Evidence for fixed charge in the nexus

Abstract

The nexus or gap junction has been characterized as a low-resistance junction as well as a highly permeable junctional membrane to many molecules^1–6. The transfer of electrical current from one cell interior to another, the aqueous solubility of dyes used to trace cell to cell communication and the fact that these molecules move across the nexus more rapidly than the plasma membrane^4–10 have led to the hypothesis of an aqueous channel in the junction. Both Ca²⁺ (ref. 11) and H⁺ (ref. 12) are thought to alter nexal membrane conductance, and a voltage-sensitive gate has been demonstrated within the junction¹³. Recently, Flagg-Newton et al.¹⁴ have concluded that mammalian junctions may contain fixed charge or be of smaller diameter than arthropod junctions. Here, we have investigated these alternatives by examining the permeability of nexuses of septa of the median giant axon of Lumbricus terrestris^1,15 with various derivatives of fluorescein. Both carboxyfluorescein and aminofluorescein were found to have depressed permeabilities relative to their predicted permeabilities based on molecular size and weight (MW). Fluorescein diffusion was significantly suppressed in axons pre-injected with aminofluorescein but carboxyfluorescein had no such effect (Table 1). These data suggest the existence of fixed anionic charge within the nexal channel which may have affinity for amino groups.