Sequence of events involving cyclic AMP as a second messenger

Hormone binds to its receptor on the outer surface of the target cell membrane. Hormone-receptor interaction stimulates the activity of adenyl cyclase on the e

Effects of cyclic AMP in cholera e * At villus: inhibits entry of water, sodium and chloride.

h * In the crypt: promotes pumping out into the gut lumen of water, sodium, chloride y si and bicarbonate.

Inositol 1,4,5-triphosphate pathway

Activated alphal adrenergic receptors act via G-proteins to stimulate phospholipase C. Phospholipase C cleaves phosphoinositide to give inositol 1,4,5-triphosphate and

1,2-diacylglycerol. Inositol 1,4,5-triphosphate mobilises Ca2+ from intracellular stores. Ca2+ and 1,2-diacylglycerol activate calmodulin kinases and protein kinase C. These in turn phosphorylate a number of important proteins (epidermal growth factor receptor, glycogen synthase)

cytoplasmic side of the membrane. Activated adenyl cyclase catalyses the conversion of ATP to cyclic AMP within the cytoplasm. Cyclic AMP activates protein kinase enzymes that are already present in the cytoplasm in an inactive state.

Activated cyclic AMP-dependent protein kinase transfers phosphate groups (phosphorylates) to other enzymes in the cytoplasm. The activity of specific enzymes is either increased or inhibited by phosphorylation. Altered enzyme activity mediates the target cell's response to the hormone.

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