Characterization of proton currents in neurones of the snail, Lymnaea stagnalis.

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Abstract
Internal perfusion voltage-clamp and inside-out patch-clamp techniques were used to study the voltage-dependent H+ currents in snail neuron cell bodies. In whole cells the voltage-activated outward H+ current was measured 60 ms after stepping to +40 mV with an internal pH (pHi) of 5.9 and no internal K+ ([K+]i = 0), and the delayed K+ current was measured 60 ms after stepping to +40 mV with pHi = 7.3 and [K+]i = 74 mM. The mean H+ and K+ current densities were 14.6 .+-. 7.8 and 38.2 .+-. 14.0 nA/nF, respectively, giving a mean ratio of the H+ to K+ current of 0.4 .+-. 0.2. There is not a strong correlation between the densities of the two kinds of outward currents found in different cells. Inside-out patch studies reveal that the H+ and K+ currents are distributed quite differently in the membrane. While 85% of all patches had K+ current, only five out of thirty-eight patches studied had H+ currents. In those five patches the H+ currents measured at +30 mV ranged from 10.7 to 21.0 pA, and the ratio of the H+ and K+ currents at +30 mV was 0.83 .+-. 0.38. The mean H+ and K+ currents for all thirty-eight patches were 1.9 .+-. 4.9 and 10.5 .+-. 7.9 pA, respectively. The current distribution patterns demonstrate that the H+ current does not flow through the delayed K+ current channels even though the two currents have similar voltage dependence and time course. The relative ability of various extracellular divalent cations to block the H+ current was found to be Cu2+ .simeq. Zn2+ > Ni2+ > Cd2+ > Co2+ > Mn2+ > Mg2+ = Ca2+ = Ba2+. Since 100 .mu.M-Zn2+ blocks the H+ current more than it blocks the Ca2+ current, it can be used to reduce the contamination of Ca2+ current measurements by the H+ current. The magnitude of the H+ current has a stronger temperature sensitivity than does the magnitude of the delayed K+ current. The Q10 of the H+ current magnitude is 2.1 .+-. 0.4, while the K+ current magnitude is 1.4 .+-. 0.04. This suggests a higher activation energy may be involved in the conduction of the H+ current than for K+ current. The smooth time course of the H+ current measured in patches indicates that the size of the unitary H+ current is very small. Although the H+ current activates between -30 and +10 mV, the variance of the currents recorded shows no significant increase in this voltage range. The unitary current at +10 mV is estimated to be less than 0.004 pA.