Intracellular distribution of calcium in cardiac muscles studied by electron microscope autoradiography

The intracellular distribution of calcium was investigated by 45Ca electron microscope autoradiography in the frog or rat ventricular muscle and guinea pig papillary muscle. The muscle was incubated for 30 min in 45Ca-Ringer's solution or high Ca solution in which 1/10--1/20 of Ca was replaced with 45Ca. The distribution of developed silver grains sensitized by the 45Ca (45Ca grains) was found to be 38% in the mitochondria, 43% in the myofibrils and 19% in the other regions (8% in the cytoplasma and 11% in the various membranous structures) in the frog muscle cell incubated in the 45Ca-Ringer's solution. After electrical stimulation (10 Hz, 5 min), the distribution was 22% in the mitochondria, 53% in the myofibrils and 25% in other regions. Similar results were obtained after high K stimulation in the frog ventricular muscles and after electrical stimulation in high Ca solution in guinea pig papillary muscles. Following the addition of 1 X 10(-6) g/ml adrenaline into the incubating solution, the grain distribution in guinea pig papillary muscles did not show significant change but the distribution in myofibril decreased after electrical stimulation, while it increased in the mitochondria. Longitudinal localization of 45Ca grains along the myofibril was also examined and it was a general tendency throughout all animals examined that the grain density at the A band and near the Z band decreased after electrical stimulation, while it increased at the I band and, especially, at the A-I junction.