Studies on developmental changes in fine structure and metabolism in flight muscle of Locusta migratoria L.

Abstract

The fine structure of the median i n direct dorsal longitudinal flight muscles has been examined throughout the first week of adult life. During this period, muscle colour changed from white to reddishbrown and the banding pattern characteristic of mature adult flight muscle was established. Associated with these changes there was an increase in myofbrilsize and the mean number of myosin filaments per myofibril,- no significant change was observed in the actin : myosin ratio. There were indications that the number of myofibrils per muscle fibre increase by " longitudinal splitting" of existing myofibrils in the first four days of adult life. A marked increase in mitochondrial size and complexity was noted with increasing age. In addition, total mitochondrial protein increased approximately 10-fold between the 9 th day of 5th instar and the 6th day of adult life. However, the increase in mitochondrial size is probably due to both the syn thesis of new mitochondrial protein and the fusion of adjacent mitochondria. The mitochondria gradually become arranged in straight columns between the myofibrils by the 5th day of adult life. The relative volume of the sarcoplasmic reticulum (SR) and T-system decreased with age. This was associated with the formation of dyadic junctions and the separation of adjacent myofibrils by sheet(s) of SR. In contrast to the situation, observed in the first few days of adult life, where dyadic junctions are situated near the Z-bands or at an oblique angle to the adjacent m of brils, in more developed flight muscle they are situated in the region of the A-bands and run parallel to the myofibrils. The distribution of the SR and T-system was affected by the penetration of tracheoles into the muscle fibres. Muscle tracheation was more-or-less fully developed within the 3rd-day of adult life. The relative volume of the tracheoles decreased with age. The physiological implications of these developmental changes in fine structure are discussed. Mitochondrial phospholipids contained five mean fatty acids at all ages studied; palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2) and linolenic acid (18:3). The relative amount of stearic acid (18:0) and the ratio of unsaturated : saturated fatty acids decreased over the period between the 9th-day of the 5th instar and the 15th day of adult life. Providing a suitaible reaction medium was used, oxidative phosphorylation was demonstrable at all ages studied with the following substrates: ɑ-glycerophosphate, pyruvate plus proline and glutamate, but not with succinate. Allosteric activation of ɑ-giycerophosphate dehydrogenase by Ca(^2+) was demonstrable at all ages studied. No such activation was observed when Mg(^2+) replaced Ca(^2+) a-Glycerophosphate dehydrogenase activity increased with age. The properties of SR-ATPase (total ATPase) of Locusta flightmuscle were similar to those reported for vertebrates skeletal muscle SR-ATPase. Total ATPase activity required Mg(^2+) and was stimulated byCa(^2+). Optimal concentrations of Ca(^2+) and Mg(^2+) for total ATPase were 3.19 X 10(^-6)M (free Ca(^2+)) and 1.5 - 3mM, respectively. The pH optimum was ca. 7.6 for total ATPase activity. The Ca(^2+) -stimulated component of ATPase activity showed similar optima to total ATPase activity. In the presence of Ca(^2+) the apparent Km for ATP was decreased from 0.643mM (Mg(^2+) present) to 0.420mM. There was an approximately 4-fold increase in SR protein per thorax and in the specific activity of total and Ca(2+)- stimulated ATPase activity. The developmental changes referred to above are discussed in relation to the improved flight performance observed during the first week of adult life in Locusta migratoria L.