Somatostatin, gastrin, and cholinergic muscarinic binding sites in rat gastric, duodenal, and jejunal mucosa

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

Immunohistochemical localization of different forms of somatostatin in the gastrointestinal tract of the calf

The presence of two peptides that belong to the somatostatin family has been investigated in the calf gut. Somatostatin-14-like and Somatostatin-28-like peptides have been localized by a light microscopic immunohistochemical method. The method employed antibodies linked to colloidal gold particles that were revealed by a silver-enhancement step. Somatostatin-14-like peptide was only present in mucosal endocrine cells, which were detectable along the entire gut with the exceptions of the abomasal gastric proper glands and caecum. The cells were most abundant in cardiac and pyloric glands. Langerhans' islets also contained this type of endocrine cell. Somatostatin-28-like-immunoreactive endocrine cells were more abundant than the former cell type. They were present in the gastric proper glands and caecum where Somatostatin-14-like-immunoreactive cells were absent. They were as numerous as the former type of cell in the endocrine pancreas. The Somatostatin-28-like peptide was also detectable in the intramural nervous components of the abomasum and the intestine, in both perikarya and terminals. Our results show a possible heterogeneity of an endocrine cell type, which synthesizes and secretes somatostatin peptides. Our results also support the hypothesis that somatostatin-14 and somatostatin-28 peptides may have distinct functional roles, particularly in different species.

Serum gastrin level and gastric somatostatin content and binding in long-term pyloromyotomized children

Since somatostatin inhibits basal and stimulated gastric acid secretion and gastrin release, it is conceivable that decreased gastric somatostatin concentration may be one of the factors responsible for gastric hypersecretion found in patients who have undergone long-term pylorotomy for hypertrophic pyloric stenosis. To investigate this proposal the somatostatin-like immunoreactivity concentration was determined in antral and fundic mucosa samples from control and long-term pyloromyotomized children. In addition, somatostatin binding to cytosol from gastric (fundus and antrum) mucosa and fasting serum gastrin levels and serum gastrin response to a standard breakfast were also studied. The mean fundic and antral somatostatin-like immunoreactivity concentrations were significantly lower in long-term pyloromyotomized children than in control children. The depletion of fundic and antral somatostatin-like immunoreactivity content was associated with an increase in the number of gastric somatostatin binding sites. The fasting serum gastrin levels and serum gastrin response to a standard breakfast (after 60 min) in long-term pyloromyotomized children was significantly higher than those in control children. Since, together with the increase of somatostatin binding to gastric mucosa, there is an increase in the gastrin serum levels, despite the inhibitory effect of somatostatin on gastrin release, the binding capacity cannot be the main factor determining the response to somatostatin in long-term pyloromyotomized children. The present results suggest that both somatostatin and gastrin have some pathophysiologic importance in long-term pyloromyotomized children.

Somatostatin binding and modulation of adenylate cyclase in ovine retina membranes

Somatostatin (SS) receptors in membranes from ovine retinas were examined using 125I-Tyr11-SS as a ligand. Receptor binding was rapid, specific, saturable, reversible and dependent on temperature and membrane concentration. Conditions of apparent equilibrium were obtained at 25 degrees C after a 45 min incubation in the presence of about 0.25 mg membrane protein/ml. Native SS competitively inhibited the binding of 125I-Tyr11-SS in the range of 0.01-10 nM, and half-maximal inhibition was observed at 0.2 nM SS. Scatchard analysis of these data suggested the existence of a single population of SS receptors with a dissociation constant of 0.23 +/- 0.03 nM and a maximum binding capacity of 84 +/- 6 fmol/mg protein. The binding of 125I-Tyr11-SS was inhibited by various synthetic SS analogs in a dose-dependent manner whereas peptides unrelated to SS did not show practically any effect even at concentrations as high as 10(-6) M. SS receptor occupancy appears to be coupled to inhibition of adenylate cyclase activity by a guanine nucleotide-binding regulatory protein, as suggested by the facts that: (a) SS noncompetitively inhibited the stimulatory effect of vasoactive intestinal peptide (VIP) (3 x 10(-7) M) on membrane adenylate cyclase activity but it did not alter basal enzyme activity; and (b) the addition of guanosine 5'-triphosphate (GTP) (10(-5) M) decreased the specific binding of 125I-Tyr11-SS to 26.6% of the control value due to a decrease in SS receptor affinity. The present results support the hypothesis that SS may contribute to the physiological regulation of the functions of the retina.

The gastrin receptor assay

Gastrin is a major physiologic regulator of gastric acid secretion and growth of the oxyntic mucosa. Biologically active radiolabelled hormones may be used to characterize and localize receptors for peptide hormones. The cellular localization of the gastrin receptor in the fundic mucosa, however, is still a matter of great debate owing to difficulties in developing a gastrin receptor binding assay. Despite considerable work in several laboratories, the criteria for true receptor binding have not yet been fulfilled. The preparation of a suitable tissue receptor material (plasma membranes or isolated cells) from the heterocellular fundic mucosa seems to be the major problem. This problem may be related to the fact that the receptors are only present on the enterochromaffin-like cells (ECL), which constitute but a minor fraction of the cells in the oxyntic mucosa. Furthermore, the second messenger of gastrin is still not known, and the poor functional responsiveness of isolated cells and the oxyntic glands to gastrin further complicates the evaluation of the gastrin receptor. In this review the different steps in the gastrin receptor assay (the labelling of gastrin, preparation of the receptor, and the incubation and correlation of the binding and biologic effect of gastrin) are discussed.

Somatostatin receptors and the effect of somatostatin on histamine-stimulated adenylate cyclase activity in isolated gastric glands of guinea pigs

To investigate the mechanism of the suppressive action of somatostatin on gastric acid secretion, we determined the somatostatin binding and adenylate cyclase activity on crude membrane fractions of isolated gastric glands of guinea pigs. The binding studies using 125I-Tyr11 somatostatin showed the presence of somatostatin receptors with a single high affinity on crude membrane fractions. The dissociation constant (KD) for somatostatin was 1.05 +/- 0.13 nM, and the number of binding sites (Bmax) was 6.98 +/- 1.27 fmol/mg protein (mean +/- SE, n = 6). The adenylate cyclase activity was increased by histamine, which was completely inhibited by 10(-3) M cimetidine. Somatostatin non-competitively suppressed the histamine-stimulated adenylate cyclase activity in the presence of guanosine 5'-triphosphate (GTP). These results suggest that somatostatin inhibits histamine-stimulated acid secretion through the inhibition of the adenylate cyclase system, via somatostatin receptor and guanine nucleotide binding protein, which is activated by GTP binding.