Intracellular Membranes as Boundaries for Ionic Distribution. In Situ Elemental Distribution in Guinea Pig Heart Muscle in Different Defined Electro-Mechanical Coupling States

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Abstract
Using X-ray microanalysis and cryo-ultramicrotomy, Ca and other diffusible elements were localized in heart muscle strips which had been shock frozen under different, defined conditions of electromechanical coupling. Guinea pig papillary muscles were shock frozen 1.5 s after paired stimulation, 5 min after rest in normal bath medium and 5 min after rest in bath medium to which noradrenaline [norepinephrine] was added. In the 1st state, high Ca concentrations of 11.5 mmol/kg d.w. [dry wt] were regularly detected in sites at the level of Z-lines, which probably correspond to the Z rete of SR [sarcoplasmic reticulum]. In the 2nd state, in which the mechanogram of the 1st contraction after rest norally 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 Ca accumulations of 77 mmol/kg d.w. These regions included JSR [junctional sarcoplasmic reticulum] and/or T-tubuli. In the last condition, in which the mechanogram of the 1st contraction after rest normally showed a retarded peak with high tension, Ca 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. The respective rates of rise of tension and time to peak tension were extremely different. Possible correlations between different contraction patterns and different Ca stores involved in the various experiments were discussed.

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