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In previous work on rat striated muscle cells a silver-reducing component was found selectively localized at the terminal cistern/transverse tubule system (Tandler and Pellegrino de Iraldi 1989). To further investigate that problem we performed the Hg-Ag argentaffin reaction on a sarcoplasmic reticulum fraction from rat skeletal muscle. Circular profiles corresponding to vesicular structures were found outlined by silver grains. The number of silver "stained" vesicles were less than the total number vesicles stained by conventional procedures. The correlation between argentaffinities in the intact muscle fiber and their subcellular organelles indicated that the Hg-Ag reactive vesicles must be those derived from the terminal cisternae of the sarcoplasmic reticulum. The silver-reducing constituent aggregates in the presence of 1 mM CaCl2 or 0.5 M K cacodylate. The state of aggregation induced by Ca2+ was not affected by incubation with 0.5% Triton X-100 or by 2 mM EDTA, thus suggesting a localization at or near the membrane of the terminal cistern vesicle facing the junctional gap. In Laemmli SDS-acrylamide gels the Hg-Ag reaction stained all proteins in a manner similar to Coomasie blue. It is suggested that the selective histochemical staining is the result of differential reactivities due to steric requirements of the chemical reaction.

Summary. This study reports the presence of a silver-reducing constituent in rat striated muscle fiber located selectively at the level of the terminal cistern/transverse tubule system. It is related to the T tubule network at or near sites that participate in junctions with terminal cisternae, i.e., at both sides of the T tubule in skeletal muscle (triad) and, predominantly, at one side in the ventricle (dyad). Little reactivity is present in the auricle due to the scarcity of those membrane systems. The longitudinal sarcoplasmic reticulum, the sarcolemma, mitochondria and myofibrils are not outlined by the reaction product. Extraction of low molecular weight substances, nucleic acids and lipids did not suppress the chemical reaction. A new argentaffin (Hg-Ag) technique is described. Ethanol or aldehyde fixed muscles were passed to water, postfixed 6-24 h with mercuric acetate (5% w/v in 1% acetic acid), washed with 1% acetic acid and distilled water, stained 12-24 h at 43 ~ C with ammoniacal silver nitrate (60% w/v) and washed in 10% sodium sulfite (three changes) and water. All steps were carried out in darkness. Postfixation with mercuric acetate proved to be essential for immobilizing the argentaffin component without interfering with its strong argentaffinity. The procedure also provides a simple method for tracing the pathway of transversally oriented membrane systems in skeletal and cardiac muscle cells.

A mixture of absolute methanol and acetic anhydride (MA) (5:1, v/v; 24 h at 25 degrees C) which is known to methylate and acetylate--respectively--free carboxyl and amino groups in proteins, was tested for its effectiveness as a blocking agent in glutaraldehyde-fixed mouse testis and skeletal muscle. In young spermatids the staining of the acrosome with either uranyl acetate (UA) or ethanolic phosphotungstic acid (PTA) was completely prevented by prior treatment with MA. Esterification of carboxyl groups with 0.1 N HCl in methanol (24 h at 30 degrees C) eliminated the UA staining without affecting that due to PTA. It is concluded that--COOH groups are responsible for UA binding and that acetylation (of amino groups) prevented PTA binding. MA also abolishes the strong affinity of PTA to the lateral elements of the synaptonemal complex in meiotic chromosomes, the axoneme and fibrous sheath of the spermatid tail and the Z band in skeletal muscle. Reactivity was diminished in nucleoli and remained unaffected in chromatin, the outer coarse fibers of the flagellum and collagen fibrils. Different functional groups may participate in PTA staining. The ultrastructure was well preserved in all cases.