Activation of the myofibrillar ATPase activity by extension of glycerolextracted insect fibrillar muscle.

Abstract

The myofibrillar ATPase activity of glycerol extracted insect fibrillar muscle fibres (Dorsolongitudinal muscle, Lethocrrus) suspended in ATP-salt solution was determined in dependence of fibre length. Little change occurred between 70 and 100%L 0. But stretching by only 2–3%L 0 nearly doubled the ATPase activity and the contractile tension. The proportional increase in tension an ATPase activity in the range of 100–103%L 0 (30 pmoles ATP split for 1 dyne by 1 cm fibre during 1 min) observed after stretching suggests an increase in the extent of actinmyosin interaction, inspite of the fact that the actin-myosin overlap zone then slightly decreases. The relation between tension increase and ATPase activity increase is nearly the same after stretching or after raising the free calcium concentration; nonetheless, the stretch induced activation is apparently not due to an increase in the calcium affinity of actomyosin (as suggested byChaplain) because stretch activation is observed even at ‘enzyme saturating’ concentrations of 10−5 M Ca2+. Furthermore the sigmoidal relationship between ATPase activity (or tension) and p Ca and in particular the calcium concentration required for 50% activation is similar in stretched and unstretched fibres, in myofibrils and in actomyosin. The fibre preparation may be stretched in ‘relaxing solution’ containing less than 10−8 M Ca++ (i.e. in the absence of actin-myosin crosslinking), but the ATPase activity increase following stretching is then only observed after increasing the [Ca++] to ≳ 10−7 M; it is correlated with passive tension in relaxing solution rather than with the extent of stretch, suggesting that the element responding to a length change is apparently the passive tension bearing myosin filament which is attached to the Z-line in insect flight muscle.