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The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 9 4581-4587
Copyright © 2004 by The Endocrine Society

Activation of Somatostatin-Receptor Subtype-2/-5 Suppresses the Mass, Frequency, and Irregularity of Growth Hormone (GH)-Releasing Peptide-2-Stimulated GH Secretion in Men

Ali Iranmanesh, Cyril Y. Bowers and Johannes D. Veldhuis

Endocrine Service, Medical Section, Salem Veterans Affairs Medical Center (A.I.), Salem, Virginia 24153; Division of Endocrinology and Metabolism, Department of Internal Medicine, Tulane University Medical Center (C.Y.B.), New Orleans, Louisiana 70112-2699; and Endocrine Research Unit (J.D.V.), Mayo Medical and Graduate Schools of Medicine, General Clinical Research Center, Mayo Clinic, Rochester, Minnesota 55905

Address all correspondence and requests for reprints to: Dr. Johannes D. Veldhuis, Division of Endocrinology and Metabolism, Department of Internal Medicine, Endocrine Research Unit, Mayo Medical and Graduate Schools of Medicine, General Clinical Research Center, Mayo Clinic, Rochester, Minnesota 55905. E-mail: veldhuis.johannes{at}mayo.edu.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Somatostatin antagonizes the stimulatory actions of GHRH and GH-releasing peptides (GHRPs). However, precisely how the inhibitory susceptibilities of the two secretagogues differ is not clear. One interpretative difficulty is that native somatostatin activates six different receptor subtypes. The present study adopts the complementary strategy of enforcing feedback inhibition via the preferential somatostatin receptor subtype 2 and 5 (SSTR-2/-5) agonist, octreotide. We postulated that putative SSTR-2/-5 agonism would unmask secretagogue-selective interactions in the control of GH secretory burst mass, frequency, and/or regularity. To this end, 10 healthy men each underwent eight randomly ordered, separate-day, fasting morning infusion sessions. Interventions comprised sc administration of octreotide (1 µg/kg), followed by bolus iv injection of saline, GHRH (1 µg/kg), GHRP-2 (1 µg/kg), or both peptides. Compared with placebo, the SSTR-2/-5 agonist reduced fasting GH concentrations from 0.27 ± 0.07 to 0.12 ± 0.02 µg/liter (P = 0.020), GH secretory burst mass from 2.7 ± 0.65 to 0.55 ± 0.11 µg/liter (P = 0.013), and basal GH secretion from 0.24 ± 0.043 to 0.11 ± 0.015 µg/liter·100 min (P = 0.0063). The foregoing outcomes were selective, because octreotide did not alter GH secretory burst frequency (3.1 ± 0.5 vs. 3.3 ± 0.21 events/3 h) or the regularity of the GH release process (approximate entropy, 0.58 ± 0.048 vs. 0.68 ± 0.064). In the GHRP-2-stimulated setting, presumptive SSTR-2/-5 agonism suppressed all three GH secretory burst masses, from 28 ± 3.2 to 18 ± 2.0 (P = 0.045); GH pulse frequency, from 3.3 ± 0.30 to 2.0 ± 0.18 (P = 0.0025); and the irregularity (approximate entropy) of GH release, from 0.648 ± 0.049 to 0.433 ± 0.047 (P < 0.01). In contrast, in the GHRH and combined GHRH/GHRP-2-stimulated contexts, octreotide decreased only GH secretory burst mass (P = 0.047). In summary, the present data indicate that GH secretory burst mass, frequency, and orderliness are subject to interactive control by at least SSTR-2/-5-dependent feedback and GHRP-dependent feedforward signals.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
SYNTHETIC GH-RELEASING peptides (GHRPs) were identified approximately 25 yr ago in the laboratory of Bowers et al. (1, 2) as derivatives of met-enkephalin that stimulate GH secretion in vitro and in vivo. The cognate receptor was cloned in 1996 (2), and an endogenous ligand (ghrelin) was identified in 1999 (3). Both the natural agonist and receptor are expressed (nonexclusively) in the hypothalamic arcuate nucleus, anterior pituitary gland, and oxyntic cells of the stomach (3, 4, 5, 6). Laboratory and clinical studies document that GHRP exerts unique hypothalamo-pituitary actions, which include 1) antagonism of somatostatinergic inhibition of hypothalamic gene expression and somatotrope cell GH release (7, 8); 2) stimulation of arcuate nucleus GHRH outflow (9, 10); 3) synergy with GHRH in healthy individuals (11); and 4) partial antagonism of negative feedback enforced by recombinant human GH (rhGH) or IGF-I (12, 13, 14).

The somatostatin receptor (SSTR) family is encoded by five genes, which give rise to six functional proteins (15). Two transcripts are particularly prominent in the hypothalamus (SSTR-1/-2), and three are prominent in somatotrope cells (SSTR-1/-2/-5) (16, 17). The native tetradecapeptide can activate each SSTR-signaling pathway. In contrast, the synthetic compound, octreotide, preferentially stimulates SSTR-2/-5 signaling (18, 19, 20). Because octreotide acts on both hypothalamus and pituitary gland, this analog provides a potential probe of feedback repression of GH secretion mediated via SSTR-2 (central nervous system and pituitary) and SSTR-5 (pituitary) (21). The present study exploits this basic laboratory background to test the clinical hypothesis that SSTR-2/-5 activation regulates feedforward by GHRH and GHRP-2 via distinguishable neuroendocrine mechanisms.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Ten healthy men participated in and completed all eight study sessions (below). To explore possible modulation of octreotide-dependent inhibition of unstimulated GH release and GHRH and GHRP-2 action by age or relative obesity, the range of ages studied intentionally included a uniform spread over 19–78 yr and a range of body mass indexes from 19–37 kg/m2.

Participants provided written informed consent approved by the local institutional review board. The protocol was reviewed by the U.S. FDA and assigned an investigator-initiated IND for experimental use of rhGHRH-(1–44)-amide and GHRP-2. Exclusion criteria included cardiac, cerebrovascular, peripheral arterial, or venous thromboembolic disease; alcohol or drug abuse; nightshift work or recent transmeridian travel exceeding three time zones work (within 10 d); significant weight change (≥2 kg in 3 wk); acute or chronic organ system disease; psychiatric illness requiring medical treatment; concomitant or recent (within six biological half-lives) use of neuroactive medications; anemia (hematocrit, <38%); and failure to provide written informed consent. Some enrollees continued to take multivitamins, ferrous sulfate, or topical ophthalmic or dermatological ointments. Inclusion criteria were an unremarkable medical history and physical examination, and normal screening laboratory tests of hepatic, renal, endocrine, metabolic, and hematological function.

Protocol design

The design was a prospectively randomized, placebo-controlled, patient-blinded, within-subject, cross-over intervention. Each subject undertook eight sampling sessions, which were scheduled at least 5 d apart. Any individual completed all admissions within 3 months.

Volunteers remained fasting overnight and until 1300 h the next day. Caffeinated beverages, smoking, sleep, and vigorous exercise were disallowed during the sampling study. At 0700 h, an indwelling iv catheter was inserted into a forearm vein. At 0800 h, subjects were given a single sc injection of saline or octreotide (1 µg/kg). Beginning 1 h thereafter, blood (1 ml) was collected every 10 min for 5 h (until 1300 h) for later determination of GH concentrations. Three hours after octreotide/saline administration, participants received a single iv bolus of saline, rhGHRH-(1–44)-amide (1 µg/kg), GHRP-2 (1 µg/kg), or both peptides.

Hormone assays

GH concentrations were measured in duplicate by modified automated ultrasensitive immunochemiluminometry (Nichols Institute Diagnostics, San Juan Capistrano, CA) using 22-kDa recombinant human GH as assay standard (22, 23). The cross-reactivity with 20-kDa GH is 30%. The sensitivity is 0.005 µg/liter (defined as 3 SD above the hypopituitary zero dose tube). Median intra- and interassay coefficients of variation were 5.2% and 6.3%, respectively. No sample concentration in the present study fell below 0.020 µg/liter.

Analysis of basal and pulsatile GH secretion

The amount of GH secreted in bursts was quantitated by modified biexponential deconvolution analysis (24). Two-component GH kinetics were defined by a rapid phase half-life of 3.5 min, a slow phase half-life of 20.8 min, and a fractional (slow/total) decay amplitude of 0.63. These values were previously determined directly (25). Pulse times were set a priori by low threshold Cluster analysis (26). The foregoing two statistical conditions are required in this deconvolution formulation to distinguish burst-like secretion (micrograms per liter per 3 h postsecretagogue) reliably from each of 1) time-invariant basal release, 2) partial overlap of prior and ongoing hormone secretory bursts, and 3) decay of hormone concentrations across the stimulus-response interval (27, 28).

Statistical comparisons

The null hypothesis posits that compared with placebo/saline, administration of octreotide and/or secretagogue is not a determinant of GH secretory burst mass. Statistical analysis comprised two-way repeated-measures ANOVA in a randomized block design comprising two factors (octreotide vs. placebo) by three factors (GHRH, GHRP-2, and both). The outcome variable was the logarithm of the within-subject difference over the placebo/saline response of octreotide-inhibited and agonist-stimulated GH secretory burst mass. For discussion purposes, fractional inhibition was calculated as the octreotide-induced decrement (algebraic difference) between secretagogue-stimulated GH secretory burst mass after the administration of placebo and octreotide divided by the response to secretagogue alone. Post hoc contrasts were assessed by Tukey’s test (overall experiment-wise protected, P < 0.05) (29). Logarithmic transformation was used to limit the dispersion of residual variance and to address the biological assumption that GH secretion is stimulated asymptotically (30).

The approximate entropy (ApEn) statistic was applied to quantitate the pattern regularity of serial GH concentrations (31, 32). Lower values of normalized ApEn (m = 1; r = 0.20) denote greater secretory orderliness, which, in turn, quantitates enhanced feedback effectiveness theoretically and empirically (33, 34).

Linear and monoexponential regression analyses were used to explore the relationship between age or body mass index and GH secretory burst mass with the Tukey adjustment for multiple comparisons (35).

Data are cited as the arithmetic mean ± SEM (median) or 95% statistical confidence intervals. Median values allow valid centering of potentially asymmetric distributions (29).


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Inhibition of mean GH concentrations

Figure 1Go illustrates eight separate GH concentration-time profiles obtained in one volunteer by sampling blood every 10 min for 5 h in the morning while fasting. Statistical contrasts revealed that octreotide suppressed (3 h mean) GH concentrations significantly in all four interventions, viz. the unstimulated state (P = 0.02) and after the infusion of GHRH (P = 0.017), GHRP-2 (P = 0.021), and both secretagogues (P = 0.045; Fig. 2Go). Expressed as median percentage decrements, the rank order of octreotide-induced inhibition of GH concentrations was saline (56%) = GHRH (54%) > GHRP-2 (32%) = combined GHRP-2/GHRH (32%) (P < 0.01, order effect by ANOVA with significantly greater inhibition of both saline and GHRH compared with either GHRP-2 or GHRP-2/GHRH).



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  		FIG. 1. Illustrative GH concentration-time series collected every 10 min for 5 h after randomly ordered separate-day injection of placebo (left) vs. octreotide (right;1 µg/kg, sc), followed in 120 min by a bolus iv infusion of saline, GHRH, GHRP-2, or both peptides (1 µg/kg each) in the fasting state (see Subjects and Methods). The eight panels reflect data from one of 10 men (age, 52 yr; body mass index, 28 kg/m2).

 


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  		FIG. 2. Mean GH concentrations after iv infusion of saline or the indicated peptidyl secretagogue(s). Responses were monitored over 3 h (during the interval 2–5 h) after sc injection of placebo or octreotide (see Subjects and Methods). The overall interventional effect was significant at P < 0.001. Post hoc contrasts between responses to octreotide and placebo for any given agonist are indicated by individual P values. Data are the mean ± SEM (n = 10 men).

 
Deconvolution analysis

Unstimulated basal and pulsatile GH secretion. Octreotide compared with placebo administration reduced 1) basal GH secretion equivalently and significantly before each stimulus (median fractional decrement, 54%; P = 0.0063), and 2) unstimulated 3-h pulsatile GH secretion by 82% (P = 0.0083; Fig. 3Go). The latter effect was due to commensurate (80%) repression of the mass of GH secreted per burst (P = 0.013).



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  		FIG. 3. Impact of putative SSTR-2/SSTR-5 activation on fasting basal (nonpulsatile) and pulsatile GH secretion (summed GH secretory burst mass per 3 h). Biexponential deconvolution analysis was applied to GH concentrations monitored during sequential administration of placebo vs. octreotide (1 µg/kg, sc) and saline. P values contrast GH responses to octreotide and placebo. Data are the mean ± SEM (n = 10 men).

 
Stimulated GH secretion. Figure 4Go summarizes the effects of octreotide vs. placebo on specific secretagogue-stimulated GH secretory burst mass (micrograms per liter per burst). Fractional decremental inhibition followed the descending rank order: saline (80%) = GHRH (62%) > GHRP-2 (36%) = both stimuli [29%; P < 0.01, by ANOVA, suppression of saline and GHRH (equivalent) exceeded that of GHRP-2 and GHRP-2/GHRH (equivalent)].



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  		FIG. 4. Effect of placebo vs. octreotide (1 µg/kg, sc) exposure on saline- and secretagogue-stimulated GH secretory burst mass (micrograms per liter per burst). P < 0.005 was the overall interventional value. Data are presented as described in Fig. 3Go.

 
GH secretory burst frequency. GH secretory burst frequency (pulses per 3 h) did not differ in the placebo setting (range of cohort means, 2.8–3.3). Pulse number also was stable after injection of octreotide and stimulation with saline, GHRH, and combined GHRP-2/GHRH (range, 3.0–3.3). In contrast, octreotide treatment before GHRP-2 stimulation decreased GH pulse frequency by 39%, viz. from 3.3 ± 0.30 to 2.0 ± 0.18 (P = 0.0025). The latter outcome was corroborated by a reciprocal 46% increase in the interburst interval, viz. from 43 ± 3.9 min (placebo) to 63 ± 4.2 min (octreotide; P < 0.01). Thus, octreotide decreased GH pulse number only in the face of GHRP-2 stimulation.

ApEn

ApEn is a model-dependent and concentration-invariant measure of pattern regularity or orderliness. Greater pattern reproducibility denotes more effective negative feedback (32, 34, 36). Octreotide compared with placebo administration did not alter ApEn of GH release after stimulation by saline, GHRH, or combined GHRH/GHRP-2. Expressed as a pooled standard deviate (SD or z-score), the median placebo-octreotide differences in GH ApEn were +0.21 (saline), +0.20 (GHRH), and +0.06 (combined GHRH/GHRP-2; P = NS each for a two-tailed test of the null hypothesis of a mean z-score difference of zero). In contrast, octreotide reduced GHRP-2-associated GH ApEn from 0.648 ± 0.049 to 0.433 ± 0.047 (z-score difference, +3.1; P < 0.01). Lower ApEn in the last setting quantitates enhanced regularity (pattern orderliness) of GH secretion.

Regression on age

Regression analysis revealed that age correlated negatively with the mass of GH secreted (micrograms per liter per 3 h) after 1) placebo/GHRH (r = –0.75; P < 0.01) and octreotide/GHRH (r = –0.70; P = 0.03; both linear models), and 2) placebo/GHRH/GHRP-2 (r = –0.85; P < 0.01; exponential model). The first two correlation coefficients did not differ significantly (see Subjects and Methods). Body mass index did not correlate with saline- or secretagogue-stimulated GH secretory burst mass or frequency in this cohort.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The present investigation unveils that putatively selective SSTR-2/-5 agonism via acute octreotide administration modulates specific facets of GHRH, GHRP-2, and combined GHRH/GHRP-2 actions in men. From a technical vantage, the current design is unique in implementing each of 1) frequent (10-min) blood sampling or a 5-h time window; 2) ultrasensitive immunochemiluminometric GH assay (threshold, 0.005 µg/liter); 3) predominantly SSTR-2/-5-specific agonism rather than pleiotropic SSTR drive; 4) modified deconvolution analysis, statistically conditioned on biexponential GH kinetics and a priori pulse times to discriminate basal and burst-like secretion; 5) objective quantitation of the regularity of stimulated GH release patterns via the ApEn statistic; and 6) inclusion of a uniform range of ages and body mass indexes to explore secondary determinants of SSTR-2/-5-dependent feedback control. From a mechanistic perspective, octreotide achieved both general and secretagogue-specific feedback regulation. Salient outcomes included suppression of 1) basal GH secretion (median, 54% decrement); 2) fasting pulsatile GH secretion (82% decrement); 3) peptide-stimulated GH secretory-burst mass (by a minimum of 29% for combined GHRH/GHRP-2 and a maximum of 62% for GHRH); 4) the frequency of GH secretory bursts under GHRP-2 feedforward only; and 5) the irregularity of GH release in response to GHRP-2 agonism only. Exploratory regression analysis disclosed that age negatively predicts GH secretory responses to GHRH after placebo and octreotide administration and to combined GHRH/GHRP-2 stimulation after placebo. These outcomes introduce the idea that significant hypothalamo-pituitary mechanisms link SSTR-2/-5 agonism to specific peptide-induced pulsatile GH secretion in the human with evident age dependency.

Deconvolution analysis was applied to dissect the mechanisms of octreotide-induced suppression of mean GH concentrations after saline and secretagogue stimulation. The interval of sampling used here is sufficient for physiological studies, and the octreotide dose mimics that used in continuous infusion experiments (19). We used a recently modified deconvolution methodology to ensure valid analytical discrimination among basal release, burst-like secretion, and hormone elimination. In the revised construct, the statistical solution is predicated upon biexponential decay kinetics and a priori pulse identification (37, 38). According to this analytical formalism, octreotide administration decreased fasting basal GH release by more than 50% before each stimulus. Significant inhibition of basal GH secretion would be consistent with earlier indirect inferences (39, 40). Presumptive SSTR-2/-5 agonism reduced GH secretory burst mass in the descending rank order of saline equals GHRH, which, in turn, exceeded both GHRP-2 and GHRH/GHRP-2. The percent suppression of the single GHRP-2 and the combined GHRH/GHRP-2 stimulus was equivalent. Albeit unlikely (type II error <15%), the cohort size of 10 individuals selected for broad representation of age and body mass index does not exclude a possible difference in octreotide-induced repression of GHRP-2 vs. dual GHRH/GHRP-2 feedforward in a subset of men.

Post hoc analyses revealed distinctive interactions between octreotide-enforced negative feedback and GHRP-2-induced feedforward, which were not shared by GHRH, viz. 1) reduction of the frequency of GH secretory bursts (resulting in 46% prolongation of the GH interburst-interval length; and 2) enhancement of the quantitative regularity of GH release patterns (ApEn decrease of 3.1 SD). These two outcomes are consistent with a postulate that feedback via SSTR-2/-5 and feedforward via GHRP-2 converge in part on hypothalamic loci of control (below).

Earlier studies using native somatostatin, which is able to activate all six SSTRs, have described either equivalent or greater suppression of peak and integrated GH concentrations stimulated by GHRH compared with GHRP (41). Laboratory analyses show that GHRP and ghrelin induce GH release directly in vitro via the somatotrope type Ia receptor and in vivo via combined hypothalamo-pituitary effects (1, 2, 42, 43). The GHRH receptor is not absolutely required for direct pituitary actions of GHRP in the human, because GHRP-2 stimulates GH secretion by 2- to 6-fold in rare patients with inactivating (truncational) mutations of the GHRH receptor (44, 45). In contrast, when hypothalamo-pituitary connectivity is intact, the effects of GHRH and GHRP are synergistic (11, 46, 47, 48, 49, 50, 51). The present analyses corroborate GHRH/GHRP synergism in men and demonstrate further that the facilitative interaction specifically entails augmentation of GH secretory burst mass. On technical grounds, secretion of more GH (micrograms) per unit distribution volume (liters) per burst would explain earlier inferences that combined GHRH/GHRP can increase each of the incremental, integrated, and peak GH concentrations (52). In relation to such measures, octreotide administration in two earlier studies also inhibited GHRH more than GHRP-stimulated GH release in respectively five and nine healthy young men (53, 54). Differences in the degree of absolute inhibition of GHRP action by octreotide presumably arise from different experimental paradigms and effector doses. In the simplest interpretation, greater suppression of responses to GHRH than GHRP-2 or combined GHRH/GHRP-2 by octreotide may reflect the unique capability of GHRP to antagonize both hypothalamic and pituitary inhibition by somatostatin (7, 8, 55).

The somatostatin receptor subtype selectivity of octreotide would imply that discrete signaling pathways participate in the amplitude and frequency-sensitive feedback interactions identified here. In the rat, SSTR-1 and SSTR-2 transcripts predominate in the hypothalamus, and SSTR-2 and SSTR-5 predominate in somatotropes (15, 56, 57). Assuming that octreotide enters the hypothalamus (21), we postulate that this agent could activate SSTR-2 in the hypothalamus and SSTR-2 and SSTR-5 in the pituitary gland. In the last regard, inactivating mutation of SSTR-5 in a patient with acromegaly was associated with lack of suppressibility of GH secretion by octreotide, consistent with tumoral autonomy and/or loss of SSTR-5-dependent inhibition (15). Selective pharmacological probes that distinguish SSTR-2 and SSTR-5 signaling will be required to clarify such facets of feedback control.

Sequential administration of octreotide and GHRP-2 was specific in decreasing GH ApEn. Decreased ApEn denotes quantitatively more orderly patterns of GH release (31, 32, 33, 36, 58). In theoretical and empirical models, a more regular secretory process signifies enhanced feedback control in an interlinked axis (34). Concomitantly, octreotide suppressed both the frequency and mass of GHRP-2-stimulated GH secretory bursts (above). Three-fold feedback effects of octreotide on stimulation by GHRP-2 would support a role for SSTR-2/-5-dependent modulation of hypothalamo-pituitary control of GH secretion.

The present data establish that octreotide inhibits analytically determined basal GH secretion (40, 59). The regulation of nonpulsatile (basal) GH release is not well understood However, a plausible mechanism of octreotide’s effect is direct activation of SSTR-2 and/or SSTR-5 inhibitory pathways in somatotrope cells (15, 57). Other recent studies suggest that continuous stimulation with GHRH, GHRP-2, or both can elevate basal GH secretion (46, 47, 60, 61, 62, 63). Thus, available data permit the hypothesis that octreotide reduces basal GH output by blocking somatotrope GH release and/or antagonizing hypothalamic peptidyl drive.

Exploratory regression analysis was used to test the relationship between body mass index or age and the degree of placebo- or octreotide-enforced suppression of GH secretory burst mass. Body mass index did not correlate significantly with inhibitory susceptibility to the SSTR-2/-5 agonist in the present cohort of volunteers. At present, we are unaware of other reports on this subject. In contradistinction, age correlated negatively with the stimulatory effects of GHRH and GHRH/GHRP-2 after placebo administration and with the effects of GHRH alone after octreotide inhibition. The negative relationship between age and GHRH efficacy in the presence of octreotide could indicate that impaired GHRH action in older adults does not exclusively reflect heightened somatostatinergic restraint.

In summary, presumptive pharmacological activation of hypothalamo-pituitary SSTR-2/-5 signaling by octreotide administration suppresses basal and pulsatile GH secretion and reduces the mass of GH secreted in bursts after individual and combined GHRH and GHRP-2 stimulation in men. In the face of GHRP-2 feedforward, octreotide also represses GH secretory burst frequency and the irregularity of GH release patterns. From a parsimonious mechanistic perspective, tripartite inhibition of GHRP-2-augumented GH secretory burst mass, frequency, and irregularity would point to convergent control of GH secretion by SSTR-2/-5- and GHRP-responsive pathways.


   Acknowledgments
 
We thank Kimberly Coulter and Gail Bierbaum for excellent assistance with manuscript preparation, Jonathan Kuipers for contributing to data analyses and illustrations, and Brenda Grisso in the Veterans Affairs Laboratory for performing the immunoassays.


   Footnotes
 
This work was supported in part by Grant MO1-RR-00585 to the General Clinical Research Center of the Mayo Clinic and Mayo Foundation from the National Center for Research Resources (Rockville, MD) and NIH Grant R01-AG-19695.

Abbreviations: ApEn, Approximate entropy; GHRP, GH-releasing peptide; rh, recombinant human; SSTR, somatostatin receptor.

Received February 5, 2004.

Accepted June 11, 2004.


   References
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 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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