Plasma Somatostatin-like Immunoreactivity Research Articles

Somatostatin decrease associated with growth hormone increase after TRH injection in constitutionally tall adolescents

12 adolescents referred for excessive height prediction were studied. Somatostatin like immunoreactivity (SLI) and plasma GH levels were measured during saline infusion and after TRH injection. SLI was measured after extraction by RIA with antibody against 14 cyclic somatostatin. In 6 subjects, TRH induced a sharp increase of GH from 2.3 ± 0.6 ng/ml to 23 ± 3.6 ng/ml; SLI dropped by 60% averaging 12.1 ± 5.8 pg/ml before and 4.8 ± 4.1 pg/ml during the 20 to 60 min after TRH injection. In 6 other patients TRH induced no significant changes of plasma GH (3.1 ± 1.1 ng/ml) nor SLI (11.3 ± 4.1 pg/ml). During the saline infusion mean levels of SLI were the same in the 2 groups of subjects (12 ± 4.8 pg/ml). 5 GH responders subjects undergone bromocriptine treatment (5 mg/day). After 3 months of this treatment the paradoxical rise of GH and the drop in SLI levels after TRH injection were suppressed. It is thus hypothesized that the GH release after TRH in tall adolescents is due to a decrease inhibitory effect of somatostatin at the pituitary level.

Role of circulating somatostatin in regulation of gastric acid secretion, gastrin release, and islet cell function. Studies in healthy subjects and duodenal ulcer patients.

Studies were designed (a) to determine whether somatostatin is released into the circulation after meals in sufficient amounts to regulate gastric or pancreatic islet function in humans and (b) to investigate the possible role of somatostatin in the pathogenesis of duodenal ulcer disease. Mean plasma somatostatin-like immunoreactivity (SLI) increased from 6.2 +/- 1.5 pg/ml to a peak level of 13.8 +/- 1.3 pg/ml in eight healthy subjects after a 1,440-cal steak meal (P less than 0.005). When somatostatin-14 was infused intravenously, basal and food-stimulated gastric acid secretion and also basal and food-simulated plasma insulin and glucagon concentrations were reduced significantly at mean plasma SLI concentrations within the range seen after a meal. Thus, the amount of somatostatin reaching the systemic circulation after a steak meal was sufficient to inhibit gastric acid secretion and islet cell function. On the other hand, basal and food-stimulated plasma gastrin concentrations were reduced by intravenous somatostatin only at plasma SLI concentrations that were several-fold greater than post-prandial SLI concentrations. Although duodenal ulcer patients had significantly higher basal, food-stimulated, and peak pentagastrin-stimulated gastric acid secretion rates than healthy controls, duodenal ulcer patients and controls had nearly identical basal and food-stimulated SLI concentrations. Moreover, food-stimulated gastric acid secretion and gastrin release were inhibited by intravenous somatostatin to the same extent in ulcer patients and controls. These studies suggest that duodenal ulcer patients release normal amounts of somatostatin into the circulation and that target cells controlling acid secretion and gastrin release are normally sensitive to somatostatin in these patients.

Brain, pancreas, and plasma somatostatin-like immunoreactivity (SLI) in fowls

Brain, pancreas, and plasma somatostatin-like immunoreactivity (SLI) in fowls

Infusion of graded concentrations of somatostatin in man: pharmacokinetics and differential inhibitory effects on pituitary and islet hormones.

The present study was undertaken to determine the plasma levels of somatostatin-like immunoreactivity (SLI) during constant infusion of graded concentrations of synthetic somatostatin-14 (S-14); to determine the half-life (t1/2) and metabolic clearance rate (MCR) of SLI; to correlate the plasma SLI levels with the degree of inhibition of pituitary and islet hormone secretion and to establish whether the plasma SLI levels capable of inhibiting pituitary and islet hormone secretion fall into the physiological range. Four normal subjects on separate occasions were each infused with saline or S-14 (25,50 and 75 micrograms/h) at a constant rate for 2 1/2 h. Thirty min following the infusions, TRH (200 micrograms) and arginine (0.5 g/kg) were given i.v. Blood samples were drawn every 15 min for measurement of GH, TSH, insulin, glucagon and SLI (by RIA of acid-ethanol extracted plasma) and at rapid intervals for 10 min after stopping the infusions for measurement of SLI disappearance. During S-14 infusions, plasma SLI rose rapidly, reached a plateau from 15-150 min and declined rapidly on cessation of the infusions with a mean t 1/2 of 2.72 +/- 0.45 min. Mean plateau SLI levels were: 149 +/- 3 pg/ml (25 micrograms/h), 465 +/- 35 pg/ml (50 micrograms/h), and 1244 +/- 71 pg/ml (75 micrograms/h). SLI was cleared rapidly but the MCR exhibited a dose-dependent decrease from 3225 +/- 699 ml/min for the 25 micrograms infusion to 1249 +/- 241 ml/min for the 75 micrograms/h infusion (P less than 0.05). The 25 micrograms/h infusion dose produced near-maximal suppression of GH secretion and inhibited insulin secretion but not TSH or glucagon secretion. The intermediate dose significantly inhibited GH, TSH, and insulin but not glucagon whereas the 75 micrograms/h infusion suppressed all four hormones. In six normal subjects endogenous plasma SLI rose from a basal value of 32.5 +/- 4.9 pg/ml to 75.5 +/- 9.0 pg/ml following ingestion of a mixed meal. This level was 50% of that resulting from the 25 micrograms/h infusion and which suppressed GH almost completely. We concluded that: Infused S-14 is cleared rapidly and decays with a short t 1/2; S-14 inhibits its own MCR; The somatotrophs are the most sensitive to S-14 inhibition, followed by the thyrotrophs and the B-cells (approximately equally) followed by the A-cells; Fluctuations in plasma SLI occurring physiologically may influence GH and possibly other S-14 sensitive cells by an endocrine mechanism.

Characterization of somatostatin in peripheral and portal plasma in the duck: in-vivo metabolism of somatostatin-28 and -14.

A sensitive and specific radioimmunoassay without an extraction step was developed for somatostatin in duck plasma. Degradation of Tyr1-125I-labelled somatostatin-14 (S-14) averaged 2% for blood collected with EDTA and zymofren. Recovery of somatostatin-like immunoreactivity (SLI), added to the plasma, averaged 91% for S-14 and 86% for S-28. Chromatographic analysis of portal plasma on Sephadex G-25 showed three peaks: one peak coeluted with cytochrome c (mol. wt 12500) in the void volume and was called 'big' somatostatin; of the two smaller forms, one coeluted with synthetic S-28 and the other with synthetic S-14; these were immunologically and physicochemically indistinguishable from synthetic S-28 and S-14. In peripheral plasma only the large form of somatostatin, 'big' somatostatin, was found. The mean portal plasma concentration of SLI was 4.1 +/- 0.41 microgram/l (n = 11, range 2.8-5.1 micrograms/l). In peripheral plasma the SLI concentration was 1.05 +/- 0.45 microgram/l (n = 11, range 0.84-1.2 microgram/l). The metabolic clearance rate, distribution volume and calculated half-life values were 63.1 +/- 14 ml/kg per min, 40.9 +/- 8.9 ml/kg and 1.06 +/- 0.19 min for S-14 compared with 45.7 +/- 7 ml/kg per min, 14.8 +/- 2.5 ml/kg and 2.14 +/- 0.54 min for S-28. These results indicated that S-28 was cleared from plasma at a slower rate than S-14 in the duck. It was concluded that: (1) portal plasma SLI was four times higher than peripheral SLI; (2) SLI in portal plasma existed as 'big' somatostatin, S-28 and S-14, whereas in peripheral plasma it existed mainly as 'big' somatostatin; (3) in-vivo studies indicated that S-28 was metabolized less rapidly than S-14.

Contribution of the pancreas to circulating somatostatin-like immunoreactivity in the normal dog.

These studies were performed to assess the contribution of the pancreas to the somatostatin-like immunoreactivity (SLI) circulating in arterial and portal venous plasma. Basal SLI concentrations in arterial, pancreatic venous, and portal venous plasma were 95 +/- 9, 277 +/- 32, and 130 +/- 12 pg/ml, (means +/- SEM), respectively. Measurement of pancreatic and portal venous blood flow (5 +/- 1 vs. 365 +/- 46 ml/min) and hematocrit allowed calculation of net, base-line SLI output from the right lobe of the pancreas (521 +/- 104 pg/min) and from the gastrointestinal tract (8,088 +/- 1,487 pg/min), which suggested that the contribution of the pancreas to circulating SLI was minor when the D cells were not stimulated. To stimulate the secretion of SLI from both pancreatic and nonpancreatic sources, isoproterenol, a beta-adrenergic agonist, was infused intravenously for 1 h into six anesthetized dogs. Arterial SLI increased by 52 +/- 9 pg/ml; superior pancreatico-duodenal venous SLI increased by 380 +/- 95 pg/ml; portal venous SLI increased by 134 +/- 14 pg/ml. Pancreatic venous blood flow remained unchanged at 5 +/- 1 ml/min, but portal venous blood flow increased to 522 +/- 62 ml/min. SLI output from the right lobe of the pancreas increased by 684 +/- 227 pg/min and that from the gastrointestinal tract increased by 23,911 +/- 3,197 pg/min, again suggesting that the pancreas was a minor source of circulating SLI even when the D cells were stimulated. We conclude that the measurement of arterial-venous SLI concentrations, in the absence of measurements of organ blood flow, can give a false impression of the organ's contributions of circulating SLI. To verify that the contribution of the pancreas was negligible, six dogs received an acute pancreatectomy and then an intravenous infusion of isoproterenol at the same rate. In these dogs, both the base-line level of SLI in arterial plasma (109 +/- 12 pg/ml) and the increment during isoproterenol (56 +/- 8 pg/ml) were similar to those of normal dogs. Likewise, in pancreatectomized dogs both the base-line level of SLI in portal venous plasma (129 +/- 16 pg/ml) and the increment during isoproterenol (174 +/- 34 pg/ml) were similar to those of normal dogs. We conclude that, in normal dogs, the pancreas makes a negligible contribution to the basal and stimulated level of SLI in arterial and portal venous plasma and therefore that these levels should not be used as an index of secretory activity of the pancreatic D cells.

Characterization and measurement of plasma somatostatin-like immunoreactivity in a sahelian lizard ( Varanus exanthematicus) during starvation

1. 1. Blood samples were withdrawn from different areas, i.e. from splenic vein (exit of primitive intrasplenic islet), pancreatic vein, portal vein and abdominal vein (peripheral venous level). 2. 2. Plasma somatostatin levels were estimated in all regional samples by radioimmunoassay. Splenic vein values (2642 ± 285 pg/ml) were markedly higher than those of pancreatic vein (1741 ± 247 pg/ml) and a negative gradient was observed between portal and peripheral plasma levels. 3. 3. Serial dilution displacement curves and gel-chromatography of extracted plasma revealed that circulating somatostatin was heterogeneous. A high molecular weight form appeared predominant in all the areas studied. This “large” form, eluted in fractions close to synthetic somatostatin-28, seemed to be the single molecular form present in splenic vein while in pancreatic and abdominal veins two components of somatostatin-28 and -14 molecular size can be observed. 4. 4. Results are discussed from a phylogenetic point of view and focused on high levels and high molecular forms of somatostatin in blood issued from primitive pancreatic structures.

Effect of naloxone on pancreatic and gastric endocrine function in response to carbohydrate and fat-rich test meals

The present study was designed to determine the effect of naloxone, a specific opiate receptor antagonist, on postprandial levels of insulin, glucagon, pancreatic polypeptide (PP), somatostatin-like immunoreactivity (SLI) and gastrin in response to carbohydrate and fat-rich test meals in a group of 6 healthy volunteers. The addition of naloxone to a meal consisting of 50 g sucrose dissolved in 200 ml water augmented the rise of plasma insulin levels significantly during the first 30 min after its ingestion and reduced the decrease of plasma glucagon. During the ingestion of a fat-rich meal in form of 200 ml cream naloxone reduced the rise in plasma insulin and pancreatic polypeptide and elevated glucagon levels during the last 30 min of the experimental period. When sucrose was dissolved in 200 ml cream the addition of naloxone augmented the postprandial rise of insulin levels between 15 and 60 min after ingestion of the meal and elicited an increase of plasma SLI and PP levels throughout the entire experimental period which indicates that post-prandial levels of insulin, glucagon, PP and SLI are modulated via endogenous opiate receptors during the ingestion of carbohydrate and fat test meals and that this effect depends on the composition of the ingested nutrients. These data raise the possibility that endogenous opiates participate in the regulation of postprandial insulin, glucagon, somatostatin and pancreatic polypeptide release not only in certain disease states as demonstrated recently for insulin secretion in type II diabetes mellitus but endogenous opiates may also be of importance under physiological conditions.

Response of plasma somatostatin-like immunoreactivity (SLI) to a 75 g oral glucose tolerance test in normal subjects and patients with impaired glucose tolerance.

To study the role of somatostatin in the pathophysiology of glucose intolerance in man, plasma somatostatin-like immunoreactivity (SLI) was measured in 8 normal subjects, 6 patients with insulin dependent diabetes mellitus (IDDM), 13 with non-insulin dependent diabetes mellitus (NIDDM), and 9 with hyperthyroidism, by extraction of plasma SLI and radioimmunoassay. The extraction method gave a recovery rate for synthetic somatostatin-14 and somatostatin-28 of 72 +/- 6 and 55 +/- 7%, respectively. No SLI corresponding to somatostatin-28 in human peripheral blood was observed. Incubation of somatostatin-28 in plasma gave a rapid decrease of immunoreactivity, and no conversion to somatostatin-14 was observed. It is speculated that SLI extracted with acid-acetone mainly represents a molecular weight similar to somatostatin-14. After oral administration of glucose (75 g), a clear and sustained rise in plasma SLI was seen in normal subjects from an initial value (+/- SEM) of 29.9 +/- 5.4 pg/ml to a peak value, at 60 min of 93.4 +/- 15.5 pg/ml. The increase of plasma SLI after 75 g glucose was also observed in IDDM and NIDDM. The peak level of SLI was significantly less than that for normal subjects. The extraction of plasma SLI with acetic acid and acetone gave reproducible results and showed a fluctuation of SLI with glucose concentration.

Somatostatin inhibits rat hepatic T4-5'-deiodinase. The effect is independent of the associated hypoinsulinemia.

Somatostatin decreases the serum 3,5,3'-triiodothyronine (T3) concentration in athyreotic subjects treated with L-thyroxine (T4). The present study was performed to determine the effect of somatostatin on T4-5'-deiodinase activity in rat tissue homogenate preparations. This enzyme is an important regulator of T3 production. Continuous somatostatin infusion at high dose (4 micrograms/kg per min subcutaneously) and low dose (0.8 micrograms/kg per min subcutaneously) for 48-72 h significantly increased (P less than 0.001) the mean aorta plasma somatostatin-like immunoreactivity concentration to 786 +/- 65 and 448 +/- 58 pg/ml, respectively compared with the normal mean of 69 +/- 17 pg/ml in the carbohydrate-fed rat (20% glucose in water ad lib.). The mean hepatic T4-5'-deiodinase activity at both 48 h (100 +/- 5 pmol/min per 100 mg protein) and 72 h (90 +/- 7 pmol/min per 100 mg protein) was significantly reduced in the high-dose group (P less than 0.005), compared with the mean enzyme activity in the glucose-fed control group (138 +/- 6 pmol/min per 100 mg protein). There was a negative correlation (r = -0.9, P less than 0.01) between the alterations in the peripheral plasma somatostatin-like immunoreactivity concentration and hepatic T4-5'-deiodinase activity. High-dose somatostatin did not consistently lower the serum T3 concentration in the glucose-fed rat. Somatostatin had no effect on pituitary T4-5'-deiodinase activity in the glucose-fed rat. High-dose somatostatin also significantly inhibited (P less than 0.01) the glucose-refeeding reactivation of hepatic T4-5'-deiodinase in the 72-h-fasted rat. The mean enzyme activity after 96 h was 96 +/- 8 pmol/min per 100 mg protein compared with 127 +/- 4 pmol/min per 100 mg protein in the refed control group. Somatostatin had a similar inhibitory effect on serum T3. There was a positive correlation (r = 0.5, P less than 0.01) between the somatostatin-induced alterations in serum T3 and hepatic T4-5'-deiodinase during refeeding. A significant positive correlation (r = 07, P less than 0.005) was noted between the somatostatin effect on hepatic T4-5'-deiodinase activity and the induced hypoinsulinemia in the fed group. In addition, a significant negative correlation (r = -0.9, P less than 0.001) was noted between the suppressed enzyme activity and the serum glucose/insulin ratio in the refed group. However, although low-dose somatostatin also induced the same degree of hypoinsulinemia (P less than 0.05) in the fed and refed groups it had no effect on hepatic T4-5'-deiodinase activity. Furthermore, despite the induction of hyperinsulinemia during refeeding, the high dose somatostatin inhibitory effect on enzyme activity persisted. Thus, somatostatin inhibited hepatic T4-5'-deiodinase activity in the carbohydrate-fed rat and prevented the carbohydrate-refeeding normalization of enzyme activity in the 72-h-fasted rat. The effect of somatostatin on enzyme activity was independent of the associated hypoinsulinemia. In the carbohydrate-fed animal the somatostatin effect was selective, as the hormone had no effect on pituitary T4-5'-deiodinase activity. These data suggest that somatostatin could play a role in the peripheral metabolism of thyroid hormones.

Circulating somatostatin concentrations in healthy and non-insulin-dependent (type II) diabetic subjects.

A technically simple radioimmunoassay for somatostatin-like immunoreactivity (SLI) in human plasma, which uses polyethylene glycol 6000 to precipitate tracer-degrading enzymes and high molecular weight interfering components, is described and validated. Fasting concentrations of SLI in peripheral plasma were similar in healthy and non-insulin-dependent diabetic (NIDD) subjects (range 3-39 pg/ml) but showed fluctuations for individuals in both groups when sampled frequently over a 60-min period. The mean coefficient of variation of SLI concentration was, however, less (8.0%; range 3-13%) in the diabetic group than in age-matched healthy subjects (19.5%; range 11-44%) (P less than 0.02). Thus the regulation of basal D cell activity or of the somatostatinergic nervous system may be altered in type II diabetic subjects. A hospital breakfast (550 kcal) was a weak stimulus for release of SLI in both healthy and diabetic subjects. The rise in plasma SLI was sustained over 120 min. An increment in plasma SLI over mean basal levels of 5.9 +/- 0.8 pg/ml was observed in young (mean age 23 yr; N = 6) control subjects, 7.9 +/- 2.0 pg/ml in older (mean age 55 yr; N = 7) control subjects and 5.9 +/- 0.6 pg/ml (mean age 62 yr; N = 7) in diabetics. Oral glucose (75 g) produced only a transient and erratic rise in plasma SLI that was not significant at any time point for either healthy or NIDD subjects. It is concluded that further investigations are necessary before hypotheses linking somatostatin deficiency with the hormonal and metabolic abnormalities of non-insulin-dependent diabetes can be accepted.

Hyperinsulinemia in obesity: Lack of relation to gastric emptying of glucose solution or to plasma somatostatin levels

To investigate some factors that may be related to the hyperinsulinemia of obesity, we measured fractional gastric emptying rates and changes of circulating levels of glucose, insulin, and somatostatin-like immunoreactivity (SLI) following the intragastric instillation of glucose in age-matched obese and nonobese Pima Indians with normal glucose tolerance. Results for the nonobese Pimas were also compared with findings for age- and weight-matched Caucasians with normal glucose tolerance. The levels of fasting plasma insulin and the integrated insulin response to glucose were significantly greater ( P < 0.01) in obese than in nonobese Pimas. Mean rates of fractional gastric emptying, both in the basal state and after the glucose load, were similar for the three groups. The fractional gastric emptying rates after a glucose load were strongly correlated with the integrated responses of both plasma glucose and insulin in the nonobese Caucasians ( r = 0.88, 0.90; P < 0.01) but not in either Pima group. There were no significant differences in peripheral plasma SLI for any of the three groups, either in the basal state or after the glucose load. These findings suggest that the hyperinsulinemia of established obesity is not mediated by alterations in the gastric emptying rate of liquids or by peripheral plasma SLI concentrations. They do not, however, exclude defects in gastric emptying of solid foods. Nor do they exclude the possibility that gastric or D-cell abnormalities exist during the period of fat accumulation but recede after obesity is established.

Inhibitory effect of intragastric glucose on gastric acid secretion and gastric emptying of liquids in man. Role of endogenous somatostatin, gastrin, and insulin.

Intragastric glucose inhibits gastric acid secretion and gastric emptying in man. To determine if these effects are mediated by somatostatin--a known inhibitor of gastric acid production, gastrin secretion, and gastric motility--the plasma somatostatin-like immunoreactivity (SLI) levels were determined in healthy human subjects after an intragastric load of 30% glucose solution. These findings were compared with results after an instillation of distilled water. Following the glucose load, the intragastric acid concentration, the acid output, and the fractional gastric emptying rate declined significantly (P less than 0.01) before either the plasma glucose or plasma insulin levels had increased. Neither the gastrin nor SLI plasma concentrations changed after the water or glucose load. These findings suggest that the suppression of gastric acid secretion and inhibition of the rate of gastric emptying that occur with intragastric glucose are mediated by factors other than changes in the peripheral circulating levels of SLI, gastrin, insulin, or glucose.

Effect of β-Casomorphins on Somatostatin Release in Dogs*

In the present study, the effects of orally administered beta-casomorphins (beta-CM) and met-enkephalin on postprandial plasma somatostatin-like immunoreactivity (SLI) were assessed in conscious dogs. The intragastric instillation of a liver extract-sucrose test meal containing 12 mg beta-CM or 10 mg met-enkephalin, respectively, augmented the postprandial rise of peripheral vein plasma SLI levels significantly. This effect was inhibited by the additional administration of the specific opiate-receptor antagonist, naloxone. When liver extract-sucrose was dissolved in fresh bovine milk the increase of plasma SLI levels was significantly greater than liver extract-sucrose dissolved in water. This milk-induced augmentation of SLI levels was also reduced by naloxone. Since these opiate-active compounds have an influence upon insulin release when given iv, the effect of beta-CM-7, beta-CM-5, beta-CM-4 beta-CM-4-amide, and met-enkephalin on SLI levels was assessed during their iv infusion at a rate of 1 nmol/kg . h during an iv background infusion of a glucose-amino acid mixture. The infusion of beta-CM-5 elicited a stimulation of peripheral vein SLI levels, whereas the infusion of met-enkephalin resulted in a significant decrease of SLI levels. beta-CM-7, beta-CM-4, and beta-CM-4-amide had no effect on plasma SLI levels at the dose employed. The present data demonstrate that in dogs the ingestion of opiate-active peptide stimulates postprandial SLI release, indicating that nutrient-contained opiate-active material (exorphins) might participate in the regulation of postprandial gastrointestinal endocrine function.

Plasma somatostatin 28 increases in response to feeding in man.

Tissue somatostatin-like immunoreactivity (SLI) consists of a number of molecular species including the cyclic tetradecapeptide or SRIF, an N-terminally extended form of SRIF termed somatostatin-28, as well as larger precursor peptides. The function and nature of circulating SLI is not well understood. In this report, we describe techniques for the definition of the components of plasma SLI in normal human plasma. Plasma SLI measured after gel filtration on Bio-gel P-6 columns was found to consist of from 1-3 peaks. The void volume peak was present in greatest concentration (34.2 +/- 8.9 pg/ml) and did not increase in response to a mixed meal. Very low levels of two additional peaks of SLI activity were found. To further characterize these peaks, 10-ml plasma samples were extracted and concentrated on octadecylsilyl silica (C-18) cartridges with subsequent fractionation on Bio-gel P-6 columns. The two peaks that coeluted with synthetic SRIF and S-28 markers, respectively, were present in concentrations of 5.4 +/- 1.4 and 4.8 +/- 1.9 pg/ml in fasting plasma. In response to a mixed meal, the SLI14 peak doubled (12.9 +/- 2.4 pg/ml) while the SLI28 peak increased to 29.9 +/- 7.2 pg/ml at 120 min. These results provide evidence that S-28 circulates in human plasma and its increase after feeding is consistent with a possible biological role for this peptide.

Somatostatin 1–28 circulates in human plasma

The gel filtration profile of immunoreactive somatostatin in human plasma in the fasting state is not well established as a consequence of insufficient sensitivity of the combined chromatography and radioimmunoassay procedures usually employed. We here report the gel filtration profiles of plasma samples after somatostatin concentration by batchwise immunoaffinity chromatography. The results clearly and reliably document the presence of a circulating peptide in human plasma with a gel permeation chromatography profile identical to the one of synthetic somatostatin 1–28. Approximately 46% of the total somatostatin-like immunoreactivity in plasma is due to this component.

Opposing actions of glucagon and insulin on splanchnic d cell function.

The effect of glucagon at various infusion rates on plasma levels of somatostatin-like immunoreactivity (SLI) was examined in conscious normal and chronic alloxan diabetic dogs. In normal dogs glucagon infused at 6, 36, and 120 ng/kg per min did not affect the peripheral venous plasma SLI levels. In diabetic dogs, however, peripheral venous plasma SLI levels in inferior vena cava rose significantly from a mean base-line value of 181+/-9 pg/ml to a peak value of 279+/-38 pg/ml during the infusion of 120 mug/kg per min of glucagon, which raised plasma immunoreactive glucagon to >5,000 pg/ml. This glucagon-mediated increase was completely abolished by coinfusion of 7 mU/kg per min of insulin, a rate that maintained the ratio of insulin to glucagon at approximately the baseline value. In anesthetized normal dogs the concentration of SLI in the venous effluent of the pancreas, the gastric fundus and the antrum increased significantly with each infusion rate of glucagon using including the lowest rate of 4 ng/kg per min, which raised the plasma level of glucagon to 395+/-19 pg/ml. This stimulatory effect on SLI secretion was completely abolished by insulin coinfusion at a rate designed to maintain the insulin to glucagon ratio at approximately the baseline value, but the effect of a high 90-ng/kg per min infusion on pancreatic and gastric SLI release was not suppressed by coinfusion of 10 mU/kg per min insulin. These results suggest that glucagon stimulates splanchnic D cells unless insulin secretion is proportionally stimulated and suggests that the splanchnic D cell is a common target upon which the two hormones exert opposing actions. The loss of insulin inhibition of glucagon-mediated somatostatin secretion may account for the hypersomatostatinemia of severe diabetes.

Fluctuations in basal levels and effects of altered nutrition on plasma somatostatin.

Rapid oscillations in basal plasma levels of insulin, C-peptide, glucagon, and glucose in rhesus monkeys have been reported previously. We now report large minute-to-minute fluctuations in plasma somatostatinlike immunoreactivity (SLI) in undisturbed, chair-adapted, and chronically cannulated male rhesus monkeys. Plasma samples were drawn from eight monkeys at 1- to 2-min intervals for 30-40 min following an overnight fast. Large fluctuations in SLI levels with periods of 4.4-9.4 min/cycle and possibly secondary periods of 14-18 min/cycle with average amplitudes of +/- 23% relative to the respective mean SLI levels were observed both within a single experiment and across monkeys under basal conditions. No consistent relationship was found between SLI fluctuations and that of insulin or glucagon. When portal and central blood samples were drawn precisely simultaneously no consistent portal-central gradient was found. Patterns of fluctuations in central and portal SLI levels were not correlated. Overfeeding significantly increased insulin levels but did not alter SLI levels. After 5 days of food deprivation basal SLI levels were unchanged. These data indicate the need to exercise care in the interpretation of single samples in the measurement of plasma SLI levels. Somatostatin secreted into the blood under basal, unstimulated conditions appears to be unrelated to nutritional balance.

Origin of peripheral venous hypersomatostatinemia in alloxan-diabetic dogs.

Basal and postprandial somatostatin-like immunoreactivity (SLI) is elevated in the peripheral venous plasma of chronic alloxan-diabetic dogs. To determine if this hypersomatostatinemia was the consequence of increased somatostatin release from the pancreas, stomach, or both, plasma SLI was measured in the pancreaticoduodenal, antral, and fundic veins and in the inferior vena cava in response to stimulation by a gastric liver test meal, followed by an intragastric HCl load. Basal and postprandial inferior vena caval plasma SLI levels were significantly higher than the control level (P less than 0.05-0.001), confirming earlier findings. Basal pancreatic venous SLI was 780 +/- 45 pg/ml in the diabetic dogs and 493 +/- 65 pg/ml in the controls (P less than 0.02). In response to the liver meal at pH 7, the incremental pancreatic venous SLI level in the diabetic dogs was 1630 +/- 95 pg/ml, and after HCl it rose to 9479 +/- 384 pg/ml compared to 938 +/- 80 and 4677 +/- 192 pg/ml, respectively, in the controls (P less than 0.02 and P less than 0.005). However, antral and fundic venous SLI levels in the diabetic dogs did not differ from the controls in either basal or stimulated states. The present data demonstrate that the pancreas and not the stomach is the probable source of the peripheral hypersomatostatinemia of alloxan-diabetic dogs.

Radioimmunoassay of human plasma somatostatin.

Human plasma somatostatin-like immunoreactivity (SLI) is heterogeneous. Moreover, human plasma contains a somatostatin-binding protein, enzymes that degrade the peptide, and other poorly characterized substances that interfere in radioimmunoassays of the peptide. Previously described extraction methods only partially circumvent these problems and generally result in less than adequate recovery. The present studies were therefore undertaken to determine whether gel filtration of plasma could be used to avoid these difficulties and provide a means for radioimmunoassay of plasma somatostatin in man. Gel filtration (Sephadex G25) of human plasma and subsequent radioimmunoassay of column eluates using five different antisera uniformly demonstrated two fractions of SLI in plasma. One fraction (peak I) eluted in the void volume and had an apparent molecular weight in excess of 150,000 daltons; this material comprised most of the plasma SLI, did not dilute in parallel with synthetic somatostatin, and decreased immunoprecipitability of both N-tyrosine- and tyrosine-11-125I somatostatin. The second fraction (peak II) coeluted with synthetic somatostatin (apparent mol. wt. 1600 daltons), diluted in parallel with synthetic somatostatin, and did not appreciably degrade either 125I-tyrosylated analogue. Synthetic somatostatin added to plasma was recovered nearly completely (95 ± 3%) in peak II SLI. Infused synthetic somatostatin increased peak II SLI but did not alter peak I SLI. These results demonstrate that gel chromatography of human plasma before assay can be used to circumvent problems of immunoheterogeneity, 0degradation, and nonspecific interference while providing adequate recovery. With this method, mean plasma somatostatin concentrations (peak II SLI) in postabsorptive normal volunteers ranged from approximately 40 to 100 pg/ml, depending on the antibody system used. These concentrations (3–6 × 10−11 M) are of similar magnitude to thosereported to affect reputed target tissues of somatostatin in vitro and are thus consistent with a possible hormonal function for the peptide.