Voltage-dependent sodium channels in synaptoneurosomes: studies with 22Na+ influx and [3H]saxitoxin and [3H]batrachotoxinin-A 20-alpha-benzoate binding. Effects of proparacaine isothiocyanate.

22Na+ influx and binding of [3H]saxitoxin ([3H]STX) and [3H]batrachotoxin-A 20-alpha-benzoate ([3H]BTX-B) were studied in guinea pig cerebral synaptoneurosomes. STX and tetrodotoxin (TTX) completely blocked the stimulation of sodium influx induced by 1 microM BTX. The IC50 values for STX and TTX closely matched the Ki values for inhibition of [3H]STX binding, suggesting that the sites labelled by [3H]STX are associated with a population of BTX-sensitive channels. BTX induced a dose-dependent stimulation of sodium influx in synaptoneurosomes (EC50 280 nM). The potency of BTX for stimulation of sodium influx was increased (EC50 24 nM) in the presence of 0.6 microgram/ml scorpion venom without any change in maximal influx. In contrast, specific binding of [3H]BTX-B to synaptoneurosomes was minimal in the absence of scorpion venom, but it was increased several fold in the presence of 60 micrograms/ml scorpion venom. With proparacaine isothiocyanate (PROPRIT), an irreversible local anesthetic, the inhibition of [3H]BTX-B binding by PROPRIT did not occur in parallel with an inhibition of sodium influx induced by BTX. Preincubation of synaptoneurosomes with 10 microM PROPRIT for 10 min resulted in approximately 70% inhibition of [3H]BTX-B binding in the presence of scorpion venom. Such preincubation did not alter BTX-induced sodium uptake in synaptoneurosomes. Preincubations of synaptoneurosomes with 100 microM PROPRIT for 10 min completely inhibited [3H]BTX-B binding, and under these conditions BTX-induced sodium influx was reduced only by 50%. The results indicate that virtual elimination of binding sites labeled by [3H]BTX-B in the presence of scorpion venom by PROPRIT has little effect on sodium influx induced by BTX.(ABSTRACT TRUNCATED AT 250 WORDS)