Oscillatory hyperpolarizations and resting membrane potentials of mouse fibroblast and macrophage cell lines.

Abstract

L cells (a mouse fibroblast cell line) and macrophages exhibit slow oscillatory hyperpolarizations and relatively low membrane potentials, when measured with glass micro-electrodes. The role of microelectrode-induced leakage in these oscillations for L cells and a mouse macrophage cell line (P388D1) is described. Both L cells and macrophages showed fast negative-going peak-shaped potential transients upon micro-electrode entry. The micro-electrode apparently introduces a leakage conductance acrosss the membrane. The peak values of these fast transients were less negative for L cells (-17 mV) than for macrophages (-39 mV), although their sustained resting membrane potentials were about equal (-13 mV). The pre-impaled membrane potential of macrophages is apparently more negative than that of L cells. Ionophoretic injection of Ca2+ into the P388D1 macrophages showed the existence of a Ca2+-dependent hyperpolarizing conductance presumed to be involved in the oscillatory hyperpolarizations of L cells and macrophages. Cells increased in size by X-ray irradiation to reduce membrane input resistances were still found to be susceptible to micro-electrode-induced leakage. Impalement transients upon entry of a 2nd electrode during a hyperpolarization evoked by a 1st electrode, were often step-shaped instead of peak-shaped due to the high membrane conductance associated with hyperpolarization. Since peak-shaped impalement transients were always seen with the 1st impalement both in oscillating and non-oscillating cells, oscillatory hyperpolarizations cannot be regarded as spontaneously occurring in the unperturbed cells but are induced by micro-electrode penetration. Since the hyperpolarizing response can be evoked by ionophoretic injection of Ca2+, and oscillatory as well as single hyperpolarizing responses are absent in a Ca2+-free medium, the Ca2+ needed intracellularly to activate the hyperpolarizing responses apparently enters the cell via the leakage pathway introduced by the measuring electrode.