The functional coupling between membrane electrical events and exocytotic release of hormones from endocrine cells is a key element in the physiology of secretion in endocrine glands. In the case of pituitary hormone secretion several membrane ion channels are involved and the molecular mechanisms coupling stimulatory and inhibitory secretogogue receptors to these ion channels are only now being revealed. Many secretogogue receptors belong to the superfamily of seven membrane spanning domain receptors signal via guanine nucleotide binding proteins (G-proteins). Among such receptors are those for dopamine (D2 type), the primary regulator of prolactin secretion, and somatostatin which governs inhibition of growth hormone release. Among the effectors to which these classes of receptor couple are potassium and calcium channels. In recent years, many elements of the signal transduction pathways linking dopamine receptors to these ion channels have been cloned. In particular the potassium channels activated by these receptors are members of a two transmembrane spanning domain superfamily of inwardly rectifying K channels. We hypothesize these G- protein coupled K channels, or GIRKs, to be the critical link between receptor activation and the inhibition of secretion. The principle objectives of the present study are to (1) examine the molecular specificity of the signaling pathway which involves GIRK both in terms of G-proteins and GIRK channels, (2) to critically test the central involvment of GIRK channels in the regulation of secretion, and (3) to examine the mechanisms and functional role of modifications to the pathway such as desensitization. To this end we will combine molecular methods to both potentiate and interfere with G-protein and channel function with electrophysiological and optical assays of channel and secretory behavior, respectively. All experiments will be performed in single, isolated pituitary cells of the rat or in mouse pituitary tumor cells. This research will lead to a clearer understanding of the events underlying regulated hormone secretion in pituitary cells. In addition, the common identity of dopamine receptor subtypes in both pituitary and brain as well as similarities in G-protein gated K channels in each tissue, suggest that these studies may provide mechanistic insight into the actions of these neurotransmitters in affective disorders and neural regulation of vegetative functions.
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