A thermogenic organ, modified from an eye muscle, warms the brain and eyes of several oceanic fish. The extraocular muscles associated with thermogenesis are composed of modified muscle cells that are structurally distinct from all other types of muscle previously described. In "heater" cells, contractile filaments are virtually absent and the cell volume is packed with mitochondria and smooth membranes. Freeze-fracture studies and negative staining of microsomal fractions treated with vanadate indicate that most of the membrane system of heater cells has a high Ca2+-ATPase density and is equivalent to skeletal muscle sarcoplasmic reticulum (SR). High voltage electron micrographs of heater cells infiltrated with the Golgi stain demonstrate that the cells also have an extensive transverse tubule system with a complicated three-dimensional structure. Junctional regions between transverse tubules and SR occur in the heater cell and contain feet protein. Activation of thermogenesis in heater cells may occur through the same protein components involved in excitation-contraction coupling and appears to be associated with the ATP-dependent cycling of calcium at the SR.

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