Ultrastructural and mechanical properties of electrically inexcitable skeletal muscle fibers of the crustacean Atya lanipes

Tubular localization of silent calcium channels in crustacean skeletal muscle fibers

Ca2+-induced Ca2+ release (CICR) in the superficial abdominal flexor muscle of the crustacean Atya lanipes appears to be mediated by a local control mechanism similar to that of vertebrate cardiac muscle, but with an unusually high gain. Thus, Ca2+ influx increases sufficiently the local concentration of Ca2+ in the immediate vicinity of the sarcoplasmic reticulum Ca2+ release channels to trigger the highly amplified release of Ca2+ required for contraction, but is too low to generate a macroscopic inward current (i.e., the Ca2+ channels are silent). To determine the localization of the silent Ca2+ Channels, the mechanical, electrophysiological and ultrastructural properties of the muscle were examined before and after formamide treatment, a procedure that produces the disruption of transverse tubules of striated muscle. We found that tubular disruption decreased tension generation by about 90%; reduced inward current (measured as Vmax, the maximum rate of rise of Sr2+ action potentials) by about 80%; and decreased membrane capacitance by about 77%. The results suggest that ca. 80% of the silent Ca2+ channels are located in the tubular system. Thus, these studies provide further evidence to support the local control mechanism of CICR in crustacean skeletal muscle.

Immunocytochemical electron microscopic study and Western blot analysis of troponin in striated muscle of the fruit fly Drosophila melanogaster and in several muscle cell types of the earthworm Eisenia foetida

BACKGROUND

There is little information about troponin in invertebrate muscles, and no previous references to this protein in annelid muscles have been found. The aim of this paper was to study the presence and distribution of troponin in different muscle cell types from the earthworm Eisenia foetida (the muscular body wall, and the inner and outer muscular layer of the pseudoheart). These results were compared with those obtained in the transversely striated muscle of Drosophila melanogaster and in skeletal and smooth muscles of the mouse.

METHODS

Immunocytochemical electron microscopic study and Western blot analysis using anti-TnT antibodies were employed in this study.

RESULTS

Troponin immunoreaction was detected in the mouse skeletal muscle, the fly flight muscle, and earthworm obliquely striated muscles (body wall musculature and inner muscular layer of the pseudoheart). Immunolabeling for TnT in all these muscle cells appeared in moderate amounts at any point along the sarcomere length, except for the central zone of the A band (H band). This suggests that troponin molecules were located along the thin filaments. The density of immunogold particles was similar in the three muscles, and thus the amount of troponin in each muscle type was proportional to the number and length of actin filaments in each. Troponin was found in neither the mouse smooth muscle nor the outer muscular layer of the earthworm pseudoheart. The latter muscle showed an ultrastructural pattern that was intermediate between obliquely striated and smooth muscle. The estimated molecular weight for TnT in the earthworm was 55 kDa; this is higher than the weight of this protein in the mouse skeletal muscle (40 kDa) but similar to that of the D. melanogaster muscle (52 kDa).

CONCLUSIONS

Troponin is present in both types of striated muscle (transversely striated and obliquely striated) of the earthworm with a distribution that is very similar to that observed in the mammalian striated muscle. As in vertebrates, troponin is absent in the smooth muscle of the earthworm. Discrepancies in the classification of some invertebrate muscles are common in the literature, and the use of distinctive markers, such as troponin, may improve our understanding of the nature and properties of many invertebrate muscles showing an ultrastructural pattern that does not resemble any of the classic muscle types.