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Effect of bombesin and related peptides on surfactant secretion by cultured fetal lung cells

Asokananthan, Nithiananthan (1996) Effect of bombesin and related peptides on surfactant secretion by cultured fetal lung cells. PhD thesis, Murdoch University.

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The fetal lung undergoes extensive biochemical and physiological maturation prior to birth in preparation for its function as an organ of gas exchange. Type II pneumocytes within the lung develop the capacity to produce surfactant, a substance which lowers surface tension in the lung and prevents collapse of the alveoli during expiration. A reduced ability to produce surfactant has been shown to result in neonatal respiratory distress syndrome (RDS). Peptides structurally related to bombesin, including gastrinreleasing peptide (GRP), have been identified in a number of mammalian tissues, including the lungs. Studies show that the GRP concentration is high in full-term infants and low in infants suffering from RDS. Furthermore, the progressive increase in the GRP concentration in lung tissue during late gestation suggests that this peptide may play a significant role in the maturation of the lung, which is essential for postnatal survival.

Gastrin-releasing peptide and bombesin both enhance the rate of secretion of surfactant-associated lipids from cultured fetal rat type II pneumocytes. This effect, which is evident within one hour of addition of the peptide, is concentration-dependent with a maximal response at 3.0 nM. When assessed in comparison with other known secretagogues, it was found that GRP was additive in its enhancement of surfactant secretion with either isoproterenol, an activator of cyclic AMP-dependent protein kinase, or thapsigargin, an activator of Ca2+/calmodulin-dependent protein kinase. In contrast, there was no response to GRP over and above that observed with phorbol 12-myristate 13-acetate, an activator of protein kinase C. This suggests that the secretory response to GRP is via protein kinase C and is independent of both cyclic AMP- and Ca2+/calmodulin-dependent protein kinases. This conclusion is supported by the observation that the GRP-induced secretion is inhibited by calphostin C, a specific inhibitor of protein kinase C, but not by either H89, an inhibitor of cyclic AMP-dependent protein kinase, or KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase.

This study also investigated the peptide specificity of this effect by measuring the response to a range of bombesin-related peptides, namely neuromedin C (GRPig-27), neuromedin B and GRPi_i6 (the N-terminal region of GRP). Whereas neuromedin C was more potent than GRP, the closely related peptide neuromedin B is significantly less effective in promoting surfactant secretion and GRP 1.15 is completely without effect. In order to assess if the active peptides acted via a common receptor, the response to these peptides was studied in the presence of the known bombesin receptor antagonist, Phel2-bombesin. Phe12-bombesin at 12 pM concentration was found to inhibit the GRP-, neuromedin C-, neuromedin B- and bombesin-mediated secretion of surfactant lipids from type II pneumocytes by about 50-70%. Binding studies revealed that 125I-GRP specifically interacts with cell membrane preparations derived from cultured fetal rat type II pneumocytes. These findings support the earlier proposal that GRP and related peptides play a significant role in the physiological maturation of the lung during gestation.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Cake, Max
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