Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The release of growth hormone (GH) from the somatotrophs of the anterior pituitary is under the control of two hypothalamic peptides, GH-releasing factor (GRF) and somatostatin (SRIF), which stimulates and inhibits release respectively. The aim of this study was to explore the role of intracellular Ca{dollar}\sp{lcub}2+{rcub}{dollar} ( (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar}) in the regulation of GH release by GRF and SRIF in acutely dispersed purified rat somatotrophs.;The role of extracellular Ca{dollar}\sp{lcub}2+{rcub}{dollar} was studied by assessing the effect of Ca{dollar}\sp{lcub}2+{rcub}{dollar}-free medium and Ca{dollar}\sp{lcub}2+{rcub}{dollar} antagonists on GRF-induced GH release. The fluorescent dye indo-1 was used to measure (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar}. Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx was evaluated by measuring {dollar}\sp{lcub}45{rcub}{dollar}Ca uptake.;GRF-stimulated GH release was inhibited in Ca{dollar}\sp{lcub}2+{rcub}{dollar}-free medium. The Ca{dollar}\sp{lcub}2+{rcub}{dollar} anatagonists nifedipine and diltiazem, inhibited basal, K{dollar}\sp{lcub}+{rcub}{dollar}- and GRF-induced GH release. GRF stimulated a triphasic increase in {dollar}\sp{lcub}45{rcub}{dollar}Ca uptake. GRF also stimulated a biphasic increase in (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar}, which was entirely dependent on Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx. SRIF decreased {dollar}\sp{lcub}45{rcub}{dollar}Ca uptake in steady and non-steady states. It also inhibited the steady state GRF-induced increase in {dollar}\sp{lcub}45{rcub}{dollar}Ca uptake. SRIF lowered baseline (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar} and inhibited the increase in (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar} stimulated by GRF. To study the underlying mechanisms involved in the control of (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar} by GRF and SRIF, the effect of K{dollar}\sp{lcub}+{rcub}{dollar} depolarization, cAMP analogues and protein kinase C (PKC) activators on Ca{dollar}\sp{lcub}2+{rcub}{dollar} fluxes and (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar}, were examined. K{dollar}\sp{lcub}+{rcub}{dollar}-dependent depolarization stimulated a nifedipine-sensitive Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx which was not inhibited by SRIF, and which resulted in an increase in (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar}. 8-(4-Chlorophenylthio)-cAMP raised (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar} by stimulating a nifedipine- and SRIF-sensitive Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx. The PKC activators 1,2-dioctanoylglycerol and the phorbol 12-myristate 13-acetate raised (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar} by stimulating a nifedipine- and SRIF-sensitive Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx.;The conclusions of this study are (a) Ca{dollar}\sp{lcub}2+{rcub}{dollar} is essential for GRF-induced GH release; (b) GRF stimulates Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx which results in a biphasic increase in (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar}; (c) SRIF lowers (Ca{dollar}\sp{lcub}2+{rcub}{dollar}) {dollar}\sb{lcub}\rm i{rcub}{dollar} by inhibiting Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx; (d) GRF could stimulate Ca{dollar}\sp{lcub}2+{rcub}{dollar} influx by depolarizing the cell or by increasing cAMP- and PKC-dependent phosphorylation.



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