ATP breakdown rate does not control output of adenosine from contracting buffer-perfused rat gracilis muscle

Abstract

Adenosine is released from skeletal muscle during contractions and contributes to exercise hyperemia, but the mechanism(s) that control the adenosine production and release remain unclear. Adenosine is derived from adenosine triphosphate (ATP) breakdown, and the rate of ATP breakdown varies, depending on the conditions for the contraction (such as velocity of shortening, muscle length, etc). In this study, we investigate the influence of the rate of ATP breakdown, modified by varying the conditions under which the muscle contracts, on the adenosine output from the perfused rat gracilis muscle in-situ. The muscle was vascularly isolated and perfused at a constant flow rate of 40-50 pl/min/g, via a muscular branch of the femoral artery, with Kreb’s bicarbonate buffer (pH 7.3-7.4), supplemented with dextran and albumin, and equilibrated against 95% 02 / 5% COZ. ATP breakdown rate was varied by altering either the length of the muscle during isometric contractions or the weight lifted during isotonic contractions. Samples of venous effluent were collected before (control) and during supramaximal electrical stimulation (2 Hz) of the muscle via electrodes placed directly on its surface. Samples were purified by organic extraction, and their adenosine concentrations were determined by reversed-phase high-performance liquid chromatography. Under isometric c conditions, muscle contractions increased venous adenosine from 30 + 9 to 1473 f 3 18 nM (n = 12; PC 0.001 Student’s t-test), but neither resting nor stimulated adenosine output was affected by reducing muscle length to 10% below 1, (the length giving maximal active tension), although both passive and active tensions were decreased to 50%. Under isotonic conditions, stimulation increased venous adenosine from 428 + 55 nM to 19 12 + 28 1 nM (n = 12; P < O.OOO 1 ), but adenosine output was not influenced by the weight lifted by the muscle (either log, equivalent to the passive tension exhibited by the muscle when stretched to 1,, or 5g). Also, the change in venous adenosine due to muscle contractions (stimulation venous-control venous) was similar for both isotonic and isometric contractions. These data suggest that the rate of ATP breakdown by a contracting muscle may not be a major influence on the output of its vasoactive breakdown product, adenosine.