PHARMACOLOGICAL PROPERTIES OF VOLTAGE-GATED NA-NEURONS FROM A HYDROZOAN JELLYFISH POLYORCHIS-PENICILLATUS( CURRENTS IN MOTOR)

The Na+ current of 'swimming motor neurones' in the hydromedusan Polyo rchis penicillatus was tetrodotoxin-insensitive. The local anaesthetic s lidocaine and procainamide caused partial, non use-dependent blockad e of the Na+ channel, Veratridine produced partial blockade of the Na channel without affecting inactivation, An order of blocking potency of di- and trivalent cations was established as: La3+ = Zn2+ = Cd2+ > Ni2+ > Mn2+ = Co2+ > Ca2+ > Ba2+ > Mg2+. All these cations, except Ba2 +, produced depolarizing shifts in the conductance-voltage curves, Eve n at relatively high concentrations, the dihydropyridines nicardipine, nitrendipine and (+)Bay K 8644 produced only weak blockade of the Na current; while nimodipine, nifedipine and (-)Bay K 8644 were ineffect ive. Diltiazem and verapamil weakly blocked the Na+ current in a dose- dependent manner with no evidence of use-dependence, The calmodulin in hibitors W7 and calmidazolium were ineffective blockers of Na+ current s, Crude Conus venoms and the Conus peptides, mu-conotoxin GIIA, mu O- conotoxin MrVIA, omega-conotoxin GVIA and omega-conotoxin MVIIC, were without effect. Capsaicin produced rapid, reversable blockade of Na+ c urrent. It has been suggested that 'primitive' Na+ channels could be e xpected to have pharmacological properties that are intermediate betwe en those of Na+ and Ca2+ channels, If such channels exist, the Na+ cha nnel described here is clearly not one of them.