Loss of water from heart muscle cells during aging of rats as measured by X-ray microanalysis

Water Loss in Aging Erythrocytes Provides a Clue to a General Mechanism of Cellular Senescence

Experimental evidence for age-dependent loss of intracellular water content as a widespread concomitant of cellular senescence is reviewed. Quantitative models are presented, indicating that an age-dependent increase in macromolecular crowding resulting from water loss may be responsible for three observed phenomena: a general age-dependent loss of intracellular protein solubility, a delayed and rapid appearance of high molecular weight aggregates, and an age-dependent transfer of intracellular protein from dilute to concentrated or condensed phases.

The measurement of water distribution in frozen specimens

There are three techniques to measure local water fractions in the cryomicroscope. First, water content may be measured by a direct analysis of oxygen in bulk samples using a windowless detector. Secondly, mass thickness may be estimated in frozen-hydrated, then frozen-dried sections. This technique offers unrivalled spatial resolution, especially if the radiation dose in the frozen-hydrated state is kept low by the use of electron scattering techniques instead of an X-ray microanalytical background determination. External water content standards can be used instead of frozen-hydrated sections and the whole analysis can even be performed exclusively on frozen-dried sections at room temperature. Thirdly, local water fractions can be evaluated from X-ray microanalytical measurements of element concentrations per mass in the frozen-hydrated and frozen-dried state. Corrections necessary for the other techniques cancel out. However, the high radiation dose required for a fully quantitative analysis excludes the use of these methods in thin or ultrathin sections.

A mitochondrial membrane hypothesis of aging

Recently it has been demonstrated that the loss of water during aging from rat liver and heart muscle tissue is solely due to water loss from the mitochondria. A hypothesis is presented here which shows the relation between free radical attack to, and water loss from, the mitochondria and the decreased energy availability in aged cells. According to it, water loss from the mitochondria is due to an increasing permeability of the inner mitochondrial membrane as a result of its changed composition and, therefore, decreasing osmotic reflexion coefficients for inorganic ions, mainly potassium. The water loss results in limited diffusion of intramitochondrial metabolites and, according to the theory of polyelectrolyte solutions, electrostatic and sterical hindrance of enzymic functions. The hypothesis is in accordance with the well-documented experimental facts, e.g. the lack of structural alterations in mitochondrial DNA and proteins, the constancy of the delta mu H+ and the decrease of state 3 respiration with only a limited number of substrates in mitochondria from old postmitotic cells.

Intracellular water and ionic shifts during growth and ageing of rats

As measured by X-ray microanalysis of frozen-dried cryosections, physiological concentrations of phosphorus, Na+, K+ and Mg2+ show a biphasic age dependence in some compartments of rat hepatocytes, especially in the regions of condensed chromatin associated with the actively transcribed genes. This could be consistent with a reactivation of the protein synthesizing apparatus in old livers. The decrease in intracellular water concentrations in hepatocytes and myocytes during ageing is shown to be solely due to a preferential decrease in the water concentration in mitochondria. As a consequence, ionic strength in mitochondria increases with age. This result could provide a rationale for a better understanding of functional declines found in mitochondria from old donors.