Role of the sarcoplasmic reticulum in glycogen metabolism. Binding of phosphorylase, phosphorylase kinase, and primer complexes to the sarcovesicles of rabbit skeletal muscle

Sarcoplasmic vesicles and beta-glycogen particles 30-40 mmicro in diameter were isolated from perfused rabbit skeletal muscle by the differential precipitation-centrifugation method. This microsomal fraction was subjected to zonal centrifugation on buffered sucrose gradients, in a B XIV Anderson type rotor, for 15 hr at 45,000 rpm in order to separate the two cytoplasmic organelles. Zonal profiles of absorbance at 280 mmicro, proteins, glycogen, and enzymatic activities (phosphorylase b kinase, phosphorylase b, and glycogen synthetase) were performed. Whereas the entire synthetase activity was found combined with the glycogen particles, 39% of phosphorylase and 53% of phosphorylase b kinase activities, present in the microsomal fraction, were recovered in the purified vesicular fraction (d = 1.175). This latter fraction consists of vesicles, derived from the sarcoplasmic reticulum, and of small particles 10-20 mmicro in diameter attached to the outer surface of the membranes. These particles disappear after alpha-amylase treatment. Incubation of the sarcovesicular fraction with (14)C-labeled glucose-1-phosphate confirms the localization of a polysaccharide synthesis at the level of the membranes. "Flash activation" of phosphorylase b, i.e. Ca "activation" of phosphorylase kinase followed by a conversion of phosphorylase b into a, was demonstrated in the purified sarcovesicular fraction. Moreover, the active enzymatic sites were detected on the membranes by electron microscopy. The presence of binding sites between the membranes of the sarcoplasmic vesicles and a glycogen-enzyme complex suggests that this association plays a role in the glycogenolysis during muscle contraction.

Ketosis in hepatic glycogenosis

The occurrence of ketosis in 41 patients with liver glycogenosis and a control group of 22 children was investigated. Fasting ketosis was present in children with a deficiency of the debranching enzyme system and in young children with a deficiency of the phosphorylase system, but never in patients with a glucose-6-phosphatase deficiency. Oral tolerance tests on a patient deficient in debranching enzyme showed that the blood levels of glucose and ketone bodies changed in the opposite direction. It is argued that, for biochemical reasons, the occurrence of ketosis in patients with a glucose-6-phosphatase deficiency is improbable.

Cell communication, calcium ion, and cyclic adenosine monophosphate

The hypothesis advanced in this article requires further validation. Undoubtedly it will require modification as our knowledge of biochemical control increases. Nevertheless, it should prove useful in focusing attention on the apparent similarity in the response of a large number of specific cell types to particular stimuli. Emphasis has been placed on a few common and apparently key elements in these responses. It is recognized that other factors are undoubtedly involved. Specifically, the changes in membrane potentials indicate the likelihood of widespread changes in the properties of the cell membrane, for example, changes in Na(+) and K(+) transport and distribution. These aspects of cellular responses may eventually prove to be of equal or greater importance than those common aspects of the system already identified.

Effect of glucagon on cyclic 3',5'-AMP, phosphorylase activity and contractility of heart muscle of the rat

The purpose of this investigation was to contrast the effect of glucagon and that of epinephrine on the concentration of cyclic adenosine 3',5'-monophosphate (cyclic AMP), the activity of phosphorylase a and the contractile amplitude of isolated perfused rat hearts. The two drugs were about equally effective except that the maximal augmentation of contractility by epinephrine (5 x 10 -9 moles) was twice that produced by an equivalent dose of glucagon with a fourfold greater increase in cyclic AMP concentration. Combination of large doses of the two drugs caused increases in the cyclic nucleotide considerably greater than those required for maximal phosphorylase activation or associated with a maximal inotropic response. The effects of glucagon also developed more slowly than those of epinephrine. An increase in cyclic AMP was not detectable until after phosphorylase a and contractile amplitude had increased.

The beta-receptor-blocking agents dichloroisoproterenol and pronethalol did not block the biochemical responses to glucagon in doses which abolished the epinephrine-induced increases in cyclic AMP and phosphorylase a . These results, along with those obtained by other investigators, indicate that glucagon can elicit the same biochemical responses in intact heart as have been obtained with epinephrine, but by action at a different receptor site.

The steps between depolarization and the increase in the respiration of frog skeletal muscle

1. For many years it has been known that when muscles are depolarized by raising [K(+)](out) there is an increase in respiration, even at levels of depolarization below the threshold for a detectable contracture.2. K(+)-stimulated respiration occurs in muscles in which protein synthesis is blocked with puromycin. Stimulation does not depend upon activation of phosphorylase kinase. In muscle poisoned with IIA and kept in N(2), depolarizations below the threshold for contracture cause a fall in creatine phosphate. Apparently an ATPase is activated by depolarization; the resulting ADP is probably the trigger for the increase in oxygen uptake.3. When the T-tubules are destroyed by the glycerol-osmotic shock method depolarization does not produce an increase in respiration.4. Caffeine is known to stimulate respiration at concentrations below the threshold for producing a contracture. Muscles that have been made refractory to stimulation by potassium are still stimulated by caffeine: the action of caffeine is not antagonized by an increase in extracellular Mg(2+). Caffeine must act on a later step in excitation-contraction coupling.5. K(+)-stimulated respiration ultimately depends on the presence of Ca(2+) in the Ringer. However, the Ca(2+) can be replaced by Ni(2+). It is known that Ni(2+) does not activate actomyosin. Ni(2+) is not sequestered by isolated fragments of the sarcoplasmic reticulum. It seems that the Ni(2+) or Ca(2+) in the extracellular solution is required for a superficial step in excitation-contraction coupling.6. Respiration is also often stimulated when muscles are placed in an isotonic sucrose solution, even though the fibres are hyperpolarized. A trace amount of Ca(2+) in the sucrose solution is probably necessary for the response.7. An interaction between Ca(2+) and a superficial membrane receptor appears to be an essential, early step in excitation-contraction coupling.