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Effects of maitotoxin on atrial natriuretic factor-mediated accumulation of cyclic GMP in PC12 cells
A Schulick1, F Gusovsky, T Yasumoto
1Laboratory of Bioorganic Chemistry, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland.
Abstract:
Maitotoxin (MTX) activates calcium channels and stimulates phosphoinositide breakdown in pheochromocytoma PC12 cells, while having no effect on basal levels of the cyclic nucleotides cAMP and cGMP. Atrial natriuretic factor (ANF) induces a dose-dependent accumulation of cGMP in PC12 cells through the activation of a membrane bound guanylate cyclase. Effects of ANF on cGMP are independent of extracellular concentrations of calcium. Since agents that activate phosphoinositide breakdown can indirectly affect cyclic nucleotide formation, the effects of MTX on ANF-mediated accumulation of cGMP was studied. MTX induces a dose-dependent inhibition of ANF-mediated accumulation of cGMP. The inhibition by MTX requires the presence of extracellular calcium, but is unaffected by the calcium channel blocker nifedipine. The inhibitory effect of MTX is not mimicked by the calcium ionophore ionomycin. A phorbol ester, PMA, which stimulates protein kinase C, also inhibits ANF-mediated accumulation of cGMP. Sodium nitroprusside induces large accumulations of cGMP in PC12 cells through the stimulation of a soluble guanylate cyclase. Neither MTX nor PMA inhibit nitroprusside-mediated accumulation of cGMP. The results indicate that in PC12 cells, protein kinase C activation, either directly with PMA, and indirectly with MTX through phosphoinositide breakdown and formation of diacylglycerol, leads to inhibition of ANF-mediated, but not nitroprusside-mediated accumulation of cGMP.
Insights
Maitotoxin (MTX) inhibits atrial natriuretic factor (ANF)-induced cyclic GMP (cGMP) accumulation in PC12 cells. This inhibition requires extracellular calcium and involves protein kinase C activation, impacting ANF signaling pathways.
Area of Science:
- Cellular signaling pathways
- Neuropharmacology
- Calcium channel activation
Background:
- Maitotoxin (MTX) activates calcium channels and phosphoinositide breakdown in PC12 cells.
- Atrial natriuretic factor (ANF) increases cyclic GMP (cGMP) via membrane-bound guanylate cyclase, independent of extracellular calcium.
- Phosphoinositide breakdown can indirectly influence cyclic nucleotide formation.
Purpose of the Study:
- To investigate the effects of MTX on ANF-mediated cGMP accumulation in PC12 cells.
- To elucidate the role of extracellular calcium and protein kinase C in MTX's inhibitory action.
Main Methods:
- Treatment of PC12 cells with MTX, ANF, phorbol ester (PMA), and sodium nitroprusside.
- Measurement of intracellular cGMP levels.
- Assessment of calcium dependency and effects of nifedipine and ionomycin.
Main Results:
- MTX inhibited ANF-mediated cGMP accumulation in a dose-dependent manner.
- This inhibition required extracellular calcium but was not affected by nifedipine or ionomycin.
- Protein kinase C activation, induced by PMA or indirectly by MTX, inhibited ANF-mediated, but not sodium nitroprusside-mediated, cGMP accumulation.
Conclusions:
- MTX indirectly activates protein kinase C, leading to inhibition of ANF-stimulated cGMP production in PC12 cells.
- The inhibitory mechanism is specific to the ANF signaling pathway involving membrane-bound guanylate cyclase.