Insulin stimulates sugar transport in giant muscle fibres of the barnacle

Insulin stimulates sugar transport in vertebrate skeletal muscle but the mechanism of insulin action is unknown. It has been reported that Na transport in giant muscle fibers of the barnacle (Balanus nubilis) is sensitive to insulin but no one has examined the sensitivity of sugar tansport to insulin in this preparation. We show here that insulin does, indeed, stimulate sugar transport in barnacle muscle. The great advantage of barnacle muscle over all other muscles used so far for investigating the mechanism of insulin action is its large size, which facilitates measurements on single cells and permits the experimenter to control the intracellular environment of the muscle fibre by the technique of internal dialysis. Using single muscle fibres it is possible to show that acceleration of sugar transport by insulin is associated with a fall in ionized Ca, a fall in cyclic AMP and a rise in cyclic GMP. Working with internally dialysed muscle fibres we find that insulin only increases sugar transport when the dialysis solution contains ATP. In the absence of insulin, sugar transport is dialysed muscle is increased by a rise in ionized Ca, a fall in cyclic AMP and, when the internal Ca is elevated, by a rise in cyclic GMP.     

Expression of a mammalian Na-H exchanger in muscle fibres of the giant barnacle

It is now well established that the internal pH (pHi) of mammalian cells is regulated by means of a plasma membrane transport system that exchanges extracellular Na+ for intracellular H+ (ref. 1). Furthermore, modulation of the activity of the Na-H exchanger seems to have a crucial role in the action of various mitogens and growth factors. The possibility that such a mammalian Na-H exchanger might be efficiently expressed in a giant invertebrate cell was suggested to us by recent results of Barnard and Miledi and colleagues, who demonstrated in frog oocytes the expression of various plasma membrane channels that presumably were encoded by the mammalian messenger RNA wih which the oocytes had been injected. We used muscle fibres of the giant barnacle, which normally have no demonstrable Na-H exchanger activity, and report here that, when injected with poly(A)+ RNA isolated from rabbit liver, the muscle fibres express a Na-H exchanger. No such expression is observed, however, when the injected material is pretreated with ribonuclease A. As hepatocytes are known to possess a Na-H exchanger, the most straightforward interpretation of our data is that a mammalian Na-H exchanger has been expressed in the muscle fibre of an invertebrate.     

Sliding distance between actin and myosin filaments per ATP molecule hydrolysed in skinned muscle fibres

Muscle contraction is generally thought to be driven by tilting of the 19-nm-long myosin head, part of the thick filament, while attached to actin, part of the thin filament. This motion would produce about 12 nm of filament sliding. Recent estimates of the sliding distance per ATP molecule hydrolysed by actomyosin in vitro vary widely from 8 nm to greater than or equal to 200 nm. The latter value is incompatible with a power stroke incorporating a single tilting motion of the head. We have measured the isotonic sliding distance per ATP molecule hydrolysed during the interaction between myosin and actin in skinned muscle fibres. We directly estimated the proportion of simultaneously attached actomyosin complexes and their ATP use. We report here that at low loads the interaction distance is at least 40 nm. This distance corresponds to the length of the power stroke plus the filament sliding while actomyosin crossbridges bear negative drag forces. If the power stroke is 12 nm, then our results indicate the drag distance to be at least 28 nm. Our results could also be explained by multiple power strokes per ATP molecule hydrolysed.     

Effects of vanadate on the activities of mice glucokinase and hexokinase

This study aimed at acquiring knowledge on the hypoglycemic mechanisms of sodium metavanadate (SMV) showed that the liver glucokinase and muscle hexokinase activities increased rapidly after oral SMV was given, and that the blood glucose level was correlated closely with the activities of the two enzymes but not with the insulin level; which indicated that SMV could improve the altered glucose phosphorylation in diabetic mice independently of stimulating insulin secretion. This was probably one of the mechanisms of hypoglycemic effects of SMV.