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Y C Patel

Publications and source records attributed to Y C Patel.

At least 91 records · Page 5Linked to original sources
PubMed1987

Antiserum to somatostatin-28 augments growth hormone secretion in the rat.

To determine directly whether somatostatin-28 (S-28) physiologically regulates GH secretion, and what its contribution is relative to somatostatin-14 (S-14), we have compared the effects of immunoneutralization with a specific S-28 antibody with those of an anti S-14/S-28 serum on GH secretory dynamics in the rat. Plasma samples were obtained every 15 min for 7 h (1000-1700 h) from conscious, chronically cannulated rats after iv administration of 1 ml of one of the following sera: 1) rabbit anti S-28 (reacts with S-28, but not with S-14; maximum binding, 297 pmol/ml), 2) nonimmune rabbit serum, 3) sheep anti-S-14/S-28 serum (maximum binding, 9.4 nmol S-14 or S-28/ml), and 4) nonimmune sheep serum. A comparison of the mean integrated plasma GH levels during peak and trough periods showed significantly higher trough GH levels in both antibody-treated groups compared to those in the corresponding controls. In the anti-S-14/S-28-treated group, the elevation of trough period GH levels (40.5 +/- 3.5 ng/ml) represented a 3.25-fold increase (P less than 0.01) compared to the control value (12.5 +/- 1.5 ng/ml). In the anti-S-28-treated group, trough period GH levels (14 +/- 1.6 ng/ml) showed a 2.3-fold increase (P less than 0.01) compared to the control value (6.1 +/- 0.9 ng/ml). Mean peak period GH levels were 1.35-fold higher (P less than 0.05) than control values in the anti-S-14/S-28-treated group; anti-S-28 serum did not change mean peak GH levels. These data provide strong evidence that circulating S-28 (like S-14) physiologically regulates trough GH secretion and that the contribution of S-28 to GH inhibition is as important as that of S-14.

Animals↗
PubMed1987

Hypersecretion of gastric somatostatin in spontaneously diabetic BB rats.

We previously reported that the BB diabetic rat is characterized by a reduction in pancreatic immunoreactive somatostatin (SLI) content, delta-cell mass, and delta-cell secretory reserve. Despite this, portal plasma SLI levels are elevated in diabetic animals and normalized by insulin therapy. These findings comprise indirect evidence for SLI hypersecretion by the gut in untreated BB rats. This study was undertaken with isolated stomach perfusions to investigate directly the secretory status of gastric delta-cells in this diabetic model. Isolated stomachs of three groups of insulin-treated diabetic, untreated diabetic, and nondiabetic control rats were perfused in situ under basal and glucagon-stimulated (5 nM) conditions. Untreated diabetic BB rats exhibited significant enhancement of basal and glucagon-stimulated gastric SLI release. Insulin treatment reduced gastric SLI release to significantly subnormal levels. More than 95% of basal and stimulated SLI released in diabetic BB and normal control rats coeluted with synthetic somatostatin-14 on Sephadex G-50 columns. We conclude that basal and stimulated gastric SLI release is increased in untreated BB rats and is suppressed with insulin therapy, gastric delta-cell hyperfunction accounts for portal vein hypersomatostatinemia characteristic of untreated diabetic BB rats, and somatostatin-14 is the main molecular form of SLI released from normal and diabetic stomachs.

Animals↗
PubMed1986

Chemical cross-linking of somatostatin receptors in rat adrenal cortex.

Adrenocortical somatostatin receptors have been shown to interact with somatostatin-14 (S-14) and somatostatin-28 (S-28). To determine whether these peptides interact with the same or different receptor proteins, we chemically cross-linked these receptors using disuccinimidyl suberate to radioligands prepared from tyrosinated S-14 and S-28 analogs. Sodium dodecylsulfate-polyacrylamide gel electrophoresis and subsequent autoradiography of [125I-Tyr11] S-14 and [Leu8, D-Trp22, 125I-Tyr25] S-28 cross-linked to their binding sites following solubilization in the presence of 50 mM DTT revealed the presence of a single labelled protein of Mr = 200,000. When the cross-linked material was treated under non-reducing conditions, this band was not observed. Furthermore, addition of excess S-14 and S-28 at the time of binding inhibited the incorporation of both radioligands into the receptor protein. These results demonstrate that adrenocortical membrane receptors for somatostatin contain a single receptor protein sub-unit or sub-units of identical size which interact with both S-14 and S-28.

Adrenal Cortex↗
PubMed1986

Somatostatin receptors on rat pancreatic acinar cells. Pharmacological and structural characterization and demonstration of down-regulation in streptozotocin diabetes.

The binding of somatostatin-14 (S-14) to rat pancreatic acinar cell membranes was characterized using [125I-Tyr11]S-14 as the radioligand. Maximum binding was observed at pH 7.4 and was Ca2+-dependent. Such Ca2+ dependence of S-14 receptor binding was not observed in other tissues. Scatchard analysis of the competitive inhibition by S-14 of [125I-Tyr11]S-14 binding revealed a single class of high affinity sites (Kd = 0.5 +/- 0.07 nM) with a binding capacity (Bmax) of 266 +/- 22 fmol/mg of protein. [D-Trp8]S-14 and structural analogs with halogenated Trp moiety exhibited 2-32-fold greater binding affinity than S-14, [D-F5-Trp8]S-14 being the most potent. [Tyr11]S-14 was equipotent with S-14. The affinity of somatostatin-28 for binding to these receptors was 50% of that of S-14. Cholecystokinin octapeptide (CCK-8) inhibited the binding of [125I-Tyr11]S-14, but its inhibition curve was not parallel to that of S-14. In the presence of 1 nM CCK-8, the Bmax of S-14 receptors was reduced to 150 +/- 17 fmol/mg of protein. Dibutyryl cyclic GMP, a CCK receptor antagonist, partially reversed the inhibitory action of CCK-8, suggesting that CCK receptors mediate the inhibition of S-14 receptor binding. GDP, GTP, and guanyl-5'-yl imidodiphosphate inhibit S-14 receptor binding in this tissue. The inhibition was shown to be due to decrease in binding capacity and not due to change in affinity. Specifically bound [125I-Tyr11]S-14 cross-linked to the S-14 receptors was found associated with three proteins of approximate Mr = 200,000, 80,000, and 70,000 which could be detected under both reducing and nonreducing conditions. Finally, pancreatic acinar cell S-14 receptors were shown to be down-regulated by persistent hypersomatostatinemia 1 week after streptozotocin-induced diabetes characterized by decreased Bmax (105 +/- 13 fmol/mg of protein) without any change in affinity. We conclude that pancreatic acinar cell membrane S-14 receptors require Ca2+ for maximal binding and thus differ from S-14 receptors in other tissues, S-14 receptors in this tissue also exhibit selective ligand specificities, these receptors are regulated by CCK-8 and guanine nucleotides, three receptor proteins of apparent Mr = 200,000, 80,000, and 70,000 specifically bind S-14, and (v) these receptors are regulated by S-14 in vivo as evidenced by decreased binding in streptozotocin diabetic rats characterized by hypersomatostatinemia.

Animals↗
PubMed1986

On the fate of centrally administered somatostatin in the rat: massive hypersomatostatinemia resulting from leakage into the peripheral circulation has effects on growth hormone secretion and glucoregulation.

We examined the effects of intracerebroventricular (icv) administration of the somatostatin peptides, S-14 and S-28, and an analog, [D-Trp22]S-28, on spontaneous GH and glucose secretion in freely moving rats bearing chronic icv and intraatrial cannulae. Normal saline icv-treated control rats exhibited the typical pulsatile pattern of GH secretion. Central injection of S-14, S-28, and [D-Trp22]S-28 (each at two different doses, 5 and 10 micrograms) caused an early significant suppression of plasma GH levels which was of longer duration after S-28 or [D-Trp22]S-28 than after S-14 injection. The icv administration of S-28 and [D-Trp22]S-28 (but not S-14) also resulted in significant hyperglycemia, which persisted for periods up to 1 h. In a second study designed to determine whether somatostatin administered via the brain ventricle can reach the peripheral circulation, we measured plasma levels of S-14-like immunoreactivity (S-14 LI) and S-28-(15-28) LI at frequent time intervals after icv injection of S-14 (5 micrograms) and S-28 (10 micrograms). Plasma S-14 LI rose from a basal value of 0.16 +/- 0.02 (+/- SE) ng/ml to a peak of 3.0 +/- 1.0 ng/ml at 1 min, and S-28 icv resulted in a 150-fold increase in plasma S-28-(15-28) LI 1 min after injection. Sephadex G-50 gel chromatography revealed that the plasma immunoreactivity consisted of a single molecular species (either S-14 or S-28) corresponding to the centrally administered counterpart. These results demonstrate that after icv administration at high doses, both forms of somatostatin cause an acute inhibition of spontaneous GH secretion and that the larger form also causes hyperglycemia. We correlate these events with massive hypersomatostatinemia resulting from leakage of the somatostatin peptides from the cerebrospinal fluid into the systemic circulation. The findings indicate that icv injected hypothalamic hormones can reach the peripheral circulation and exert significant biological actions on distant target organs; thus, they have important implications for the design and interpretation of experiments in which peptides are administered via the cerebral ventricles.

Animals↗
PubMed1986

Quantitative in vivo autoradiographic localization of [125I-Tyr11]somatostatin-14- and [Leu8,D-Trp22-125I-Tyr25]somatostatin-28-binding sites in rat brain.

Quantitative in vivo autoradiography was used to identify and compare the regional distribution of specific binding sites for blood-borne [125I-Tyr11]somatostatin-14 (S-14 section) and [Leu8,D-Trp22-125I-Tyr25]somatostatin-28 (S-28 section) in the rat brain. Rats were given intracardiac injections of 17 pmol S-14 section (with or without unlabeled S-14) or 17 pmol S-28 section (with or without unlabeled S-28). After whole body perfusion and fixation, brains were processed for light microscopic autoradiography of S-14 section- and S-28 section-binding sites. Of the peripheral tissues, the adrenal glands showed the highest uptake of S-14 section and S-28 section (determined by counting) and were subsequently processed for autoradiography for comparison with brain. Specific autoradiographic grains (ARG) associated with both radioligands were identified only in the circumventricular organs (CVOs). The highest ARG density associated with S-14 section was found in the area postrema, followed in decreasing order by the subfornical organ and the organum vasculosum lamina terminalis region. Median eminence (ME) contained virtually no specific S-14 section ARG. As with S-14 section, the highest ARG density of S-28 section-binding sites was also found in the area postrema, which labeled approximately equally with the two radioligands. This was followed by the ME, subfornical organ, and organum vasculosum lamina terminalis. The overall patterns of labeling of the CVOs with S-14 section and S-28 section showed significant differences, especially in the ME. Within the ME, labeled S-28 section was concentrated in a broad band throughout the external zone in a location identical to that of immunoreactive S-14. Analysis of dose-response curves obtained with 0.3-30 nmol unlabeled S-14 or S-28 revealed IC50 values for S-14 3- to 6-fold lower than those for S-28 for all labeled CVOs. With both S-14 section and S-28 section, the labelling density of the adrenal glands was double that of the area postrema. Adrenal binding of the radioligands was confined to the cells of the zona glomerulosa. We conclude: specific high affinity binding sites for S-14 section and S-28 section exist in the CVOs and the adrenal glomerulosa; and the 3- to 6-fold higher affinity of binding of S-14 to CVOs compared to S-28 together with the dissimilar patterns of labelling of the different CVOs by the two radioligands suggest the existence of separate populations of S-14 and S-28 receptors.

Animals↗
PubMed1985

Somatostatin receptors.

It is now well established that the biological actions of tetradecapeptide somatostatin (somatostatin-14, S-14) are receptor-mediated. These receptors were first quantified in GH4C pituitary tumor cells using [125I-Tyr1] S-14 as radioligand which was found to exhibit high non-specific binding to membrane receptor preparations from normal tissues. Our studies have shown that [125I-Tyr11] S-14 in which the radiolabel is situated away from the N-terminus exhibits significantly lower non-specific binding and therefore is more suitable for S-14 receptor studies. In the CNS, highest concentration of S-14 receptors was found in the cerebral cortex, followed by thalamus, hypothalamus, striatum, amygdala and hippocampus while medulla-pons, cerebellum and spinal cord exhibited negligible binding. Outside the CNS membrane receptors for S-14 have been characterized in pituitary, adrenal cortex and pancreatic acini. In all these tissues a single class of high affinity binding sites for S-14 were present, the receptors in pancreatic acinar cells exhibiting significantly greater affinity for binding S-14 than in other tissues.

Adrenal Cortex↗
PubMed1985

Approaches to the study of somatostatin biosynthesis.

Our current knowledge of the processes regulating somatostatin biosynthesis is still scarce. Approaches used for the direct investigation of somatostatin biosynthesis in different tissues include 1) analysis of incorporation of labeled amino acids into somatostatin-like immunoreactivity (SLI), 2) cell-free translation of mRNA isolated from SLI producing tissues and 3) analysis of mRNA coding for somatostatin by cDNA blot hybridization. Amino acid incorporation into SLI has been studied in a variety of systems, including anglerfish and rat pancreas, frog retina, rat dorsal root ganglia, cerebral cortex and hypothalamus. We have studied the neuronal biosynthesis of somatostatin using monolayer cultures of neonatal rat hypothalamic cells. Following pulse labeling with [3H]phenylalanine, the cellular extracts contained material that bound specifically to an immobilized anti-somatostatin antibody. Analysis of the bound label by gel chromatography and HPLC provided evidence for the presence of labeled somatostatin-14 (S-14), somatostatin-28 (S-28) and a precursor molecule of Mr 15,000 (15 K SLI). Pulse-chase experiments demonstrated a transfer of label from 15K SLI to material corresponding to S-28 and S-14. Using cloned cDNAs complementary to somatostatin mRNA, the presence of somatostatin mRNA has been demonstrated in anglerfish pancreas and intestine, rat hypothalamus and antrum, as well as in a rat medullary thyroid carcinoma and a rat pancreatic cell line. We have recently studied the developmental regulation of somatostatin gene expression in the rat brain and stomach. Messenger RNA hybridizing specifically to a rat somatostatin cDNA probe was already clearly detectable in tissue extracts derived from brains of one week old rat fetuses. A marked increase of somatostatin mRNA occurred between day 14 and day 21 of embryonic life. By contrast, in tissue extracts derived from stomach, somatostatin mRNA remained undetectable until shortly before birth. These marked differences in the tissue specific regulation of somatostatin gene expression during ontogenesis may reflect basic differences in the developmental regulation of somatostatin gene expression in neural vs. nonneural tissues or may be related to the onset of functional activity in the organs studied.

Animals↗
PubMed1985

Effects of cysteamine and antibody to somatostatin on islet cell function in vitro. Evidence that intracellular somatostatin deficiency augments insulin and glucagon secretion.

In this study we have characterized the effects of cysteamine (CHS) on the cellular content and release of immunoreactive somatostatin (S-14 LI), insulin (IRI), and glucagon (IRG) from monolayer cultures of neonatal rat islets. Incubation of cultures with 0.1-10 mM CHS for 1 h led to an apparent, dose-dependent reduction of cellular S-14 LI that was 50% of control at 0.3 mM, 87% at 1 mM, and 95% at 10 mM. IRI content was unaffected by CHS up to 1 mM, but at 10 mM 90% loss of IRI occurred. All concentrations were without effect on IRG content. The loss of S-14 LI and IRI was completely reversible with time, but with different recovery rates for the two hormones (48 h for S-14 LI, and 72 h for IRI). Released S-14 LI rose progressively with increasing doses of CHS from 21 +/- 2.5 pg/ml per hour to 41 +/- 1.4 pg/ml per hour at CHS concentrations of 5 mM and 10 mM. IRI and IRG secretion were both also significantly enhanced (by 55% and 88%, respectively), despite the elevated medium S-14 LI. Since CHS reduced cellular S-14 LI but augmented medium S-14 LI, the relative effects of CHS (1 mM) and immunoneutralization with antibody to S-14 LI on IRI and IRG secretion were tested. Anti S-14 LI alone stimulated basal IRG (67%) but not IRI. Cultures rendered S-14 LI deficient with both CHS and anti-S-14 LI exhibited threefold and 2.3-fold potentiation of IRG and IRI secretions, respectively, greater than that expected from the separate effects of the two agents. Increasing medium glucose from 2.8 mM to 16.7 mM stimulated IRI release by 86% and suppressed IRG by 53%. CHS (1 mM) and anti-S-14 LI further augmented stimulated IRI release, by 30%; although 16.7 mM glucose suppression of IRG was still maintained under these conditions, the quantitative IRG response was significantly greater. These results suggest that CHS induces an apparent loss of islet S-14 LI, and at high doses, of IRI as well, but has no effect on A cells. Complete islet S-14 LI deficiency augments IRI and IRG secretion over a wide range of glucose concentrations, suggesting a physiological role of D cells on B cell and A cell regulation. D cell modulation of B cells requires cellular but not extracellular S-14 LI, being mediated probably though direct intracellular communication, whereas the A cells seem to be regulated by both direct contact as well as through locally secreted S-14 LI.

Animals↗
PubMed1985

Cysteamine induces a loss of tissue somatostatin-28 when measured as somatostatin-28(15-28)-like immunoreactivity but not when assessed as somatostatin-28(1-14)-like immunoreactivity: evidence for the importance of the disulfide bond for cysteamine action.

The reported loss of somatostatin-14 (S-14)-like immunoreactivity (LI) by cysteamine (CSH) could be mediated through an action on the S-14 disulfide bond. If so, then in the case of somatostatin-28 (S-28) (a linear 14 amino acid N-terminally extended form of S-14), it should be possible to demonstrate with region-specific antisera, a selective effect of CSH on the disulfide bond containing C-terminal half of the molecule. To obtain evidence for this, we have determined by RIAs, the effect of CSH on S-28 concentration measured separately as S-28(15-28) LI and S-28(1-14) LI in the jejunal mucosa, a tissue rich in S-28. Four hours after a single sc injection of CSH to rats, mucosal S-28(15-28) LI was reduced from 16.4 +/- 0.6 to 4.6 +/- 0.51 pmol/mg protein (P less than 0.01). By contrast, S-28(1-14) LI sustained no loss and in fact increased from 27.6 +/- 1.9 to 41.6 +/- 2.2 pmol/mg protein (P less than 0.01). On Sephadex G-50 columns (in 6 M urea) approximately 70% of S-28(15-28) LI and S-28(1-14) LI coeluted with synthetic S-28 marker. These data suggest that CSH acts on the 15-28 segment of the S-28 molecule and renders it nonimmunoreactive probably through interaction with the disulfide bond. This mechanism probably also accounts for CSH-induced S-14 loss.

Animals↗
PubMed1985

Somatostatin receptors in the rat adrenal cortex: characterization and comparison with brain and pituitary receptors.

Specific receptors for tetradecapeptide somatostatin (S-14) in rat adrenal cortical membranes were quantitated by direct binding studies using [125I-Tyr11]S-14. Competitive inhibition of this radioligand by S-14 showed that these receptors constitute a single class of high affinity binding sites [dissociation constant (Kd) = 1.08 nM and maximum binding capacity (Bmax) = 0.35 pmol/mg membrane protein]. Structural analogs of S-14 with halogenated Trp8 moiety exhibited 4- to 46-fold greater binding affinity than S-14, [D-F5-Trp8]S-14 being the most potent. [Tyr11]S-14 and [des-Ala1]S-14 bound to these receptors with reduced affinity whereas [Phe4]S-14 exhibited 1.5-fold greater affinity than S-14. Somatostatin-28 (S-28) and S-14 were equipotent, whereas the N-terminal fragments of S-28 [S-28(1-14) and S-28(1-12)] were inactive. High affinity binding sites were also quantitated using a radioligand prepared from the tyrosinated S-28 analog, [Leu8, D-Trp22, Tyr25]S-28 (Kd = 1.2 nM; Bmax = 0.21 pmol/mg membrane protein). Both S-14 and S-28 exhibited comparable relative potencies for inhibiting the specific binding of this radioligand and [125I-Tyr11]S-14. Extracts of whole adrenal or the adrenal medulla and cortex contained very low levels of S-14-like immunoreactivity (2.4 pg/mg protein). These studies confirm the presence of specific receptors for S-14 in the adrenal cortex and suggest that 1) with respect to S-14 biological activity, Trp8-modified S-14 analogs should be more potent than S-14, S-28 equipotent with S-14, and N-terminal fragments of S-28 inactive in this tissue. 2) Direct binding studies using radioiodinated [Tyr11]S-14 and [Leu8,D-Trp22, Tyr25]S-28 appear to quantitate the same receptor sites in adrenocortical tissue. 3) The ligand specificity of the adrenocortical S-14 receptor differs from that previously reported for the pituitary and brain providing further evidence for the heterogeneity of the S-14 receptor. 4) In view of the very low concentrations of endogenous S-14-like immunoreactivity, the adrenal actions of S-14 and S-28 are probably mediated through an endocrine mechanism.

Adrenal Cortex↗
PubMed1985

Characterization of tissue and releasable molecular forms of somatostatin-28[1-12]-like immunoreactivity in rat median eminence.

The main purpose of the present study was to characterize the tissue and releasable molecular forms of somatostatin-28[1-12]-like immunoreactivity (S-28[1-12]LI) in rat median eminence (ME) fragments and to compare them with somatostatin-14-like immunoreactivity (S-14 LI) forms. Acetic acid extracts of ME were fractionated on Sephadex G-50 columns (in 6 M urea). The column eluate was monitored for S-28[1-12] LI by RIA with antibody R21 which detects S-28[1-12], S-28, and higher molecular weight forms of S-28[1-12] LI, but not S-14. The S-14 LI RIA utilized recognizes S-14, S-28, and prosomatostatin (pro-S). Rat ME contained 221 +/- 25 pmol S-14 LI/mg protein and 407 +/- 51 pmol S-28[1-12] LI/mg protein. By gel filtration S-14 LI was resolved into three peaks corresponding to S-14, S-28, and a higher mol wt form (14,000) corresponding to pro-S. S-28[1-12] LI consisted of at least five forms corresponding to pro-S, S-28, S-28[1-12], a form which represented pro-S without the S-14 sequence, and a form slightly smaller than S-28[1-12]. Pools of 20 ME incubated in 56 mM K+ solution showed 4.6-fold Ca++-dependent release of S-14 LI and 4-fold release of S-28[1-12] LI. Gel chromatographic analysis of the released material showed all three tissue S-14 LI forms and each of the tissue S-28[1-12] LI forms. HPLC analysis and RIAs further confirmed the release of S-14, S-28, S-28[1-12], and the S-28[1-12] LI form smaller than S-28[1-12]. These data suggest the presence of at least six molecular forms of somatostatin in ME. The release of this large number of peptides, presumably from mature secretory granules in ME in response to depolarization, suggests that they are products of the normal posttranslational processing of pro-S.

Animals↗
PubMed1985

Hepatic metabolism of somatostatin-14 and somatostatin-28: immunochemical characterization of the metabolic fragments and comparison of cleavage sites.

Previous studies have shown that somatostatin-14 (S-14) is rapidly metabolized in the liver through the action of aminopeptidases and endopeptidases, resulting in separate cleavages at the N-terminus and the cyclized (ring) portion of the molecule. In the present study we have characterized the hepatic metabolism of somatostatin-28 (S-28) and compared it with that of S-14 to determine whether S-28 is degraded by a process similar to that for S-14, and additionally, whether the hepatic metabolism of S-28 results in significant conversion to S-14. Isolated rat livers were perfused with synthetic S-28, somatostatin-25[(S-25), an N-terminal metabolite of S-28], C- and N-terminally radioiodinated analogs of S-28, S-14, and des-Ala1-S-14[(S-13), an N-terminal metabolite of S-14]. The metabolic products were characterized by separate N-terminally directed S-14 and S-28 RIAs, a common ring-directed RIA for S-14, S-28, S-13, and S-25, immunoprecipitation, gel chromatography, and HPLC. Hepatic extractions of S-28 and S-25, monitored as ring-directed immunoreactivity, were equivalent, but both occurred 4 times more slowly than that of S-14 or S-13. By contrast, the N-terminal metabolism of S-14 and S-28 monitored by specific N-terminal RIAs occurred at similar rates (hepatic extraction of 54% and 44%, respectively). Both S-14 and S-28 were degraded significantly more rapidly at the N-terminus than at the ring segment. Immunochemical characterization of the radioactive metabolites of N- and C-terminally radioiodinated S-28 analogs confirmed the more rapid N-terminal cleavage of S-28 compared with its ring breakdown. Gel chromatography of S-28 perfusates followed by RIA of the column fractions for N-terminal and ring-reactive metabolites, showed a time-dependent conversion of S-28 to a peak coeluting with S-14 (27% conversion by 60 min). That S-14 was a significant metabolite of S-28 was further confirmed by HPLC analysis of the hepatic perfusate. The main hepatic metabolite of S-28 coeluted with S-28 on Sephadex columns but showed reduced N-terminal reactivity compared to intact S-28. This product thus appeared to be a N-terminally modified form of S-28 as also suggested by HPLC analysis where it coeluted with synthetic S-25. These data have demonstrated that the hepatic metabolism of S-28 occurs via three separate processes, two of which are similar to those for S-14. These include 1) endopeptidase cleavage through the cyclized (ring) segment; 2) N-terminal aminopeptidase cleavage to yield metabolites such as S-25; and 3) tryptic-like cleavage of the Arg-Lys region of S-28 to generate S-14.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Somatostatin metabolism by the isolated rat lung.

We have previously reported that somatostatin-14 (S-14) and somatostatin-28 (S-28) are degraded by the rat liver. Since pulmonary enzymes known to degrade peptides such as angiotensin resemble hepatic somatostatin-degrading peptidases, the present studies were undertaken to investigate and characterize somatostatin metabolism by the lung. Isolated rat lungs were perfused in situ with synthetic S-14 or S-28. Degradation of the cyclic (ring) portion common to S-14 and S-28 was monitored by RIA as disappearance of S-14-like immunoreactivity, (S-14 LI) (ring-directed immunoreactivity). N-Terminal cleavage of S-28 was separately assessed by an RIA directed against S-28[1-14] (S-28[1-14]LI). In addition, S-28 perfusates were analyzed by gel chromatography and RIA (S-14 LI). Both S-14 and S-28 were significantly metabolized by the lung. The ring portion of S-28 was found to be degraded significantly more slowly (t 1/2 = 56.1 +/- 5.2 min) than the ring of S-14 (t 1/2 = 23.8 +/- 2.1 min) and the N-terminus of S-28 (t 1/2 = 32.2 +/- 3.0 min). S-14 was demonstrated to be a minor pulmonary metabolite of S-28. The lung was shown to release S-14 and S-28 ring-degrading enzymes into the perfusate. These data suggest that the lung is capable of metabolizing S-14 and S-28 by enzymatic mechanisms which are similar to those reported for the liver.

Amino Acid Sequence↗
PubMed1984

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.

Adult↗
PubMed1984

Somatostatin, glucagon, and insulin secretion from perfused pancreas of BB rats.

We reported previously that pancreatic somatostatin and glucagon content and D and A cells are reduced in spontaneously diabetic BB rats while portal plasma levels of these hormones are elevated but normalized by insulin therapy. Presently, we have characterized the basal and stimulated (glucose, theophylline, arginine) release of islet hormones from perfused pancreases of three groups: nondiabetic, untreated diabetic, and insulin-treated diabetic rats. Maximal glucagon and somatostatin release were significantly reduced in untreated diabetics. Treatment normalized glucagon but further reduced somatostatin secretion. Thus, hyperglucagonemia and hypersomatostatinemia cannot result from pancreatic hypersecretion but are of extrapancreatic (probably gut) origin. A theophylline-induced paradoxical inhibition of somatostatin secretion that was normalized by insulin was found in insulin-openic diabetic rats. This likely represents a secondary effect of insulin deficiency. One animal with the rare finding of spontaneous recovery from insulin-dependent diabetes was characterized as normoglycemic, hypoinsulinemic, hypersomatostatinemic, and normoglucagonemic. Normal basal and hyperglycemic insulin release was exhausted by more potent secretagogues. Somatostatin release was markedly exaggerated, whereas secretagogues had no significant effect on elevated basal glucagon output.

Animals↗