Absence Of Somatostatin Research Articles

N-(2-methoxyphenyl) benzenesulfonamide, a novel regulator of neuronal G protein-gated inward rectifier K+ channels

G protein-gated inward rectifier K+ (GIRK) channels are members of the super-family of proteins known as inward rectifier K+ (Kir) channels and are expressed throughout the peripheral and central nervous systems. Neuronal GIRK channels are the downstream targets of a number of neuromodulators including opioids, somatostatin, dopamine and cannabinoids. Previous studies have demonstrated that the ATP-sensitive K+ channel, another member of the Kir channel family, is regulated by sulfonamide drugs. Therefore, to determine if sulfonamides also modulate GIRK channels, we screened a library of arylsulfonamide compounds using a GIRK channel fluorescent assay that utilized pituitary AtT20 cells expressing GIRK channels along with the somatostatin type-2 and -5 receptors. Enhancement of the GIRK channel fluorescent signal by one compound, N-(2-methoxyphenyl) benzenesulfonamide (MPBS), was dependent on the activation of the channel by somatostatin. In whole-cell patch clamp experiments, application of MPBS both shifted the somatostatin concentration-response curve (EC50 = 3.5nM [control] vs.1.0nM [MPBS]) for GIRK channel activation and increased the maximum GIRK current measured with 100nM somatostatin. However, GIRK channel activation was not observed when MPBS was applied to the cells in the absence of somatostatin. While the MPBS structural analog 4-fluoro-N-(2-methoxyphenyl) benzenesulfonamide also augmented the somatostatin-induced GIRK fluorescent signal, no increase in the signal was observed with the sulfonamides tolbutamide, sulfapyridine and celecoxib. In conclusion, MPBS represents a novel prototypic GPCR-dependent regulator of neuronal GIRK channels.

Gene expression profiling reveals a possible role for somatostatin in the innate immune response of the liver.

Somatostatin is a neuropeptide hormone that inhibits pituitary growth hormone (GH) release. Using microarray analysis of gene expression in the livers of wildtype control and somatostatin knockout mice, we have previously identified a panel of genes whose GH-dependent and sexually dimorphic expression patterns are significantly altered by the absence of somatostatin (1). Here, we provide methodological and analytical details of that study, the raw data of which is deposited in the Gene Expression Omnibus as data set GSE56520. In addition, we performed further gene ontology analysis of the data and found that the differential expression of a second subset of genes in the livers of somatostatin-knockout mice versus wildtype controls is likely independent of GH signaling and involved in the innate immune response.

Somatostatin inhibits activation of dorsal cutaneous primary afferents induced by antidromic stimulation of primary afferents from an adjacent thoracic segment in the rat

To investigate the effect of somatostatin on the cross-excitation between adjacent primary afferent terminals in the rats, we recorded single unit activity from distal cut ends of dorsal cutaneous branches of the T10 and T12 spinal nerves in response to antidromic stimulation of the distal cut end of the T11 dorsal root in the presence and absence of somatostatin and its receptor antagonist applied to the receptive field of the recorded nerve. Afferent fibers were classified based upon their conduction velocity. Mean mechanical thresholds decreased and spontaneous discharge rates increased significantly in C and Aδ but not Aβ fibers of the T10 and T12 spinal nerves in both male and female rats following antidromic electrical stimulation (ADES) of the dorsal root from adjacent spinal segment (DRASS) indicating cross-excitation of thin fiber afferents. The cross-excitation was not significantly different between male and female rats. Microinjection of somatostatin into the receptive field of recorded units inhibited the cross-excitation. This inhibitory effect, in turn, was reversed by the somatostation receptor antagonist cyclo-somatostatin (c-SOM). Application of c-SOM alone followed by ADES of DRASS significantly decreased the mechanical thresholds and increased the discharge rates of C and Aδ fibers, indicating that endogenous release of somatostatin plays a tonic inhibitory role on the cross-excitation between peripheral nerves. These results suggest that somatostatin could inhibit the cross-excitation involved in peripheral hyperalgesia and have a peripheral analgesic effect.

Characterization of the intracellular mechanisms mediating somatostatin and lanreotide inhibition of DNA synthesis and growth hormone release from dispersed human GH-secreting pituitary adenoma cells in vitro.

Somatostatin is an endogenous inhibitor of hormone secretion and cell proliferation. Treatment with somatostatin analogues in humans causes a reduction in size and secretory activity of endocrine tumours, including GH-secreting pituitary adenomas. This study was aimed to characterize the intracellular mechanisms mediating the in vitro antiproliferative and antisecretory effects of somatostatin and its analogue lanreotide, on primary cultures of GH-secreting pituitary adenoma cells. Thirteen GH-secreting pituitary adenoma postsurgical specimens were analysed for somatostatin receptor (SSTR) mRNA expression and a subset of them was analysed in vitro for the effect of somatostatin on cell proliferation, assessed by means of [3H]-thymidine uptake, and GH release, using an immunoradiometric assay. Moreover, the intracellular signalling involved in such effects has been studied. All the adenomas analysed expressed at least one somatostatin receptor subtype mRNA. SSTR2 mRNA was identified in 77% of the adenomas, SSTR1 and SSTR3 in 69% and SSTR5 in 60%. Somatostatin and lanreotide inhibited cell proliferation in phorbol ester (PMA)-stimulated conditions (10/13 adenomas), as well as after fetal calf serum (3/3 adenomas) or IGF-I stimulation (2/2 adenomas). Conversely, GHRH or forskolin treatments did not significantly affect DNA synthesis in adenoma cells in the presence or absence of somatostatin (2/2 and 4/4 adenomas, respectively). Vanadate pretreatment reversed somatostatin inhibition of PMA-induced DNA synthesis suggesting an involvement of tyrosine phosphatase in this effect (2/2 adenomas); this was confirmed by the direct induction of tyrosine phosphatase activity in two adenomas after somatostatin treatment. Somatostatin and also lanreotide caused significant inhibition of phorbol ester, forskolin, GHRH and KCl-dependent increase of GH secretion in the culture medium. Moreover, voltage-sensitive calcium channel activity induced by 40 mm KCl depolarization in microfluorimetric analysis, was significantly reduced (5/5 adenomas). These data show that somatostatin and lanreotide inhibit human GH-secreting pituitary adenoma cell proliferation and hormone release in vitro, and suggest that the activation of tyrosine phosphatases may represent intracellular signals mediating the antiproliferative effects and that the inhibition of the voltage-dependent calcium channels and adenylyl cyclase activities may control GH secretion.

Somatostatin Receptor Subtype 2 Gene Therapy Inhibits Pancreatic Cancer in Vitro

Background. Most human pancreatic adenocarcinoma cells do not express somatostatin receptors and somatostatin does not inhibit the growth of these cancers. We have demonstrated previously that somatostatin inhibits the growth of pancreatic cancers expressing somatostatin receptor subtype 2 (SSR2) but not receptor-negative cancers. SSR2 expression may be an important tumor suppressor pathway that is lost in human pancreatic cancer. We hypothesized that SSR2 gene transfer would restore the growth inhibitory response of human pancreatic cancer to somatostatin.Methods. We created adenoviral constructs containing the SSR2 or Lac-Z gene and transfected somatostatin receptor-negative human pancreatic cancer cells (Panc-1). Presence of functional cell surface SSR2 protein was assessed by whole-cell competitive binding assays. Parental cells, Lac-Z-transfected, and SSR2-transfected cells were cultured in the presence and absence of somatostatin. The rate of cell growth was determined by direct cell counting using a hemacytometer (n = 8 wells/group). Cells were analyzed for expression of tumor suppressor proteins by Western blot.Results. Panc-1 cells transfected with the SSR2 transgene demonstrated high-affinity specific binding of 125I-somatostatin at physiologic concentrations. Expression of somatostatin receptors caused 60% inhibition of cell growth compared with the Lac-Z virus-treated controls (P < 0.05 by Kruskal–Wallis/Bonferroni). There was no additional inhibition of cell proliferation with exogenous somatostatin. Furthermore, addition of somatostatin ligand antibody did not diminish the effect of SSR2 expression on cell proliferation. Western blot analysis revealed an upregulation of the cyclin-dependent kinase inhibitor p27 in the SSR2-transfected cells.Conclusions. Expression of SSR2 by human pancreatic cancer causes significant slowing of cell division by a mechanism independent of somatostatin. The mechanism may involve upregulation of known tumor suppressor proteins. Restoration of SSR2 gene expression deserves further study as a potential gene therapy strategy in human pancreatic cancer.

Nonhepatic response to portal glucose delivery in conscious dogs.

The glycemic and hormonal responses and net hepatic and nonhepatic glucose uptakes were quantified in conscious 42-h-fasted dogs during a 180-min infusion of glucose at 10 mg. kg(-1). min(-1) via a peripheral (Pe10, n = 5) or the portal (Po10, n = 6) vein. Arterial plasma insulin concentrations were not different during the glucose infusion in Pe10 and Po10 (37 +/- 6 and 43 +/- 12 microU/ml, respectively), and glucagon concentrations declined similarly throughout the two studies. Arterial blood glucose concentrations during glucose infusion were not different between groups (125 +/- 13 and 120 +/- 6 mg/dl in Pe10 and Po10, respectively). Portal glucose delivery made the hepatic glucose load significantly greater (36 +/- 3 vs. 46 +/- 5 mg. kg(-1). min(-1) in Pe10 vs. Po10, respectively, P < 0.05). Net hepatic glucose uptake (NHGU; 1.1 +/- 0. 4 vs. 3.1 +/- 0.4 mg. kg(-1). min(-1)) and fractional extraction (0. 03 +/- 0.01 vs. 0.07 +/- 0.01) were smaller (P < 0.05) in Pe10 than in Po10. Nonhepatic (primarily muscle) glucose uptake was correspondingly increased in Pe10 compared with Po10 (8.9 +/- 0.4 vs. 6.9 +/- 0.4 mg. kg(-1). min(-1), P < 0.05). Approximately one-half of the difference in NHGU between groups could be accounted for by the difference in hepatic glucose load, with the remainder attributable to the effect of the portal signal itself. Even in the absence of somatostatin and fixed hormone concentrations, the portal signal acts to alter partitioning of a glucose load among the tissues, stimulating NHGU and reducing peripheral glucose uptake.

Evidence for autoregulation of cholecystokinin secretion during diversion of bile pancreatic juice in rats

Background & Aims: The mechanism regulating cholecystokinin (CCK) secretion during prolonged diversion of bile pancreatic juice (BPJ) is unknown. We examined the hypothesis that the decrease of plasma CCK levels after prolonged diversion of BPJ is mediated by an increase in plasma somatostatin levels evoked by hypercholecystokinemia and somatostatin in turn inhibits CCK-releasing peptide (CCK-RP) bioactivity and decreases plasma CCK levels. Methods: Pancreatic secretion, plasma CCK levels, and somatostatin levels were monitored for 7 hours after diversion of BPJ in anesthetized rats. Secretion of CCK-RP bioactivity during diversion of BPJ was examined in the presence or absence of somatostatin. Results: Diversion of BPJ for 2 hours caused a 13- and 2.5-fold increase in plasma CCK and somatostatin levels. The increase in somatostatin levels was blocked by the CCK antagonist L364,718. At 5 hours after diversion of BPJ, plasma CCK and somatostatin levels and luminal CCK-RP bioactivity decreased to basal levels. The decrease in plasma CCK levels was prevented by the administration of a specific somatostatin antagonist. We also showed that the stimulatory effect of the CCK-RP bioactivity was eliminated when the donor rat was pretreated with somatostatin. Conclusions: Autoregulation of CCK secretion occurs during the diversion of BPJ and this is mediated by somatostatin, which inhibits the secretion of CCK-RP bioactivity and decreases plasma CCK levels.

Lack of effect of hyperglycemia on lipolysis in humans

To examine whether hyperglycemia is an independent regulator of adipose tissue lipolysis, we measured palmitate flux ([3H]palmitate) on two occasions in eight volunteers with insulin-dependent diabetes. On one. occasion, euglycemia was maintained for 4 h continuously; on a different occasion, hyperglycemia (plasma glucose, 12 mmol/l) was induced after 2 h of euglycemia. Palmitate flux decreased from 1.39 +/- 0.22 to 1.25 +/- 0.18 mumol.kg-1 x min-1 during sustained euglycemia and from 1.43 +/- 0.24 to 1.13 +/- 0.19 mumol.kg-1 x min-1 during the transition from the euglycemic to the hyperglycemic study intervals. There were no significant differences between the changes in palmitate flux from the first to the second study interval on the control (euglycemia-euglycemia) and experimental (euglycemia-hyperglycemia) study days and no difference between palmitate flux on different study days. Thus, in the face of euinsulinemia, euglucagonemia, and the absence of somatostatin, no effect of hyperglycemia on free fatty acid metabolism could be detected in humans.

Effects of phospholipase A2 inhibitors on coupling of alpha 2-adrenoceptors to inwardly rectifying potassium currents in guinea-pig submucosal neurones.

1. Noradrenaline hyperpolarizes guinea-pig submucosal neurones by opening inwardly rectifying potassium channels. Intracellular recordings were made from submucosal neurones and the possible involvement of the phospholipase A2 pathway in this response was examined. 2. The non-specific phospholipase A2 inhibitors, quinacrine (10 microM) and 4-bromophenacyl bromide (4-BPB, 10 microM) inhibited nerve-evoked inhibitory synaptic potentials (i.p.s.ps) and hyperpolarizations to somatostatin and UK 14304. Quinacrine and 4-BPB also blocked the inward rectification present in current-voltage curves in the absence of somatostatin or UK 14304. 3. The more selective phospholipase A2 inhibitor, cyclosporin A (10 microM) and the lipoxygenase and cyclo-oxygenase inhibitor, eicosatetraynoic acid (ETYA, 20 microM) and nordihydroguairetic acid (NDGA, 20 microM) did not alter i.p.s.ps or hyperpolarizations to UK 14304. 4. Exogenously applied arachidonic acid (1-300 microM) did not mimic the i.p.s.p. or the hyperpolarization to UK 14304. 5. We conclude that arachidonic acid or its eicosanoid metabolites produced by phospholipase A2 stimulation are unlikely to be involved in the receptor G-protein coupled activation of potassium currents in submucosal neurones. The inhibition of the noradrenaline-induced hyperpolarization by quinacrine and 4-BPB is most likely due primarily to blockade of the basal inwardly rectifying potassium conductance present in these neurones.

Canine Parietal Cell Binding by Antibodies to the Complementary Peptide of Somatostatin

Using antibodies to a complementary peptide of somatostatin, putative somatostatin binding proteins were characterized on canine parietal cells. A synthetic peptide (S-C1) was derived from the complementary mRNA sequence for somatostatin-14. Antiserum containing antibodies to S-C1 inhibited competitively 125I-Tyr11-somatostatin binding to canine oxyntic mucosal membranes. Canine parietal cell preparations were incubated with carbachol in the presence or absence of somatostatin and antisera to S-C1. Antibodies to S-C1 produced a decrease in carbachol-stimulated 14C-aminopyrine uptake comparable with that produced by 10(-6) M somatostatin. In immunocytochemical studies by light microscopy, antibodies to S-C1 produced positive staining of parietal cells throughout the oxyntic gland area. By electron microscopy using immunogold techniques, binding by antibodies to somatostatin C-1 was localized ultrastructurally to basolateral and intracellular membranes and to secretory canalicular membranes of parietal cells. These studies support the conclusion that antibodies to the somatostatin complementary peptide demonstrate properties similar to those of somatostatin in that they inhibit carbachol-stimulated aminopyrine uptake and 125I-somatostatin binding. Furthermore, these antibodies localize to specific regions on plasma membranes of parietal cells, which may represent somatostatin binding sites.

Common ionic mechanisms of excitation by substance P and other transmitters in guinea-pig submucosal neurones.

1. Intracellular recordings were made from submucosal neurones and single-electrode voltage-clamp methods were used to record membrane currents. The actions of substance P (SP), 5-hydroxytryptamine (5-HT), muscarine, vasoactive intestinal polypeptide (VIP), forskolin and nerve stimulation were studied. 2. Substance P, 5-HT (in the presence of 5-HT3 receptor antagonists), muscarine, VIP, forskolin and slow excitatory synaptic transmission all produced identical responses: an inward current associated with a membrane conductance decrease at the resting potential. The actions of any one occluded the actions of any other and all responses were pertussis-toxin insensitive. 3. These agonists produced a voltage-independent decrease in a 'leak' potassium conductance between -40 and -120 mV in 14% of neurones. 4. These agonists decreased a voltage-dependent, calcium-activated potassium conductance between -40 and -80 mV in all other (86%) neurones. The agonists still evoked an inward current without apparent conductance change at potentials between -90 and -130 mV. 5. In a low calcium solution containing cobalt or cadmium, the agonists produced an inward current associated with a conductance increase from -40 to -120 mV. Ion replacement studies indicated this current was due to an increase in a cation-selective (mainly sodium) conductance. 6. The agonists also reduced the inwardly rectifying potassium current that is activated by somatostatin and alpha 2-adrenoceptor agonists in these neurones. The agonists did not alter the inwardly rectifying potassium current that is present in these neurones in the absence of somatostatin or alpha 2-agonists. 7. Thus, SP, 5-HT, muscarine, VIP and the release of slow excitatory transmitters all appear to act through a common intracellular transduction pathway, an increase in adenylate cyclase. This results in an activation of a sodium-selective cation current and an inhibition of three distinct potassium conductances: the background potassium conductance, the calcium-activated potassium conductance and the inwardly rectifying potassium conductance activated by somatostatin and alpha 2-adrenoceptor agonists.

Somatostatin effects on cultured human fetal epiphyseal chondrocytes.

Somatostatin effects on cultured human fetal epiphyseal chondrocytes were evaluated by studying the effects of somatostatin on DNA synthesis. Cultured epiphyseal chondrocytes from human fetuses (12-40 wk old) were incubated for 48 h in Ham's F-12 serum-free medium. After this, the medium was replaced by MCDB-104 serum-free medium and the cells were incubated for an additional 48 h in the presence or absence of somatostatin 1 pM to 10 microM, with the addition of 3H-thymidine (5 microCi/mL) for the last 24 h of incubation. A significant (p < 0.02) inhibitory effect of somatostatin (1 nM to 10 microM) on 3H-thymidine DNA incorporation was observed in cultured chondrocytes from fetuses of all gestational ages studied (12-40 wk), with no significant differences among fetal ages. In conclusion, our results show that somatostatin exerts a biologic effect on cultured human fetal epiphyseal chondrocytes, as it does in its target cells. These results suggest that somatostatin could regulate human skeletal growth not only by growth hormone secretion regulation, but also by acting directly on chondrocyte metabolism. However, the physiologic significance of the latter remains to be elucidated.

Inhibition of DNA synthesis by somatostatin in rat hepatocytes stimulated by hepatocyte growth factor or epidermal growth factor

The antiproliferative effects of somatostatin on hepatocytes stimulated by hepatocyte growth factor (HGF) or epidermal growth factor (EGF) were investigated using primary cultures of adult rat hepatocytes. Somatostatin inhibits HGF-induced (at a dose of 10 ng/mL) or EGF-induced (at a dose of 100 ng/mL) 3H-thymidine incorporation into hepatocytes in a dose-dependent manner (10−10 to 10−8 M). This inhibition was confirmed by autoradiography. The effect of somatostatin was nontoxic as judged by preserved albumin synthesis, a marker for differentiated hepatocyte function. In the presence or absence of somatostatin, neither HGF nor EGF significantly altered intracellular cyclic adenosine monophosphate (cAMP). We conclude that somatostatin is a potent inhibitor of HGF-or EGF-induced deoxyribonucleic acid synthesis in adult rat hepatocytes. The mechanism of this inhibition appears to be independent of cAMP. The significance of somatostatin in liver regeneration has yet to be

The interaction of growth hormone releasing hormone and somatostatin in the generation of a GH pulse in man

To study the regulation of the growth hormone (GH) response to growth hormone releasing hormone (GHRH) in the presence or absence of somatostatin pretreatment. Seven healthy adult male volunteers of normal height and weight and aged between 19 and 29 years underwent four separate studies (each containing three cycles in one day) in random order. The studies were separated from each other by at least a week. On day 1, three consecutive cycles (between 0800 and 2000 hours) consisted each of a saline infusion for 3 hours which was stopped prior to a bolus injection of saline and followed by 60 minutes of more intensive blood sampling. On day 2, the bolus injections were of GHRH given after saline infusion. On days 3 and 4 somatostatin infusions were administered instead of saline over the 3-hour periods followed by bolus injections of saline or GHRH respectively. In all studies, samples were collected for the measurement of serum GH concentration at 15-minute intervals from time 0 to 180 minutes and then at 5-minute intervals for a further 60 minutes, this cycle being repeated three times. Serum GH concentrations were analysed by serial array averaging. Prompt release of GH was observed in response to GHRH given against a saline background (day 2, cycle 1) (mean at 60 minutes 49.2 +/- 14.7 mU/l) but the responses observed during the second and third cycles were attenuated (mean at 60 minutes 17.2 +/- 4.0 mU/l; P = 0.025). GH release between somatostatin infusions (somatostatin withdrawal; day 3) occurred twice as often as that observed during saline infusions (62% day 3: 29% day 1). The response, although qualitatively similar to that induced by GHRH, was reduced in amplitude and the time of onset variable (5-45 minutes). On day 4, the administration of GHRH as a bolus injection combined with somatostatin withdrawal led to consistent and repeatable GH responses (mean at 60 minutes, cycle 1, 39.7 +/- 10.8 mU/l; cycles 2 and 3, 37.4 +/- 9.4 mU/l) which were similar to those observed with GHRH alone (day 2, cycle 1) (mean 39.7 +/- 10.8 mU/l) (P = NS). These data suggest that endogenous somatostatin secretion is important in determining the ability of the somatotroph to respond to repeated growth hormone releasing hormone stimulation and that for regular GH pulse generation a close interplay between growth hormone releasing hormone and somatostatin is required.

Role of insulin in the blunted glucose metabolic response of septic rats to epinephrine

Epinephrine produces smaller incremental increases in plasma glucose concentration and rate of glucose appearance (Ra) in septic rats compared with nonseptic animals. In the present study, we investigated the role of insulin in the diminished response of septic rats to epinephrine-induced increases in glucose turnover. Glucose kinetics were assessed by the infusion of [6- 3H]-glucose in conscious catheterized rats made septic by subcutaneous injections of live Escherichia coli. Epinephrine was infused at 1 μg/min/kg for 2 hours in the presence and absence of somatostatin and mannoheptulose (SRIF + MH). In comparison to nonseptic control animals, epinephrine-induced increases in plasma glucose concentration and glucose Ra were blunted by more than 50% in the septic rats. Infusion of SRIF + MH with epinephrine restored the blunted response to normal. During the infusion of epinephrine alone, the plasma insulin concentration in the septic rats was 2.8-fold higher than the nonseptic controls. SRIF + MH lowered the plasma insulin concentrations in both the nonseptic and septic rats to less than 10 μU/mL. SRIF + MH reversed the sepsis-induced hyperglucagonemia, but did not prevent a slight increase in glucagon levels during the epinephrine infusion in the nonseptic rats. In a second study, septic rats infused with SRIF + MH and replacement insulin showed a smaller increase in glucose concentration and glucose production in response to epinephrine than did septic animals administered SRIF + MH and no insulin. These results indicate that insulin plays an important role in the diminished response of septic rats to epinephrine.

Requirement of GTP on somatostatin-induced K+ current in human pituitary tumor cells.

The role of GTP on somatostatin-induced K+ current increase was examined in dissociated human pituitary tumor cells obtained from three acromegalic patients. Pituitary cells in culture were voltage-clamped by using the patch clamp technique in the whole-cell configuration. Somatostatin (100 nM) increased the membrane permeability to K+ ions and inhibited hormone secretion. A current-voltage relation of the somatostatin-induced K+ current showed an inward rectification when the concentration of extracellular K+ ions was increased. The amplitude of the somatostatin-induced K+ current decreased during recording when the patch pipette solution did not contain GTP; addition of 100 microM GTP to the patch pipette solution prevented this reduction. Intracellular application of 100 microM guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S] evoked an inward rectifying K+ conductance in the absence of somatostatin. After the GTP[gamma S]-induced K+ conductance reached a steady level, application of somatostatin did not further increase the K+ conductance. In pertussis toxin-treated cells GTP[gamma S] did not evoke K+ conductance. It was concluded that somatostatin-induced K+ channels were regulated by a GTP-binding protein.

Hyperglycemic effect of neurotensin.

Mechanisms of hyperglycemic action of neurotensin were investigated in anesthetized dogs. The intravenous administration of neurotensin 1.0 microgram/kg for 5 min induced an immediate decrease in arterial blood pressure and increases in levels of blood glucose, glucagon and insulin. Although blood levels of glucagon and insulin were greatly reduced and not elevated by neurotensin in the presence of somatostatin, the response of blood glucose to neurotensin was similar to that in the absence of somatostatin. The rise in blood glucose produced by intraportal injection of neurotensin was not greater than that produced by injection of the same dose of neurotensin into the femoral vein. The increments of glucagon and insulin secretion caused by the intraportal injection were also the same as those produced by the peripheral injection. Participation of antihypotensive mechanisms in the neurotensin-induced hyperglycemia was investigated by use of alpha-adrenoceptor blockade and baroceptor denervation. Only the combination of somatostatin and alpha-adrenoceptor blockade or the denervation of baroceptors could suppress the hyperglycemic response to neurotensin. Stimulation of the secretion of anterior pituitary hormones by neurotensin infusion could not be recognized in the present experiments. These results suggested the following: 1) both glucagon and catecholamines may contribute to neurotensin-induced hyperglycemia, 2) neurotensin does not directly act on the liver, 3) catecholamine response could be mediated by baroceptor stimulation through hypotension, and 4) the hyperglycemic effect of anterior pituitary hormones does not participate in neurotensin-induced hyperglycemia.

Importance of glucagon in mediating epinephrine-induced hyperglycemia in alloxan-diabetic dogs.

In normal dogs epinephrine stimulates glucose production (Ra) independently of glucagon. To investigate the role of this interaction in diabetes, epinephrine (0.1 micrograms . kg-1 . min-1) was infused for 90 min in five alloxan-diabetic dogs in the presence or absence of somatostatin (0.1 micrograms . kg-1 . min-1). In response to epinephrine, glycemia rose by 40% reflecting a near maximal (122%) increase in Ra. Plasma glucagon (IRG) rose to 953 pg/ml, whereas insulin (IRI) increased minimally. When somatostatin was infused with epinephrine to prevent the rise of IRG and IRI, there was only a marginal increase of glucose concentration (12%) and production (38%). The effect of somatostatin was reversed by infusing glucagon (10 ng . kg-1 . min-1) together with epinephrine and somatostatin into five additional alloxan-diabetic dogs. Increments in IRG, glycemia, and Ra were fully reestablished. A 100% FFA increase was observed in all three groups, indicating that the lipolytic effect of epinephrine was independent of glucagon. In conclusion, in diabetic dogs, in contrast to normal dogs, epinephrine induced a marked and prolonged increase in glucose concentration and production mostly through a stimulation of IRG secretion.

Lack of effect of somatostatin on the glucagon-induced alterations of hepatic metabolism of [1- 14C]oleate

Livers from normal fed male rats were perfused in a recycling system in vitro. Glucagon was infused in varying quantities to give final concentration in the cell-free perfusate of 4.9 · 10 −10−4.9 · 10 −7 M after 3 h of perfusion, assuming no degradation of the hormone. Where indicated, cyclic somatostatin was infused simultaneously to give a final concentration of 3.0 · 10 −6 M. In the absence of somatostatin, glucagon at a concentration as low as 4.9 · 10 −10 M increased the release of glucose and increased ketogenesis, but impaired the synthesis and release of perfusate triacylglycerol and very low density lipoprotein lipids. Somatostatin did not affect these actions of glucagon. Somatostatin alone, however, did reduce the output of very low density lipoprotein. It is suggested that the alteration of fatty acid metabolism by somatostatin in vivo results from modulation of pancreatic glucagon secretion, not from interference by somatostatin of the action of glucagon on the liver.

Glycogen Synthase, Synthase Phosphatase, and Phosphorylase Response to Glucose in Somatostatin-Pretreated Intact Rats

The response of the glycogen synthase and phosphorylase systems of the liver to intravenous glucose in the presence and absence of short-term somatostatin blockade of insulin secretion was determined in fed and 20-h fasted rats. These enzyme systems regulate glycogen synthesis and degradation, respectively. In the presence of somatostatin, intravenous glucose (1.0 g/kg), promptly (5 min) increased the proportion of synthase in the I (active) form, and the increase was similar to that in animals that had not received somatostatin. In the same animals, phosphorylase a also was decreased, and the decrease was similar in all groups. When a smaller dose of glucose (250 mg/kg) was used that only modestly increased plasma glucose (139 mg/dl) and produced a less than maximal synthase response, insulin (1 U/kg) did not potentiate glucose activation of synthase either in the presence or absence of somatostatin. Phosphorylase a did not change significantly in any group. Glucose, in both the presence and absence of somatostatin, also rapidly (2 min) converted synthase phosphatase from a form inhibited by EDTA to a form not inhibited by EDTA. These data indicate that the synthase and phosphorylase systems in vivo respond primarily to a rise in plasma glucose and not to a simultaneous elevation in plasma insulin. Thus, glucose is regulating its own storage as glycogen in the liver. The effect of glucose on the synthase may be mediated through a conversion of the synthase phosphatase to a form that is not dependent on divalent cation for activity. This form presumably is more active in vivo.