Changes in the viscosity of the plasma membrane of flounder (Platichthys flesus L.) erythrocyte induced by varying the content of membrane cholesterol or by benzyl alcohol. Correlation of the activity of the intrinsic Mg2+-ATPase and the viscosity

The order parameter S and the correlation time tau from the ESR-spectra of 5-DS and 5-DD, respectively, are used as measures of the viscosity of the plasma membrane. It is suggested that the membrane undergoes a thermotropic phase transition which correlates with a change of the activation energy of the (Na+ + K+)-ATPase but not of the Mg2+-ATPase. The viscosity varies almost proportionally with the amount of cholesterol in the membrane when 0 less than mg C/mg PL less than 0.5. It is suggested that cholesterol is uniformly distributed in the membrane when mg C/mg PL less than 0.5 and that cholesterol-rich domains appear when mg C/mg PL greater than 0.5. Benzyl alcohol decreases the viscosity of the membrane. The effect depends on the presence of cholesterol in the membrane. The onset of the effect of benzyl alcohol on the native membrane is delayed--in terms of alcohol concentration--relative to the cholesterol-depleted membrane. Benzyl alcohol affects only the outer part of the membrane bilayer. Changes of the viscosity and of the activity of the Mg2+-ATPase are proportional. The effect of cholesterol-depletion on the activity of the Mg2+-ATPase cannot be ascribed only to the simultaneous lowering of the viscosity.     

Myofibrillar M-band proteins in rat skeletal muscles during development.

The distribution of three myofibrillar M-band proteins, myomesin, M-protein and the muscle isoform of creatine kinase, was investigated with immunocytochemical techniques in skeletal muscles of embryonic, fetal, newborn and four-week-old rats. Furthermore, muscles of newborn rats were denervated and examined at four weeks of age. In embryos, myomesin was present in all myotome muscle fibres of the somites, whereas M-protein was detected only in a small proportion of the myotome muscle fibres and muscle creatine kinase was not detected at all. In fetal and newborn muscles, all fibres contained all three M-band proteins. At four weeks of age, when fibre types (type 1 or slow twitch fibres and type 2 or fast twitch fibres) were clearly discernable, the pattern was changed. Myomesin and muscle creatine kinase were still observed in all fibres, whereas M-protein was present only in type 2 fibres. On the other hand, in muscle fibres denervated at birth all three M-band proteins were still detected. Our results suggest 1) that during the initial stages of myofibrillogenesis expression and incorporation of myomesin into the M-band precede that of M-protein and muscle creatine kinase; 2) that expression and incorporation of all three M-band proteins during fetal development is nerve independent and non coordinated to the expression of different forms of myosin heavy chains, and 3) that the suppression of M-protein synthesis during postnatal development is nerve dependent and reflects the maturation of slow twitch motor units.