ALPHA-BUNGAROTOXIN BLOCKS THE NICOTINIC RECEPTOR-MEDIATED INCREASE INCELL NUMBER IN A NEUROENDOCRINE CELL-LINE

Exposure of H69 small cell lung carcinoma cells to nicotinic agonists resulted in a significant increase (up to 100%) in cell number after 6 to 12 days. The effect of nicotine (10(-8) M to 10(-4) M) was both do se and time dependent as was that of another nicotinic agonist cytisin e (10(-6) M to 10(-4) M). Interestingly, both the nicotine and cytisin e induced increases in H69 cell number were blocked by alpha-bungaroto xin, as well as d-tubocurarine a nicotinic blocker which appears to in teract with most nicotinic receptors. These results suggest that the n icotine induced increase in cell number is mediated through an interac tion at the nicotinic alpha-bungarotoxin receptor. This idea is furthe r supported by experiments which show (1) that H69 cells possess high affinity alpha-bungarotoxin sites (K-d = 25 nM, B-max = 10.4 fmol/10(6 ) cells) with the characteristics of a nicotinic alpha-bungarotoxin re ceptor and (2) that the potencies of nicotinic receptor ligands in the alpha-bungarotoxin binding assay were similar to those observed in th e functional studies. Northern analysis showed that mRNA for alpha 7, a putative nicotinic alpha-bungarotoxin binding subunit, and for alpha 5 were present in H69 cells. The present data provide further evidenc e that nicotine increases cell number in small cell lung carcinoma and are the first to show that this effect is mediated through an interac tion at the nicotinic alpha-bungarotoxin receptor population. These re sults suggest that the alpha-bungarotoxin site may be involved in modu lating proliferative responses in neuroendocrine derived SCLC cells.