Glucagon stimulates the cardiac Ca2+ current by activation of adenylyl cyclase and inhibition of phosphodiesterase

Article Abstract:

Glucagon, a hormone produced by cells in the pancreas, changes the storage form of the sugar molecule, glycogen, to glucose, which can be used by the cell for energy. Glucagon causes an increase in heart muscle contraction and affects the regularity of the heartbeat. The effects of glucagon are associated with stimulation of the enzyme system adenyl cyclase, which is involved in many types of cellular signals, including channels in the cellular membranes involved in the transport of calcium ions. The effects of glucagon on the flow of calcium ions in heart muscle was studied. In heart muscle cells from the rat, glucagon stimulated adenyl cyclase in a manner that is similar to the action of beta-adrenergic agonist drugs, such as isoprenaline. However in the frog, glucagon had a smaller effect on the flow of calcium in heart muscle and adenyl cyclase was not stimulated. In the frog, glucagon inhibited an enzyme known as cAMP phosphodiesterase. Many drugs that are cardiotonic (increase the efficiency of the heart muscle contractions), such as milrinone and Ro-20-1724, appear to act in way which is similar to the effect of glucagon on frog heart tissue, by inhibiting phosphodiesterase activity, but not effecting adenyl cyclase. This study demonstrates that glucagon can activate the flow of calcium by two different mechanisms, the stimulation of adenyl cyclase and the inhibition of phosphodiesterase activity. Compounds that affect cardiac function, such as neuropeptides and hormones, could use either pathway for stimulating the flow of calcium. Further study is needed to discern the different pathways of various compounds. (Consumer Summary produced by Reliance Medical Information, Inc.)

Models, Heart beat, Heart rate, Calcium, Calcium (Chemical element), Heart conduction system, Glucagon

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Structure of the adenylyl cyclase catalytic core

Article Abstract:

The mammalian adenylyl cyclase has two conserved regions, C(sub 1) and C(sub 2), which are responsible for forskolin- and G-protein-stimulated catalysis. The C(sub 2) catalytic region of type II rat adenylyl cyclase contains an alpha/beta class fold that has a central cleft. At the end of the cleft are two forskolin molecules that bind in hydrophobic pockets while at the center are charged residues implicated in adenosine triphosphate binding. Forskolin seems to activate adenylyl cyclase by promoting the assembly of the active dimer and by direct interaction within the catalytic cleft. Other adenylyl cyclase regulators work at the dimer interface or on a flexible C-terminal region.

Research

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Rhodopsin phosphorylation as a mechanism of cyclic GMP phosphodiesterase regulation by S-modulin

Article Abstract:

Rods in the vertebrate eye are desensitized and light response is hastened during light-adaptation. Cyclic GMP is hydrolyzed as soon as a phosphodiesterase is activated by the light absorption of the rods. Part of the light-adaptation process is the decrease in cytoplasmic calcium concentration. Evidence is presented to support the notion that s-modulin prolongs the lifetime of active cyclic GMP-phosphodiesterase (PDE) at high calcium concentrations. When active PDE lifetime is prolonged, rhodopsin phosphorylation is also inhibited by s-modulin.

Phosphorylation, Photoreceptors

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