A CNS antagonist to bombesin

692

SELECTED

GASTROENTEROLOGY

SUMMARIES

addition, folate absorption in the normal adult is complex and involves a number of steps. Folate absorption has been the subject of a number of reviews (N Engl J Med 1975:293:1303-E, Clin Haematol 1976;5:589-618, Am J Clin Nutr 1979;32:846-55, Annu Rev Med 1980;31:79-87. Fed Proc 1984;43:2423-91. Brieflv. before absorption dietary folates, which exist predominatily as reduced and conjugated pteroyl polyglutamates with various substitutions on the pteridine ring, must be hydrolyzed to the monoglutamate form by mucosal conjugase, or perhaps also by pancreatic conjugase (Am J Clin Nutr 1983;37:262-7). Subsequent transport, which may involve one or more metabolic conversions by the intestinal mucosa, probably occurs by an active transport process that is markedly sensitive to mucosal surface pH, with an optimum near pH 6-the so-called “pH microclimate” (Gut 1978;19:735-42, J Lab Clin Med 1979;93:428-36, Pflugers Arch 1981:392:29-33, Am J Physiol 1981;240:G97-101, Biochim Biophys Acta 1984;775:402-8). Folate absorption is considered to be maximum in the proximal jejunum. 5-Methyltetrahydrofolate, a naturally occurring folate derivative which has been used in a number of studies of folate absorption, is taken up by the same structure-specific mechanism responsible for the active transport of folic acid and other monoglutamyl folates (Am J Physiol 1984;246:G515-20). Furthermore, a recent in vitro study also describes no reduction in absqrption of this compound by rat jejunum from senescent compared with adolescent rats (Digestion 1984;30:231-5). Absorption of dietary folates will decrease not only with mucosal disease, but also with ingestion of alcohol (Gastroenterology 1981:80:99-102, Clin Gastroenterol 1981; 10:263-93, Annu Rev Med 1982;32:345-54) and many drugs (Curr Concepts Nutr 1983;12:73-87 and 129-38). The chronic polypharmacy of the elderly, with both prescription and over-thecounter drugs, is well known. Few of the earlier reported studies of folate absorption in the elderly have adequately characterized habits of study subjects with regard to their alcohol or drug ingestion, or both. Theoretically, dietary folate absorption in the

elderly could still be impaired by age-related asymptomatic Am J hypochlorhydria (Stand J Gastroenterol 1966;1:199-206, Dig Dis 1970:15:783-9, Int J Nurs Stud 1978;15:107-14). A significant reduction in jejunal mucosal surface area has also been described in a group of 10 well-nourished elderly patients aged 60-73 yr when compared with controls aged 16-30 yr (Lancet 1978;ii:849-501, and this could influence folate absorption. Agerelated motility changes of the stomach have been described, although the significance of this is doubtful. One can choose whether there is no change in gastric emptying or decreased or increased gastric emptying with age (Gastroenterology 1983; 85:1444, Clin Sci 1984;67:213-83. Constipation and fecal stasis in old age is well recognized and, not surprisingly, total intestinal transit time increases with age (Gerontol Clin 1969;11:293-300). However, normal age-related changes in small bowel motility and in the capacity of the splanchnic circulation to respond to a meal remain poorly explored. The best clinical test to assess blood flow in the presence of normal renal function is purported to be the dxylose clearance test (Texter EC. The splanchnic circulation and nutrient absorption in the aging patient. In: Texter EC, ed. The aging gut. New York: Masson, 1983:13-20). Intestinal absorption of d-xylose does appear to decrease beyond 70 yr of age (J Gerontol 1982;37:300-5). As I said earlier-the process of aging is a complex phenomenon, but we can now be reassured that at least our ability to absorb folate will not decrease in the years to come. E. URBAN. M.B.B.S.

A CNS ANTAGONIST Yachnis AT, Crawley JN, Jensen TW (Department of Biochemistry,

TO BOMBESIN RT, McGrane MM, Moody The George Washington

Vol.

89. No. 3

University School of Medicine and Health Sciences, Washington, D.C.; Psychobiology Branch, The National Institute of Mental Health, and Digestive Disease Branch, National Institute of Digestive and Kidney Disease, Bethesda, Maryland) The antagonism of bombesin in the CNS by substance P analogues. Life Sci 1984;35:1963-9. The authors report a study that examines the antagonism by substance P analogues of the binding of bombesin to rat brain slices and on the behavioral effects of bombesin. The study was conducted as a result of previous studies on dispersed guinea pig pancreatic acini which demonstrated the antagonism of bombesin-induced amylase secretion by substance P analogues. The substance P analogues examined were (D-Pro’, D-Phe’, D-Trp”) substance P, (D-Pro”, D-Trp7.‘) substance P, and (D-Arg’, D-Trp7,“, Leu”) substance P (spantide). Slices of rat brain provided the source of central receptors for specific binding of substance P, bombesin, or vasoactive intestinal polypeptide (VIP). Specific binding was achieved using radioiodinated ligands. ‘2”I-physalaemin, and ‘““I-VIP. The (lz51-Tyr4) bombesin, potency of the substance P antagonists was examined by determining their ability to inhibit specific ‘2”I-physalaemin binding. Spantide and (D-Pro’, D-Trp7,“) substance P were equally potent antagonists, whereas the third antagonist was about five times less potent. Peptides structurally unrelated to substance P, e.g., VIP, secretin, bombesin, or gastrin-releasing peptide (GRP), did not inhibit ““I-physalaemin binding. ‘251-Vasoactive intestinal polypeptide binding was inhibited by unlabeled VIP but not by unrelated peptides, including the substance P antagonist spantide. (lz51-Tyr) bombesin binding was inhibited by bombesin and the structurally related GRP, but not by physalaemin, substance P, VIP, or secretin. Spantide, however, did inhibit binding to central receptors for bombesinlike peptides with about the same affinity as it did to central substance P receptors. Spantide was examined for its ability to inhibit bombesin-induced hypothermia and grooming. Bombesin lowered the core temperature in coldexposed rats. This bombesin-induced hypothermia was reversed when it was coadministered with :! pg spantide. Spantide itself had no effect on core temperature. Similarly, spantide reversed the bombesin-induced grooming. Thus spantide, a substance P antagonist, with no structural similarity to bombesin is able to inhibit central receptor binding of bombesin and the biologic actions of bombesin in the central nervous system.

Comment.

Bombesin is a tetradecapeptide which was initially isolated in frog skin but has been found to be biologically active in the central and enteric nervous systems of mammals. Like many neuropeptides, its physiologic function is unknown. Studies to investigate this role have been hampered by the lack of specific antagonists. The presence of a specific antagonist that has no agonist action allows the indirect determination of the role of a peptide in a physiologic phenomenon. Should the physiologic response be altered by the presence of the antagonist, one can postulate that the endogenous ligand plays a role in mediating the response. The lack of a specific antagonist to bombesin has resulted in much description of the effects of exogenously administered bombesin without a means to determine if these effects are physiologic. Substance P, a structurally unrelated peptide. is also

September

SELECTED SCIMMAKIES

1985

a neuropeptide found in the brain and gut. Analogues have been synthesized in the attempt to obtain a specific antagonist (Eur J Pharmacol 1984:97:179-89). As seen in this study, one of the most effective antagonists at this time appears to be spantide. The possibility of antagonism of bombesin by a substance P antagonist was raised by studies indicating that substance P antagonists would reverse the bombesin-stimulated amylase secretion from dispersed guinea pig pancreatic acini (Biochim Biophys Acta 1984:804:181-91). The studies reported here were carefully executed and demonstrated that the antagonist effect on the central binding of bombesin was specific for spantide, as substance P itself did not affect binding of bombesin. There does not appear to be any similarity between the substance P and bombesin receptors, as bombesin also does not affect substance P binding. The authors then proceeded to investigate whether the ability of the substance P antagonist, spantide. to inhibit central binding of bombesin was paralleled by inhibition of a response that is mediated by the activation of central bombesin receptors. After central injection, bombesin induces hypothermia in cold-exposed rats and grooming at ambient temperature (Science 1977;196:9981001, Brain Res 1980;193:209-20). It has been postulated that these responses are mediated physiologically by endogenous bombesinlike peptides in the central nervous svstem and pharmacologically by the binding of exogenously administered bombesin to central receptors. This study was able to demonstrate that these responses to centrally administered bombesin were inhibited by spantide, and bombesin-induced hypothermia was not inhibited by substance P, which also had no antagonist properties at central bombesin receptors. This is an important study for several reasons. It establishes that a substance P antagonist such as spantide is an antagonist for bombesin receptor binding in the central nervous system as well as in the pancreas, suggesting that this property exists at all bombesin receptors. It demonstrates that a physiologic response to bombesin is inhibited by this antagonist, providing the best data to date that central bombesin receptors mediate this response. This in turn strongly suggests that the pathways exist in the brain to mediate this response endogenously, and indicate that the bombesinlike peptide, which has been demonstrated by immunohistochemical techniques to be present in the rat brain, mediates this physiologic response. It is surprising that an analogue of an apparently structurally unrelated peptide, substance P, would be an effective antagonist of bombesin. In addition to demonstrating that one cannot assume that analogues of specific peptides will only be antagonists to their parent peptide, this study underscores the importance of multiple controls in studies examining the effects of peptides where one is using incompletely characterized antagonists. Finally, the demonstration that these analogues of substance P interact with the bombesin receptor. whereas other analogues and substance P itself do not, may provide some clues as to the physicochemical structure of the bombesin receptor itself.

A. OIJYANG.M.D.

THE DIAGNOSIS PERITONITIS

OF BACTERIAL

Garcia-Tsao G, Corm HO, Lerner E (Veterans Administration Medical Center, West Haven, Connecticut, and Yale University School of Medicine, New Haven, Connecticut) The diagnosis of bacterial peritonitis: comparison of pH, lactate concentration and leukocyte count. Hepatology 1985;5:91-6.

693

During the past 20 yr since Corm first defined bacterial peritonitis (BP) in patients with ascites, this condition has become recognized as an important and life-threatening complication. If untreated, there is a high mortality rate. The clinician who is caring for a patient with ascites is faced with several difficult decisions. How can he decide when a paracentesis is indicated and how does he interpret the initial fluid analysis before the bacteriologic results are available. The question of when to do a paracentesis remains unanswered. The present study addresses the interpretation of paracentesis fluid findings. The number of polymorphonuclear neutrophilic leukocytes (PMNs) has been reported to be the most useful parameter in detecting BP. In Hoefs’ study (Hepatology 1982;2:399407) of 43 patients with BP, all had a PMN count >25Oi m13. There has been dispute, however, as to what the required cutoff point for PMNs should be in order to establish this diagnosis with certainty. Both false-positive and false-negative results have been reported when using this 2591m13 PMN level, and this has led to a search for other determinants of bacterial infection. Gitlin (Hepatology 1982;2:408-11) reported that an ascites pH C7.32 strongly suggested infection. Kao and Reynolds (Hepatology 1983;3:275) reported that they could not confirm the association between low ascitic pH and BP, although their data suggested an increased gradient between arterial pH and the pH of the ascites in infected patients. The present study was based on 70 patients with ascites who had determination of PMN count, as well as pH and lactate levels in ascites and arterial blood. Fourteen patients were considered to have BP, I2 of whom had positive cultures. In 11 of these patients the peritonitis was considered spontaneous. The 51 noninfected specimens had a mean PMN count of 122 but 7 of them had levels >250, thereby raising the question of possible BP. The pH of this non-BP group had a mean value of 7.45. There was almost no difference between arterial and ascites pH. The mean lactate level in the ascites of non-BP patients was 15 mgidl, with a minimal difference between arterial blood and ascites (3 mgidl). The 14 patients with BP all had significantly different results. The ascitic fluid contained a mean of 2686 PMN/m13. The mean pH of the infected ascites was 7.24, with a gradient of 0.22 between ascites and arterial blood. Lactate levels showed a mean of 45 mgi dl in the ascites with a gradient of 13 mgidl between ascites and arterial blood. There was no correlation between the organisms cultured and any of these determinations. The authors conclude that the PMN count remains the single most sensitive test for BP if 500 PMNlml” is accepted as the cutoff point. A pH ~7.35 or a gradient between ascites and peripheral blood of >O.lO pH units are highly suggestive of BP, although not as sensitive a parameter as the PMN count. Lactate levels >25 mgidl in ascites are also highly accurate predictors of BP. Neither glucose levels nor total white blood cell count were of much practical use in detecting BP. Thus, if either the PMN level is >500/m13 or the pH level is ~7.35 in ascitic fluid, BP can be diagnosed with great accuracy (98%). If both these determinants are normal, BP is virtually excluded. Lactate determinations are almost as good as pH but, because they are more time consuming and less available, their routine use is not required.