Properties of endogenous somatostatin-like immunoreactivity and synthetic somatostatin in dog plasma

The effects of the antimuscarinic drugs pirenzepine and atropine on plasma portal levels of somatostatin and gastrin in the dog

The effects of the antimuscarinic drugs pirenzepine and atropine on somatostatin and gastrin portal levels under basal conditions and during bethanechol infusion have been investigated in anesthetized dogs. Iv bolus administration of pirenzepine (1 mg/kg) or atropine (0.1 mg/kg), decreased gastrin concentrations, but did not affect basal somatostatin levels. During 120 min of bethanechol infusion (160 micrograms/kg/h) gastrin levels increased but somatostatin levels were unchanged. Pirenzepine (1 mg/kg iv bolus), administered at the 60th min of bethanechol infusion, decreased the gastrin concentrations, and markedly enhanced somatostatin levels. Under the same conditions atropine (0.1 mg/kg iv bolus) decreased gastrin levels, but had little or no effect on somatostatin levels. These results indicate that muscarinic receptors with similar affinity for pirenzepine and atropine mediate excitatory cholinergic influences on gastrin release. By contrast, muscarinic receptors with higher affinity for pirenzepine seem to be involved in the cholinergic inhibition of somatostatin release: by selectively blocking these receptors, pirenzepine may increase portal somatostatin levels.

Protein-binding of secretin in human plasma

Protein-binding of endogenous plasma secretin and of 125I-labelled secretin incubated with charcoal-treated plasma examined by gel filtration on a Sephacryl S-200 Superfine column (16 X 980 mm) showed that secretin in plasma appears both to be bound to at least two different plasma proteins where albumin appears to be the major binding protein, and also to occur as a free molecular form. In addition, protein-binding studied by incubating 125I-labelled secretin with charcoal-treated plasma under various conditions followed by charcoal separation of bound from free label indicated the presence of more specific secretin-binding sites on the plasma proteins with an avidity comparable to that otherwise reported for albumin as a binding protein. The protein-binding of 125I-labelled secretin was optimal or reached equilibrium after 2 days incubation at 20 degrees C and first after 8 days incubation at 4 degrees C. Also, the protein-binding of 125I-labelled secretin was higher at an incubation temperature of 20 than of 4 degrees C; was optimal at pH 7.4; increased with increasing amounts of charcoal-treated plasma up to an amount of 800 microliters in our assay system before levelling off; and increased in a constant and predictable manner with increasing amounts of 125I-labelled secretin at least with the amounts of labelled secretin examined here.

Muscle cell electrical hyperpolarization and reduced exercise hyperkalemia in physically conditioned dogs

Contracting muscle cells release K ions into their surrounding interstitial fluid, and some of these ions, in turn, enter venous plasma. Thereby, intense or exhaustive exercise may result in hyperkalemia and potentially dangerous cardiotoxicity. Training not only reduces hyperkalemia produced by exercise but in addition, highly conditioned, long-distance runners may show resting hypokalemia that is not caused by K deficiency. To examine the factors underlying these changes, dogs were studied before and after 6 wk of training induced by running on the treadmill. Resting serum [K] fell from 4.2 +/- 0.2 to 3.9 +/- 0.3 meq/liter (P less than 0.001), muscle intracellular [K] rose from 139 +/- 7 to 148 +/- 14 meq/liter (P less than 0.001), and directly measured muscle cell membrane potential (Em) in vivo rose from -92 +/- 5 to -103 +/- 5 mV (P less than 0.001). Before training, resting Em of isolated intercostal muscle in vitro was -87 +/- 5 mV, and after incubation in 10(-4) M ouabain, Em fell to -78 +/- 5 mV. After training, resting Em of intercostal muscle rose to -95 +/- 4, but fell to -62 +/- 4 mV during incubation in 10(-4) M ouabain. The measured value for the Em was not completely explained by the increased ratio of intracellular to extracellular [K] or by the potassium diffusion potential. Skeletal muscle sarcolemmal Na,K-ATPase activity (microM inorganic phosphate mg-1 protein h-1) increased from 0.189 +/- 0.028 to 0.500 +/- 0.076 (P less than 0.05) after training, whereas activities of Mg2+ -dependent ATPase and 5'nucleotidase did not change. In untrained dogs, exercise to the point of exhaustion elevated serum [K] from 4.4 +/- 0.5 to 6.0 +/- 1.0 meq/liter (P less than 0.05). In trained dogs, exhaustive exercise was associated with elevation of serum [K] from 3.8 +/- 0.3 to 4.2 +/- 0.4 (NS). The different response of serum [K] to exercise after training was not explainable by blood pH. Basal insulin levels rose from 7.0 +/- 0.7 microU/ml in the untrained dogs to 9.9 +/- 1.0 microU/ml (P less than 0.05) after training. Although insulin might have played a role in the acquired electrical hyperpolarization, the reduced exercise-produced hyperkalemia after training was not reversed by blockade of insulin release with somatostatin. Although the fundamental mechanisms underlying the cellular hyperpolarization were not resolved, our observations suggest that increased Na-K exchange across the sarcolemmal membrane, the increase of Na,K-ATPase activity and possibly increased electrogenicity of the sodium pump may all play a role in the changes induced by training.

Arginine infusion increases peripheral plasma somatostatin in man

A somatostatin (SRIF) radioimmunoassay is described, using an antiserum raised in rabbit, reacting more with SRIF-14 than -28. Glass tubes were employed for the assay because our tracer, 125I 1-Tyr-SRIF, was adsorbed to plastic (23% non specific binding). Vycor extraction was used, and following Sephadex G-50 chromatography, the plasma extract showed two forms, coeluting with SRIF-28 and -14. Fifteen healthy subjects, eight women and seven men, 21-39 years old, received an infusion of arginine chloride (2.38 mmol/kg) for 20 min, or saline. An immediate rise in plasma SRIF from the mean basal level at 2 min was shown. Maximal value was 24.0 +/- 3.0 pmol/l (P less than 0.01) after 10 min followed by a rapid descent towards the basal level after the infusion. A temporal relationship was observed between the SRIF, insulin and glucagon responses following arginine infusion, while the GH levels increased only after the SRIF levels had declined, indicating an inhibition of GH. This is further supported by the high degree of correlation (r = 0.87) between SRIF- and GH increments. It is suggested that plasma SRIF measurement during arginine infusion gives an estimate of the pancreatic SRIF releasing capacity.

Somatostatin release and plasma molecular somatostatin components in man

The present paper describes a sensitive, precise and specific radioimmunoassay method for measurements of plasma somatostatin; significant rises in plasma somatostatin following a test meal, intraduodenal infusion of fat and HCl, and intravenous injection of insulin; and separation of immunoreactive plasma somatostatin into two components probably representing bound and free molecular forms of somatostatin both in fasting and postprandial human plasmas.

On the accuracy of radioimmunological determination of somatostatin in plasma

We performed the following experiments to evaluate the accuracy of our newly developed radioimmunoassay for somatostatin: (1) Recovery of synthetic somatostatin added to human, porcine, and canine plasma with or without extraction with 67% acetone or 76% ethanol, using 3 different region-specific antibodies and, where applicable, 125I-labelled Tyr-1- or Tyr-11-substituted somatostatin or 125I-N-Tyr-somatostatin as tracers. The recovery of somatostatin corrected for losses inherent in the extraction procedure was close to 100%, and independent of species, antibody and tracer. Somatostatin 1-28 was extracted slightly less efficiently. Unextracted plasma interfered massively in the assay. (2) Pharmacokinetic experiments with infusion of somatostatin into 14 pigs and determination of metabolic clearance rate (MCR) and T-1/2. MCR was 27-38 ml/kg per min, independent of infusion rate (6.1 or 13 pmol/kg per min), extraction procedure or tracer. T-1/2 was 1.9 min. The infused somatostatin was not measurable in unextracted plasma. (3) Characterization of endogenous and exogenous, labelled and unlabelled somatostatin 1-14 in human plasma, using Sephadex G-50 columns at pH 7.5 and 9.0. Human plasma showed excess immunoreactivity eluting at the void volume whereas synthetic somatostatin was recovered quantitatively at the position of marker somatostatin when added to the plasma. The immunoreactivity of the tracers was decreased (125I-Tyr-11-somatostatin) or abolished (125I-N-Tyr- or 125I-Tyr-1-somatostatin) after incubation with plasma or void volume fractions of plasma subjected to gel filtration. Extracted plasma did not contain void volume immunoreactivity, but like whole plasma, small amounts of components which coeluted with intact somatostatin.

Hepatic and renal metabolism of somatostatin-like immunoreactivity. Simultaneous assessment in the dog

The hepatic and renal metabolism of somatostatin-like immunoreactivity (SLI) was assessed simultaneously in an in vivo dog model. The hepatic extraction of this peptide was 29.4 +/- 2.3% and was similar for endogenous and infused exogenous SLI. The renal extraction was 62.3 +/- 5%. The renal clearance of SLI was significantly greater than that of inulin indicating that the peptide is handled by peritubular uptake from postglomerular blood in addition to glomerular filtration. In both organs SLI extraction was not saturable even at arterial concentrations in excess of 100 times physiological range. The overall metabolic clearance rate of SLI was 19.7 +/- 1.6 ml/kg per minute of which 32.7 +/- 4.6% was contributed by hepatic and 37 +/- 4.9% by renal uptake mechanisms. The plasma half disappearance time of exogenously infused SLI was 1.9 +/- 0.3 min. The studies indicate that in the dog, the liver and kidney are both major sites of SLI metabolism, together accounting for 70.0 +/- 8.7% of the metabolic clearance of the peptide.

Plasma somatostatin-like immunoreactivity during the interdigestive period in the dog

To study possible physiologic relationships between somatostatin and the gastric interdigestive contractions (GIC), gastric motor activity, and plasma somatostatin-like immunoreactivity (SLI) concentration were determined simultaneously in four conscious dogs, each of which was studied on two separate occasions. Plasma SLI level was highest during the GIC period and lowest 60 and 80 min after the cessation of the GIC; the mean difference in plasma SLI was 41 +/- 6 pg/ml. When synthetic motilin, a known stimulus of GIC, was infused at a physiologic rate during the period in which plasma SLI levels were low, SLI rose to approximately the same values observed during the contraction period and GIC similar to those that occur spontaneously were observed. When synthetic somatostatin, a known inhibitor of endogenous motilin release, was infused at a rate that raised the plasma SLI to approximately the levels observed during the contraction period (0.1 microgram/kg per h), the appearance of the subsequent GIC was significantly delayed. These results are consistent with a physiological role for somatostatin in the regulation of GIC in dogs and suggest an interrelationship between motilin and somatostatin.

Somatostatin binding factor from chicken pancreas

A soluble somatostatin binding factor was detected in cell-free extracts from chicken pancreas. For binding measurements Tyr1-somatostatin was radio-labeled with 125I by the lactoperoxidase technique. Specific radioactivity of about 18.5 MBq/nmol was achieved. Maximal total binding is approximately 0.17 (B/T) in the presence of 30 mg/l pancreatic protein. The specific binding is 0.10 and is suppressed by addition of 1 mg/l synthetic cold cyclic somatostatin. The dose-response curve of synthetic cyclic somatostatin is in the range of 0.6-600 nmol/l. Ca2+ and reduced thiol-reagents inhibit the specific binding. Insulin, glucagon and corticotropin show a low, and luliberin and reduced somatostatin a high cross-reactivity. Molecular weight was estimated by gel filtration and the specific binding molecule was eluted at a Kav = 0.2 on an Ultrogel (AcA 54) column. This corresponds to Mr 40 000. Electrophoretic properties of the binding complex and semipurification by polyacrylamide disc gel electrophoresis: relative mobility of the 125I-Tyr-somatostatin binding complex is about 0.6. Relative mobilities of binding-protein fractions are 0.71 and 0.74. Highest relative specific binding was detected in the (100 000 g) cytosol fractions. Binding with cell-free extracts from the splenic lobe area was 4-fold higher than that from other parts of the chicken pancreas.

Clearance of immunoreactive somatostatin by perfused rat liver

Other investigators have demonstrated that concentrations of immunoreactive somatostatin (IRS) are higher in blood from the hepatic portal vein or its tributaries than in blood from the hepatic or peripheral systemic veins of man and animals. This suggests that there is hepatic extraction of IRS from the portal system in vivo. In the rat, portal vein plasma IRS is reported to be heterogeneous and to contain, in part, a 1,600 mol wt form of IRS which is immunochemically similar to synthetic somatostatin and not significantly bound to high molecular weight plasma protein. Our study was undertaken to determine directly whether unbound synthetic cyclic somatostatin was cleared by the rat liver perfused through the hepatic portal vein in vitro with a recirculating, plasma-free, erythrocyte-containing perfusate. At 37 degrees C and pH 7.40, perfusate IRS, at initial concentrations (1,728 pg/ml) within the range previously reported in rat portal venous blood, was removed by the liver at a rate commensurate with first-order kinetics. Hepatic clearance was 0.84+/-0.04 ml/min per g postperfusion wet weight (SE). Hepatic extraction was 36+/-2%, and t((1/2)) was 20.0+/-1.3 min. Recovery of IRS from the perfusate without the liver was >85%, excluding significant degradation by the medium. Clearance, extraction, and t((1/2)) of IRS were not changed by an unphysiologic IRS concentration (621,500 pg/ml), or by pharmacologic concentrations of insulin (8.2 muM) or glucagon (2.9 muM). The t((1/2)) was prolonged significantly to 28.2+/-1.9 and 45.6+/-4.7 min during perfusions at liver temperatures of 25 degrees and 16 degrees C, respectively. At 37 degrees C, the t((1/2)) was also significantly increased to 28.7+/-3.2 and 24.2+/-1.1 min at perfusate pH 7.06 and 6.78, respectively. These studies indicate that the rat liver clears unbound IRS from the perfusate by a first-order kinetic process that is (a) unsaturable at pharmacologic concentrations, (b) temperature-sensitive and, to a lesser extent, influenced by lowered pH, and (c) not affected by insulin and glucagon. The liver would appear to play an important role in the metabolism of the 1,600 mol wt form of somatostatin. Clearance of endogenous IRS by the liver should be considered in the interpretation of IRS concentrations in the peripheral systemic veins.

Role of histamine H2-receptors in gastric and pancreatic release of somatostatin-like immunoreactivity during the gastric phase of meal

The present study was designed to determine the role of H2-receptors in the postprandial release of somatostatin-like immunoreactivity (SLI) from the gastric fundus and antrum and from the pancreas. In dogs subjected to laparotomy, the pylorus was bisected and a gastric fistula was created, following which 250 ml 20% liver extract (LE) at pH 7 or 2 were instilled intragastrically. In the fundic vein the incremental SLI rise in response to LE at pH 7 was 2423 plus or minus 540 pg/ml during a control infusion of saline and 4780 plus or minus 863 pg/ml during the infusion of cimetidine (1 mg/kg per h) (P less than 0.05). In the antral vein the incremental SLI in response to LE at pH 7 was 2182 plus or minus 530 pg/ml during the saline control but did not rise significantly during cimetidine infusion. In the pancreatic vein the incremental SLI level after LE at pH 7 was 1953 plus or minus 358 pg/ml in the control experiments and 4430 plus or minus 1024 pg/ml during cimetidine infusion (P less than 0.025). The incremental inferior vena cava SLI level was approximately 925 pg/ml in both groups (not significant). The instillation of LE at pH 2 during the saline control lowered fundic vein SLI by 500 pg/ml; this decline was abolished during cimetidine infusion. In the antral vein the incremental SLI level of 15 750 plus or minus 2514 pg/ml during saline was lowered to only 6728 plus or minus 2257 pg/ml during cimetidine (P less than 0.025). After LE at pH 2 the incremental pancreatic vein SLI level of 5641 plus or minus 1175 pg/ml during the control infusion was also significantly reduced to 2392 plus or minus 559 pg/ml by cimetidine (P less than 0.05). The incremental SLI in the inferior vena cava was reduced from 1270 plus or minus 280 pg/ml during saline to 680 plus or minus 190 pg/ml when cimetidine was infused (P less than 0.05). The present data suggest a histaminergic influence via stimulation of H2-receptors upon the regulation of gastric and pancreatic somatostatin release during the gastric phase of a meal.

Evidence for a role of free fatty acids in the regulation of somatostatin secretion in normal and alloxan diabetic dogs

To investigate the effect of acute elevation of plasma free fatty acids (FFA) on the secretion of splanchnic somatostatin-like immunoreactivity (SLI), the peripheral venous, pancreatic, and gastric venous effluent levels of SLI were measured in normal and chronic alloxan diabetic dogs before and after the infusion of a fat emulsion supplemented with heparin. In normal conscious dogs heparin injected during the infusion of a fat emulsion elevated FFA levels from a mean (+/-SE) base-line level of 0.7+/-0.1 meq/liter to a peak value of 1.5+/-0.1 meq/liter (P < 0.001) and plasma SLI rose from a mean (+/-SE) base-line value of 145+/-7 pg/ml to a peak of 253+/-44 pg/ml (P < 0.05). Neither the infusion of glycerol, of fat emulsion without heparin, of heparin alone nor of saline itself had an effect on either the plasma level of FFA or SLI. In another group of anesthetized dogs with surgically implanted catheters the administration of fat emulsion plus heparin was accompanied by more than a two-fold rise in the concentration of SLI in the venous effluent of the pancreas and of the gastric fundus and antrum in association with an elevation of FFA levels. In a group of conscious diabetic dogs fat emulsion plus heparin raised FFA from a mean base-line level of 1.2+/-0.2 to 1.6+/-0.3 meq/liter (P < 0.05) and SLI rose from a mean base-line level of 185+/-9 pg/ml to a peak value of 310+/-44 pg/ml (P < 0.01). Although SLI levels were significantly greater than in normal dogs at several time points after the rise in FFA, the magnitude of the increment in diabetic dogs did not differ from normal. These results demonstrate that a rise in FFA levels is a potent stimulus for SLI secretion from the pancreas and stomach and raise the possibility that FFA is an important physiological regulator of SLI secretion.

Molecular forms of somatostatin in normal subjects and in patients with pancreatic somatostatinoma

Two patients with somatostatin-secreting pancreatic tumours are described, one presenting with hypoglycaemia due to hyperinsulinism, and the other with Cushing's syndrome due to ectopic ACTH production. When plasma from these patients was subjected to gel chromatography under conditions designed to prevent somatostatin binding to larger proteins, a peak of monomeric immunoreactive somatostatin was observed as well as several large molecular weight forms. These larger forms of somatostatin could be dissociated into monomeric somatostatin by dithiothreitol. Similar studies on plasma obtained from normal subjects also showed heterogeneity of circulating somatostatin. Extracts of tumour tissue from both patients contained predominantly monomeric somatostatin, but only small amounts of high molecular weight somatostatin which differed from the profile seen in plasma. The site(s) of origin of the large molecualr weight forms of somatostatin seen in plasma and their relative biological activities remain to be established.

Splanchnic somatostatin: a hormonal regulator of nutrient homeostasis

Free (approximately 1600 daltons) somatostatin-like immunoreactivity was identified in arterial plasma of dogs that had received a test meal. Neutralization of circulating somatostatin while the dogs were consuming a fatty meal increased the plasma concentrations of triglycerides, gastrin, pancreatic polypeptide, and insulin after the meal. It is concluded that, in the dog, somatostatin is a true hormone that regulates the movement of nutrients from the gut to the internal environment.

Somatostatin and gastrin release from canine stomach

Somatostatin and gastrin release into the veins draining the stomach was studied in 27 anaesthetized dogs. Basal somatostatin-like immunoreactivity (SLI) in corpus veins (136 +/- 36 pg/ml) was significantly higher than in antrum veins (83 +/- 20 pg/ml; p less than 0.05) and the femoral artery (58 +/- 15 pg/ml; p less than 0.02). During peptone, pH 6.5, perfusion of the stomach, SLI concentration increased significantly in the corpus veins to approximately four times basal and in the antrum veins to three times basal, whereas SLI levels in the peripheral circulation remained constant. Peptone, pH 3.5, and sodium oleate did not stimulate gastric SLI. Gastric distension increased significantly SLI release from the corpus. In gel filtration studies 50%--70% of SLI from gastric vein plasma samples but greater than 90% from femoral artery samples eluted in the void volume of Sephadex G-25 columns. Gastrin secretion was stimulated significantly only by peptone, pH 6.5.

Measurement, characterization, and source of somatostatin-like immunoreactivity in human amniotic fluid

Somatostatin-like immunoreactivity (SLI) is widely distributed in tissues and biological fluids. To determine whether SLI is also present in amniotic fluid, samples obtained by amniocentesis from 30 normal and 27 abnormal pregnancies were studied by radioimmunoassay. Direct incubation of [(125)I-Tyr(1)]tetradecapeptide somatostatin (SRIF) with amniotic fluid resulted in 89% tracer degradation. Damage was reduced to <5% when samples were acidified and boiled before the assay. With this technique, SLI was detectable in all normal amniotic fluid samples; the mean level at 15-20 wk of gestation (320+/-55 pg/ml, n = 15) being 4.5 times higher than the mean at 32-43 wk (70+/-12 pg/ml, n = 15) (P < 0.001). In cases of preeclampsia (n = 6), gestational diabetes (n = 5), anencephaly (n = 1), and meningomyelocele (n = 1), SLI values were in the normal range, but in one juvenile diabetic and one patient with chronic renal failure, SLI was undetectable (<10 pg/ml). In a pair of monochorionic diamniotic twins, SLI levels were very different (33 and 197 pg/ml), which suggests that fetal factors are more important than materno-placental ones in determining amniotic fluid SLI. Serial dilutions of amniotic fluid showed parallelism with standard SRIF. When concentrates of pooled amniotic fluid were chromatographed on Sephadex G-25 columns, all SLI eluted in the void volume ahead of SRIF even after treatment with 8 M urea and dithiothreitol. This "big" SLI incubated in amniotic fluid showed 100% stability over 24 h at 37 degrees C, whereas SRIF was rapidly inactivated (t((1/2)) congruent with 7 min). Extracts of placenta and fetal membranes contained no SLI, but small amounts (6-20% of total amniotic fluid SLI) were found in cells from fresh fluid. Radioimmunoassay of SLI in extracts of seven paired cord arterial and venous plasma samples showed no arteriovenous gradient consistent with fetal origin of cord blood SLI. It is concluded that (a) amniotic fluid contains SLI which is of fetal origin and (b) normal levels vary with gestational age. The SLI has a higher molecular weight (>/=5,000) and is more stable in amniotic fluid than SRIF.

Somatostatinoma syndrome. Biochemical, morphologic and clinical features

Diabetes mellitus, steatorrhea, cholelithiasis and a tumor distorting the duodenum prompted a work-up for somatostatinoma in a 52-year-old man. The responses of pancreatic B-cells but not of A-cells to nutrient stimuli were inhibited, and growth-hormone release was suppressed, suggesting somatostatin resistance in some target tissues. Plasma somatostatin-like immunoreactivity ranged from 9000 to 13,000 pg per milliliter (normal: 88+/-8, mean +/- S.E.M.) and was distributed in four molecular forms, including free somatostatin. The primary tumor contained 5 microgram of somatostatin-like immunoreactivity per milligram of wet tissue, distributed in three of the molecular forms noted in plasma. Plasma calcitonin was also elevated (4650 pg per milliliter; normal: less than 120). Immunocytochemical studies showed that cells of the primary tumor contained somatostatin and calcitonin but no other peptide hormones. Only somatostatin was present in the metastases. Somatostatin was localized electron microscopically in all secretory granules, irrespective of size and shape, whereas calcitonin was present only within a single subpopulation of small granules in the same cells.