Introduction

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Somatostatin is a well-known physiological inhibitor of gastric acid secretion (Lloyd and Debas, 1994), acting in a paracrine rather than in an endocrine fashion (Short et al., 1985; Schubert et al., 1987; Makhlouf and Schubert, 1990). In the stomach, somatostatin is synthesized and secreted by endocrine D-cells of the fundic and antral mucosa (Lucey and Yamada, 1989) and is released from cytoplasmic processes (Larsson et al., 1979) that are closely associated with parietal cells, ECL cells and G-cells. Results from in vivo and ex vivo experiments indicate that somatostatin inhibits acid secretion directly (Bech, 1986; Michelangeli et al., 1988; Sandvik and Waldum, 1988; Schubert et al., 1989; Yang et al., 1990) and indirectly by reducing concentrations of circulating histamine (Sandvik and Waldum, 1988; Payne and Gerber, 1992) and gastrin (Bloom et al., 1974; Saffouri et al., 1979; Chiba et al., 1981; Jansen and Lamers, 1981; Short et al., 1985; Wolfe and Reel, 1986; Makhlouf, 1987; Yang et al., 1990; McIntosh et al., 1991; Holst et al., 1992). In addition, somatostatin has been shown in vitro to block histamine release from ECL cells (Chuang et al., 1993; Prinz et al., 1994a), gastrin release from G-cells (Giraud et al., 1987) and, at higher doses, [¹⁴C]aminopyrine uptake by parietal cells (Park et al., 1987).

Recently, the molecular characterization and physiology of somatostatin receptors have been reviewed (Reisine, 1995), and guidelines defining their nomenclature have been recommended (Hoyer et al., 1995). Five distinct somatostatin receptor subtypes have been cloned, including sst₁ and sst₂ (Yamada et al., 1992a), sst₃ (Yasuda et al., 1992; Yamada et al., 1992b) and sst₄ and sst₅ (Yamada et al., 1993). Tissue distribution of mRNA coding for each somatostatin receptor includes the stomach, which expresses all receptor isoforms (Bruno et al., 1993). At least three of the somatostatin receptors are important in regulating stomach function, because i.v. administration of SMS-201-995, a relatively long-acting synthetic peptide analog of somatostatin that binds with higher affinity to sst₂, sst₃ and sst₅ receptors than to either sst₁ or sst₄ receptors (Raynor et al., 1993a,b), causes potent inhibition of gastric acid secretion (Whitehouse et al., 1986).

Several synthetic peptide analogs of somatostatin-14, with 1,000- to 10,000-fold differences in in vitro binding affinity, have been used pharmacologically to characterize individual somatostatin receptors (Raynor et al., 1993a,b). Previous studies using somatostatin analogs relatively selective for sst₂, sst₃ and sst₅ receptors revealed that pentagastrin-stimulated acid secretion could be blocked by activation of peripheral sst₂ receptors (Rossowski et al., 1994; Lloyd et al., 1995) and basal acid secretion could be reduced by activation of sst₅ and sst₂ receptors in the central nervous system (Martinez et al., 1995, 1996). Part of the acid inhibition observed may be due to regulation of histamine release, because in vitro somatostatin blocks gastrin-stimulated histamine release from gastric ECL cells in culture (Chuang et al., 1993) by activation of sst₂ receptors (Prinz et al., 1994a). Therefore, to determine which somatostatin receptor is involved in regulation of gastric acid secretion and histamine release, we administered i.v., in anesthetized rats, synthetic peptide analogs of somatostatin with relatively selective binding affinity for either sst₂ (NC-8-12) (Rossowski et al., 1994), sst₃ (BIM-23058) or sst₅ (BIM-23052) (Raynor et al., 1993a,b) receptors.

	Methods

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Drugs. Pentagastrin (Ayerst, New York, NY), bethanechol (Urecholine; Merck Sharp & Dohme, West Point, PA) and histamine (Sigma Chemical Co., St. Louis, MO) were diluted in 0.9% saline for i.v. infusion. Somatostatin-14 (Peninsula Laboratories, Belmont, CA), which binds all five somatostatin receptors, and SMS-201-995 (Sandostatin; Sandoz Laboratories, Basel, Switzerland), which is relatively selective for sst₂, sst₃ and sst₅ receptors, were dissolved in 0.1% CSA (Sigma Chemical Co., St. Louis, MO) in saline. Somatostatin peptide analogs NC-8-12, BIM-23058 and BIM-23052 (table 1), with relative selectivity for sst₂, sst₃ and sst₅ receptors, respectively (obtained from Dr. D. Coy, Tulane University), were dissolved in 0.01% acetic acid/0.1% CSA (1:9) for i.v. infusion.

Animals. Rats were prepared for acid secretion experiments as previously described (Lloyd et al., 1992). Adult male Sprague-Dawley rats (Harlan Laboratory, San Diego, CA) weighing 300 to 350 g were housed in group cages, under conditions of controlled temperature (22-24°C) and illumination (12-hr light cycle starting at 6:00 A.M.), for at least 7 days before experiments. Rats were maintained on Purina Laboratory Chow (Ralston Purina, St. Louis, MO), available ad libitum, and tap water. Experiments were performed on rats that had been deprived of food for 24 hr but given free access to water up to the beginning of the study.

Rat model of gastric acid secretion. Fasted rats were anesthetized with sodium pentothal (50 mg/kg i.p.), the trachea was cannulated to ensure a clear airway and the cervical esophagus was ligated. The abdomen was opened through a ventral median celiotomy, the pylorus was ligated and the stomach was flushed with 0.15 M NaCl through an incision in the nonglandular forestomach. A 1-cm-diameter, double-lumen, plastic gastric cannula was secured in the stomach and exited the abdomen through the midline incision.

Gastric acid secretion and portal venous histamine release. Gastric effluent was collected every 10 min by flushing through the gastric cannula twice with 5 ml of 0.9% saline under gravity drainage and once with 5 ml of air under slight positive pressure. Gastric samples were back-titrated to pH 7.0 with 0.1 N NaOH, using an automatic titrator (Radiometer, Copenhagen, Denmark). After a 30-min basal period, acid secretion was stimulated for 2 hr by an i.v. infusion of either pentagastrin (24 µg/kg/hr), bethanechol (0.2 mg/kg/hr) or histamine (5 mg/kg/hr). The doses of these acid secretagogues were previously established to maximally stimulate acid secretion (Lloyd et al., 1992). During the second 1 hr of acid stimulation, either vehicle (CSA), somatostatin-14 (10 nmol/kg/hr), SMS-201-995 (10 nmol/kg/hr) or increasing doses of NC-8-12 (0.1, 1, 10 or 100 nmol/kg/hr), BIM-23058 (10 or 100 nmol/kg/hr) or BIM-23052 (10 or 100 nmol/kg/hr) were administered by i.v. infusion for 1 hr. Doses were selected based on previous experiments in rats (Rossowski et al., 1994; Lloyd et al., 1995).

Statistical evaluation. Acid output data are presented as mean ± S.E.M. and as a percentage of maximum stimulated acid output. Percent of maximum stimulated acid output was calculated by dividing the sum of each rat's integrated acid output during the last 30 min of the second 1 hr of pentagastrin infusion by the sum of each rat's integrated acid output during the last 30 min of the first 1 hr of pentagastrin infusion, multiplying the quotient by 100 and finally calculating the mean of each group. To compensate for variations between groups, portal venous histamine concentrations were analyzed as a percent of maximum stimulated histamine release, which was calculated in a fashion similar to that for percent of maximum acid output. Within-group differences in percent of maximum acid output or portal venous histamine concentration over time were assessed via repeated-measures analysis of variance, and the Tukey least-significant difference criterion was used to determine significance between any two groups of rats at the P < .05 level.

	Results

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Gastric Acid Secretion

Histamine. Basal acid output was 17.6 ± 1.3 µmol/30 min and was increased 3-fold to 43.4 ± 5.0 µmol/30 min during the first 1 hr of i.v. histamine (5 mg/kg/hr; n = 9). During the second 1 hr, somatostatin-14 (10 nmol/kg/hr; n = 4) inhibited histamine-stimulated acid output to 68 ± 10% of maximum stimulated acid output (fig. 1). Activation of sst₂, sst₃ and sst₅ receptors using SMS-201-995 (10 nmol/kg/hr; n = 5) decreased acid output to 67 ± 5% of maximum. The sst₂ receptor agonist NC-8-12 (10 nmol/kg/hr; n = 6) inhibited acid output to 76 ± 3% of maximum, with an IC₅₀ of 3.8 nM. In contrast, the sst₃ receptor agonist BIM-23058 (n = 4) and the sst₅ receptor agonist BIM-23052 (n = 4) had no significant inhibitory effect (fig. 2). At the highest dose administered (100 nmol/kg/hr), NC-8-12 inhibited histamine-stimulated acid secretion to not less than 50% of maximum acid output (fig. 3) . 

View larger version (26K): [in this window] [in a new window]   Fig. 1.   Gastric acid output stimulated by a 2-hr i.v. infusion of histamine (5 mg/kg/hr) after a 30-min basal period in separate groups of rats that received either vehicle (0.1% CSA in saline, n = 9) () or 10 nmol/kg/hr levels of either somatostatin-14 (n = 4) (), SMS-201-995 (n = 5) () or NC-8-12 (n = 6) () i.v. for 1 hr.

View larger version (27K): [in this window] [in a new window]   Fig. 2.   Percent of maximum histamine-stimulated acid output before (basal, control) and after i.v. administration of either vehicle (0.1% CSA in saline, n = 9) or 10 nmol/kg/hr levels of either somatostatin-14 (SS-14) (n = 4), SMS-201-995 (n = 5), NC-8-12 (n = 6), BIM-23058 (n = 4) or BIM-23052 (n = 4). *P < .05.

View larger version (13K): [in this window] [in a new window]   Fig. 3.   Percent of maximum histamine-stimulated acid output in response to increasing doses of either NC-8-12 (0.1, 1, 10 and 100 nmol/kg/hr, n = 6), BIM-23058 (10 and 100 nmol/kg/hr, n = 4) or BIM-23052 (10 and 100 nmol/kg/hr, n = 4).

Bethanechol. Basal acid output was 20.3 ± 1.7 µmol/min and was increased 3.5-fold to 71.4 ± 3.0 µmol/30 min during the first 1 hr of i.v. bethanechol (0.2 mg/kg/hr; n = 6). During the second 1 hr, somatostatin-14 (10 nmol/kg/hr; n = 4) inhibited bethanechol-stimulated acid output to 26 ± 6% of maximum stimulated acid output (fig. 4). SMS-201-995 (10 nmol/kg/hr; n = 5) decreased acid output to 14 ± 5% of maximum. NC-8-12 (10 nmol/kg/hr; n = 7) inhibited acid output to 30 ± 5% of maximum, with an IC₅₀ of 9.4 nM, whereas BIM-23058 (n = 9) and BIM-23052 (n = 6) were significantly less effective (fig. 5). NC-8-12 induced a dose-dependent inhibition of acid secretion to basal levels, whereas the acid-inhibitory responses to doses of BIM-23058 and BIM-23052 were 100-fold less effective (fig. 6).

View larger version (27K): [in this window] [in a new window]   Fig. 4.   Gastric acid output stimulated by a 2-hr i.v. infusion of bethanecol (0.2 mg/kg/h) after a 30-min basal period in separate groups of rats that received either vehicle (0.1% CSA in saline, n = 6) () or 10 nmol/kg/hr levels of either somatostatin-14 (n = 4) (), SMS-201-995 (n = 5) () or NC-8-12 (n = 7) () i.v. for 1 hr.

View larger version (26K): [in this window] [in a new window]   Fig. 5.   Percent of maximum bethanecol-stimulated acid output before (basal, control) and after i.v. administration of either vehicle (0.1% CSA in saline, n = 6) or 10 nmol/kg/hr levels of either somatostatin-14 (SS-14) (n = 4), SMS-201-995 (n = 5), NC-8-12 (n = 7), BIM-23058 (n = 9) or BIM-23052 (n = 6). *P < .05.

View larger version (15K): [in this window] [in a new window]   Fig. 6.   Percent inhibition of maximum bethanecol-stimulated acid output in response to increasing doses of either NC-8-12 (0.1, 1, 10 and 100 nmol/kg/hr, n = 7), BIM-23058 (10 and 100 nmol/kg/hr, n = 9) or BIM-23052 (10 and 100 nmol/kg/hr, n = 6).

Pentagastrin. Basal acid output was 21.5 ± 1.5 µmol/min and was increased 3.5-fold to 68.0 ± 7.9 µmol/30 min during the first 1 hr of i.v. pentagastrin (24 µg/kg/hr; n = 5). During the second 1 hr, somatostatin-14 (10 nmol/kg/hr; n = 7) inhibited pentagastrin-stimulated acid output to 34 ± 3% of maximum stimulated acid output (fig. 7). SMS-201-995 (10 nmol/kg/hr; n = 4) decreased acid output to 19 ± 6% of maximum. As demonstrated previously (Lloyd et al., 1995), NC-8-12 (10 nmol/kg/hr; n = 4) inhibited acid output to 47 ± 8% of maximum, whereas BIM-23058 (n = 5) and BIM-23052 (n = 3) were ineffective (fig. 8).

View larger version (27K): [in this window] [in a new window]   Fig. 7.   Gastric acid output stimulated by a 2-hr i.v. infusion of pentagastrin (20 µg/kg/hr) after a 30-min basal period in separate groups of rats that received either vehicle (0.1% CSA in saline, n = 5) () or 10 nmol/kg/hr levels of either somatostatin-14 (n = 7) (), SMS-201-995 (n = 4) () or NC-8-12 (n = 4) () i.v. for 1 hr.

View larger version (25K): [in this window] [in a new window]   Fig. 8.   Percent of maximum pentagastrin-stimulated acid output before (basal, control) and after i.v. administration of either vehicle (0.1% CSA in saline, n = 5) or 10 nmol/kg/hr levels of either somatostatin-14 (SS-14) (n = 7), SMS-201-995 (n = 4), NC-8-12 (n = 4), BIM-23058 (n = 5) or BIM-23052 (n = 3). *P < .05.

Portal Venous Histamine Release

The net increase in portal venous histamine concentration was 338 ± 22 nmol/ml after the first 1 hr of bethanecol (0.2 mg/kg/hr) stimulation. Neither somatostatin-14, NC-8-12, BIM-23058 nor BIM-23052 significantly reduced the net histamine response to bethanecol (fig. 9a). In contrast, the net increase in histamine concentration was 297 ± 66 nmol/ml after the first 1 hr of pentagastrin (24 µg/kg/hr) stimulation, which was approximately 10-fold greater than pentagastrin-stimulated histamine release from an isolated, perfused, rat stomach model (Sandvik and Waldum, 1988). Somatostatin-14 (10 nmol/kg/hr) and NC-8-12 (10 nmol/kg/hr) inhibited histamine release to 50 ± 14% and 60 ± 12%, respectively, of the net histamine response to pentagastrin (fig. 9b).

View larger version (22K): [in this window] [in a new window]   Fig. 9.   Percent of maximum bethanecol-stimulated (n = 3-7) (a) and pentagastrin-stimulated (n = 4) (b) histamine release after i.v. administration of either vehicle (0.1% CSA in saline) or 10 nmol/kg/hr levels of either somatostatin-14 (SS-14), NC-8-12, BIM-23058 or BIM-23052. *P < .05.

	Discussion

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Somatostatin-14 potently inhibits gastric acid secretion in anesthetized rats. Using a peptide analog of somatostatin, SMS-201-995, we showed that acid inhibition by somatostatin-14 occurs by activation of sst₂, sst₃ and sst₅ receptors (Raynor et al., 1993b; Bruno et al., 1993; Rossowski et al., 1994). We then administered individual somatostatin analogs with relatively selective binding affinity for either sst₂, sst₃ or sst₅ receptors, to determine which are involved in regulating acid secretion. One of these analogs, NC-8-12, which binds with relatively higher affinity to the sst₂ receptor than to either the sst₃ or sst₅ receptor (Raynor et al., 1993a,b), reduced acid output stimulated by either pentagastrin, bethanecol or histamine. At the same dose (10 nmol/kg/hr), NC-8-12 inhibited bethanecol-stimulated and pentagastrin-stimulated acid secretion nearly to basal levels but inhibited histamine-stimulated acid secretion only to 76% of maximum acid output. Activation of sst₂ receptors alone could account for the acid-inhibitory effect of somatostatin. In contrast, activation of sst₃ and sst₅ receptors by BIM-23058 and BIM-23052, respectively, had little inhibitory effect, compared with either somatostatin, SMS-201-995 or NC-8-12. Our results are consistent with previous findings in rats (Rossowski et al., 1994; Lloyd et al., 1995), in which sst₂ receptors appear to be the principal somatostatin receptor subtype involved in regulation of stimulated acid secretion.

The somatostatin receptor family (Reisine, 1995) shows only limited homology with one other receptor family, the opioid receptor family (Reisine and Bell, 1993). No one has demonstrated that any of the somatostatin analogs used in this study are agonists at either gastrin, histamine or acetylcholine receptors, although SMS-201-995 has been shown to be an antagonist at the µ-opioid receptor (Reisine and Bell, 1993). Instead, these agonists are relatively selective for different somatostatin receptors (Raynor et al., 1993a,b; Rossowski et al., 1994), although they show significantly greater differences in binding affinity for different somatostatin receptors in vitro than what is apparent from our data and from previous studies in vivo (Rossowski et al., 1994; Lloyd et al., 1995; Martinez et al., 1995, 1996).

NC-8-12 reportedly is at least 1000-fold more avid for sst₂ receptors than for either sst₃ or sst₅ receptors (Rossowski et al., 1994; Raynor et al., 1993b), and BIM-23058 and BIM-23052 are approximately 1,000- to 10,000-fold less avid for sst₂ receptors in vitro (Raynor et al., 1993a,b). We found that NC-8-12 is between 100- and 1000-fold more effective than either BIM-23058 or BIM-23052 at inhibiting acid secretion by 50% in vivo (table 2). Furthermore, the calculated IC₅₀ values of NC-8-12 for inhibition of histamine-stimulated acid secretion (IC₅₀ = 3.8 nM) and inhibition of bethanecol-stimulated acid secretion (IC₅₀ = 9.4 nM) are slightly greater than those calculated for inhibition of pentagastrin-stimulated acid secretion in rats (IC₅₀ = 1.26 or 2.5 nM) (Rossowski et al., 1994; Lloyd et al., 1995). These differences in in vitro binding affinity, compared with in vivo inhibitory activity, of the somatostatin receptor agonists could be partially explained by individual susceptibility to metabolic degradation pathways and clearance rates in vivo (Bunnett et al., 1988; Gu et al., 1992). This is an important consideration, because it is difficult to assess what tissue concentration of analog is achieved after i.v. administration. It is known that somatostatin analogs are more resistant than somatostatin to metabolism by degradative enzymes (Gu et al., 1992). Our preliminary experiments in vitro (data not shown) indicate that the three analogs used in this study are equally resistant to metabolic degradation by neutral endopeptidase 24.11. Under the conditions of our experiment, the apparent proteolytic stability did not have a substantial effect on their activity, because i.v. infusion of a 10 nmol/kg/hr dose of NC-8-12 caused a similar level of inhibition as did somatostatin.

Although histamine and pentagastrin stimulate acid secretion by a direct action on parietal cells (Soll, 1978; Soll et al., 1984), pentagastrin also can promote histamine release from ECL cells (Brenna and Waldum, 1991; Prinz et al., 1994b), which potentiates gastrin stimulation of parietal cells (Soll, 1982). In the present study, equimolar doses (10 nmol/kg/hr) of somatostatin, SMS-201-995 and NC-8-12 were less effective inhibitors of histamine-stimulated acid secretion than of pentagastrin-stimulated acid secretion. These results are consistent with studies in isolated cells, in which 100-fold higher concentrations of somatostatin were required to inhibit [¹⁴C]aminopyrine uptake by parietal cells in vitro (Park et al., 1987) than histamine release from ECL cells stimulated by gastrin (Chuang et al., 1993; Prinz et al., 1994a). Somatostatin-induced acid inhibition by activation of sst₂ receptors appears to be more effective against acid-stimulatory pathways that converge on parietal cells than against acid-stimulatory pathways directly on parietal cells.

Because histamine is an important mediator of the acid response to secretagogues, we investigated whether activation of sst₂ receptors also regulates histamine release. Activation of sst₂ receptors by NC-8-12 inhibited pentagastrin-stimulated histamine release coincident with inhibition of pentagastrin-stimulated acid secretion. Few studies have measured histamine release in response to gastrin in vivo; one study using vascularly perfused rat stomach showed that somatostatin inhibits gastrin-stimulated histamine release (Sandvik and Waldum, 1988). This effect presumably occurs at the gastrin-responsive histamine stores in the stomach, because the somatostatin receptors found on ECL cells of the fundic mucosa (Reubi et al., 1992) have been identified as being sst₂ receptors (Prinz et al., 1994a). Furthermore, somatostatin is known to block gastrin-induced histamine release in vitro from isolated dog (Chuang et al., 1993) and rat (Prinz et al., 1994b) gastric ECL cells, but not from mast cells (Chuang et al., 1993). Therefore, it is likely that the predominant effect of somatostatin on gastrin-stimulated acid secretion is mediated primarily by activation of sst₂ receptors on ECL cells and inhibition of histamine release.

In contrast to pentagastrin, doses of somatostatin and NC-8-12 that produced nearly maximal inhibition of bethanecol-stimulated acid output had no significant effect on bethanecol-stimulated histamine release. Although in some assays activation of sst₂ receptors potently inhibits neural, perhaps cholinergic, activity (McKeen et al., 1994; Feniuk et al., 1995), to our knowledge there is little evidence demonstrating that somatostatin is an effective inhibitor of cholinergically stimulated histamine release from ECL cells (Sandor et al., 1995). Unlike gastrin and histamine, cholinergic pathways are thought to stimulate acid secretion (Schubert et al., 1992), in part by inhibiting somatostatin release (Schubert et al., 1987, 1989; Makhlouf and Schubert, 1990). Furthermore, weak inhibition of bethanecol-stimulated histamine release despite strong inhibition of bethanecol-stimulated acid output suggests that parietal cells are the likely target of somatostatin-induced inhibition of cholinergic stimulation (Park et al., 1987). This may be explained by the fact that in parietal cells acetylcholine activates a muscarinic type 3-G protein complex that promotes calcium conductance and stimulates acid secretion (Kajimura et al., 1992), whereas somatostatin activates a sst₂-G protein complex to block calcium conductance and inhibit acid secretion (Reisine, 1995).

Although histamine release in response to bethanecol was not measurably different than that in response to pentagastrin, ECL cells are thought to be principally activated by gastrin (Chuang et al., 1992). In contrast, cholinergic agonists tend to have a weaker effect than gastrin, although this has been little studied (Brenna and Waldum, 1991; Sandor et al., 1995). We believe that, in contrast to gastrin, cholinergic stimulation of acid secretion is mediated principally by a direct effect on parietal cells and less by an indirect mechanism through the release of histamine. Consistent with this are findings of a previous study that showed that an H₂ antagonist was a poor inhibitor of bethanecol-stimulated acid secretion (Lloyd et al., 1992).

We believe that, using our experimental model, acid inhibition by somatostatin and the somatostatin analogs was caused by a direct effect on the acid secretory apparatus of the stomach (e.g., parietal cells and ECL cells), rather than an indirect effect on vascular smooth muscle resulting in vasoconstriction and a decrease in gastric mucosal blood flow. Although we did not measure gastric mucosal blood flow in our study, an earlier report by Leung and Guth (1985) in anesthetized rats, using the hydrogen gas-clearance technique, demonstrated that i.v. infusion of somatostatin in doses that mimic those we used had no effect on basal corpus or antral mucosal blood flow and instead increased corpus mucosal blood flow while inhibiting pentagastrin-stimulated acid secretion. Differences in results from the study by Leung and Guth and other studies showing opposite effects were explained by lower doses of somatostatin, the timing of the observations, the method of measurement (e.g., [¹⁴C]aminopyrine clearance, which is related to changes in the hydrogen gradient as well as changes in blood flow) and species differences. Furthermore, other investigators (Schubert et al., 1989; Schubert and Hightower, 1989) corroborated the assumption that somatostatin alters acid secretion without affecting mucosal blood flow, by showing in isolated, luminally perfused, mouse stomach that somatostatin inhibited histamine-stimulated acid secretion.

The role of somatostatin and activation of sst₂ receptors in the regulation of stomach function is complex. For example, sst₂ gene expression increases during fasting and achlorhydria in the antral and corpus mucosa of rats, although somatostatin synthesis and release increases and decreases, respectively, under these conditions (Sandvik et al., 1995). Furthermore, functional characterization of two isoforms of the sst₂ receptor, sst_2a (Yamada et al., 1992a) and sst_2b (Vanetti et al., 1992), and possibly a third truncated isoform (Sandvik et al., 1995) in the stomach that arise by post-translational processing remains elusive because of the lack of selective agonists that could be used to determine their activity.

The changes in acid secretion and histamine release induced by the somatostatin receptor agonists are likely due to peripheral effects and not central actions. It is generally believed that the somatostatin analogs do not cross the blood-brain barrier when administered i.v. and instead elicit their effects in the periphery. When analogs are administered systemically in doses similar to those used in our study, the effects are often different than when the analogs are given centrally. For example, activation of sst₃ receptors by BIM-23056 given centrally increases acid output but has no significant effect when the analog is given in a much larger i.v. dose (Rossowski et al., 1994; Martinez et al., 1995). Furthermore, substantially higher doses of somatostatin given centrally are needed to cross the blood-brain barrier and to evoke effects peripherally (Tannenbaum and Patel, 1986). In addition, centrally administered NC-8-12 potentiates the inhibitory effect of BIM-23052 on basal acid output in rats, which by itself has no effect (Martinez et al., 1995, 1996). Because acid secretion is a physiological response mediated by central and peripheral pathways, it is possible that simultaneous administration of combinations of different somatostatin receptor agonists in the brain and the periphery may help to define more precisely the action of somatostatin in the regulation of stomach function.