Increase in Ca2 + current by sustained cAMP levels enhances proliferation rate in GH3 cells

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Abstract

Aims:

Ca2 + and cAMP are important intracellular modulators. In order to generate intracellular signals with various amplitudes, as well as different temporal and spatial properties, a tightly and precise control of these modulators in intracellular compartments is necessary. The aim of this study was to evaluate the effects of elevated and sustained cAMP levels on voltage-dependent Ca2 + currents and proliferation in pituitary tumor GH3 cells.

Main methods:

Effect of long-term exposure to forskolin and dibutyryl-cyclic AMP (dbcAMP) on Ca2 + current density and cell proliferation rate were determined by using the whole-cell patch-clamp technique and real time cell monitoring system. The cAMP levels were assayed, after exposing transfected GH3 cells with the EPAC-1 cAMP sensor to forskolin and dbcAMP, by FRET analysis.

Key findings:

Sustained forskolin treatment (24 and 48 h) induced a significant increase in total Ca2 + current density in GH3 cells. Accordingly, dibutyryl-cAMP incubation (dbcAMP) also elicited increase in Ca2 + current density. However, the maximum effect of dbcAMP occurred only after 72 h incubation, whereas forskolin showed maximal effect at 48 h. FRET-experiments confirmed that the time-course to elevate intracellular cAMP was distinct between forskolin and dbcAMP. Mibefradil inhibited the fast inactivating current component selectively, indicating the recruitment of T-type Ca2 + channels. A significant increase on cell proliferation rate, which could be related to the elevated and sustained intracellular levels of cAMP was observed.

Significance:

We conclude that maintaining high levels of intracellular cAMP will cause an increase in Ca2 + current density and this phenomenon impacts proliferation rate in GH3 cells.

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