The biological activity of catfish pancreatic somatostatin

Catfish pancreatic somatostatin, which contains eight additional amino acids on the amino terminus of a tetradecapeptide with considerable homology to tetradecapeptide somatostatin (SRIF), is a naturally occurring homology of the hypothalamic peptide. The purpose of these studies was to determibe the biological activity of this somatostatin homolog. Inhibition of ¹²⁵I-labelled tyr¹-SRIF binding to bovine pituitart plasma membranes by catfish pancreatic somatostatin was approximately 33% that of SRIF. Pancreatic somatostatin has full biological activity measured by inhibition of growth hormone release from isolated rat pituitary cells, but 0.01–0.1% the potency of SRIF. Pancreatic somatostatin at 100 ng/ml produced a 50–60% inhibition of insulin and glucagon secretion from perfused rat pancreas, while SRIF produced comparable inhibition at 10 ng/ml. This report demonstrates that a larger molecular form and natural homolog of SRIF, isolated from fish pancreas, has the same (but reduced) biological activities in rat assay systems as somatostatin originally isolated from sheep hypothalamus.     

Identification of the subunits of GTP-binding proteins coupled to somatostatin receptors.

Somatostatin (SRIF) induces its biological effects by interacting with membrane-bound receptors that are linked to cellular effector systems via G proteins. We have studied SRIF receptor-G protein associations by solubilizing the SRIF receptor from rat brain and AtT-20 cells and immunoprecipitating the receptor-G protein complex with peptide-directed antisera against the different subunits of the G protein heterotrimer. Antiserum 8730, which selectively interacts with all Gi alpha subtypes, maximally and specifically immunoprecipitated SRIF receptor-Gi alpha complexes. To identify the subtypes of Gi alpha that are coupled to SRIF receptors, the subtype-selective antisera 3646, 1521, and 1518, which specifically interact with Gi alpha 1, Gi alpha 2, and Gi alpha 3, respectively, were used to immunoprecipitate SRIF receptor-Gi alpha complexes. Antiserum 3646 immunoprecipitated SRIF receptor-Gi alpha 1 complexes from both brain and AtT-20 cells. Antiserum 1521 immunoprecipitated Gi alpha 2 from both brain and AtT-20 cells but did not immunoprecipitate SRIF receptors from these tissues. Antiserum 1518 immunoprecipitated AtT-20 cell SRIF receptors but uncoupled brain SRIF receptor-G protein complexes. This result was confirmed with another peptide-selective antiserum, SQ, directed against Gi alpha 3. The findings from these studies indicate that Gi alpha 1 and Gi alpha 3 are coupled to SRIF receptors, whereas Gi alpha 2 is not. Even though brain and AtT-20 cell SRIF receptors were both coupled to Gi alpha, the receptors from these tissues differed in their coupling to Go alpha. Antiserum 2353, which is directed against Go alpha, immunoprecipitated SRIF receptors from AtT-20 cells, but did not immunoprecipitate or uncouple SRIF receptor-G protein complexes from rat brain. To determine the beta subunits associated with the SRIF receptor, antisera directed against G beta 36 and G beta 35 were used to immunoprecipitate SRIF receptor-G protein complexes from brain. Peptide-directed antiserum against G beta 36 selectively immunoprecipitated solubilized brain SRIF receptors. However, antiserum directed against the G beta 35 subunit did not immunoprecipitate brain SRIF receptors, suggesting that brain SRIF receptors may preferentially associate with G beta 36. In addition to coimmunoprecipitating with Gi alpha and G beta, brain SRIF receptors coimmunoprecipitated the G protein gamma subunits, G gamma 2 and G gamma 3. These results provide the first evidence that SRIF receptors are coupled to different subunits of G proteins and suggest that selectivity exists in the association of different G protein subunits with the SRIF receptor.     

The effects of substance P and met5-enkephalin in dog ileum

Substance P initiated tonic contraction of dog ileum when administered in doses from 1 pg to 20 micrograms intraarterially (ED50 = 67 ng). Low doses acted to excite cholinergic postganglionic neurones since atropine or tetrodotoxin (TTX) increased the ED50 of substance P about 25-fold, while hexamethonium and local field stimulation had only a small effect to increase the ED50. Also atropine and tetrodotoxin effects were not additive. Higher doses apparently acted to stimulate smooth muscle directly, but no evidence was obtained that local field stimulation could release substance P to act on smooth muscle. Substance P tachyphylaxis prevented substance P actions on cholinergic nerves, but it did not affect responses to intraaterial acetylcholine or block distal inhibition from proximal distention or field stimulation. Met-enkephalin given intraarterially, was also excitatory in doses from 1 ng to 20 micrograms; the amplitude of tonic and phasic contractions produced was significantly decreased by TTX and atropine but was not diminished by hexamethonium or substance P tachyphylaxis. Partial tachyphylaxis to met-enkephalin was produced but was not diminished by hexamethonium or substance P tachyphylaxis. Partial tachyphylaxis to met-enkephalin was produced without affecting the ED50 for substance P. We conclude that substance P acts in small amounts on receptors in myenteric nerves to release acetylcholine by a mechanism, presumably involving postganglionic cholinergic nerves, while met-enkephalin also apparently may act at least in part through a similar TTX- and atropine-sensitive mechanism. These peptides also caused activation of other receptors, probably on smooth muscle by noncholinergic. TTX-insensitive mechanisms. Also the receptors for each peptide which are located on nerves were distinct and independent since tachyphylaxis could be produced to each without affecting the response to the other.     

Electrophysiological actions of somatostatin (SRIF) in hippocampus: Anin vitro study

The electrophysiological actions of somatostatin (somatotropin release inhibiting factor; SRIF) were investigated in the in vitro hippocampal slice preparation. Intracellular recordings were obtained from pyramidal neurons in area CA1 in slices of hippocampus from guinea pigs and rabbits. Somatostatin, applied via micropressure ejection to CA1 pyramidal-cell somata, was primarily excitatory. The effects, however, were quite variable, with nearly all cells displaying pronounced tachyphylaxis. A majority of cells was depolarized by SRIF, but hyperpolarizations or biphasic depolarization/hyperpolarization responses were also recorded. Only minimal conductance changes were associated with the SRIF-induced voltage changes. Depletion of SRIF, by injection of the intact animal with cysteamine several hours before preparing slices, resulted in no obvious abnormalities in hippocampal slice electrophysiology. Our results obtained with application of exogenous SRIF are consistent with the concept that SRIF acts as an excitatory neurotransmitter/neuromodulator in hippocampus. However, our attempts to demonstrate endogenous SRIF action have thus far been unsuccessful.     

Electromagnetic stimulation blocks isometric twitch contraction in the frog sciatic nerve-gastrocnemius muscle preparation

The effect of electromagnetic stimulation (EMS) on nerve conduction and muscle twitch contraction was studied in isolated sciatic nerve-gastrocnemius muscle of the frog. Electrical stimulation (ES) of the sciatic nerve produced twitch contractions in the gastrocnemius muscle and these responses were reduced and eventually blocked by EMS, applied to the nerve simultaneously with ES, from a d.c. source at a certain frequency and duration of the induced current. The EMS-induced inhibition of the twitch contractions was reversible, and this depended on the induced current and its duration. The possibility that other factors may have contributed to the inhibition of twitch contractions, such as a rise in temperature, was also investigated. It was concluded that EMS inhibited indirectly-elicited twitch contractions produced by ES in the frog nerve-muscle preparation.     

Somatostatin and GABA correlate with cervical autonomic nerve activity

The mode of inhibitory action of centrally administered SRIF on the efferent activity of autonomic nerves was investigated in the rat by assessing the SRIF-induced change in the activity of the superior laryngeal nerve with or without pretreatment with various drugs. After picrotoxin or bicuculline treatment, the inhibition of nerve activity by SRIF was abolished while reserpine and atropine failed to abolish the SRIF effect. The centrally administered GABA inhibited the activity of the superior laryngeal nerve and the cervical sympathetic trunk. However, SRIF did not affect the sympathetic trunk. Arterial blood pressure was increased by SRIF while GABA produced hypotension.

These data provide evidence for a GABAergic system as the mediator of SRIF action in the brain and for the selectivity of SRIF action on the particular intermediary GABAergic neurones.     

Somatostatin inhibits follicle-stimulating hormone-induced adenylyl cyclase activity and proliferation in immature porcine Sertoli cell via sst2 receptor

The potential involvement of somatostatin (SRIF) in testicular function was studied by using as a model primary cultures of purified immature porcine Sertoli cells. In the present report we show that Sertoli cells express mRNA for sst2 SRIF receptor and display SRIF-sensitive adenylyl cyclase. Sensitivity of adenylyl cyclase to SRIF and its analogues is compatible with the pharmacological profile of this receptor type. Relevant cAMP production is similarly inhibited by SRIF in both basal and stimulated (by gonadotropin FSH or by forskolin) conditions. Moreover, the observed SRIF actions on Sertoli cells require functional coupling of specific membrane receptors to adenylyl cyclase via Gi proteins because pertussis toxin prevents SRIF-dependent inhibition of adenylyl cyclase in either basal or FSH-stimulated conditions. Given the potent antiproliferative actions of SRIF in other cell types, we further assessed the possible SRIF-dependent modulation of [(3)H]thymidine incorporation by Sertoli cells. Our data point to SRIF-mediated inhibition of both basal and FSH-stimulated [(3)H]thymidine uptake. This inhibition of Sertoli cell proliferation is, at least in basal conditions, also blocked by pertussis toxin pretreatment. Altogether, these data suggest that SRIF may play a role as an (local) inhibitor of FSH actions in testicular development.     

Characterization of 17-beta-estradiol-dependent and -independent somatostatin receptor subtypes in rat anterior pituitary

Previous studies from this laboratory showed that treatment with 17-beta-estradiol (E2) caused an acquisition of inhibitory effect of somatostatin (SRIF) on prolactin release with an increased number of SRIF-binding sites in the rat anterior pituitary. The aim of this study was to characterize the E2-dependent SRIF receptor in comparison with the E2-independent one, which was expressed in ovariectomized rats. The following observations were obtained: 1) both of the E2-dependent and E2-independent SRIF receptors, measured with 125I-Tyr11-SRIF as a radiolabeled ligand, were enriched in the plasma membrane fraction of the cells, displaying a single class of binding site (E2-dependent: Kd, 32 pM, Bmax, 2.3 pmol/mg protein; E2-independent: Kd, 83 pM, Bmax, 0.26 pmol/mg protein). The ligand binding to both receptors was sensitive to monovalent and divalent cations, and GTP. 2) Among the SRIF analogs tested, the relative potencies of SRIF28 and its analog and cyclosomatostatin compared with SRIF were lower in the E2-dependent receptor than in the E2-independent one. 3) A cross-linking study with N-hydroxysuccinimidyl-4-azido-benzoate revealed that the molecular weight of the cross-linked E2-dependent receptor was approximately 94,000, whereas that of the E2-independent one was 82,000, irrespective of the presence of a reducing reagent. The molecular weight of SRIF receptor from normal male or female rat pituitary was similar to the E2-independent type. 4) Both types of the cross-linked SRIF receptors were solubilized by sucrose monolaurate, adsorbed to a wheat germ agglutinin-agarose column, and eluted with N-acetyl-glucosamine. 5) SRIF inhibited the forskolin-stimulated adenylate cyclase activity in the pituitary membranes from E2-treated rats, but it did not in the E2-depleted membranes. These results demonstrate that there are at least two subtypes of SRIF receptor in the rat anterior pituitary, one of which is exclusively expressed by the treatment with E2, and that these subtypes are distinct with respect to ligand binding specificity, molecular weight, and coupling to adenylate cyclase inhibition.     

Analogues of Somatostatin Bind Selectively to Brain Somatostatin Receptor Subtypes

Somatostatin (SRIF) is a neurotransmitter that produces its multiple effects in the CNS through interactions with membrane-bound receptors. Subtypes of SRIF receptors are found in the CNS that are distinguished by their sensitivities to the cyclic hexapeptide MK-678, such that SRIF1 receptors are sensitive to MK-678 and SRIF2 receptors are insensitive to MK-678. In the present study, we further examined the selectivities of a series of structurally diverse SRIF analogues for SRIF receptor subtypes. SRIF receptors were labeled by 125I-Tyr11-SRIF, which has indistinguishable affinities for SRIF receptor subtypes. The inhibition by MK-678 was incomplete, indicating this peptide is highly selective for a subtype of SRIF receptor that we have termed the SRIF1 receptor. The binding of 125I-MK-678 to SRIF1 receptors was monophasically inhibited by SRIF, the octapeptides (such as SMS-201-995), and the hexapeptides (such as MK-678), consistent with the highly selective labeling of a subtype of SRIF receptor. In contrast, the smaller CGP-23996-like analogues did not inhibit 125I-MK-678 binding to SRIF1 receptors. The binding of 125I-CGP-23996 to SRIF receptors was inhibited by SRIF and the octapeptides with Hill coefficients of less than 1, indicating that 125I-CGP-23996 labels multiple SRIF receptor subtypes. The hexapeptides and CGP-23996-like compounds produced only partial inhibitions of 125I-CGP-23996 binding, which were additive, indicating selective interactions of these compounds with the different receptor subpopulations labeled by 125I-CGP-23996. 125I-Tyr11-SRIF binding and 125I-CGP-23996 binding to SRIF receptors were likewise only partially affected by 100 microM guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), a concentration that completely abolishes specific 125I-MK-678 binding to SRIF1 receptors. The component of 125I-CGP-23996 labeling that was sensitive to GTP gamma S was also MK-678 sensitive. Thus, two subpopulations of SRIF receptors exist in the CNS. The SRIF1 receptor is sensitive to cyclic hexapeptides such as MK-678 and to GTP gamma S but insensitive to smaller CGP-23996-like compounds. The SRIF2 receptor is sensitive to the CGP-23996-like compounds and can be selectively labeled by 125I-CGP-23996 in the presence of high concentrations of the hexapeptides or GTP gamma S because, unlike the SRIF1 receptor, the SRIF2 receptor is insensitive to these agents. The SRIF receptor subtype-selective peptide analogues will be useful in the future characterization of the functions mediated by SRIF receptor subtypes in the CNS.     

Pre- and postsynaptic effects of nicotine on the mouse phrenic nerve-diaphragm preparation

Nicotine at less than or equal to 33 microM enhanced the single twitch response to indirect stimulation but potentiated the blocking effect of tubocurarine. Failure of tetanic contraction (tetanic fade) occurred on stimulation at 100 Hz. At 76 microM, nicotine induced a first phase rapid (10 min) inhibition of twitch response followed later (60-90 min) by a second phase complete block. Neostigmine partially restored the response at either phase of block whereas diaminopyridine completely antagonized the blockade. The end-plate was depolarized maximally by only 10-15 mV within 30 min with 43 microM nicotine. The depolarization was maintained but was antagonized by tubocurarine. The twitch response induced by direct stimulation was unchanged indicating no depolarization block ensued. The amplitudes of both EPP (0.7 Hz) and MEPP were markedly depressed in parallel indicating a curare-like postsynaptic inhibition without an effect on the release of transmitter. It is concluded that nicotine blocks the neuromuscular transmission by a dual mechanism by its partial agonist action. At higher frequencies of transmission, nicotine (greater than or equal to 22 microM) also produced a remarkable run-down of EPP just like other receptor antagonists suggesting that the nerve terminal acetylcholine receptors are not particularly sensitive to nicotine as those on the autonomic ganglia.     

Effects of somatostatin and adrenergic blockade on glucagon, insulin and glucose in exercising sheep

This study was conducted to characterize the mechanisms of hyperglycaemia in exercising sheep. Sheep were run on a treadmill for 45 min (5.5 km h-1, 8% incline) during adrenergic blockade (propranolol or phentolamine mesylate infusions) and during suppression of the rise in glucagon by infusion of somatostatin (SRIF). Propranolol did not alter the glucagon, insulin or glucose responses, except it tended to increase the metabolic clearance of glucose, presumably as a result of blocking the beta-adrenergic inhibition of glucose uptake. Phentolamine mesylate administration was associated with a suppression of the rise in glucagon concentrations, a reversal of alpha-adrenergic inhibition of insulin release and a reduction in glucose appearance during exercise. SRIF prevented the rise in glucagon and reduced insulin concentrations to below resting values. Propranolol and phentolamine mesylate did not alter the glucagon, insulin or glucose response to SRIF. However, SRIF prevented the insulin rise that occurred during phentolamine administration. The increment in glucose appearance produced in response to exercise was the same for SRIF, plus phentolamine mesylate and phentolamine mesylate in the first 25 min of exercise, but was significantly less than in the controls. During the last 20 min of exercise, glucose appearance was not significantly different from the control for any of the groups. The depression by SRIF and alpha-adrenergic blockade of the increment in glucose appearance due to exercise was associated with an impairment of the glucagon response. It appears, therefore, that glucagon may stimulate glucose production early in exercise in sheep directly, as well as by having a permissive effect.     

Energetics of Shortening Muscles in Twitches and Tetanic Contractions : I. A Reinvestigation of Hill''s Concept of the Shortening Heat

Shortening heat was defined by Hill as the "difference between heat produced when shortening occurs and that produced in a similar contraction without shortening." For the tetanus the "similar contraction" was an isometric one at or near l_o. By contrast, in a twitch the "similar contraction" was one in which only activation heat was produced. The applicability of Hill's concept of the shortening heat has been reexamined in both the twitch and tetanus of Rana pipiens semitendinosus muscles. Results of this investigation confirm the existence of an extra heat production accompanying shortening in the twitch and tetanus. In both cases, this shortening heat was proportional to distance shortened and relative afterload. However, at a given afterload the amount of shortening heat produced per distance shortened was greater in the twitch than the tetanus. This difference suggests that the base lines or "similar contractions" employed for the twitch and tetanus are not equivalent. The discrepancy is not remedied by utilizing in the tetanus the activation heat as the myothermic baseline and suggests that some heat producing factor(s) has been omitted in Hill's formulation of the shortening heat. Finally, the existence of Hill's feedback heat, an energy liberation associated with the presence of tension during mechanical relaxation, was not confirmed. This result strongly indicates that relaxation is energetically passive.     

Reduced Somatostatin in Hypothalamus of Young Male Mouse Increases Local but not Circulatory GH

The release of growth hormone (GH) from the pituitary gland is primarily inhibited by somatostatin (SRIF) from the hypothalamus via interactions with five types of SRIF receptors (SSTRs). However, the inhibition mechanism of SRIF on GH has not been fully examined. In this study, we repressed the hypothalamic SRIF in young male mice by stereotaxic injection of the lentiviral-shRNA against SRIF to investigate the role of hypothalamic SRIF on hormone secretion in the GH/IGF-1 axis. We found that the reduction of SRIF in hypothalamus was associated with an increase in the protein, but not the mRNA level, of the GH in the pituitary where SSTR 2 and SSTR 5 act importantly. Interestingly, the level of blood circulatory SRIF, GH, IGF-1 and the body weight were not significantly influenced by the downregulation of hypothalamic SRIF. Our findings provide insights into the mechanisms underlying the inhibition of SRIF on GH secretion.     

Somatostatin receptors in brain and pituitary

Somatostatin (SRIF) actions in the brain and pituitary are mediated by specific receptors. Using radioiodinated ligands it has been possible to characterize the kinetics of specific binding sites in the brain and pituitary, and to determine their cellular localization by autoradiography. At the pituitary level, the inhibition of growth hormone, prolactin and thyrotropin secretions induced by SRIF is mediated through a single binding site which is coupled to the inhibition of adenylate cyclase. In the brain, SRIF receptors are localized on neurons and glial cells and are also coupled to adenylate cyclase inhibition. Two sites are differentiated in the brain with an analogue of somatostatin, SMS 201995. In humans, SRIF-binding sites have been related to a number of pathologies. At the pituitary level, it has been shown that the number of binding sites was negatively correlated to growth hormone levels in acromegaly. Furthermore, SRIF-binding sites were undetectable in a patient which did not respond to SMS 201995 therapy. In the brain, meningiomas and gliomas are rich in SRIF binding sites. This suggests a possible role for SRIF on glia. In neurodegenerative diseases, cortical SRIF concentrations are decreased in Alzheimer's and Parkinson's disease associated with dementia while SRIF-binding sites are only affected in Alzheimer's disease. In conclusion, the physiological role of SRIF in the brain and pituitary can be evaluated by studying the receptors of the peptide. Such studies allow to question the implication of SRIF in endocrine and neuropathologies.     

Effect of somatostatin in the Syrian hamster bearing a transplantable islet-cell tumor.

The influence of somatostatin (SRIF) on blood glucose, plasma insulin and plasma glucagon was studied in hamsters bearing a transplantable islet-cell tumor secreting insulin and glucagon as well as in normal controls. Fed anesthetized animals were infused intraperitoneally either at a dose of 10 microgram in 15 min or of 150 microgram in 30 min, and intravenously at a dose of 250 microgram in 30 min. Blood was withdrawn from the jugular vein before and after infusion. Before the infusions, tumor bearing animals (TB) had lower blood glucose, markedly elevated plasma glucagon and slightly lower plasma insulin by comparison with normal hamsters (N). Both doses of somatostatin infused by the intraperitoneal route produced a slight but significant hypoglycemia in TB hamsters but not in normals. Ten microgram SRIF did not affect insulin and plasma glucagon levels whereas 150 microgram SRIF significantly depressed plasma insulin in both types of hamsters (N and TB). This latter dose of SRIF decreased plasma glucagon in normal but not in tumor-bearing hamsters. Intravenous infusion of 250 microgram SRIF did not reduce the hyperglucagonemia of TB hamsters either. These results indicate that somatostatin does not reduce the hyperglucagonemia due to the transplantable islet-cell tumor but nevertheless decreases blood glucose and plasma insulin.     

Simultaneous measurement of changes in the membrane potential and the intracellular Ca2+ concentration caused by somatostatin in human GH-producing pituitary tumor cells.

Changes in the membrane potential and the intracellular Ca2+ concentration ([Ca2+]i) caused by somatostatin (SRIF) were simultaneously measured in human GH-producing pituitary tumor cells, by means of the nystatin-perforated whole cell clamp technique and Fura-2 AM. An application of 10(-8) M SRIF hyperpolarized the membrane and arrested Ca(2+)-dependent spontaneous action potentials. [Ca2+]i concurrently decreased during membrane hyperpolarization. When the membrane potential was clamped below the threshold for voltage-gated Ca2+ channels, [Ca2+]i decreased and SRIF did not further reduce [Ca2+]i. In cells which did not show spontaneous action potentials, SRIF hyperpolarized the membrane but it affected [Ca2+]i little. From these results it was concluded that the reduction in [Ca2+]i caused by SRIF was ascribed to the decrease in Ca2+ influx through voltage-gated channels during membrane hyperpolarization. The effect of SRIF on the voltage-gated Ca2+ channel current was also examined under the perforated whole cell clamp. SRIF (10(-8) M) inhibited the Ca2+ channel current to 80.8 +/- 15.4% (n = 5) of the control. Because SRIF-induced inhibition of the voltage-gated Ca2+ channel current was not prominent, it was considered that membrane hyperpolarization is the major cause of the reduction in [Ca2+]i in human GH-producing cells.     

In vivo behavior of muscle fascicles and tendinous tissues of human gastrocnemius and soleus muscles during twitch contraction.

The present study investigated the differences between the human medial gastrocnemius (MG) and soleus (SOL) muscles in length changes of muscle fascicles and tendinous tissues during twitch contraction induced by an electrical nerve stimulus. Also, the time-course characteristics of twitch torque were related with changes in the length of muscle fascicles and tendinous tissues. No significant difference was observed between MG and SOL in contraction and half relaxation times of the changes in lengths and velocities of both muscle fascicles and tendinous tissues. The time-course of changes in twitch torque was nearly identical to that of the length of muscle fascicles and tendinous tissues. It was suggested that the behavior of MG and SOL during twitch contraction is practically similar in spite of their known physiological and architectural differences, and that the time-course of twitch torque is greatly influenced by the changes in the length of muscle fascicles and tendinous tissues.     

Biologic activity of the tachykinins on lamb and sheep gallbladder

In this study, we examined the activity of the tachykinins (TKs) on lamb and sheep isolated gallbladder and whether the TKs are involved in the capsaicin-induced activity in these tissues. Substance P (SP) and physalaemin (PHYS) contracted lamb gallbladder, PHYS-induced striking tachyphylaxis. This tissue was nearly insensitive to neurokinin A (NKA), neurokinin B (NKB), septide, and capsaicin. As in lamb tissues, SP and PHYS both contracted sheep gallbladder although PHYS induced no tachyphylaxis. At doses that had no effect on lamb tissue, NKA, NKB, septide, and capsaicin contracted sheep gallbladder. Our findings indicate that TK receptors differ in adult and young ovine gallbladder. The activity of PHYS on lamb gallbladder could depend on the existence of an unusual binding site, carrying one or more residues critical for the N-terminal sequence present in PHYS but not in SP.     

Adenylyl Cyclase/cAMP System Involvement in the Antiangiogenic Effect of Somatostatin in the Retina. Results from Transgenic Mice

Neoangiogenesis is a response to retinal hypoxia that is inhibited by somatostatin (SRIF) through its subtype 2 receptor (sst2). Using a mouse model of hypoxia-induced retinopathy, we investigated whether inhibition of adenylyl cyclase (AC) is involved in SRIF anti-angiogenic actions. Hypoxia increased AC responsiveness in wild type (WT) retinas and in retinas lacking sst2, but not in sst2-overexpressing retinas. Hypoxia also altered AC isoform expression with different patterns depending on sst2 expression level. The AC VII isoform mRNA and protein resulted the most affected. Indeed, in hypoxia AC VII expression was enhanced in WT retinas and it was further increased in sst2-lacking retinas, whereas in sst2 overexpressing retinas the increase of AC VII was lower than in WT retinas. These data suggest an involvement of AC/cAMP in mediating both hypoxia-evoked retinal neoangiogenesis and SRIF protective actions. The AC VII isoform is a candidate to a main role in these mechanisms.