Pertussis toxin differentiates between two mechanisms of attenuation of cyclic AMP accumulation by muscarinic cholinergic receptors.

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Abstract
Muscarinic cholinergic receptor-mediated attenuation of cAMP accumulation occurs through activation of phosphodiesterase in 1321N1 human astrocytoma cells. Pertussis toxin, which ADP-ribosylates the guanine nucleotide regulatory protein involved in receptor-mediated inhibition of adenylate cyclase (Ni), was utilized to further differentiate between the mechanism of cholinergic regulation of cAMP metabolism in 1321N1 cells and the mechanism involving inhbition of adenylate cyclase in other tissues. Muscarinic receptor-mediated regulation of cAMP accumulation in NG108-15 neuroblastoma-glioma cells occurs through inhibition of adenylate cyclase. Pretreatment of these cells with pertussis toxin completely blocked the capacity of carbachol to attenuate cAMP accumulation. Concentrations of pertussis toxin 2-3 orders of magnitude higher than those effective in NG108-15 cells had no effect on muscarinic receptor-mediated attenuation of cAMP accumulation in 132N1 cells. No effect of pertussis toxin was observed either on the control rate or the carbachol-stimulated rate of cAMP degradation measured directly in intact 1321N1 cells. A 41,000 MW protein previously proposed to be the .alpha. subunit of Ni was labeled during incubation of a plasma membrane fraction from 1321N1 cells with [32P]NAD and pertussis toxin. Pertussis toxin is apparently active in 1321N1 cells, since this protein substrate was not labeled in plasma membrane preparations from cells previously incubated with toxin. Functional activity of Ni was demonstrated by the observation that guanosine 5''-[.gamma.-thio]triphosphate- and GTP-mediated inhibition of forskolin-stimulated adenylate cyclase activity occurred in cell-free preparations from 1321N1 cells. The inhibitory activity of these guanine nucleotides was lost in membrane preparations from pertussis toxin-treated cells. Thus, adenylate cyclase apparently is not involved in cholinergic action in 1321N1 cells and Ni apparently is not involved in muscarinic receptor-mediated activation of phosphodiesterase in these cells. Pertussis toxin can be used to differentiate between 2 mechanisms of cholinergic regulation of cAMP metabolism.