Release of Contraction and Changes in Birefringence Caused by Adenosine Triphosphate in Isolated Cross Striated Muscle Fibres.1

Abstract

Summary. 1 Micro-application of adenosine triphospbate (a 99 per cent preparation) in iso-osmotic Ringer solution releases in threshold amounts of 1–2 times 10-5μg a brief tetanus-like activity in non-curarised and curarised isolated striated muscle fibres. 2 A mechanical response following electric stimulation lasts considerably longer when a small fibre bundle has previously been treated with adenosine triphosphate than it does before application of the substance. 3 Application of adenosine triphosphate causes a reversible fall in birefringence. This decrease is abolished by poisoning with iodoacetate. 4 The fall in birefringence amounts to 10–30 per cent, increasing with increasing concentration of adenosine triphosphate, and decreases with increasing stretch of the fibre. 5 While the initial fall in birefringence following electrical stimulation occurs more rapidly than after application of adenosine triphosphate, the time of restitution is 10–20 min, being identical in both cases. The fibre has regained its resting condition, when birefringence reaches its minimum, and the protracted return is regarded as an expression of restitutional processes in muscle proteins. 6 Adenosine diphosphate and adenosine diphosphate + ortho-phosphate act like adenosine triphosphate both as regards initiation of contraction and effect on birefringence. In the case of adenosine diphosphate × orthophosphate the decrease in birefringenee is, however, essentially less than after application of adenosine triphosphate and adenosine diphosphate. 7 Adenylic acid has no effect on birefringence or contraction. Adenylic acid + pyrophosphate release contraction and cause a decrease of birefringence which reaches its minimum more rapidly than after application of adenosine diphosphate. 8 Inorganic sodium triphosphate releases contraction, while its influence on birefringence is absent or slight. 9 Sodium pyrophosphate causes contracture-like mechanical responses. Threshold amounts for the release of contraction are accompanied by irreversible changes in birefringence. 10 Sodium orthophosphate has no influence on birefringence and acts, even in large concentrations inconstantly as a chemical stimulus; the response, if present, is protracted. Birefringence remains unchanged. 11 The energy-rich phosphate bond in the form of adenosine-triphosphate is not only the immediate source of energy recharge, but also the agent for the release of contraction.