Influence of caffeine, Ca2+, and Mg2+ on ryanodine depression of the tension transient in skinned myocardial fibers of the rabbit

Mild stress of caffeine increased mtDNA content in skeletal muscle cells: the interplay between Ca2+ transients and nitric oxide

Caffeine increases mitochondrial biogenesis in myotubes by evoking Ca(2+) transients. Nitric oxide (NO) also induces mitochondrial biogenesis in skeletal muscle cells via upregulation of AMP-activated protein kinase (AMPK) activity and PGC-1α. However, the interplay and timing sequence between Ca(2+) transients and NO releases remain unclear. Herein, we tested the hypothesis that caffeine-evoked Ca(2+) transients triggered NO production to increase mtDNA in skeletal muscle cells. Ca(2+) transients were recorded with Fura-2 AM and confocal microscopy; mtDNA staining, mitochondrial membrane potential and NO level were determined using fluorescent probes PicoGreen, tetramethylrhodamine methyl ester (TMRM) and DAF-FM, respectively. In primary cultured myotubes, a subtle and moderate stress of caffeine increased mtDNA exclusively. Mitochondrial membrane potential and mtDNA were increased by 1 mM as well as 5 mM caffeine, whereas 10 mM caffeine did not change the fluorescence intensity of PicoGreen and TMRM. NO level in myocytes increased gradually following the first jump of Ca(2+) transients evoked by caffeine (5 mM) till the end of recording, when Fura-2 indicated that Ca(2+) transients recovered partly and even disappeared. Importantly, nitric oxide synthase (NOS) inhibitor (L-NAME) suppressed caffeine-induced mtDNA biogenesis, whereas NO donor (DETA-NO) increased mtDNA content. These data strongly suggest that caffeine-induced mtDNA biogenesis is dose-sensitive and dependent on a certain level of stress. Further, an increasing level of NO following Ca(2+) transients is required for caffeine-induced mtDNA biogenesis. Additionally, Ca(2+) transients, a usual and first response to caffeine, was either suppressed or attenuated by L-NAME, DETA-NO, AICAR and U0126, suggesting an inability to control [Ca(2+)](i) in these treated cells. There may be an important interplay between NO and Ca(2+) transients in intracellular signaling system involving NOS, AMPK and MEK.

Caffeine ingestion and metabolic responses of tetraplegic humans during electrical cycling

Normally, caffeine ingestion results in a wide spectrum of neural and hormonal responses, making it difficult to evaluate which are critical regulatory factors. We examined the responses to caffeine (6 mg/kg) ingestion in a group of spinal cord-injured subjects [7 tetraplegic (C5-7) and 2 paraplegic (T4) subjects] at rest and during functional electrical stimulation of their paralyzed limbs to the point of fatigue. Plasma insulin did not change, caffeine had no effect on plasma epinephrine, and there was a slight increase (P < 0. 05) in norepinephrine after 15 min of exercise. Nevertheless, serum free fatty acids were increased (P < 0.05) after caffeine ingestion after 60 min of rest and throughout the first 15 min of exercise, but the respiratory exchange ratio was not affected. The exercise time was increased (P < 0.05) by 6% or 1.26 +/- 0.57 min. These data suggest that caffeine had direct effects on both the adipose tissue and the active muscle. It is proposed that the ergogenic action of caffeine is occurring, at least in part, by a direct action of the drug on muscle.

Sarcoplasmic reticulum function in determining atrioventricular contractile differences in rat heart

The relationships between the contractile characteristics and the sarcoplasmic reticulum (SR) function of rat atrial and ventricular trabeculae were compared. The isometric developed tension (DT) and the rates of contraction (+ dT/dt) and relaxation (-dT/dt) normalized to cross-sectional area were 3.7, 2.2, and 1.8 times lower, respectively, in intact atrial strips compared with ventricular strips, whereas + dT/dt and -dT/dt (normalized to DT) were 2.3 and 2.8 times higher, respectively, in atria. Atria exhibited a maximal potentiation of DT after shorter rest periods than ventricles and a lower reversal for prolonged rest periods. Caffeine-induced tension transients in saponin-permeabilized fibers suggested that the Ca2+ concentration released in atrial myofibrils reached a lower maximum and decayed more slowly than in ventricular preparations. However, the tension-time integrals indicated an equivalent capacity of sequestrable Ca2+ in SR from both tissues. In atrial, as in ventricular myocardium, the SR Ca2+ uptake was more efficiently supported by ATP produced by the SR-bound MM form of creatine kinase (CK; MM-CK) than by externally added ATP, suggesting a tight functional coupling between the SR Ca2+ adenosinetriphosphatase (ATPase) and MM-CK. The maximal rate of oxalate-supported Ca2+ uptake was two times higher in atrial than in ventricular tissue homogenates. The SR Ca(2+)-ATPase 2a mRNA content normalized to 18S RNA was 38% higher in atria than in ventricles, whereas the amount of mRNA encoding the alpha-myosin heavy chain, calsequestrin, and the ryanodine receptor was similar in both tissues. Thus a lower amount of readily releasable Ca2+ together with a faster uptake rate may partly account for the shorter time course and lower tension development in intact atrial myocardium compared with ventricular myocardium.

Ca2+ uptake by cardiac sarcoplasmic reticulum ATPase in situ strongly depends on bound creatine kinase

The role of creatine kinase (CK) bound to sarcoplasmic reticulum (SR), in the energy supply of SR ATPase in situ, was studied in saponin-permeabilised rat ventricular fibres by loading SR at pCa 6. 5 for different times and under different energy supply conditions. Release of Ca2+ was induced by 5 mM caffeine and the peak of relative tension (T/Tmax) and the area under isometric tension curves, ST, were measured. Taking advantage of close localisation of myofibrils and SR, free [Ca2+] in the fibres during the release was estimated using steady state [Ca2+]/tension relationship. Peak [Ca2+] and integral of free Ca2+ transients (S[Ca2+]f) were then calculated. At all times, loading with 0.25 mM adenosine diphosphate, Mg2+ salt (MgADP) and 12 mM phosphocreatine (PCr) [when adenosine triphosphate (ATP) was generated via bound CK] was as efficient as loading with both 3.16 mM MgATP and 12 mM PCr (control conditions). However, when loading was supported by MgATP alone (3.16 mM), T/Tmax was only 40% and S[Ca2+]f 31% of control (P < 0.001). Under these conditions, addition of a soluble ATP-regenerating system (pyruvate kinase and phosphoenolpyruvate), did not increase loading substantially. Both ST and S[Ca2+]f were more sensitive to the loading conditions than T/Tmax and peak [Ca2+]. The data suggest that Ca2+ uptake by the SR in situ depends on local ATP/ADP ratio which is effectively controlled by bound CK.

Effects of halothane, isoflurane and sevoflurane on calcium-related contraction in porcine coronary arteries

BACKGROUND

Volatile anaesthetics have a direct inhibitory effect upon epicardial coronary arterial smooth muscles (1-4). The site and mode of their action at the cellular level need to be clarified, which was the purpose of our study. The present investigation attempted to answer the question in what way volatile anaesthetics influence Ca(2+)-related contraction in isolated porcine epicardial coronary to understand their intracellular mechanism.

METHODS

Isolated helical strips of porcine epicardial coronary artery without endothelium were suspended for isotonic contraction recordings in Krebs-Ringer's solution. 9.4 x 10(-2) MK+, 2.5 x 10(-3) M Ca(2+)-induced shortening of the strips was regarded as the reference value (100%). After incubation in Ca(2+)-free solution with 10(-3) M ethlene glycol bis (beta-aminoethyl ether)-N, N-tetraacetic acid (EGTA) for 60 minutes, the muscle strips were exposed to increasing Ca2+ concentrations (10(-4)-10(-2)) either in the presence or absence of 1.5 or 2.5 minimum alveolar concentration (MAC) halothane, isoflurane or sevoflurane, with 9.4 x 10(-2) M K+ bath solution.

RESULTS

All three drug groups produced apparent biphasic effects with a cumulative increase of Ca2+ concentration compared with control groups. An initial increase at low Ca2+ concentration was followed by a decrease of Ca(2+)-activated contractions. Isoflurane affected Ca(2+)-induced contraction significantly more than halothane and sevoflurane.

CONCLUSIONS

The results imply that volatile anaesthetics influence Ca(2+)-dependent activity of coronary smooth muscle by complex mechanisms, which involve promotion of intracellular Ca2+ release and other mechanisms that alter sensitivity to calcium.

Modulation of the ryanodine receptor sarcoplasmic reticular Ca2+ channel in skinned fibers of fast- and slow-twitch skeletal muscles from rabbits

This study was performed to compare skinned fibers from rabbit adductor magnus (AM) and soleus (SL) muscles with regard to the influence of caffeine, Ca2+ and Mg2+ on the depressive effects of ryanodine (RYA) on the caffeine-induced tension transients. Single skinned fibers were immersed in solutions to load Ca2+ into, and release Ca2+ from the SR (a load-release cycle). Three cycles were sequentially performed in each skinned fiber: (1) a control (no RYA), (2) a conditioning period in which activation was carried out in the presence of ryanodine plus various concentrations of the modulators, i.e. caffeine, Ca2+ or Mg2+, and (3) a test (no RYA) which monitored the release activity retained after the conditioning cycle. The depressive effect of RYA was found to be a function of [ryanodine], [caffeine], or [Ca2+], and an inverse function of [Mg2+], where [] denotes concentration. The half-maximal effects of RYA in AM (5 microM RYA) and SL (10 microM RYA), respectively, occurred at a pCa50 of 5.32 versus 5.43 without caffeine, or pCa50 of 7.24 versus 6.88 and pMg50 of 3.29 versus 3.61 with 25 mM caffeine, at a [caffeine] of 4.96 versus 7.29 mM, and at a [ryanodine] of 31.0 versus 101.6 microM. Thus, the RYA depression in skinned muscle fibers is modulated by caffeine, Ca2+, and Mg2+ in both muscle types, and AM is at least two- to fourfold more sensitive than SL.

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.

Modulation of sarcoplasmic reticulum Ca(2+)-release channels by caffeine, Ca2+, and Mg2+ in skinned myocardial fibers of fetal and adult rats

Ryanodine causes depression of the caffeine-induced tension transient (ryanodine depression) in skinned muscle fibers, because it blocks the sarcoplasmic reticulum (SR) Ca(2+)-release channels [Su, J. Y. (1988) Pflügers Arch 411:132-136, 371-377; (1992) Pflügers Arch 421:1-6]. This study was performed to examine the sensitivity of SR Ca(2+)-release channels to ryanodine in fetal compared to adult myocardium and to investigate the influence of Ca2+, caffeine, and Mg2+ on ryanodine depression in skinned fibers. Ryanodine (0.3 nM-1 microM) caused a dose-dependent depression in skinned myocardial fibers of the rat, and the fetal fibers (IC50 approximately 74 nM) were 26-fold less sensitive than those of the adult (IC50 approximately 2.9 nM). The depression induced by 0.1 microM or 1 microM ryanodine was a function of [caffeine], or [Ca2+] (pCa < 6.0), which was potentiated by caffeine, and an inverse function of [Mg2+]. At pCa > 8.0 plus 25 mM caffeine, a 20% ryanodine depression was observed in both the fetal and adult fibers, indicating independence from Ca2+. Ryanodine depression in skinned fibers of the fetus was less affected than that seen in the adult by pCai, [caffeine]i, or 25 mM caffeine plus pCai or plus pMgi (IC50 approximately pCa 4.5 versus 5.1; caffeine 12.7 mM versus 2 mM; pCa 6.7 versus 7.3; and pMg 3.9 versus 3.3 respectively). The results show that the SR Ca(2+)-release channel in both fetal and adult myocardium is modulated by Ca2+, caffeine, and Mg2+.(ABSTRACT TRUNCATED AT 250 WORDS)