Photometric observations of rat liver microsomal suspensions

Abstract

The application of photometric measurements to determinations of microsomal volume changes has been investigated, with particular consideration of the suitability of utilizing such measurements in permeability studies. The relationships among optical density, medium osmolality, and medium refractive index observed with microsomes suspended in sucrose-raffinose media are comparable to relationships previously defined for mitochondrial suspensions. Together with the observation that these effects are osmotically reversible, such evidence is consistent with the interpretation that the microsomes exhibit osmotic behavior in sucrose-raffinose media. Other unidentified changes in optical density take place. Comparisons of observations of suspensions in sucrose-raffinose media to observations of suspensions in media containing lower molecular weight nonelectrolytes indicate that monosaccharides and smaller compounds penetrate the microsomes rapidly. These findings necessitated a reevaluation of some earlier published studies of optical density relationships of microsomal suspensions in mannitol-raffinose media. The data indicate that penetrant solutes alter microsomal light scattering by causing osmotic swelling and, in addition, by increasing the anhydrous mass of the particles. At comparable osmolalities and refractive indices, optical densities of microsomal suspensions in media containing mixtures of sucrose and a sugar polymer (dextran or Ficoll) differ from optical densities of suspensions in sucrose-raffinose media. The occurrence of events presently not accounted for emphasizes the need for caution in interpreting photometric data in experiments with microsomal suspensions.