ATP FORMATION AND ATP HYDROLYSIS DURING FATIGUING, INTERMITTENT STIMULATION OF DIFFERENT TYPES OF SINGLE MUSCLE-FIBERS FROM XENOPUS-LAEVIS

This report describes changes of the rate of ATP hydrolysis in single, intact muscle fibres during the development of fatigue induced by int ermittent tetanic stimulation. High (type 3) and low (type 1) oxidativ e muscle fibres dissected from the iliofibularis muscle of Xenopus lae vis were studied at 20 degrees C. The rate of ATP hydrolysis was calcu lated during different time intervals from changes in the content of n ucleotides, creatine compounds and lactate, as well as lactate efflux and oxygen uptake. During the first phase of intermittent stimulation, phosphocreatine is fully reduced while the rate of oxygen consumption increases to its maximum, the lactate content increases to a maximum level, and a small amount of IMP is formed; the rate of ATP hydrolysis in type 3 fibres is constant while force decreases, whereas the rate decreases approximately in proportion to force in type 1 fibres. After the first phase, the rate of ATP hydrolysis in type 3 fibres decrease s slightly and the fibres reach a steady metabolic state in which the rates of ATP formation and hydrolysis are equal; in type 1 fibres a dr astic change of the rate of ATP hydrolysis occurs and a steady metabol ic state is not reached. On the basis of the time courses of the metab olic changes, it is concluded that the rate of ATP hydrolysis in type 3 fibres is reduced by acidification and/or a reduced calcium efflux f rom the sarcoplasmic reticulum, whereas in type 1 fibres inorganic pho sphate and/or acidification inhibit the rate initially and ADP is a li kely candidate to explain the drastic fall of the rate of ATP hydrolys is during late phases of fatiguing stimulation.