor antagonist, on vasodilator responses to ADM and the structurally related peptide, calcitonin gene-related peptide (CGRP), were investigated in the hindlimb vascular bed of the cat under constant-flow conditions. ADM-(22-52) had no significant effect on hindlimb perfusion pressure when injected in doses up to 120 nmol; after administration of ADM-(22-52), vasodilator responses to ADM were unchanged, whereas vasodilator responses to CGRP were inhibited. The inhibitory effects of ADM-(22-52) on responses to CGRP were selective and reversible and were similar to the inhibitory effects of the CGRP antagonist CGRP-(8-37). Hindlimb vasodilator responses to CGRP and to ADM were increased in duration by the adenosine 3',5'-cyclic monophosphate (cAMP) phosphodiesterase inhibitor rolipram but were not altered by inhibitors of guanosine 3',5'-cyclic monophosphate phosphodiesterase, nitric oxide synthetase, K(+)-ATP channels, the cyclooxygenase pathway, or the adrenergic nervous system. These results demonstrate that ADM-(22-52) is a selective CGRP receptor antagonist in the hindlimb vascular bed of the cat. The present data suggest that vasodilator responses to CGRP and ADM are mediated by different receptors but that these peptides dilate the hindlimb vascular bed of the cat by a similar cAMP-dependent mechanism.

Analysis of cardiovascular responses to PACAP-27, PACAP-38, and vasoactive intestinal polypeptide

Responses to pituitary adenylate cyclase polypeptide (PACAP)-27, PACAP-38, and vasoactive intestinal peptide (VIP) were compared in the peripheral and pulmonary vascular beds of the cat and in the isolated perfused neonatal pig heart. Intravenous injections of PACAP-27 and PACAP-38 produced biphasic changes in systemic arterial pressure whereas iv injections of VIP caused only decreases in arterial pressure. When blood flow to the hind limb and mesenteric vascular beds was maintained constant, PACAP-27 and PACAP-38 caused dose-related biphasic changes in perfusion pressure, whereas VIP only decreased perfusion pressure. PACAP-27 was approximately threefold more potent than PACAP-38, and the pressor component of the biphasic response was blocked by alpha-adrenergic antagonists and adrenalectomy. PACAP-27, PACAP-38, and VIP produced decreases in pulmonary vascular resistance, and all three peptides had significant vasodilator activity in the isolated perfused neonatal pig heart. Although all three peptides decreased coronary vascular resistance, only PACAP-27 and PACAP-38 increased left ventricular contractility, with PACAP-27 approaching isoproterenol in potency. The results of these experiments show that PACAP-27, PACAP-38, and VIP have significant effects on vasomotor tone that depend on the vascular bed studied and the contribution of adrenal catecholamines.

Catecholamine release mediates pressor effects of adrenomedullin-(15-22) in the rat

Human adrenomedullin, a novel hypotensive peptide, contains a six-member ring structure similar to that found in calcitonin gene-related peptide and pancreatic amylin. Unlike the full-sequence peptide, human adrenomedullin-(15-22) [hADM-(15-22)], which contains the ring structure, increases systemic arterial pressure in the rat but not the cat. We undertook the present study to investigate the mechanism by which hADM-(15-22) increases systemic arterial pressure in the rat. Injection of hADM-(15-22) in doses of 10 to 300 nmol/kg i.v. increased systemic arterial pressure in a dose-dependent manner and was threefold less potent than norepinephrine when doses were compared on a nanomole basis. However, the ring structures of human calcitonin gene-related peptide and human amylin, human calcitonin gene-related peptide-(1-8) and human amylin-(1-8), respectively, had no significant effect on systemic arterial pressure in the rat. Pressor responses to hADM-(15-22) were reduced significantly after administration of phentolamine or reserpine. Responses to hADM-(15-22) were not altered by the angiotensin type 1 blocking agent DuP 753 or the endothelin-A/endothelin-B receptor blocking agent bosentan, and responses to hADM-(15-22) and the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) were reduced after bilateral adrenalectomy. Pressor responses to DMPP were reduced by hexamethonium, whereas the nicotinic blocking agent had no effect on the pressor response to hADM-(15-22). These data suggest that increases in systemic arterial pressure in response to hADM-(15-22) in the rat are mediated by the activation of alpha-adrenergic receptors by catecholamines released from the adrenal medulla. The present data suggest that hADM-(15-22) releases catecholamines from the adrenal medulla by a noncholinergic mechanism.

Role of endogenous bombesin-peptides during vagal stimulation of gastric acid secretion in the rat

The stimulatory effect of exogenous bombesin and its related mammalian peptides on gastric acid secretion and gastrin release has been examined in detail, while the regulatory role of endogenously released bombesin-like peptides is largely unknown. Accordingly we have determined the effect of a specific bombesin receptor antagonist during vagal stimulation of gastric acid secretion and gastrin release. In anesthetized rats electrical stimulation of the vagal nerves (10 V, 10 Hz, 1 ms) significantly increased plasma gastrin levels by 82 +/- 11 pg/20 min (P < 0.01) and gastric acid output by 99.4 +/- 9.9 mueq/20 min (P < 0.01). Intravenous infusion of the specific bombesin receptor antagonist D-Phe6-BN(6-13)OMe (400 nmol/kg/h) significantly reduced vagally induced increase of plasma gastrin levels by 70% to 29 +/- 8 pg/20 min (P < 0.05 vs control) and vagally stimulated gastric acid output by 40% to 57.4 +/- 10.6 mueq/20 min (P < 0.05 vs control). To demonstrate that the residual gastrin and acid response is due to non-bombesinergic mechanisms and not to an inadequate dose of the receptor antagonist, the latter was tested against gastrin-releasing peptide (GRP) at the maximally effective concentration of 300 pmol/kg/h, which resulted in an even 50% higher increase of plasma gastrin levels compared to vagal stimulation. The dose of the antagonist employed (400 nmol/kg/h) was sufficient to abolish GRP-induced stimulation of gastrin and gastric acid secretion. Previously it has been postulated that endogenous bombesin-peptides can stimulate acid secretion via gastrin-independent mechanisms. To investigate this possibility further the effect of the antagonist was examined on vagally induced acid secretion while gastrin levels were restored to the range of the respective control experiments. In presence of the antagonist the infusion of gastrin-17 (15 pmol/kg/h) in addition to vagal stimulation elevated plasma gastrin to levels not different from those during vagal stimulation alone. With identical plasma gastrin levels the bombesin receptor antagonist had no effect on vagally stimulated acid secretion (86.3 +/- 10.7 mueq/20 min vs 99.4 +/- 9.9 mueq/20 min in the controls; n.s.). In conclusion, the present data demonstrate for the first time that in rats in vivo endogenous bombesin peptides contribute to vagal stimulation of gastrin release and gastric acid secretion. Furthermore, endogenous bombesin-peptides exert their action on parietal cell function via an increase of gastrin release, while non-gastrinergic mechanisms are unimportant under the experimental conditions employed.

Somatostatin receptor subtype 2 mediates inhibition of gastrin and histamine secretion from human, dog, and rat antrum

BACKGROUND & AIMS

In gastric antrum, somatostatin exerts a tonic inhibitory influence on gastrin and histamine secretion. Five different subtypes of somatostatin receptors, designated sst1-5, have been identified. sst2, sst3, and sst5 subtypes have been localized to the stomach by molecular biological techniques. The aim of this study was to identify the sst subtype regulating gastrin and histamine secretion in human, dog, and rat stomach.

METHODS

Mucosal segments from human, dog, and rat antrum were superfused with various concentrations of somatostatin 14 and somatostatin analogues selective for sst2, sst3, and sst5. Gastrin and histamine were measured by radioimmunoassay.

RESULTS

Somatostatin 14 and the sst2 agonist EC 5-20 inhibited gastrin and histamine secretion from all three species in a concentration-dependent manner, whereas the sst3 and sst5 agonists had no significant effect. Neutralization of endogenous somatostatin with the somatostatin antibody increased gastrin and histamine secretion. The increases were not affected by the gastrin antagonist but were abolished by the sst2 agonist, implying that the inhibitory influence of ambient somatostatin is exerted directly on the histamine cell rather than indirectly via changes in gastrin secretion.

CONCLUSIONS

Somatostatin inhibits gastrin and histamine secretion in human, dog, and rat antrum by activating sst2 receptors on gastrin and histamine cells.

Somatostatin receptor-mediated signaling in smooth muscle. Activation of phospholipase C-beta3 by Gbetagamma and inhibition of adenylyl cyclase by Galphai1 and Galphao

In COS-7 cells, all five cloned somatostatin receptors are coupled via inhibitory G proteins to activation of an unidentified phospholipase C-beta (PLC-beta) isozyme and inhibition of adenylyl cyclase. In the present study, intestinal smooth muscle cells (SMC) that express only one receptor type, sstr3, and possess a full complement of G proteins and PLC-beta isozymes were used to identify the PLC-beta isozyme and the G proteins coupled to it and to adenylyl cyclase. Somatostatin-14 bound with high affinity to intestinal SMC; stimulated D-myo-inositol-1,4,5-trisphosphate (IP3) formation, Ca2+ release, and contraction; and inhibited forskolin-stimulated cAMP formation in a pertussis toxin-sensitive fashion. Somatostatin also stimulated phosphoinositide hydrolysis in plasma membranes. Only those somatostatin analogs that shared a high affinity for sstr3 receptors elicited muscle contraction. IP3 formation, Ca2+ release, and contraction in permeabilized SMC and phosphoinositide hydrolysis in plasma membranes were inhibited (approximately 80%) by pretreatment with antibodies to PLC-beta3 but not other PLC-beta isozymes, and by antibodies to Gbeta but not Galpha. Inhibition of cAMP formation was partially blocked by antibody to Galphai1 or Galphao and additively blocked by a combination of both antibodies. Somatostatin-stimulated [35S]GTPgammaS-Galpha complexes in plasma membranes were bound selectively by Galphai1 and Galphao antibodies. We conclude that in smooth muscle sstr3 is coupled to Gi1 and Go; the alpha subunits of both G proteins mediate inhibition of adenylyl cyclase, while the betagamma subunits mediate activation of PLC-beta3.

Pituitary adenylate cyclase-activating polypeptide stimulates cholecystokinin secretion in STC-1 cells

Secretion of cholecystokinin (CCK) from the endocrine cells of small intestinal mucosa and the murine intestinal tumor cell line STC-1 is known to involve both adenosine 3',5'-cyclic monophosphate (cAMP)-and Ca(2+)-dependent signal transduction pathways. However, the endogenous stimulant(s) that acts through the cAMP-dependent cascade has not been identified. We determined the effect of pituitary adenylate cyclase-activating polypeptide (PACAP) on CCK secretion and cAMP production and its interaction with other CCK secretagogues in STC-1 cells. At concentrations > 10 nM, PACAP-27 stimulated the release of large intestinal CCK from STC-1 cells in a time- and dose-dependent manner. The stimulatory effect of PACAP-27 was enhanced by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). PACAP-27, PACAP-38, and vasoactive intestinal polypeptide (VIP), with or without IBMX, were equally effective and potent to elicit CCK release with similar half-maximal doses and maximal levels of stimulation. Both forms of PACAP and VIP stimulated a transient but not significant increase in the cellular cAMP level. In the presence of IBMX, all three peptides increased significantly the cellular cAMP level between 2 and 5 min, but PACAP produced a two times higher level than VIP. The stimulatory effect of PACAP-27 on CCK release was also potentiated by bombesin and KCl but without a synergistic production of cAMP. With or without IBMX, PACAP-27-stimulated CCK secretion was not affected by the Ca2+ channel blocker diltiazem (1 microM), the cell-permeable Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM; 25 microM), or by downregulation of protein kinase C. The stimulatory effects of KCl and bombesin were either reduced or abolished by these treatments. The synergistic effect of bombesin with PACAP was abolished by diltiazem and BAPTA-AM but not by downregulation of protein kinase C, whereas KCl remained synergistic with PACAP after these treatments. Taken together, these results indicate that PACAP may be a neuromodulator of CCK secretion that acts through activation of adenylate cyclase and may function as a coregulator with other CCK secretagogues that are known to increase intracellular Ca2+ concentration.

Somatostatin receptor subtype 2 mediates somatostatin inhibition of ion secretion in rat distal colon

BACKGROUND & AIMS

Somatostatin peptides are potent inhibitors of intestinal ion secretion, providing the rationale for their use in treating secretory diarrhea. However, the nature of the receptors that mediate these effects is unclear. The aims of this study were to investigate expression of somatostatin receptor subtypes (SSTRs) 1-5 in rat colonic epithelium and to identify which subtype(s) mediate inhibition of adenosine 3', 5'-cyclic monophosphate (cAMP)-activated secretion using SSTR-selective analogues.

METHODS

SSTR expression was determined by reverse-transcription polymerase chain reaction and immunoblotting. Effects of somatostatin analogues on electrogenic ion secretion were studied in isolated colonic mucosa mounted in Ussing chambers.

RESULTS

Crypt epithelium expressed messenger RNA for SSTR1 and SSTR2 and low levels of SSTR5. A splice variant of SSTR2 (SSTR2B) was also detected. The SSTR2 selective analogue NC-812 was a potent inhibitor of forskolin-activated secretion and cAMP accumulation. In contrast, peptides selective for SSTR3 (DC-25/12) and SSTR5 (DC-23/99) were weak inhibitors of secretion. NC-812 also inhibited dibutyryl cAMP-activated secretion, indicating a site of action distal to cAMP production. Immunoblot analysis confirmed expression of a 93-kilodalton SSTR2 protein in crypt cell membranes.

CONCLUSIONS

SSTR2 receptors expressed by colonocytes mediate somatostatin's antisecretory actions in rat colon. Somatostatin analogues directed to specific SSTRs may provide the basis for more selective antidiarrheal drugs.

Receptor-specific somatostatin analogs: correlations with biological activity

A number of cyclic and linear somatostatin (SRIF) analogs have now been found to have promising levels of selectivity for rodent somatostatin receptors (rsst2,3,5), but not sst1 and sst4. Comparisons between binding affinities for these and transfected human receptors are just beginning to emerge and we present results from a comparison of affinities of several key families of peptides for sst2 present on rat AR42J cells and on cells transfected with human (h)sst2. The typical cyclic octapeptide analogs, octreotide, lanreotide, and RC-160, exhibited similar affinities to SRIF for rsst2, but somewhat lower affinities for the human receptor. Affinities of several analogs for transfected hsst5 were also measured. As with the rat receptor, octreotide-related analogs had low affinity for hsst5. The highly specific rsst5 analog, DC-23-99, was less so for the human receptor; however, a D-Tyr1 version of DC-23-99 had subnanomolar affinity (Ki, 0.68 nmol/L) and high selectivity. A new extended-ring analog, BIM-23268D, showed superior affinity to DC-23-99 and even to SRIF and SRIF-28 for hsst5 (K(i), 0.38 nmol/L), and had the highest sst5/sst2 selectivity ratio of any analog that we have tested thus far.

Hindbrain GRP receptor blockade antagonizes feeding suppression by peripherally administered GRP

Bombesin (BN)-like peptides injected peripherally or centrally suppress food intake in rats. The relationship between the central and peripheral actions of BN is unknown. However, experimental evidence supports a critical role for the caudal hindbrain in mediating the feeding effects of BN. To investigate this relationship further, we examined the ability of fourth ventricular infusion of a specific gastrin-releasing peptide (GRP) antagonist, [D-F5, Phe6, D-Ala11]BN-(6-13) methyl ester (BN-ME), to block suppression of glucose intake (0.5 kcal/ml) produced by intraperitoneal administration of GRP-(18-27) in 5-h food-deprived male Sprague-Dawley rats (n = 10). We found that fourth ventricular administration of 10, 32, and 100 ng BN-ME blocked the suppression of glucose intake produced by peripherally administered 10 nmol/kg GRP-(18-27). The most effective dose of BN-ME (100 ng) blocked the ability of peripheral injection of GRP-(18-27) to inhibit glucose intake but had no effect on intake when given alone. These results demonstrate that the availability of caudal hindbrain GRP receptors is necessary for peripherally administered GRP-(18-27) to reduce food intake in rats.

[MPR14]-rADM(14-50), a novel analog of adrenomedullin, possesses potent vasodilator activity in the hindlimb vascular bed of the cat

Responses to [Mpr14]-ADM(14-50), a novel analog of adrenomedullin, were investigated in the hindlimb vascular bed of the cat under conditions of controlled blood flow. Intraarterial injections of [Mpr14]-rADM(14-50) in doses of 0.003-1 nmol caused dose-related decreases in hindlimb perfusion pressure. In terms of relative vasodilator activity, [Mpr14]-rADM(14-50) was more potent than human synthetic adrenomedullin (hADM) in doses of 0.003-0.1 nmol. The recovery half-times (T 1/2) for the vasodilator response to [Mpr14]-rADM(14-50) were significantly greater than the recovery half-times for hADM in all doses studied. Decreases in hindlimb perfusion pressure in response to [Mpr14]-rADM(14-50) were not altered by the calcitonin gene-related peptide receptor antagonist rCGRP(8-37) at the same time vasodilator responses to calcitonin gene-related peptide were significantly reduced. The present data demonstrate that [Mpr14]-(14-50) has potent and long-lasting vasodilator activity when compared to hADM, and that vasodilator responses to [Mpr14]-rADM(14-50) are not dependent on the activation of CGRP receptors in the hindlimb vascular bed of the cat.

Pulmonary vasodilator responses to adrenomedullin are reduced by NOS inhibitors in rats but not in cats

Responses to and the mechanism of action of adrenomedullin (ADM), the carboxy-terminal fragments of ADM, and calcitonin gene-related peptide (CGRP), a structurally related peptide, were investigated in the pulmonary vascular bed of the rat. Under conditions of elevated tone and controlled pulmonary blood flow in the isolated blood-perfused rat lung, injections of ADM, the 15-52 amino acid carboxy-terminal ADM analogue (ADM15-52), and CGRP caused dose-related decreases in pulmonary arterial perfusion pressure. In contrast, the carboxy-terminal 22-52 and 40-52 amino acid fragments had no consistent vasodilator activity. After administration of the nitric oxide synthase inhibitors, N omega-nitro-L-arginine benzyl ester or N omega-nitro-L-arginine methyl ester (L-NAME), pulmonary vasodilator responses to ADM, to ADM15-52, to CGRP, to acetylcholine, and to bradykinin were significantly decreased in the rat, whereas vasodilator responses to isoproterenol and nitroglycerin were not changed. However, in the pulmonary vascular bed of the cat, L-NAME had no significant effect on vasodilator responses to ADM in doses that attenuated vasodilator responses to acetylcholine and bradykinin. L-NAME had no effect on responses to isoproterenol or nitric oxide. When the relative vasodilator activity of the active peptides was compared, ADM15-52 was approximately three-fold less potent than ADM, and ADM was threefold less potent than CGRP in decreasing pulmonary vascular resistance in the rat lung. When vasodilator responses were compared in the rat and cat, ADM was threefold more potent in decreasing pulmonary vascular vascular resistance in the cat than in the rat, and vasodilator responses to ADM were independent of the intervention used to raise tone in the rat. The present data demonstrate that ADM and ADM15-52 have significant vasodilator activity in the pulmonary vascular bed of the rat, and that responses to ADM, ADM15-52, and CGRP are dependent on the release of nitric oxide in the rat. The present results indicate that pulmonary vasodilator responses to ADM are not dependent on the release of nitric oxide in the cat and suggest that responses to the peptide are mediated by different mechanisms in the pulmonary vascular bed of the rat and cat.

Intracerebroventricular injection of somatostatin sst5 receptor agonist inhibits gastric acid secretion in rats

Somatostatin and its analogs act in the brain to influence gastric acid secretion. Five different somatostatin receptor subtypes have been characterized (sst1 to sst5). We studied the influence of somatostatin (0.18-0.6 nmol/rat) and selective sst2, sst3 and sst5 receptor ligands on basal gastric acid secretion in conscious rats equipped with chronic gastric and intracerebroventricular (i.c.v.) cannulae. Somatostatin-14 (0.36 nmol/rat), the sst2, sst3 and sst5 receptor agonist, Des-AA1,2,4,5,12,13-[D-Tryp8,D-Cys14]somatostatin (SMS 201-995) (0.18-0.36 nmol/rat) and the sst5 receptor agonist, BIM-23052, (0.8-1.2 nmol/rat) injected i.c.v. inhibited gastric acid secretion. Maximal inhibition reaching 42%, 60% and 42% was induced by somatostatin-14 (0.36 nmol/rat), SMS 201-995 (0.18 nmol/rat) and BIM-23052 (0.8 nmol/rat) respectively. The sst2 receptor agonist, DC 32-87 (0.2-0.8 nmol/rat) and sst3 receptor agonist, BIM-23056 (0.2-1.2 nmol/rat), did not modify gastric acid secretion, except the sst3 receptor agonist at 0.4 nmol/rat which increased acid output at 20 min post-injection. The sst2 receptor agonists (0.4 nmol/rat) co-injected i.c.v. with a subthreshold dose of sst5 (0.4 nmol/rat) inhibited gastric acid secretion. These results show that i.c.v. injection of somatostatin-14 inhibits basal gastric acid secretion in conscious rats through an action on sst5 receptor subtype which can be potentiated by sst2 receptor subtype.

Proadrenomedullin NH2-terminal 20 peptide has cAMP-mediated vasodilator activity in the mesenteric vascular bed of the cat

Responses to proadrenomedullin NH2-terminal 20 peptide (hPAMP), a truncated analogue [hPAMP(12-20)], and adrenomedullin (hADM) were investigated in the mesenteric vascular bed of the cat. Under constant-flow conditions, injections of hPAMP, hPAMP(12-20), and hADM caused dose-related decreases in mesenteric perfusion pressure. hADM was 100-fold more potent than hPAMP, and 1000-fold more potent than hPAMP(12-20). Vasodilator responses to hPAMP and hADM were not altered by adrenergic-blocking agents, were similar in innervated and denervated preparations, and were similar when tone was increased by sympathetic nerve stimulation or phenylephrine infusion. Vasodilator responses to hPAMP and hADM were increased in duration by rolipram, a cAMP phosphodiesterase inhibitor. The present data suggest that vasodilator responses to the hPAMP and hADM are mediated by an increase in cAMP and that an interaction with the adrenergic nervous system is not involved.

Renal responses to intra-arterial administration of adrenomedullin in dogs

The present study was undertaken to evaluate the renal responses to intra-arterial infusion (12.5 pmol.kg-1.min-1) for 30 min of adrenomedullin (ADM) in denervated kidneys of anesthetized dogs (n = 8). Total renal blood flow (RBF) was measured with an electromagnetic flow probe, and two single-fiber laser-Doppler needle flow probes were used to measure relative changes in cortical (CBF) and medullary blood flow (MBF). ADM administration resulted in decreases in renal vascular resistance (32.3 +/- 4.9 to 22.3 +/- 3.0 mmHg.ml-1.min.g) and increases in RBF (4.5 +/- 0.5 to 6.0 +/- 0.6 ml.min-1.g-1), CBF (33 +/- 1.3%, n = 41, and MBF (32 +/- 5.7%, n = 5) without changes in glomerular filtration rate. There were also substantial increases in urine flow (11.3 +/- 1.9 to 28 +/- 4.8 microliters.min-1.g-1), as well as in sodium excretion (1.8 +/- 0.2 to 4.2 +/- 0.6 microliter.min-1.g-1). After cessation of ADM infusion, partial recovery of these responses was observed within a 30-min period. Responses to ADM were attenuated during inhibition of nitric oxide (NO) synthesis by nitro-L-arginine (50 micrograms.kg-1.min-1). These findings indicate that renal vasodilator, diuretic, and natriuretic responses to ADM may be mediated by the release of endogenous NO and suggest that ADM could play a role in regulating water and electrolyte excretion by the kidney.

Comparison of the peptide structural requirements for high affinity interaction with bombesin receptors

Recently it has been established that both a gastrin-releasing peptide (GRP)-preferring bombesin receptor and a neuromedin B-preferring bombesin receptor mediate the mammalian actions of bombesin-related peptides. Because many tissues used for studies of the structure-activity relationship of these peptides possess both receptor subtypes and none possess only the neuromedin B-preferring subtype, there is minimal information on the peptide structural features determining receptor selectivity and it is unknown whether the determinants of agonism at both bombesin receptor subtypes are similar. In the present study we have used native cells either possessing only one bombesin receptor subtype or stably transfected with one subtype to study in detail the peptide structural requirements for interacting and activating each receptor subtype. For the naturally occurring agonists, at the GRP-preferring bombesin receptor the relative affinities were litorin = ranatensin = bombesin > GRP >> neuromedin B, phyllolitorin and at the neuromedin B-preferring bombesin receptor were litorin = neuromedin B = ranatensin > bombesin, phyllolitorin >> GRP. For the GRP-preferring bombesin receptor the heptapeptide and for the neuromedin B-preferring bombesin receptor the octapeptide was the minimal carboxyl fragment interacting with the receptor/or causing biologic activity, and the nonapeptide and full decapeptide, respectively, were the minimal required for full affinity. Making neuromedin B more bombesin- or GRP-like by replacing amino acids in position 3, 6, and 9 demonstrated that position 3 was the most important, followed by position 9 for receptor subtype selectivity. A conformationally restricted GRP analogue, [D-Cys6,D-Ala11,Cys14]bombesin-(6-14) had a significantly higher affinity for GRP-preferring bombesin receptor than NMB receptor. These results demonstrate that: (1) the structure-function relations for the two mammalian bombesin receptors have important differences; (2) suggest that the active conformation of neuromedin B must differ markedly from the beta-sheet model proposed for GRP; and (3) suggest that one important function of the NH2 terminus of GRP and neuromedin B is determining receptor subtype selectivity.

Distinct receptors mediate gastrin-releasing peptide and neuromedin beta-induced delay of gastric of liquids in rats

The present study was carried out to define which bombesin receptors are involved in the delay of gastric emptying induced by bombesin-like peptides. Adult male rats were fitted with gastric and jugular vein cannulas. Gastric emptying was determined 5 min after a 3-ml intragastric load of 0.9 M NaCl using phenol red as a marker. Mammalian bombesin-like peptides gastrin-releasing peptide-10 and neuromedin B both induced a delay of gastric emptying. When [Phe6]bombesin-(6-13)-methyl ester, a selective antagonist of the gastrin-releasing peptide-preferring subtype of bombesin receptors, was injected 5 min before the agonists, the effect of gastrin-releasing peptide-10 was competitively inhibited, whereas that of neuromedin B remained unaffected. Our results indicate that gastrin-releasing peptide-10 and neuromedin B delay gastric emptying by acting on distinct receptors in rats, in vivo.

Galanin inhibits cholecystokinin secretion in STC-1 cells

Neuropeptides such as gastrin releasing peptide and pituitary adenylate cyclase activating polypeptide (PACAP) stimulate CCK secretion from CCK producing cells. We hypothesized that in addition to somatostatin, galanin may also play an inhibitory role on CCK secretion. The effect of galanin on CCK secretion was studied in a CCK-producing murine neuroendocrine tumor cell line, STC-1. Galanin below 10 nM did not affect basal CCK secretion but dose- and time-dependently inhibited KCl-stimulated CCK secretion. Galanin also inhibited forskolin-, bombesin- and PACAP- but not dibutyryl cAMP- or beta-TPA-stimulated CCK secretion. The inhibitory effect of galanin was reduced partially by a blocker of ATP-sensitive K+ channel (K+ ATP), glibenclamide, and prevented by pretreatment of the cells with PTX. The results indicated galanin regulates CCK secretion by modulation of K+ ATP and cAMP production through receptors coupled to a PTX-sensitive G protein.

Pituitary adenylate cyclase activating peptide receptors regulate the growth of non-small cell lung cancer cells

We have identified pituitary adenylate cyclase activating peptide (PACAP) receptors on small cell lung cancer cell line NCI-N417 in a previous study. In this study, the role of PACAP in the growth and signal transduction of non-small cell lung cancer cells was investigated. Northern blot analysis with a full-length human PACAP receptor cDNA probe revealed a major 7.5-kb hybridizing transcript when total RNA extracted from NCI-H838 cells was used. PACAP bound with high affinity (Kd = 1 nM) to a single class of sites (Bmax = 14,000/cell) when NCI-H838 cells were used. Specific 125I-labeled PACAP binding was inhibited with high affinity by PACAP-27 and PACAP-38, with moderate affinity by PACAP(6-38), and with low affinity by vasoactive intestinal polypeptide, PACAP(28-38), and PACAP(16-38). PACAP-27 elevated cAMP in a dose-dependent manner, and the increase in cAMP caused by PACAP was reversed by PACAP(6-38). PACAP-27, but not vasoactive intestinal polypeptide, elevated cytosolic Ca2+ in individual NCI-H838 cells. PACAP-27 stimulated arachidonic acid release, and the increase caused by PACAP was reversed by PACAP(6-38). PACAP-27 stimulated colony formation in NCI-H838 cells, whereas the PACAP antagonist PACAP(6-38) reduced colony formation in the absence or presence of exogenous PACAP-27. In nude mice bearing NCI-H838 xenografts, PACAP(6-38) slowed tumor growth significantly. These data suggest that biologically active type 1 PACAP receptors are present on human non-small cell lung cancer cells, which exhibit dual signal transduction pathways and regulate cell proliferation.

Expression of somatostatin receptor genes and their role in inhibiting Cl- secretion in HT-29cl.19A colonocytes

Recent studies in a cultured model of the intestinal epithelium (HT-29cl.19A) have shown that somatostatin-14 (SS-14) inhibits the Cl- secretory process by acting at multiple G protein-dependent sites. These actions may underlie the antidiarrheal properties of SS peptides. This study has investigated the expression of specific SS receptor subtypes (SSTR) in HT-29cl.19A and examined their role in mediating SS antisecretory actions. Two predominant SSTR, SSTR1 and SSTR2, were identified by reverse transcriptase-polymerase chain reaction (RT-PCR) of mRNA from polarized HT-29cl.19A monolayers. Receptor binding studies showed evidence of two distinct populations of binding sites consistent with the known properties of SSTR1 and SSTR2. The role of SSTR in inhibition of secretion was investigated by comparing the effectiveness of native and synthetic SS peptides on adenosine 3',5'-cyclic monophosphate (cAMP)-dependent Cl- secretion. Secretion stimulated by the receptor-mediated agonist prostaglandin E2 (PGE2) was inhibited > 70% by SS-14 with a 50% effective concentration (EC50) of 32 nM. In contrast, SMS-201-995 (SMS) and RC-160 exhibited little or no antisecretory activity (maximum inhibition of 15 +/- 1.9 and 2.8 +/- 1.9%, respectively, at 100 microM; EC50 > 1.5 microM). Similar effects on PGE2-stimulated cAMP accumulation were also observed. SS-14, but not SMS, also inhibited secretion stimulated by dibutyryl cAMP, which acts independently of changes in cellular cAMP. Pretreatment with pertussis toxin reversed the antisecretory effects of SS peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide structural requirements for antagonism differ between the two mammalian bombesin receptor subtypes

Recently it has been established that both a gastrin-releasing peptide (GRP) receptor and a neuromedin B (NMB) receptor mediate the actions of bombesin-related peptides in mammals. Five different classes of peptides that function as GRP receptor antagonists have been identified; however, it is unknown whether similar strategies will yield antagonists for the closely related NMB receptor. In the present study we have used either native cells possessing only one bombesin (Bn) receptor subtype or cells stably transfected with one subtype to determine whether using the strategies that were used successfully for GRP receptors would allow NMB receptor antagonists to be identified. [DPhe12]Bn analogs; des Met14 amides, esters and alkylamides; psi 13-14 Bn pseudopeptides; and D-amino acid-substituted analogs of substance P (SP) or SP(4-11) were all synthesized and each functioned as a GRP receptor antagonist. All of these antagonists had low affinity for the NMB receptor. Application of similar strategies to NMB by formation of [DPhe8]NMB, [psi 9-10]NMB pseudopeptides, des-Met10 NMB amides, alkylamide or esters did not result in any potent NMB receptor antagonists. D-Amino acid SP and SP(4-11) analogs were weakly selective NMB receptor antagonists. No COOH-terminal fragments of NMB or GRP functioned as a GRP or NMB receptor antagonist. These results demonstrate that none of the known strategies used to prepare peptide GRP receptor antagonists are successful at the NMB receptor, suggesting that a different strategy will be needed for this peptide, such as the formation of somatostatin octapeptide or D-amino acid-substituted substance P analogs. These results suggest that even though there is a close homology between GRP and NMB and their receptors, their structure-function relations are markedly different. These results indicate that the development of receptor subtype-specific peptide agonists or peptide antagonists for newly discovered receptor subtypes of gastrointestinal hormones/neurotransmitters may be difficult because the strategies developed for one well-studied subtype may not apply to the other even though it is structurally closely related.

Three-dimensional quantitative structure-activity relationships of somatostatin analogues. 1. Comparative molecular field analysis of growth hormone release-inhibiting potencies

Somatostatin is a hypothalamic hormone that inhibits the release of growth hormone (GH). It has also been shown to inhibit the release of a broad range of hormones including insulin, glucagon, and gastrin. Presently, five different receptor subtypes of somatostatin have been characterized and cloned. Our previous work on the structure-activity relationship of somatostatin and that of many others has generated a large database of analogues with different biological activities and receptor affinities. This present work is an investigation of the growth hormone release-inhibiting potencies of somatostatin analogues by the three-dimensional quantitative structure-activity paradigm, comparative molecular field analysis (CoMFA). A total of 64 analogues were modeled in SYBYL using structural information from two NMR studies. The molecules were aligned by a root-mean-square fit of atoms and field-fit of the steric and electrostatic molecular fields and the resulting databases analyzed by partial least squares analysis with cross-validation to extract the optimum number of components. The analysis was then repeated without cross-validation to give the final QSAR models. Preliminary investigations with the CoMFA models led to the synthesis of a new somatostatin analogue. This compound together with five other newly synthesized compounds not included in the original training sets were used to test the predictive ability of the CoMFA models. Two models with good predictive powers are presented.

Somatostatin receptor subtype 3 mediates the inhibitory action of somatostatin on gastric smooth muscle cells

Previous functional studies show that somatostatin (SS) interacts with specific receptors to inhibit relaxation in gastric smooth muscle cells. There are no ligand binding studies, and it is unknown which of the five subtypes of SS receptors mediates the action. Dispersed gastric smooth muscle cells from guinea pig bound both 125I-labeled SS-14 and 125I-D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Cys-Nal-NH2 (where Nal indicates N-naphthylalanine) (cyclo-SS-8), a synthetic peptidase-resistant octapeptide SS analogue. SS-28 and SS-14, cyclo-SS-8, and SS analogue D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-ol [SMS-(201-995) (octreotide)] inhibited 125I-cyclo-SS-8 binding with relative potencies of SS-28 = cyclo-SS-8 = SMS-(201-995) (octreotide), and the binding was not affected by the addition of protease inhibitors. SS-14 caused inhibition only in the presence of protease inhibitors. Ligand analysis demonstrated a two-binding-site model. Analysis of the relationship between biological function and binding suggested the high-affinity sites mediated the relaxant action of SS. 5'-Guanylylimidodiphosphate [Gpp-(NH)p] inhibited binding by reducing the affinity of the high-affinity site. Six SS-8 analogues that distinguish SS subtypes showed that 125I-SS-14 bound to somatostatin receptor subtype 3 (SSTR3). The results demonstrated that gastric smooth muscle cells possess distinct receptors for SS of the SSTR3 subtype. Occupation of these sites inhibits relaxation in gastric smooth muscle cells. Modulation between the high- and low- affinity binding states of SSTR3 is at least partially mediated by activation of guanine nucleotide regulatory proteins.

Fourth ventricular injection of the bombesin receptor antagonist [D-Phe6]bombesin(6-13)methyl ester, but not BW2258U89, increases food intake in rats

To investigate the role of endogenous bombesin-like peptides in the caudal brainstem for the short-term control of food intake, we evaluated the effects of fourth-ventricular injections of two different bombesin (BN) receptor antagonists, [D-Phe6]BN(6-13) methyl ester and BW2258U89, on intake of sweetened, condensed milk in male rats. Although fourth-ventricular administration of BW2258U89 (0.125-20 ng) had no effect on food intake, fourth-ventricular injections of 1.0-20.0 ng of [D-Phe6BN(6-13) methyl ester resulted in an inverted U-shaped, dose-response curve with a maximal effect at 2.5 ng. Microstructural analysis of the licking behavior indicated that the increase in intake was primarily the result of an increased number of licks and an increase in lick efficiency. Behavioral time sampling demonstrated that these changes in intake occurred without the appearance of any competing behavior or significant change in the overall pattern of behavior. Because [D-Phe6BN(6-13) methyl ester appears to be a preferential antagonist at the GRP-preferring receptor, the increased intake that occurred after its administration suggests that an endogenous GRP-mechanism in the caudal brainstem is necessary for the normal, short-term control of sweet milk intake under these conditions.

Influence of intracisternal injection of somatostatin analog receptor subtypes 2, 3 and 5 on gastric acid secretion in conscious rats

Somatostatin (SST) and its analogs injected into the CSF induce different gastric acid response patterns. Five SST receptors have been characterized (SSTR-1 to -5). We studied the influence of selective SSTR-2, -3 and -5 ligands on basal gastric acid secretion after intracisternal (i.c.) injection in conscious rats equipped with chronic gastric and i.c. cannulae. Compared with pre-injection level, the SSTR-3 agonist, BIM-23056 (1 microgram) increased acid secretion by 274 +/- 43% while the SSTR-2 agonist, DC 32-87 (1 microgram) inhibited acid secretion by 50.7 +/- 13.3%. SST-14 (1 microgram), the SSTR-5 agonist, BIM-23052, (0.5-1 microgram), SSTR-3 (0.5 micrograms) and -2 (0.5 microgram) or vehicle injected i.c. did not modify basal acid secretion. These results show that the activation of brainstem SSTR-3 receptors stimulate and SSTR-2 inhibit basal gastric acid secretion in conscious rats with chronic gastric fistulae.

A comparison of the effects of human and ovine corticotropin-releasing hormone on the pituitary-adrenal axis

To compare the clinical efficacy of ovine and human sequence corticotropin-releasing hormone (CRH), we examined the effects of both peptides on ACTH and cortisol secretion in normal human volunteers, obese subjects, and patients with pituitary-dependent (Cushing's disease) and adrenal-dependent Cushing's syndrome. All subjects in each group were studied twice in random order. One hundred micrograms of CRH were administered as an iv bolus through an in-dwelling forearm cannula at 0930 h, and thereafter, blood was drawn every 15 min for 2 h for the measurement of ACTH and cortisol. In the normal subjects, the peak ACTH, peak incremental ACTH, and mean area under the curve after CRH treatment were greater with ovine CRH than human CRH, although there was no difference in the cortisol response, however it was analyzed. There was no difference in the ACTH or cortisol response to the two preparations in the obese subjects, and no significant difference was found, for either cortisol or ACTH, between obese subjects and normal volunteers. With both varieties of CRH, Cushing's disease resulted in greater responses for ACTH and cortisol than those seen in the other 2 groups (P < 0.001 for all comparisons), but there was no difference between the sequences. However, a significant cortisol response, defined as an increase of greater than 4 times the coefficient of variation of the assay (24%), was seen in all 10 Cushing's patients with human CRH, but in only 8 with ovine CRH. In 3 patients with adrenal tumors, serum cortisol did not change after the administration of either preparation, whereas plasma ACTH remained undetectable throughout the study. We suggest that although ovine sequence CRH causes more prolonged and greater ACTH, and possibly cortisol, secretion compared to human CRH, the discriminatory value of the CRH test, in terms of either the diagnosis or differential diagnosis of Cushing's syndrome, is comparable for the two peptide sequences.

Comparison of responses to adrenomedullin and adrenomedullin analogs in the mesenteric vascular bed of the cat

Responses to adrenomedullin, a newly discovered hypotensive peptide isolated from human pheochromocytoma cells, and the carboxy terminal 15-52 (adrenomedullin-(15-52)) and 22-52 (adrenomedullin-(22-52)) amino acid fragments of adrenomedullin were investigated in the mesenteric vascular bed of the cat. Under constant flow conditions, injections of adrenomedullin, adrenomedullin-(15-52), and calcitonin gene-related peptide (CGRP) in doses of 0.003-1 nmol into the perfused superior mesenteric artery caused significant dose-related decreases in mesenteric arterial perfusion pressure. Mesenteric vasodilator responses to adrenomedullin and adrenomedullin-(15-52) were similar in magnitude and duration, while vasodilator responses to CGRP were greater in magnitude and longer in duration than those produced by adrenomedullin or adrenomedullin-(15-52) when these agents were injected in doses of 0.1-1 nmol. Adrenomedullin-(22-52) caused no significant change in mesenteric arterial perfusion pressure when injected in doses up to 10 nmol. These results suggest that amino acids 15-52 and the six-membered ring structure of adrenomedullin are important for the expression of vasodilator activity in the mesenteric vascular bed of the cat.