Selective modulation of cellular voltage-dependent calcium channels by hyperbaric pressure-a suggested HPNS partial mechanism

Abstract

Professional deep sea divers experience motor and cognitive impairment, known as High Pressure Neurological Syndrome (HPNS), when exposed to pressures of 100 msw (1.1 MPa) and above, considered to be the result of synaptic transmission alteration. Previous studies have indicated modulation of presynaptic Ca(2+) currents at high pressure. We directly measured for the first time pressure effects on the currents of voltage dependent Ca(2+) channels (VDCCs) expressed in Xenopus oocytes. Pressure selectivity augmented the current in CaV1.2 and depressed it in CaV3.2 channels. Pressure application also affected the channels' kinetics, such as ƮRise, ƮDecay. Pressure modulation of VDCCs seems to play an important role in generation of HPNS signs and symptoms.

Keywords: conformational change; high pressure; high pressure neurological syndrome; hyperbaric helium pressure; synaptic transmission; voltage dependency; voltage dependent calcium channels.

Figure 1

Currents recorded in Ca_V1.2 (C,E,G) and Ca_V3.2 (D,F,H) channels. (A) Depolarization steps for (C,E). (B) Depolarization step for (D,F). (C,D) Ba²⁺ currents at 0.1 MPa. (E,F) Ba²⁺ currents at 5.0 MPa. Note current increase in (E) at HP and decrease in (F). (G,H) Superimposed single current traces under normo- and hyperbaric conditions, in response to identical depolarization to 0 mV. (I) Indication of the corresponding sections of the currents used to calculate the inactivation and kinetics parameters.

Figure 2

I-V curves of maximal currents. (A,C,E) Ca_V1.2, (B,D,F) Ca_V3.2 channels. (A,B) I-V curve of a single oocyte. (C,D) Pooled data from 9 to 17 (C) and 7 to 9 (D) oocytes exposed to 0.5–5.0 MPa pressure (color indicated). Holding potential is adjusted so that 0 indicates the potential at which maximal current is obtained (V_Imax). (E,F) Normalized-to-max I-V curves. Each curve is normalized to its own maximal value and corresponds to its pertinent curve in (C,D). Holding potential is adjusted as in (C,D). Statistical significance for each point on the curve is indicated by corresponding color asterisks (p < 0.05). Dec indicates decompression.

Figure 3

Temperature, time, Cl⁻_Ca channel blocker, and Ca²⁺ ion control experiments in Ca_V1.2 channel. (A) I–V curves measured at 5.0 MPa repeatedly while the preparation's adiabatic temperature change is subsiding. (B) I-V curves measured in a different oocyte for 1 h at 5.0 MPa after control temperature was regained. (C) I-V curves measured in a solution containing 9-AC. (D) I-V curves of compression and decompression performed with 9-AC. The expected HP effect (augmentation) is evident. (E) Example of current traces recorded with Ca²⁺ or Ba²⁺ ions in the solution at the same depolarization to 0 mV. Note the stronger and faster inactivation with Ca²⁺. (F) Compression to 0.5 MPa when Ba²⁺ ions were replaced by Ca²⁺ ions. Replacement of ions caused a decrease of the currents at 0.1 MPa, but compression to 0.5 MPa still augmented the Ca²⁺ current in a similar manner to Ba²⁺ experiments.

Figure 4

Channels' conductance at various pressures. (A,C,E) In Ca_V1.2 and (B,D,F) in Ca_V3.2 channels. (A,B) Conductance measured in a single oocyte. (C,D) Pooled data of the channels. (E,F) Normalized conductance: each curve is normalized to its own maximal value and corresponds to its pertinent curve in (C,D). A Boltzmann fit was used in (C–F). Pressures are color indicated. Statistical significance for each point on the curve is indicated by corresponding color asterisks. Holding potential is expressed as in Figure 2. Dec indicates decompression.

Figure 5

Voltage- and time-dependent current inactivation (I_end/I_max) at various pressures. (A,C,E) In Ca_V1.2 and (B,D,F) in Ca_V3.2 channels. (A,B) Inactivation measured in a single oocyte. (C,D) Pooled data of the channels. (E,F) Normalized inactivation: each curve is normalized to its own minimal value and corresponds to its pertinent curve in (C,D). Pressures are color indicated. Statistical significance for each point on the curve is indicated by corresponding color asterisks. Holding potential is expressed as in Figure 2. Dec indicates decompression.

Figure 6

Time to current peak (TTP, from stimulus onset) at various pressures. (A,C,E) in Ca_V1.2 and (B,D,F) in Ca_V3.2 channels. (A,B) TTP measured in a single oocyte. (C,D). Pooled data of the channels. (E,F) Normalized TTP: each curve is normalized to its own minimal value and corresponds to its pertinent curve in (C,D) Pressures are color indicated. Statistical significance for each point on the curve is indicated by corresponding color asterisks. Holding potential is expressed as in Figure 2. Dec indicates decompression.

Figure 7

Time constant of current activation (Ʈ_Rise) in Ca_V1.2 channel. (A) Ʈ_Rise measured in a single oocyte. (B) Pooled data of Ʈ_Rise from 7 to 16 oocytes. (C) Normalized Ʈ_Rise: each curve is normalized to its own minimal value and corresponds to its pertinent curve in (B). Pressures are color indicated. Statistical significance for each point on the curve is indicated by corresponding color asterisks. Holding potential is expressed as in Figure 2. Dec indicates decompression.

Figure 8

Fast time constant of voltage- and time-dependent current inactivation (Ʈ_{Decay Fast}). (A,C,E) in Ca_V1.2 and (B,D,F) in Ca_V3.2 channels. (A,B) Ʈ_{Decay Fast} measured in a single oocyte. (C,D) Pooled data of the channels. (E,F) Normalized Ʈ_{Decay Fast}: each curve is normalized to its own minimal value and corresponds to its pertinent curve in (C,D). Pressures are color indicated. Statistical significance for each point on the curve is indicated by corresponding color asterisks. Holding potential is expressed as in Figure 2. Dec indicates decompression.

Figure 9

Slow time constant of voltage- and time-dependent current inactivation (Ʈ_{Decay Slow}). (A,C,E) in Ca_V1.2 and (B,D,F) in Ca_V3.2 channels. (A,B) Ʈ_{Decay Slow} measured in a single oocyte. (C,D) Pooled data of the channels Ʈ_{Decay Slow}. (E,F) Normalized Ʈ_{Decay Slow}: each curve is normalized to its own maximal value and corresponds to its pertinent curve in (C,D). Pressures are color indicated. Statistical significance for each point on the curve is indicated by corresponding color asterisks. Holding potential is expressed as in Figure 2. Dec indicates decompression.

Figure 10

Tail current time constant (Ʈ_Tail) in Ca_V1.2 channel. (A) Ʈ_Tail measured in a single oocyte. (B) Pooled data of Ʈ_Tail from 7 to 16 oocytes. Only 5.0 MPa has affected (reduced) Ʈ_Tail throughout the channels' activity range. (C) Normalized Ʈ_Tail: each curve is normalized to its own minimal value and corresponds to its pertinent curve in (B). Pressures are color indicated. Statistical significance for each point on the curve is indicated by corresponding color asterisks. Holding potential is expressed as in Figure 2. Dec indicates decompression.