Two structurally distinct calcium storage sites in rat cardiac sarcoplasmic reticulum: an electron microprobe analysis study.

Abstract

The elemental composition of subcellular organelles in resting rat papillary muscle was measured by electron probe x-ray microanalysis of cryosections of flash-frozen tissue. Nonmitochondrial electron-dense structures (50-100 nm in diameter) with a phosphorous concentration larger than 375 mmol/kg dry wt were identified in the interfibrillar spaces of the I band region. They were not visible in the proximity of transverse tubules. The sodium, magnesium, phosphorus, sulfur, chlorine, and potassium content of the electron dense structures showed a normal distribution, consistent with a uniform composition of a specific subcellular organelle. However, the distribution of the calcium concentrations in these electron-dense structures was bimodal, suggesting that they are composed of at least two subpopulations. One subpopulation had relatively high calcium (up to 53 mmol/kg dry wt) content with a mean value of 12.5 +/- 1.1 mmol/kg dry wt, while the other one had a relatively low calcium content with a mean value of 2.8 +/- 0.3 mmol/kg dry wt. The mean calcium concentration in the junctional sarcoplasmic reticulum (j-SR) in rat papillary muscle with calcium concentrations larger than 6 mmol/kg dry wt was 14.6 +/- 2.0 mmol/kg dry wt. We propose that the electron-dense structures described above correspond to nonjunctional sarcoplasmic reticulum and that the population containing relatively high calcium concentrations is calsequestrin-containing corbular sarcoplasmic reticulum (c-SR) confined to the I band region, while the population containing relatively low calcium concentrations corresponds to anastomosing regions of the network sarcoplasmic reticulum that lack calsequestrin.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitation of Membrane Fusion During Exocytosis and Exocytosis-Coupled Endocytosis and Acceleration of ``Ghost' Detachment in Paramecium by Extracellular Calcium. A Quenched-Flow/Freeze-Fracture Analysis

“…2 values of ∼50 mM [58,59], assuming 20% dry weight. Similar variation ranges were reported for SR, i.e., between 6 mM [24], 15 [23], 20 [22] and 33 mM [60], or for ER in different cells, i.e., between 1 mM [6], 1.5 to 2.5 [11], 3 [36], and 9.5 mM [7]. In sum, if one disregards excessively high values reported in older references for all of these systems, it appears reasonable to assume a total of ∼5 mM Ca 2+ in alveolar sacs.…”

Relationship between calcium loading and impaired energy metabolism during Na+, K+ pump inhibition and metabolic inhibition in cultured neonatal rat cardiac myocytes.

“…These findings are consistent with the documented propensity of mitochondria and sarcoplasmic reticulum to accumulate calcium when cells are presented with an excess calcium load (1,31,32,(35)(36)(37)(38). EPMA measurements also showed that the elevated calcium in the mitochondrial and extramitochondrial sites was associated with increased phosphorus, indicating the formation of calcium phosphate precipitates (10,31,32).…”

Total and free myoplasmic calcium during a contraction cycle: x‐ray microanalysis in guinea‐pig ventricular myocytes.

“…However, in multicellular ventricular preparations, elevated diastolic YCamito has not been reported up to now. Not only unpotentiated ventricular trabeculae have a XCamito below the detection limit (Wendt-Gallitelli, 1986;Joergensen et al 1988). In guinea-pig ventricular trabeculae potentiated with paired pulses (Wendt-Gallitelli, 1986) or in hamster hearts beating at 4 Hz (Bond et al 1989), ECamito was not significantly different from zero.…”

“…Using EPMA, ECaSR can be shown to have the properties expected from the functional Ca2+ store (Wendt-Gallitelli, Jacob & Wolburg 1982;Wendt-Gallitelli, 1985, 1986Wheeler-Clark & Tormey, 1987;Joergensen et al 1988;Bond et al 1989). During diastole, XCaSR in junctional SR and corbular SR was significantly higher than ECamyo; this difference disappeared during early systole and reappeared during late systole.…”