Ryanodine and theophylline-induced depletion of energy stores in amphibian muscle.

The effects of high and low levels of ryanodine on theophylline-induced energy depletion were studied in isolated frog sartorius muscle. Whereas low concentrations of ryanodine (1-10 microM) did not change high energy phosphate contents (PE) after 60 min, high levels (100 microM) reduced resting energy contents by 60% after 60 min. Subcontracture levels of theophylline (2 mM), in the presence of high ryanodine, produced an 80% PE depletion, suggesting possible additive or synergistic effects of these two agents. In contrast to theophylline-induced depletion, neither the ryanodine-induced depletion nor the theophylline-plus-ryanodine-induced depletion of PE seemed sensitive to inhibition by 1 mM procaine. This suggests that there may be differences in the mechanisms whereby methylxanthines and ryanodine deplete energy stores and evoke contractures in amphibian skeletal muscle.

Involvement of transport of H+ equivalents in activation-induced alkaline shift in frog muscle.

Previous studies have shown that "activation" of frog skeletal muscle by low concentrations of caffeine (less than 2.5 mM) and K+ (5-20 mM) causes a steady-state alkaline shift and the appearance of H+ in the medium. Both responses are sensitive to blockers of calcium release from sarcoplasmic reticulum. A variety of conditions that inhibit H+-equivalent transport processes in several different tissues were tested for effects on the activation-induced alkaline shift. Blockers of anion transport (4-acetoamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, furosemide, acetazolamide, and Cl- replacement) had no effect on the alkaline shift. Replacement of Na+ by Mg2+, tris(hydroxymethyl)aminomethane, or choline decreased the alkaline shift. Replacement of Na+ by Li+ had no effect. As little as 5 mM external Na+ gave 50% of the alkaline shift. In 15 mM sodium Ringer solution amiloride inhibited the alkaline shift. Lowering extracellular pH (pHe) inhibited the alkaline shift. At pHe less than 7.0 there was a small constant alkaline shift, whereas at pHe greater than 7.0 the shift was dependent on pHe. This pHe sensitivity was independent of changes of intracellular Ca2+ as determined by twitch potentiation or stimulation of Ca2+ flux. It was concluded that the alkaline shift is probably due to activation of a "H+" transport process requiring Na+ and sensitive to external or internal pH.

Twitch potentiation and caffeine contractures in isolated rat soleus muscle.

1. Electrically-evoked twitch and tetanic tension were measured in isolated rat soleus muscle after exposure to caffeine. 2. Between 0.01 and 2.5 mM caffeine twitch tension was potentiated, reaching a peak of 150% of Resting Tension at 0.5 mM. 3. Biphasic Tension development with relaxation was observed at 2.5 mM caffeine with maximal contractures (110% tetanic tension) occurring at 20 mM. 4. Creatine phosphate and ATP stores were maintained throughout the period of tension development and relaxation. 5. In contrast with amphibian muscle, the isolated soleus is very sensitive to low doses of caffeine and produces biphasic caffeine contractures which relax in the presence of caffeine.

Energy coupling to sodium transport in frog skeletal muscle.

In addition to a strophanthidin-sensitive (SS) sodium efflux, a large component of the sodium efflux in freshly isolated frog skeletal muscle is sodium-activated and strophanthidin-insensitive (SASI). The amount of metabolic energy associated with sodium movement by each of these components was measured and the coupling between sodium movement and adenosine 5'-triphosphate (ATP) hydrolysis in muscle was calculated. Energy production was blocked by iodoacetate and cyanide. Energy turnover was estimated from the change in creatine phosphate (CrP) and ATP contents and expressed as potential energy (PE = CrP + 2ATP). After metabolic poisoning a linear fall of PE occurred (6.3 mumol/g.h). Metabolic poisoning had no effect on the magnitude of the SS or SASI components of sodium efflux. In 2 h the sodium moved, and PE change due to the SS component was 4.35 and 1.66 mumol/g.h, respectively, which gave a coupling factor of 2.6. The amount of sodium moved by the SASI component was similar to that moved by the SS component in 2 h whereas no energy change was observed. It was, therefore, concluded that sodium movement by the SASI component requires no energy input.

Theophylline action on sodium fluxes in frog striated muscle.

The action of theophylline on both sodium efflux and influx was measured using freshly isolated frog sartorius muscles. In normal Ringer's fluid, 2 mM theophylline increased sodium efflux by 35% whereas it decreased sodium influx by about 10%. The percent increase in sodium efflux produced by 2 mM theophylline was not significantly altered in sodium-free, lithium-containing solutions. Strophanthidin prevented the stimulation of sodium efflux by 2 mM theophylline in both normal, sodium-containing Ringer's fluid and sodium-free, lithium-containing solutions. Hence, the major effect of theophylline seems to be stimulation of active sodium transport and the enhanced rate of sodium exit induced by theophylline does not seem to require the presence of external sodium. An interesting and unexplained findings is that 2 mM theophylline, which does not produce a maximal stimulation of sodium efflux, prevents the increased sodium efflux induced by saturating doses of epinephrine.