Are axoplasmic microtubules necessary for membrane excitation?

Abstract

The excitability of the squid giant axon was studied as a function of transmembrane hydrostatic pressure differences, the latter being altered by the technique of intracellular perfusion. When a KF solution was used as the internal medium, a pressure difference of about 15 cm water had very little effect on either the membrane potential or excitability. However, within a few minutes after introducing either a KCl-containing, a KBr-containing, or a colchicine-containing solution as the internal medium, with the same pressure difference across the membrane, the axon excitability was suppressed. In these cases, removal of the pressure difference restored the excitability, indicating that the structure of membrane was not irreversibly damaged. Electron-microscopic observations of these axons revealed that the perfusion with a KF solution or colchicine-containing solution preserves the submembranous cytoskeletal layer, whereas perfusion with a KCl or KBr solution dissolves it. These results suggest that the submembranous cytoskeletons including microtubules provide an important mechanical support to the excitable membrane but are not essential elements in channel activities.

Stretch-Activation and Stretch-Inactivation of Shaker-IR, a Voltage-Gated K+ Channel

“…Like TREK and NMDA channels (Wan et al, 1999;Paoletti and Ascher, 1994) which are most mechanosusceptible after the cortical cytoskeleton deteriorates, voltage-gated channels in squid axon show evidence of feeling tension when the membrane skeleton is destroyed. Using electron microscopy and electrophysiology plus chaotropic solutions on squid axon, Terakawa and Nakayama (1985) demonstrated that submembranous cytoskeleton dissolves with KCl and KBr, but maintains its integrity with nonchaotropic KF. Inflation of KF-perfused axons has little impact on the action potential or on voltage clamp currents, whereas inflation of KCl-or KBr-perfused axons reversibly depolarizes them and alters their Na ϩ and K ϩ channel currents.…”

Membrane Stretch Affects Gating Modes of a Skeletal Muscle Sodium Channel

“…Whether these specific linkages are designed so that the membrane skeleton and extracellular matrix spare the bilayer from mechanical loads is still unknown. Terakawa and Nakayama (1985) have, however, provided evidence that voltage-gated Na ϩ and K ϩ channels rely on the submembranous cytoskeleton for mechanoprotection. Electronmicroscopy plus current and voltage clamp of internally perfused squid axons showed that after removal of the submembranous skeleton by chaotropic anions (e.g., Cl Ϫ , as KCl), transient inflation (stretch) reversibly abolished the action potential.…”

Mechanical, thermal, and optical changes of the nerve membrane associated with excitation.

“…The authors concluded that at least a portion of the birefringence change is associated with the sodium permeability change. The above experiment is somewhat difficult to interpret because recently colchicine has been found to enhance the effect of hydrostatic pressure of the intracellular perfusate on the membrane excitability [72]. The relationship between membrane potential and retardation response might change with the application of colchicine especially when the hydrostatic pressure applied to the internal perfusate is excessive.…”