Calcium Independence of Phosphoinositide Hydrolysis-Induced Increase in Cyclic AMP Accumulation in SK-N-SH Human Neuroblastoma Cells

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Abstract
Previous work has shown that stimulation of muscarinic receptors in various cell lines increases intracellular cyclic AMP (cAMP) levels. This unusual response has been hypothesized to be mediated by stimulation of calcium/calmodulin-sensitive adenylate cyclase, secondary to inositol trisphosphate (IP3)-mediated calcium mobilization. To test this hypothesis, we stimulated muscarinic receptors in SK-N-SH human neuroblastoma cells while blocking the IP3-mediated rise in intracellular calcium concentration using two different methods. Loading cells with the intracellular calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) abolished the carbachol-mediated intracellular calcium release without abolishing the carbachol-mediated increase in cAMP level. Similarly, in cells preexposed to carbachol, the agonist-induced change in intracellular calcium level was blocked, but the cAMP response was not. Thus, both of these methods failed to block the muscarinic receptor-mediated increase in cAMP level, thereby demonstrating that this cAMP level increase is not mediated by a detectable rise in intracellular calcium concentration.