Involvement of phosphodiesterase-cGMP-PKG pathway in intracellular Ca2+ oscillations in pituitary GH3 cells

The present study investigates the potential role of the Ca²⁺-calmodulin-dependent type I phosphodiesterase (PDE)-cGMP-protein kinase G (PKG) pathway in spontaneous [Ca²⁺](i) oscillations in GH₃ cells using fura-2 single cell videoimaging. Vinpocetine (2.5-50 μM), a selective inhibitor of type I PDE, induced a concentration-dependent inhibition of spontaneous [Ca²⁺](i) oscillations in these pituitary cells, and at the same time produced an increase of the intracellular cGMP content. The cell permeable cGMP analog N²,2'-O-dibutyryl-cGMP (dB-cGMP) (1 mM) caused a progressive reduction of the frequency and the amplitude of spontaneous [Ca²⁺](i) oscillations when added to the medium. KT5823 (400 nM), a selective inhibitor of cGMP-dependent protein kinase (PKG), produced an increase of baseline [Ca²⁺](i) and the disappearance of spontaneous [Ca²⁺](i) oscillations. When KT5823 was added before vinpocetine, the PKG inhibitor counteracted the [Ca²⁺](i) lowering effect of the cGMP catabolism inhibitor. Finally, the removal of extracellular Ca²⁺ or the blockade of L-type voltage-sensitive calcium channels (VSCC) by nimodipine produced a decrease of cytosolic cGMP levels. Collectively, the results of the present study suggest that spontaneous [Ca²⁺](i) oscillations in GH₃ cells may be regulated by the activity of type I PDE-cGMP-PKG pathway. Copyright (C) 1999.