SOME EFFECTS OF SHORT-CHAIN PHOSPHOLIPIDS AND N-ALKANES ON A TRANSIENT POTASSIUM CURRENT (I-A) IN IDENTIFIED HELIX NEURONS

Many effects of short-chain phospholipids and n-alkanes on the squid a xon sodium current (I-Na) are consistent with mechanisms involving cha nges in membrane thickness. Here, we suggest that the actions of short -chain phospholipids on an A-type potassium current (I-A) in two-micro electrode voltage clamped Helix D1 and F77 neurons are incompatible wi th such simple mechanisms. Diheptanoyl phosphatidylcholine (diC(7)PC, 0.2 and 0.3 mM) caused substantial (58 and 79%), and in some cases par tially reversible, increases in I-A amplitude. These were correlated w ith hyperpolarizing shifts of up to -7 mV in the voltage dependence of current activation. The voltage dependence of steady-state inactivati on was also moved in the hyperpolarizing direction. These effects are the opposite of those described for squid I-Na 0.5 Saturated n-pentane and saturated n-hexane caused significant (-3 and -6 mV) hyperpolariz ing shifts in the voltage dependence of I-A inactivation, qualitativel y consistent with their effects on squid I-Na, while the voltage depen dence of activation was moved slightly to the left or unchanged. Hydro carbons had variable effects on peak current amplitude, although satur ated n-pentane produced a clear suppression. DiC(7)PC caused a 25% inc rease in the time constant of macroscopic I-A inactivation (tau(b)) bu t 0.5 saturated n-pentane and saturated n-hexane reduced tau(b) by 40% . The effects of these agents on current-clamped cells were broadly co nsistent with their opposing actions on tau(b)-phospholipids tended to reduce excitability and n-alkanes tended to increase it. Possible mec hanisms of I-A perturbation are discussed.