A new device is described which allows the study of optical properties of a small area (< 2500 μm2) of a single fiber of skeletal muscle or of thin myocardial trabeculae, under controlled electrophysiological conditions (voltage or current clamp). The intensity of scattering by myocardial trabeculae is less prominent than that of skeletal fibers. In both tissues, the absorbance after correction, corresponds to the components of the preparations. Studies with polarized u.v. light indicated the presence of oriented compounds with positive dichroism ratios around 280 nm and negative ratio around 260 nm. In both tissues, scattering and absorbance increase during a contraction. An absorbance change in the visible part of the spectrum is recorded during a single twitch and suggests early biochemical changes in the contracting fibers. An absorbance increase around 285 nm preceeds the initiation of mechanical events recorded with a mechanotranducer and changes in the dichroic ratios would correspond to an increase in the fraction of oriented aromatic aminoacids and to a decrease in bound nucleotides. Control measurements indicated that the optical phenomena were intrinsic properties of the preparations.

The optical properties of myocardial fibers and skeletal fibers were compared at rest and during activity. Scattering by myocardial trabeculae is more important than by skeletal fibers but birefringence of myocardial trabeculae is less important than that of skeletal fibers. The absorbance curves of both isolated myocardial trabeculae and skeletal fibers in vivo reflect their main components; during contraction the absorbance increases mainly at 285 nm and this increase starts earlier than contraction recorded with a mechanotransducer. An absorbance change in the spectral range of 420-440 nm is also reported which could represent very early biochemical change in the contractile fibers. The technique used, combined with electrophysiological data, allows one to study different problems related to the biophysics and biochemistry of contraction.

The main components of the excitable membranes are protein and lipoproteins absorbing in the ultraviolet. Contractile proteins in muscle also absorb ultraviolet light. If polarized light is used, it becomes possible to distinguish the orientation of the absorbing groups either at rest or during activity. A system of microspectrophotometry in vivo using polarized ultraviolet light has been established to study the portion of myocardial trabeculae or of skeletal fibers in the central compartment of a double sucrose gap apparatus. Thus the system also allows electrophysiological studies. On both preparations, a relation between the absorbance change at 280 nm and contractile activity has been measured. These preparations also contain a fraction of oriented compounds absorbing at 280 nm and another at 260 nm. During mechanical activity, the amount of oriented compounds increases at 280 nm and decreases at 260 nm. These changes were assigned to an increase in the ratio of oriented amino acids, tyrosine and tryptophan, and to a decrease of oriented adenosine triphosphate.