Release Of Immunoreactive Somatostatin Research Articles

Release of immunoreactive somatostatin from hypothalamic cells in culture: inhibition by gamma-aminobutyric acid.

Primary cultures of dispersed hypothalamic cells were prepared from embryonic rats to study the release of immunoreactive somatostatin. The immunoreactive somatostatin content of these cultures increased during the first 2 weeks after plating and was readily measurable for several weeks thereafter; this material was characterized by gel permeation and reverse-phase chromatography. Depolarization of the cells with 60 mM K+ or with veratridine resulted in a calcium-dependent release of immunoreactive somatostatin which cochromatographed with synthetic somatostatin on reverse-phase chromatography. Tetrodotoxin blocked the veratridine-evoked release. However, even in the absence of exogenous stimuli, immunoreactive somatostatin was released by the cells into the medium. More than 70% of this tonic release was found to be calcium dependent and to be inhibited by tetrodotoxin, indicating that spontaneous electrical activity in the cultures leads to a release of immunoreactive somatostatin. gamma-Aminobutyric acid inhibited the tonic release of immunoreactive somatostatin and this was reversed by bicuculline. These findings support the hypothesis that gamma-aminobutyric acid inhibits somatostatin release in vivo.

Acetylcholine inhibits the release of somatostatin from rat hypothalamus in vitro.

Acetylcholine, at concentrations of 10(-10)--10(-7) M, inhibited the release of immunoreactive somatostatin (SRIF) from rat hypothalamic segments which had been maintained in short term culture for 24 h. Neostigmine (10(-6) M), an anticholinesterase, also inhibited the release of SRIF, whereas atropine (10(-6) M), a muscarinic anticholinergic, had no effect on basal SRIF release but blocked the inhibition caused by acetylcholine (10(-8) M). However, hexamethonium (10(-6) M), a nicotinic antagonist, did not abolish the inhibition induced by acetylcholine. Potassium depolarization (56 mM KCl) caused stimulation of SRIF release, which was dependent on the presence of calcium in the incubation medium. SRIF was measured by a RIA sensitive to 1 pg/tube. Authenticity of immunoreactive SRIF released was suggested by immunological parallelism and chromatographic criteria using gel and high pressure liquid systems. These results suggest that muscarinic cholinergic mechanisms may have a regulatory role modulating the secretion of SRIF and, consequently, GH through actions at a hypothalamic level.

The effect of glucose and tolbutamide on immunoreactive somatostatin release from perifused pancreatic islets of normal and streptozotocin diabetic rats.

The patterns of immunoreactive somatostatin (IRS) responses to glucose and tolbutamide were investigated in perifused pancreatic islets of both normal and diabetic rats.The incubation chamber of the perifusion apparatus for the islets was devised with a 2.5 ml disposable syringe. By the method of Lacy, the pancreatic islets were isolated from diabetic rats about 30 days after a single intravenous injection of streptozotocin (STZ, 60 mg/kg body weight) and from normal rats of comparable body weight. The fasting blood glucose was 437±3 mg/100 ml in the diabetic rats and 120±5 mg/100 ml in normal ones. The concentration of IRS in each effluent was measured by the specific double antibody radioimmunoassay for somatostatin.During the initial 20 min incubation period with the medium containing 2.8 mM glucose, the levels of IRS were 11.3±1.0 pg/100 islets/2 min in normal islets and 32.0±3.4 pg/100 islets/2 min in diabetic islets. The latter value was significantly higher than the former (p<0.001). IRS levels gradually rose following 16.7 mM glucose and reached the maximal values with 30.2±5.7 pg/100 islets/2 min at 66 min in normal islets and with 56.8±13.3 pg/100 islets/2 min at 88 min in diabetic islets. The levels of IRS in diabetic islets were significantly higher than those of normal islets (p<0.001) throughout the experimental period. In the case of tolbutamide the IRS levels increased promptly to 51.9±6.6 pg/100 islets/2 min in diabetic islets and to 23.7±4.6 pg/100 islets/2 min in normal islets 2 min after initiation of the infusion. The former value was significantly higher than the latter (p<0.001). The levels of immunoreactive glucagon (IRG) in response to tolbutamide were significantly higher in the diabetic islets than in normal islets (p<0.001). These results indicate that there exists a hypersecrecretion of IRS from STZ-induced diabetic rats during the infusion with glucose and tolbutamide.

Enhancement of immunoreactive somatostatin release into hypophysial portal blood by electrical stimulation of the preoptic area in the rat.

Electrical stimulation of the preoptic area resulted in a significant increase in the secretion rate of immunoreactive somatostatin (IRS) and its concentration in the hypophysial portal blood of urethane-anesthetized rats. In contrast, stimulation of the ventromedial nucleus did not cause any changes in portal immunoreactive somatostatin. The flow rate of portal blood also increased significantly by electrical stimuli to the preoptic area but not to the ventromedial hypothalamus. These findings support the view that the preoptic area plays an inhibitory role in the regulation of GH secretion by increasing the release of somatostatin into hypophysial portal vessels in the rat.

Immunoreactive somatostatin release from rat spinal cord in vitro

A calcium-dependent release of immunoreactive somatostatin from rat spinal cord in vitro in response to two depolarising stimuli (60 mM KCl and 75 μM veratrine) has been demonstrated. Released somatostatin immunoreactivity comprised 0.53% of total tissue content, showed parallelism when serial dilutions were compared to the immunoassay dose-response curve and eluted similarly to synthetic somatostatin on Sephadex G-25 (f) chromatography. T These results provide further evidence for a neurotransmitter or neuromodulator role for somatostatin in mammalian spinal cord.

Stimulation by growth hormone of somatostatin release from the rat hypothalamus in vitro.

Growth hormone (GH) has been shown to cause a dose-dependent increase in the release of immunoreactive somatostatin from the rat hypothalamus in vitro, thus providing further evidence that GH may be involved in a "short loop" feedback, controlling its own secretion via hypothalamic somatostatin release.

Potassium stimulated calcium dependent release of immunoreactive somatostatin from incubated rat hypothalamus.

Somatostatin (somatotropin release inhibiting factor, SRIF) is present in the median eminence of the hypothalamus in high concentration (Kronheim et al., 1976), is visualized in nerve endings (Hökfelt et al., 1974) and has been found to be concentrated in the synaptosome fraction of hypothalamic homogenates (Epelbaum et al., 1977; Berelowitz et al., 1978), suggesting a true neurosecretory role. To further explore this possibility we have studied the release of immunoreactive SRIF from the incubated rat hypothalamus (Bradbury et al., 1974: Rotsztein et al., 1977), basally and in response to depolarising concentrations of potassium, and have assessed the calcium dependence of this release.

Effects of glucose and arginine on the release of immunoreactive somatostatin from the isolated perfused rat pancreas

Effects of glucose and arginine on the release of immunoreactive somatostatin from the isolated perfused rat pancreas

The effects of gastrin, gastric inhibitory polypeptide, secretin, and the octapeptide of cholecystokinin upon immunoreactive somatostatin release by the perfused canine pancreas.

The effects of gastrin, gastric inhibitory polypeptide, secretin, and the octapeptide of pancreozymin-cholecystokinin on immunoreactive somatostatin release were studied in the isolated perfused dog pancreas. Gastrin at a concentration of 65 ng/ml and the octapeptide of pancreozymin-cholecystokinin at a concentration of 25 ng/ml produced a prompt, but transient statistically significant, twofold rise in mean somatostatin concentration. Secretion at a concentration of 0.3 U/ml and gastric inhibitory polypeptide concentration of 58 ng/ml produced a prompt two- to threefold rise in mean somatostatin release, which persisted throughout the perfusion period. With all four polypeptides the pattern of the somatostatin response resembled that of insulin. It appears that pancreatic somatostatin release is stimulated by gastrointestinal hormones that influence the secretion of insulin and glucagon.

Release of immunoreactive somatostatin from the pancreas in response to glucose, amino acids, pancreozymin-cholecystokinin, and tolbutamide.

The effects of glucose, amino acids, pancreozymin-cholecystokinin, and tolbutamide upon the release of immunoreactive somatostatin (IRS) from the isolated perfused pancreas were studied. In seven experiments in which glucose was perfused either at a concentration of 100 or 350 mg/dl or at 25 mg/dl, IRS levels were significantly greater at the higher glucose concentrations. In three dose-response experiments in which the perfusing glucose concentration was increased at 30-min intervals from an initial concentration of 25 mg/dl to a final concentration of 300 mg/dl, progressive increases in IRS release were noted at glucose concentrations of 100 mg/dl and above. Perfusion of a 20 mM mixture of 10 amino acids also elicited a prompt and significant biphasic IRS rise in each of six experiments. In five experiments, 20 mM leucine evoked a similar response in mean IRS. Perfusion with 0.075 Ivy U/ml of pancreozymin-cholecystokinin, with or without the presence of a 1 mM 10-amino acid mixture, elicited a prompt rise in IRS with a pattern resembling that of insulin in a total of six experiments. Tolbutamide (0.75 mg/min) also stimulated IRS release in five of six challenges. The IRS responses to nutrients and to pancreozymin and their similarity to the insulin responses raise the possibility that, like insulin, pancreatic somatostatin may have an endocrine role related to nutrient homeostasis.

Pancreatic immunoreactive somatostatin release.

The location of the somatostatin-containing D-cells of the pancreatic islets between the A- and B-cells suggests that their function might be to inhibit insulin and/or glucagon secretion by these neighboring cells. To determine if insulin and/or glucagon, in concentrations that might be present in the extracellular space surrounding the D-cells, stimulate immunoreactive somatostatin (IRS) release, we perfused 10 microng of glucagon or 10 milliunits of insulin per ml in 11 isolated dog pancreases, for 40 min in seven experiments and for 100 min in four experiments. In eight of the nine experiments in which glucagon was perfused, a prompt and significant rise in mean IRS release, ranging from 71 to 128% above the control level, was observed. In the eight experiments in which insulin was perfused. IRS did not increase during the first 40 min; in the two 100-min insulin experiments, it did rise during the final 50 min, however. To determine the effect of an A- and B-cell secretogogue on IRS release, we perfused 20 mM arginine for 60 min in six experiments. In all, IRS rose within 3 min and reached a level 71-465% above the control, remaining significantly elevated throughout the perfusion, while glucagon and insulin rose to peak levels at 2 min and then declined somewhat despite continuing arginine perfusion. The results indicate that perfusion of the normal dog pancreas with high doses of glucagon or arginine is accompanied by a prompt increase in IRS release and are compatible with a local feedback circuit involving A- and D-cells. Insulin appears not to augment IRS release, at least not promptly, but IRS stimulated by local endogenous glucagon could inhibit the B-cell response to locally secreted glucagon and thereby influence the composition of the insulin/glucagon secretion mixture.

Stimulation of immunoreactive somatostatin release from hypothalamic synaptosomes by high (K+) and dopamine.

Effects of high (K+) and dopamine on the release of immunoreactive somatostatin from isolated hypothalamic synaptosomes were studied in rats. High (K+) (60 mM) and dopamine (10(-6) M) in the incubation media stimulated the release of immunoreactive somatostatin and the former effect was completely abolished by the removal of Ca++ from the media. These suggest that hypothalamic somatostatinergic synaptosomes preserved at least one of the important basic properties of secretory cells. Although it is of interest to note that dopamine stimulated the release of somatostatin. Its physiological significance awaits further studies.