Electrophysiological and electron microscopic correlations concerning the effects of neuraminidase on canine heart cells

Abstract

To assess the importance of the glycocalyx in cardiac cells, isolated preparations of sinus node (SN), atrial working muscle (AM), false tendon (FT), and ventricular working muscle (VM) were studied electrophysiologically with intracellular electrodes and then structurally with the electron microscope. Twenty isolated canine right atria with attached ventricular tissue were arterially perfused (SN artery, FT's were superfused) and at the close of each experiment, ruthenium (Ru) red in a glutaraldehyde fixative solution was substituted for the normal perfusate; this step arrested all cardiac cell activity. The Ru red was found aggregated in a thick (> 500 Å) layer outside the external leaflet of the cell membrane in SN, AM, FT, and VM cells; this layer corresponded to the location of the glycocalyx. Similar deposits of Ru red were found inside caveolae, transverse tubules, and intercellular junctions. In the SN, the glycocalyx demarcated by Ru red was different, in that it surrounded the peripheries of P cell clusters rather than individual P cells. Neither the junction between two P cells nor that between P and transitional cells was invaded. Fifteen complete sets of cardiac tissues were treated with neuraminidase (1.0 U/ml) for 1 hour or more before the addition of Ru red. In nine different preparations, the Ru red-positive layer became virtually absent after this treatment in SN, AM, and VM cells, but the glycocalyx in FT cells remained normal in appearance. Intracellular electrodes in each tissue sample recorded the electrophysiological changes during neuraminidase treatment. Functional importance of the glycocalyx in AM and VM cells was demonstrated by their inability to conduct impulses after neuraminidase treatment. The same treatment in SN cells ultimately abolished their automaticity, whereas, in quiescent Ft cells, it evoked spontaneous firing. Thus, the glycocalyx (or sialic acid removed by neuraminidase) may play a different role in each of the two types of automatic cells. These electrophysiological and ultrastructural results support an important role for the glycocalyx in the canine heart. Removal of part or all of it by neuraminidase promotes aberrant electrical activity in each different type of canine cardiac cell studied.