Biochemical properties of subsarcolemmal and interfibrillar mitochondria isolated from rat cardiac muscle

TLDR

Cardiac muscle contains two distinct mitochondrial populations: subsarcolemmal mitochondria beneath the sarcolemma and interfibrillar mitochondria between myofibrils. Subsarcolemmal mitochondria were isolated by Polytron homogenization, whereas interfibrillar mitochondria were released by nagarse digestion of the remaining tissue. Both mitochondrial types retained structural integrity during isolation, but interfibrillar mitochondria exhibited higher succinate dehydrogenase and citrate synthase activities, similar CPT and GPDH levels, and oxidized substrates about 1.5 times faster than subsarcolemmal mitochondria, highlighting functional differences linked to their cellular locations.

Abstract

Two populations of mitochondria were observed upon ultrastructural examination of cardiac muscle tissue, one located directly beneath the sarcolemma (subsarcolemmal mitochondria) and another between the myofibrils (interfibrillar mitochondria). Subsarcolemmal mitochondria were released by treatment of heart muscle with a Polytron tissue processor, while interfibrillar mitochondria were released by nagarse digestion of the remaining tissue. These results were supported by electron microscopy of Polytron-treated heart tissue showing rupture and loss of sarcolemma with release of the underlying mitochondria but with retention of intact mitochondria between the myofibrils. Electron microscopy of the isolated mitochondria indicated that both mitochondrial types maintained their structural integrity throughout the isolation procedure. Specific activities of succinate dehydrogenase and citrate synthase were higher in the interfibrillar mitochondria as compared to the subsarcolemmal mitochondria, while those of carnitine palmitoyltransferase and alpha-glycerophosphate dehydrogenase were nearly the same in both. Interfibrillar mitochondria oxidized all substrates tested approximately 1.5 times faster than did the subsarcolemmal mitochondria. Thus the two mitochondrial types differed not only in their respective locations in the cell, but also in certain biochemical properties.