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Title: Stimulus-secretion coupling in gland cells
Author: Gray, Deborah C.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1984
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The work described in this thesis is divided into three main sections- The first section concerns the role of calcium in the control of fluid secretion. When cockroach salivary glands are bathed in calcium-free medium the basal rate of fluid secretion increases from about 1nl/min to about 10nl/min; maintained dopamine stimulation elicits a further rise in secretory rate which gradually declines. Evidence is presented which suggests that magnesium is unable to substitute for calcium in this system. When calcium is returned to the bathing solution after a period of calcium deprivation there is a transient increase in secretory rate. The calcium ionophore A23187 also elicits an increase in secretory rate which is maintained in the presence of extracellular calcium but declines in calcium-free medium. Stimulation of the glands in certain conditions which inhibit the secretory response leads to some kind of calcium-dependent active state in the secretory cells which can outlast the interaction of the agonist with its receptors. The conclusion drawn from this section of the work is that stimulus-secretion coupling in this gland involves a calcium-dependent second messenger system. The second section is concerned with investigating the existence and possible location of a cellular store of calcium involved in stimulus-secretion coupling. The fluorescent compound chlorotetracycline (CTC) enters the cells of the cockroach salivary gland. The acinar peripheral cells and non-secretory duct cells become preferentially labelled by CTC. Microscopic examination of the intracellular distribution of CTC indicates that this compound labels the highly folded apical plasma membranes of the peripheral cells and the deep infolds of the basal membranes of the non-secretory duct cells. Lanthanum blocks the entry of CTC into all of the gland cells and in this condition the CTC labels the basal surfaces of the acini and ducts. The results of this investigation support the idea that CTC labels calcium ions in the vicinity of plasma membranes. A comparison was made between glands in which the calcium stores had been depleted by dopamine stimulation in calcium-free medium, prior to CTC labelling, and glands which had not been stimulated prior to labelling. No apparent reduction in the labelling of stimulated , and glands compared to unstimulated glands was noted. The third section of this work is concerned with the role of cyclic AMP in the control of fluid secretion. Exposure of the glands to exogenous cyclic AMP causes an increase in fluid secretion; maximal responses to cyclic AMP virtually match the maximal responses of the same glands to dopamine. The secretory response to maintained exogenous cyclic AMP declines in calciumfree medium. The calcium-dependence of the active state which arises in the cells during stimulation in conditions preventing fluid secretion cannot be bypassed by application of exogenous cyclic AMP. Forskolin, which reputedly stimulates adenyl cyclase, does not consistently stimulate fluid secretion. The adenyl cyclase inhibitor MDL 12,330A blocks the secretory responses to dopamine, calcium readmission and A23187. Application of the phosphodiesterase inhibitor IBMX causes an increase in fluid secretion,which declines in calcium-free medium and is blocked by MDL 12,330A. It is concluded from this section that an increase in intracellular cyclic AMP is an important step in stimulussecretion coupling in the cockroach salivary gland. Preliminary experiments to determine intracellular cyclic AMP concentration using radio-immunoassay have been carried out. In conclusion a model is presented of stimulus-secretion coupling in the cockroach salivary gland, in which calcium and cyclic AMP act as synarchic second messengers regulating fluid secretion.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Human anatomy & human histology