Guinea Pig Cardiac Muscle Research Articles

The ultrastructure of the intercalated discs of frog, mouse and guinea pig cardiac muscle

The intercalated discs of cardiac muscle of the frog, mouse and guinea pig consist of transversally oriented cell boundaries and of about 0.1 μ broad zones of dense cytoplasm associated with the plasma membranes at these boundaries. The plasma membranes are represented by an opaque osmiophilic layer, the o-layer, which in certain regions appears triple-layered. The two o-layers of the two cells at the boundary are separated by a less osmiophilic interspace, the l-space. This space varies in thickness in different parts of the boundary and in different species. It is interpreted as representing the lipid layers of the two plasma membranes in close contact. This arrangement is assumed to ensure a mechanically firm contact between the cells. It is possible structurally to distinguish between interfibrillar and intersarcoplasmic regions along the disc. The density of the cytoplasm in the interfibrillar regions is due to a dense network that connects the I-band part of the myofilaments with the o-layer of the plasma membrane. In the intersarcoplasmic regions, zones of dense cytoplasm contribute to the differentiation of specialized regions of cellular contact, the S-regions according to the terminology used here.

THE NEUTRALIZATION OR DESTRUCTION OF DIPHTHERIA TOXIN BY TISSUE

As determined by the intracutaneous test in guinea pigs, diphtheria toxin is not altered in the presence of cardiac tissue obtained from the fetal or from the adult heart of the guinea pig. Tissue cultures were apparently uninjured by the presence of the toxin in the dilutions used in these experiments, and, when washed with embryo extract after removal of the diluted toxin, continued to grow. Embryonic guinea pig cardiac muscle tissue growing in cultures in vitro possesses the power of neutralizing, binding, or destroying diphtheria toxin so that it is no longer toxic for normal guinea pigs. Such neutralization takes place through the intervention of growing tissue and is a property which is lacking in similar surviving tissue not in a state of cultivation. Thus, it appears that the living, growing cells of the tissues neutralize or destroy limited quantities of toxin; only when the quantity of toxin exceeds a certain limit is its action injurious.