This investigation aims at the role and regulation of receptor- coupled phospholipase C. We found that in a clonal neurotumor hybrid NCB-20, these cells expresses a variety of receptors of neurotransmitters and neuromodulators that are coupled to phosphoinositide (PI) hydrolysis. Carbachol, a muscarinic cholinergic receptor agonist markedly increased the accumulation of inositol monophostphate (IP1) in the presence of lithium (Li+). This increase was time and dose dependent. The formations of inositol bisphosphate and trisphosphate were also increase by this agonist but to a lesser extent and with a faster time course. Antagonist specificity suggests that this effect is mediated by the M cholinergic receptors. This activation was associated with a rapid increase in the efflux of 145 Ca from cells. NCB-20 cells also contain histamine-sensitive PI turnover system. Antagonists effect indicated that this is a histamine H1 receptor-mediated response. The neuropeptide, bradykinin, also caused a robust (more than 10-fold) increase of 3H-IP1 accumulation. The effects of bradykinin, histamine and carbachol were additive to each other. The carbachol-induced 3H-IP1 accumulation was completely inhibited by veratridine at concentrations that inhibit the voltage-sensitive sodium channel. The basal accumulation was increased 2-fold by veratridine, while the activities increased by histamine and bradykinin were unaffected. The responses of both bradykinin and carbachol were desensitized by preexposure to bradykinin and carbachol, respectively. Biologically active phorbol esters markedly attenuated the bradykinin and carbachol- receptor mediated responses and significantly inhibited the basal phospholipase C activity, suggesting that either phospholipase C or some phospholipase C regulatory proteins are substrates of phorbol ester-activated protein kinase C. Pertussis toxin only attenuated in part the carbachol and bradykinin-induced PI turnover. The concentration of lithium required to maximally increase the receptor-activated 3H-IP accumulation in cultured NCB-20 cells was about 60-80 mM which was about 10-time greater than those found in brain slices and other neurotumor cell lines such as NG 108-15 neurohybrid. This unusual requirement of high lithium may suggest that NCB-20 cells express a novel type of inositol-1-phosphatase and might lead to some clinical implication for the mechanisms of the therapeutic effect of lithium in the treatment of manic depression.
