Biosynthesis of somatostatin in pancreatic islets of Wistar rats.

Pancreatic islets of Wistar rats were isolated by collagenase digestion and incubated with [3H]-L-phenylalanine. Using a specific somatostatin antiserum radioactivity was found in the antibody-antigen-complex. The radioactivity was displaced by unlabelled somatostatin. These findings give the first evidence for the biosynthesis of somatostatin or somatostatin-like peptides in mammalian pancreatic islets.     

Effects of somatostatin on pancreatic exocrine function. Interaction with secretin.

The release of hydrolases from rat pancreas was stimulated in vivo and in vitro by somatostatin. In vitro this hypersecretion was accompanied by a moderate but significant rise in intracellular cyclic AMP. In addition, the tetradecapeptide inhibited rises of cyclic AMP provoked by secretin. The existence of the same sequence of four amino acid residues in the two peptides suggests that somatostatin's activation of the exocrine pancreas depends on its interaction with secretin receptors.     

Characterization of covalently cross-linked pancreatic somatostatin receptors

The receptor for somatostatin present in rat pancreatic plasma membranes was characterized by affinity labeling with [125I-Tyr11]somatostatin utilizing three different heterobifunctional cross-linking agents: N-5-azido-2-nitrobenzoyloxy-succinimide, N-succinimidyl 6-(4-azido 2'-nitrophenylamine)hexanoate, and N-hydroxysuccinimidyl 4-azido-benzoate. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed a broad band of Mr = 92,000 when any of the three cross-linkers was used; N-succinimidyl 6-(4-azido 2'-nitrophenylamine), however, was most efficient. Labeling of the Mr = 92,000 protein band was not affected by reducing agents but was sensitive to somatostatin and guanine nucleotides, particularly GTP gamma S, at concentrations which reduced binding to the receptor. The affinity-labeled protein could be solubilized completely with Zwittergent 3-12, partially with Triton X-100 and 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid, and poorly with Zwittergent 3-08 and digitonin. When exposed to agarose-coupled lectins, the detergent solubilized, labeled Mr = 92,000 protein was completely adsorbed to wheat germ agglutinin, partially to ricin communis II, and not at all to concanavalin A or lotus or lentil lectin. The Mr = 92,000 protein bound to wheat germ agglutinin-agarose was not eluted by N-acetylglucosamine but was by triacetylchitotriose, providing a considerable purification of the somatostatin receptor. These data allow us to conclude that the somatostatin receptor is a monomeric glycoprotein with an Mr = 90,000 binding subunit which probably contains a polymeric arrangement of N-acetylglucosamine residues.     

Size-related and size-unrelated functional heterogeneity among pancreatic islets.

Functional heterogeneity of pancreatic islets was systematically analyzed for the first time using freshly isolated single rat pancreatic islets. First, 60 islets were sequentially exposed to 3, 9.4, 15.6, and 24.1 mM glucose for 30 min each in incubation experiments: 36 (60%) responded in a concentration-dependent and 19 (32%) in an all-or-none manner, and 5 (8%) islets did not respond to high glucose. As a group, the larger the islet, the higher the beta cell glucose sensitivity. However, glucose-stimulated elevation of [Ca2+]i in the beta cell. insulin/glucagon ratio in the islet, and expression of glucose transporter 2, glucokinase, and pancreatic duodenal homeobox factor-1 in the beta cell were not significantly related to islet size. Second, 50 islets were stimulated with 16.7 mM glucose in perifusion. A biphasic insulin release was found in 39 (78%), and no or little first phase response in 11 (22%) islets, irrespective of the islet size. Nevertheless, when the response was plotted as a group, it was clearly biphasic. Islet size, insulin content and the amount of insulin release were positively correlated with each other. In conclusion, there are size-related and size-unrelated functional diversity among pancreatic islets. The reason for such heterogeneity remained to be determined.     

Why ageing stops: heterogeneity explains late-life mortality deceleration in nematodes

While ageing is commonly associated with exponential increase in mortality with age, mortality rates paradoxically decelerate late in life resulting in distinct mortality plateaus. Late-life mortality plateaus have been discovered in a broad variety of taxa, including humans, but their origin is hotly debated. One hypothesis argues that deceleration occurs because the individual probability of death stops increasing at very old ages, predicting the evolution of earlier onset of mortality plateaus under increased rate of extrinsic mortality. By contrast, heterogeneity theory suggests that mortality deceleration arises from individual differences in intrinsic lifelong robustness and predicts that variation in robustness between populations will result in differences in mortality deceleration. We used experimental evolution to directly test these predictions by independently manipulating extrinsic mortality rate (high or low) and mortality source (random death or condition-dependent) to create replicate populations of nematodes, Caenorhabditis remanei that differ in the strength of selection in late-life and in the level of lifelong robustness. Late-life mortality deceleration evolved in response to differences in mortality source when mortality rate was held constant, while there was no consistent response to differences in mortality rate. These results provide direct experimental support for the heterogeneity theory of late-life mortality deceleration.     

Conversion of big pancreatic somatostatin without peptide bond cleavage into somatostatin tetradecapeptide

Urea treatment of the big form of somatostatin obtained from rat pancreas resulted in a conversion into the small form of somatostatin. Further dissociation does not occur with mercaptoethanol. The results indicate that the existence of big somatostatin is dependent upon the formation of a non-covalent bond of the tetradecapeptide with another peptide fragment.     

Branching tree model with fractal vascular resistance explains fractal perfusion heterogeneity

Perfusion heterogeneities in organs such as the heart obey a power law as a function of scale, a behavior termed "fractal." An explanation of why vascular systems produce such a specific perfusion pattern is still lacking. An intuitive branching tree model is presented that reveals how this behavior can be generated as a consequence of scale-independent branching asymmetry and fractal vessel resistance. Comparison of computer simulations to experimental data from the sheep heart shows that the values of the two free model parameters are realistic. Branching asymmetry within the model is defined by the relative tissue volume being fed by each branch. Vessel ordering for fractal analysis of morphology based on fed or drained tissue volumes is preferable to the commonly used Strahler system, which is shown to depend on branching asymmetry. Recently, noninvasive imaging techniques such as PET and MRI have been used to measure perfusion heterogeneity. The model allows a physiological interpretation of the measured fractal parameters, which could in turn be used to characterize vascular morphology and function.     

Chromatophore heterogeneity explains phenomena seen in Rhodobacter sphaeroides previously attributed to supercomplexes

It is generally considered that metabolic reactions are well described by homogeneous kinetic models in which the reaction phase is statistically uniform. In membranes, especially in photosynthetic systems where the protein complement is high, it has recently been recognized that effects of local heterogeneity might contribute additional factors that perturb the kinetic behavior, and require more extensive treatment. We show in this paper that statistical heterogeneity in vesicular systems can also contribute to quite marked discrepancies from the behavior expected from standard kinetic and thermodynamic models, for reactions involving free diffusion in the aqueous phase. We explain the kinetic and thermodynamic effects observed in studies of photosynthetic electron transfer in cells and chromatophores from Rhodobacter sphaeroides previously attributed to supercomplexes, in terms of a model based on heterogeneity in distribution of electron transfer components among the chromatophore population. We discuss examples of data inconsistent with the supercomplex model, but well explained by the heterogeneity model.     

Sex-different response in growth traits to resource heterogeneity explains male-biased sex ratio

In dioecious plants, differences in growth traits between sexes in a response to micro-environmental heterogeneity may affect sex ratio bias and spatial distributions. Here, we examined sex ratios, stem growth traits and spatial distribution patterns in the dioecious clonal shrub Aucuba japonica var. borealis, in stands with varying light intensities. We found that male stems were significantly more decumbent (lower height/length ratio) but female stems were upright (higher height/length ratio). Moreover, we found sex-different response in stem density (no. of stems per unit area) along a light intensity gradient; in males the stem density increased with increases in canopy openness, but not in females. The higher sensitivity of males in increasing stem density to light intensity correlated with male-biased sex ratio; fine-scale sex ratio was strongly male-biased as canopy openness increased. There were also differences between sexes in spatial distributions of stems. Spatial segregation of sexes and male patches occupying larger areas than female patches might result from vigorous growth of males under well-lit environments. In summary, females and males showed different growth responses to environmental variation, and this seemed to be one of possible causes for the sex-differential spatial distributions and locally biased sex ratios.     

Solubilization and characterization of guinea-pig pancreatic somatostatin receptors

The solubilization of somatostatin receptors from guinea-pig pancreas by different non-denaturing detergents was investigated after stabilization of the receptors by prior binding of 125I-[Tyr11]somatostatin or its analogue 125I-[Leu8,DTrp22,Tyr25]somatostatin 28, to pancreatic plasma membranes. The somatostatin-receptor complexes were solubilized in a high yield by Zwittergent 3-14 (3-[tetradecyldimethylammonio]-1-propanesulfonate), a zwitterionic detergent. Other detergents, digitonin, Triton X-100, Chaps (3-[cholamidopropyldimethylammonio]-1-propanesulfonate) and octyl beta-D-glycopyranoside, achieved only partial solubilization. The recovery of receptor complexes was increased by glycerol. In order to characterize solubilized somatostatin-receptor complexes, membranes receptors were covalently labelled using N-5-azido-2-nitrobenzoyloxysuccinimide as cross-linking reagent before solubilization. Gel filtration chromatography analysis resulted in the identification of a major protein component of apparent Mr = 93,000 which interacted with the two radioligands. In addition, a similar component of Mr = 88,000 was characterized after analysis by SDS-PAGE of membrane receptors covalently cross-linked with 125I-[Leu8,DTrp22,Tyr25]somatostatin 28 by different heterobifunctional reagents: N-5-azido-2-nitrobenzoyloxysuccinimide, N-hydroxysuccinimidyl 4-azidobenzoate, N-succinimidyl 6-(4'-azido-2'-nitrophenylamino)hexanoate. Optimal cross-linking results were obtained with N-5-azido-2-nitrobenzoyloxysuccinimide. The solubilized somatostatin-receptor complex was adsorbed to wheat-germ agglutinin-agarose column and eluted by specific sugars. We concluded that the guinea-pig pancreatic somatostatin receptor in the membrane and in the non-denaturing detergent solution behaves as a protein monomer of apparent Mr approximately 85,000-90,000. The somatostatin receptor is a glycoprotein which contains complex-type carbohydrate chains.     

Unexpected growth-stimulatory effect of somatostatin analogue on cultured human pancreatic carcinoid cells.

Therapeutic efficacy of a synthetic somatostatin analogue for the treatment of carcinoid tumors is still controversial. In vivo studies performed in our laboratory showed that a somatostatin analogue, SMS 201-995, significantly inhibited growth of human pancreatic carcinoid (BON) tumors xenotransplanted into athymic nude mice. In the present study, however, SMS 201-995 did not inhibit in vitro growth of BON cells, but rather SMS 201-995 stimulated growth in a dose-dependent fashion. The growth-stimulatory effect was likely mediated through the reduction of cyclic AMP production. Unsuccessful treatment of certain types of carcinoid tumor with SMS 201-995 may be partly due to the direct growth-stimulatory effect of SMS 201-995 on carcinoid cells.     

Long-acting somatostatin analogues in pancreatic islet cell carcinoma

In secreting islet cell carcinoma, the long-acting somatostatin analogue, Sandostatin, reduces symptoms of endocrine secretion both by inhibiting peptide secretion and by acting on the target organs. It could be used during the initial evaluation of patients with such tumors and thereafter when the tumor cannot be found or is metastatic. Its efficacy depends upon the tumor type and probably the presence of somatostatin receptors on the tumoral cells. It could decrease with time, especially in VIPomas. Side effects are few and usually mild. Hypoglycemia attacks in insulinoma could be worsened during treatment by the complete inhibition of hyperglycemic hormones. The inhibition of tumoral growth, based on animal models, appears infrequent in clinical practice.     

Effect of somatostatin on glucose-induced 45Ca uptake in the pancreatic islets.

This study was designed in an attempt to elucidate a mechanism of somatostatin inhibition of glucose-induced Ca+ uptake by rat pancreatic islets. Rat pancreatic islets were perifused with Krebs-Ringer bicarbonate (KRB) buffer containing 16.7 mM of glucose with somatostatin (2 micrograms/ml) or/and diltiazem HCl (2 x 10(-5) M). Somatostatin inhibited preferentially the early phase of glucose-induced insulin release, whereas diltiazem HCl inhibited the late one. And the concomitant presence of the submaximal concentration of somatostatin (2 micrograms/ml) and diltiazem HCl (2 x 10(-5 M) provided the completely additive inhibition of glucose-induced insulin release. Rat pancreatic islets were incubated with KRB buffer supplemented with 16.7 mM of glucose and 45CaCl2 (10 muCi/ml) for 5--60 min and the biphasic 45Ca uptake by pancreatic islets was obtained. Somatostatin (500 ng/ml-4 micrograms/ml) gave the suppressive effect on the early phase of glucose-induced 45Ca uptake, but the higher concentration (2 micrograms/ml) of somatostatin did not impair the late phase of 45Ca uptake by pancreatic islets. On the other hand, diltiazem HCl did suppress the late phase of glucose-induced 45Ca uptake dose-dependently, but did not suppress the early phase (2 x 10(-5) M). These data indicate that somatostatin suppresses the early phase of glucose-induced Ca2+ uptake preferentially to the late one and has a different action mechanism from Ca antagonist on glucose-induced insulin release.