Small cell carcinoma of the lung (SCC) expresses several characteristics of neuronal cells, including voltage-gated Ca2+ channels (VGCC), and also expresses muscarinic acetylcholine receptors (mAChR). In testing the possibility that VGCC may be functionally coupled to mAChR in SCC cell lines, we found that depolarization-dependent Ca2+ influx was inhibited by carbachol (IC50 = 0.78 μM) and oxotremorine (IC50 = 0.69 μM). Equilibrium dissociation constants for several mAChR antagonists indicated that a mAChR of M3 subtype was involved. Exposure of SCC to carbachol induced the hydrolysis of phosphoinositides and increased the cytosolic free Ca2+ concentration ([Ca2+](i)). The carbachol-mediated inhibition of depolarization-dependent Ca2+ influx did not directly correlate with increased [Ca2+](i) but did correlate with inositol polyphosphate generation. The protein kinase C activators phorbol 12-myristate 13-acetate or 1-oleoyl-2-acetyl-sn-glycerol neither mimicked nor amplified the inhibitory effect of carbachol on Ca2+ influx. However, phorbol 12-myristate 13-acetate suppressed the carbachol-induced inositol polyphosphate generation and inhibition of depolarization-dependent Ca2+ influx. The inactive compound 4α-phorbol had no effect. These data suggest that the inhibition of VGCC caused by carbachol is not due to protein kinase C activation, but rather is due to events mediated by inositol polyphosphates. This is the first documentation of a role for phosphoinositide hydrolysis in the functional coupling of mAChR and VGCC. The expression of M'3 mAChR functionally coupled to VGCC could have therapeutic implications for SCC, in light of recent demonstrations that cell proliferation can be influenced by activation of neurotransmitter receptors.
|Original language||English (US)|
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|State||Published - 1990|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology