Current density as routine parameter for description of ionic membrane current: is it always the best option?

Area of Science:

• Electrophysiology
• Cellular physiology
• Ion channel research

Background:

• Current density (J) is standard for characterizing ionic membrane currents.
• J calculation assumes ionic current (I) is proportional to cell membrane capacitance (C).
• This assumption implies a strong positive correlation between I and C, with regression intersecting near the origin.

Purpose of the Study:

• To validate the presumption of I-C proportionality in various ionic currents.
• To determine if converting whole-cell current (I) to current density (J) is always beneficial.
• To investigate the impact of impaired I-C proportionality on data interpretation.

Main Methods:

• Analysis of the I-C relationship in rat atrial and ventricular myocytes.
• Measurement of specific potassium currents: I_K1, I_K(Ach)CONST, I_K(Ach)ACH, and I_to.
• Quantification of I-C correlation using Pearson's r and a novel coefficient (k) for regression intercept deviation.

Main Results:

• Satisfactory I-C proportionality (r ≥ 0.6, k ≤ 0.2) was observed in I_K1 and I_K(Ach)ACH.
• Weak positive I-C correlation (r = 0.42) was found in I_K(Ach)CONST.
• Virtually no I-C correlation (r = 0.04) was present in I_to, suggesting channel expression heterogeneity.

Conclusions:

• The conversion of whole-cell current (I) to current density (J) is not universally applicable.
• Avoid calculating J when I-C proportionality is lacking, as seen in I_K(Ach)CONST and I_to.
• Failure to assess I-C proportionality can lead to significant misinterpretation of electrophysiological data.