Calmodulin inhibitors activate glycogen phosphorylase B to a conversion in C6 glioma cells
Norman, J.A.; Staehelin, M.
Molecular Pharmacology 22(2): 395-402
1982
ISSN/ISBN: 0026-895X PMID: 6183569 Document Number: 190889
Some neuroleptic drugs were described as calmodulin antagonists in vitro, yet their usefulness as inhibitors of calmodulin in intact cell systems remains unclear. C6 glioma cells were preincubated with these drugs (0.1-0.5 mM) to determine whether they could block Ca2+ ionophore (calcimycin, A23187)-induced conversion of glycogen phosphorylase from the B to the A form. Of the 10 neuroleptic drugs tested, 7 converted 36% of the phosphorylase from the B to the A form by themselves; this activation could not be enhanced by the addition of A23187. The cAMP phosphodiesterase inhibitors 3-isobutyl-1-methylxanthine (IBMX) and Ro 20-1724 [4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone] at concentrations of 0.1 mM converted 25 and 33% of the phosphorylase to the A form, respectively; however, this activation could be further stimulated by A23187 in a partially additive manner. In C6 cell homogenates the phosphodiesterase inhibitors IBMX and Ro 20-1724 inhibit both Ca2+-dependent and Ca2+-independent phosphodiesterase activity, with IC50 values of 0.1-0.3 mM, whereas the neuroleptic drugs inhibit on the Ca2+-dependent form of phosphodiesterase, with IC50 values of 0.09-0.13 mM. Despite this, in intact C6 cells the neuroleptic drugs decreased the cAMP concentration whereas both of the phosphodiesterase inhibitors (IBMX and Ro 20-1724) increased the cAMP concentration. The neuroleptic drugs, unlike the phosphodiesterase inhibitors, activate phosphorylase conversion through a mechanism that does not involve cAMP. The limitations of these drugs as probes for calmodulin are discussed.