Intracellular membranes as boundaries for ionic distribution. In situ elemental distribution in guinea pig heart...

Using x-ray microanalysis and cryoultramicrotomy, calcium and other diffusible elements were localized in heart muscle strips which had been shock frozen under different, defined conditions of electromechanical coupling. Guniea pig papillary muscles were shock frozen: 1) 1/2 seconds after paired stimulation, 2) 5 minutes after rest in normal bath medium and 3) 5 minutes after rest in bath medium to which noradrenaline was added. In 1) high calcium concentrations of 11.5 mmol/kg d.w. were regularly detected in sites at the level of Z-lines, which probably correspond to the Z rete of SR. In 2) in which the mechanogram of the fist contraction after rest normally showed a small and retarded peak, the cell stores seemed to be nearly empty, with exception of a few regions between the mitochondria which revealed calcium accumulations of 77 mmol/kg d.w. These regions included JSR and/or T-tubuli. In 3) in which the mechanogram of the first contraction after rest normally showed a retarded peak with high tension, calcium was found in several cell structures. The highest amount, 25 mmol/kg d.w., was detected over the cell membrane. Measurable amounts were also detected over Z-lines and sarcomeres. In the present experiments, the respective rate of rise of tension, and time to peak tension, were extremely different. Possible correlations between different contraction patterns and different calcium stores involved in the various experiments have been discussed.