Gafni A, Yuh K C
Institute of Gerontology, University of Michigan, Ann Arbor 48109.
Mech Ageing Dev. 1989 Aug;49(2):105-117. doi: 10.1016/0047-6374(89)90094-8.
Sarcoplasmic reticulum (SR) vesicles isolated from skeletal muscle of Sprague-Dawley rats ranging in age from 4 months to 28 months were studied and compared. A marked decline, with age, was observed in the amount of (total) SR proteins isolated per gram of muscle tissue used. This decline is in line with the known loss of muscle fiber mass and size with advancing age; however, whether the magnitudes of these two effects are indeed identical, remains to be studied. In contrast, no analogous age-related change was detected in the amount of SR protein per unit mass of rat cardiac muscle. The calcium contents, per mg protein, in SR vesicles isolated from rats of all age groups studied did not differ significantly, and represented only a small fraction of the total capacity of the vesicles for this cation. This capacity was found to decline at old age and this effect, combined with the age-related decrease in the concentration of SR proteins in the tissue, indicate a significant decline in calcium sequestration ability in old muscle. Both basal (Ca2+ independent) and calcium stimulated ATPase activities were found not to be affected by age. In contrast, the efficiency of Ca2+ transport across the SR membrane, as reflected by the number of calcium ions pumped into the vesicles per ATP cleaved, declined from a value of 0.37 at 3-4 months to 0.15 at 24 months. This change may represent an age-related reduction in the fraction of coupled SR vesicles, possibly due to alterations in the membrane. SR vesicle preparations from both young and old rats displayed strongly biphasic inactivation kinetics when incubated at 37 degrees C. This may reflect the heterogeneity of muscles in the tissue used, or be due to the presence of a mixture of coupled and uncoupled vesicles in the SR preparations. The rate of the first step in the ATPase inactivation, in which about 75% of the activity is lost, was found to be affected by age, the old SR vesicles being markedly more labile than their young counterparts. In contrast, no difference was detected between the inactivation kinetics of young and old ATPase proteins dissolved in Triton X-100 and the inactivation was monophasic down to less than 6% of the original activity. These results indicate that the age-related modifications in the stability of the SR calcium pump system involve the membrane but not the ATPase protein. The inactivation of the SR ATPase is believed to proceed via dissociation of the dimeric enzyme to (unstable) subunits. It is therefore likely that changes in the SR membrane in old muscle render the ATPase more dissociable.
