Entrance of colloidal ThO2 tracer into the T tubules and longitudinal tubules of the guinea pig heart

Summary Colloidal ThO₂ particles (diameter of 60 Å) were used as electron-opaque markers to trace the intracellular compartments continuous with the bulk interstitial fluid of guinea pig ventricular muscle. Beating and quiescent hearts in a Langendorff preparation were perfused for 30 min with oxygenated Ringer solution containing 1% ThO₂. The hearts were immediately fixed by perfusing with glutaraldehyde solution. The colloidal ThO₂ particles entered into many of the T tubules and into longitudinallyrunning tubules. No differences in distribution of ThO₂ were observed in a heart which was not exposed to ThO₂ until after it was fixed. Tracer did not penetrate into the intercalated disk clefts in the guinea pig hearts and one frog heart used for comparison. Tubular profiles filled with ThO₂ were not seen in frog heart, an observation which confirms the absence of T tubules in this amphibian. It is concluded that, in mammalian cardiac muscle, the lumens of the longitudinal tubules are continuous with the lumens of the T tubules, forming an extensively interconnected T-L tubular system. Hence, every myofibril has close access to a fluid-filled space which is continuous with the interstitial fluid and which may be of similar cationic composition; such an arrangement should facilitate excitation-contraction coupling.Supported by grants from the American Heart Association and from the Public Health Service (HE-11155, HE-05815 and HE-10384). The authors wish to acknowledge the expert technical assistance of Mrs. Jan Redick and to thank Dr. James Smith of Marquette University for the supply of Thorotrast used in these studies.