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 47,337 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The pharmacokinetics and effects of creatine and caffeine administration on anaerobic and aerobic performance of 7 trained athletes were studied in a randomized, placebo-controlled, double-blind crossover design. The treatments were: placebo (PLA), a single oral dose (7 mg x kg(-1)) of caffeine (CAF), repeated oral doses (3 x 100 mg x kg(-1) x day(-1)) of creatine for 3 days (CRE), or the combination of caffeine and creatine (CAF + CRE) before physical exercise. In one session CAF was administered without exercise. Drug administration was followed by 3 repetitive 1-minute exercise bouts on a bicycle ergometer at maximal speed (anaerobic exercise) starting 70 min after drug administration. Anaerobic exercise was followed by 45 min of cycling at constant pedalling speed and workload (aerobic exercise). CRE and CAF, alone or in combination, did not improve maximal pedalling speed (rpm), maintenance of maximal speed (rpm) or total work output (kJ) during the 1 -minute bouts, when compared with PLA. In addition, no statistically significant differences in heart rate or blood lactate were observed between the treatments either during anaerobic or aerobic exercise bouts. Creatine was rapidly and efficiently absorbed, as reflected by plasma concentrations. The mean +/-SEM value for creatine Cmax was 1.22+/-0.14 mmol x l(-1), tmax 92+/-7 min and plasma half-life (t1/2beta) 172+/-21 min. Caffeine pharmacokinetics were not affected by concomitant administration of creatine or by physical exercise. In conclusion, neither maximal performance and subsequent recovery nor aerobic performance were enhanced by oral creatine supplementation in the study.
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PMID:Creatine and caffeine in anaerobic and aerobic exercise: effects on physical performance and pharmacokinetic considerations. 962 89

Isometric ATP consumption and force were investigated in mechanically skinned fibres from iliofibularis muscle of Xenopus laevis. Measurements were performed at different [Ca2+], in the presence and absence of caffeine (5 nM). In weakly Ca2+-buffered solutions without caffeine, spontaneous oscillations in force and ATPase activity occurred. The repetition frequency was [Ca2+]-and temperature-dependent. The Ca2+ threshold (+/- SEM) for the oscillations corresponded to a pCa of 6.5 +/- 0.1. The maximum ATP consumption associated with calcium uptake by the sarcoplasmic reticulum (SR) reached during the oscillations was similar to the activity under steady-state conditions at saturating calcium concentrations in the presence of caffeine. Maximum activity was reached when the force relaxation was almost complete. The calculated amount of Ca2+ taken up by the SR during a complete cycle corresponded to 5.4 +/ 0.4 mmol per litre cell volume. In strongly Ca2+-buffered solutions, caffeine enhanced the calcium sensitivity of the contractile apparatus and, at low calcium concentrations, SR Ca uptake. These results suggest that when the SR is heavily loaded by net Ca uptake, there is a massive calcium-induced calcium release. Subsequent net Ca uptake by the SR then gives rise to the periodic nature of the calcium transient.
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PMID:Calcium handling by the sarcoplasmic reticulum during oscillatory contractions of skinned skeletal muscle fibres. 974 51

This study assessed the effects of regular coffee drinking on 24-hour ambulatory blood pressure (ABP) in normotensive and hypertensive older men and women. Twenty-two normotensive and 26 hypertensive, nonsmoking men and women, with a mean age of 72.1 years (range, 54 to 89 years), took part in the study. After 2 weeks of a caffeine-free diet, subjects were randomized to continue with the caffeine-free diet and abstain from caffeine-containing drinks or drink instant coffee (5 cups per day, equivalent to 300 mg caffeine per day) in addition to the caffeine-free diet for a further 2 weeks. Change in systolic and diastolic blood pressures (SBP, DBP) determined by 24-hour ambulatory BP monitoring showed significant interactions between coffee drinking and hypertension status. In the hypertensive group, rise in mean 24-hour SBP was greater by 4.8 (SEM, 1.3) mm Hg (P=0.031) and increase in mean 24-hour DBP was higher by 3.0 (1.0) mm Hg (P=0.010) in coffee drinkers than in abstainers. There were no significant differences between abstainers and coffee drinkers in the normotensive group for 24-hour, daytime, or nighttime SBP or DBP. In older men and women with treated or untreated hypertension, ABP increased in coffee drinkers and decreased in abstainers. Restriction of coffee intake may be beneficial in older hypertensive individuals.
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PMID:Effects of coffee on ambulatory blood pressure in older men and women: A randomized controlled trial. 1008 1

Even if the effects of caffeine on some physiological parameters are well known, its influence on circadian rhythmicity had not yet been investigated. This possible influence is of particular importance, introducing a possible bias in chronopharmacological studies. Thus, the aim of this study was to evaluate the effects of repeated caffeine administration on the circadian rhythms of heart rate "H," body temperature "T," and motor activity "A" in unrestrained rats maintained under controlled conditions (LD 12:12, light from 0600 to 1800 h) by using radiotelemetry transmitters. The study was divided into three 7-day observation spans: a 1-week control span "P1," a 1-week treatment span "P2," and a 1-week recovery span "P3." P1 was performed for assessing baseline measurements of H. T, and A. During P2, four rats received caffeine (25 mg/kg) at 0900 h, while four rats received saline in the same conditions every day of the observation span. H, T, and A were continuously monitored and plotted every 10 min. For P1, P2, and P3. a power spectrum analysis (Fourier transform) was applied to determine the dominant period of rhythmicity. If H, T, and A circadian rhythms were detected, the characteristics of these rhythms, i.e., mesors, amplitudes and acrophases, were determined by cosinor analysis, expressed as means +/- SEM and compared by analysis of variance. Our results indicated that caffeine did not suppress the circadian rhythmicity of H, T, and A, but significantly increased mesors and decreased amplitudes of the three rhythms and advanced acrophases of temperature and activity compared to the control group.
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PMID:Caffeine-induced modifications of heart rate, temperature, and motor activity circadian rhythms in rats. 1046 32

Skinned muscle fibers are ideal model preparations for the investigation of Ca2+ -regulatory mechanisms. Their internal ionic milieu can be easily controlled and distinct physiological states are well defined. We have measured the total Ca content in the terminal cisternae of such preparations using imaging electron energy-loss spectroscopy (Image-EELS) as a new approach for quantification of sub-cellular element distributions. Murine muscle fibers submitted to a standardized calcium-loading procedure were cryo-fixed with a combined solution exchanger/plunge freezing device. Energy-filtered image series were recorded from ultrathin freeze-dried cryosections of samples immobilized in either relaxed or caffeine-contracted state. From these image series, electron energy-loss spectra were extracted by digital image-processing and quantitatively processed by multiple-least-squares-fitting with reference spectra. The calculated fit coefficients were converted to Ca-concentrations by a calibration obtained from Ca-standards. Total Ca-contents in the terminal cisternae of skinned skeletal muscle fibers decreased upon caffeine-induced Ca-release from 123+/-159 (+/-11) to 73+/-102 (+/-8) mmol/kg d.w. (weighted mean +/- SD (+/-SEM)).
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PMID:Quantification of total calcium in terminal cisternae of skinned muscle fibers by imaging electron energy-loss spectroscopy. 1055 69

In skeletal muscle, there is a length dependence of staircase potentiation for which the mechanism is unclear. In this study we tested the hypothesis that abolition of this length dependence by caffeine is effected by a mechanism independent of enhanced Ca2+ release. To test this hypothesis we have used caffeine, which abolishes length dependence of potentiation, and dantrolene sodium, which inhibits Ca2+ release. In situ isometric twitch contractions of rat gastrocnemius muscle before and after 20 s of repetitive stimulation at 5 Hz were analyzed at optimal length (Lo), Lo - 10%, and Lo + 10%. Potentiation was observed to be length dependent, with an increase in developed tension (DT) of 78 +/- 12, 51 +/- 5, and 34 +/- 9% (mean +/- SEM), at Lo - 10%, Lo, and Lo + 10%, respectively. Caffeine diminished the length dependence of activation and suppressed the length dependence of staircase potentiation, giving increases in DT of 65+/-13, 53 +/- 11, and 45 +/- 12% for Lo - 10%, Lo, and Lo + 10%, respectively. Dantrolene administered after caffeine did not reverse this effect. Dantrolene alone depressed the potentiation response, but did not affect the length dependence of staircase potentiation, with increases in DT of 58 +/- 17, 26 +/- 8, and 18 +/- 7%, respectively. This study confirms that there is a length dependence of staircase potentiation in mammalian skeletal muscle which is suppressed by caffeine. Since dantrolene did not alter this suppression of the length dependence of potentiation by caffeine, it is apparently not directly modulated by Ca2+ availability in the myoplasm.
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PMID:Length dependence of staircase potentiation: interactions with caffeine and dantrolene sodium. 1077 63

The ultrastructure of the parathyroid gland and the SEM appearances of the tibia were studied in hamsters with and without administration of caffeine. Caffeine was treated orally each day at either 2.5 mg (low dose) or 10 mg (high dose) per 100 g body weight for a period of 17 or 32 days. Statistical analysis showed no significant differences among all groups examined regarding the serum calcium level. Transmission electron microscopy of the parathyroid gland revealed that the volume densities occupied by the mitochondria, Golgi complexes and rough endoplasmic reticulum of caffeine-treated groups were found significantly higher when compared with controls. The number of secretory granules observed close to the cell membrane per total amount of these granules revealed significant increase in all caffeine-treated animals. The bone mineral content (BMC) values were closely related to body weight. In the high dose caffeine-treated hamsters increment of the mean BMC and body weight values was significantly lower than those of the controls after 32 days. In the scanning electron microscopic studies of the tibia, no alteration in the morphometric parameters was demonstrated. It is considered that the synthesis and release of parathyroid hormone is stimulated following caffeine consumption. Our data suggest that although chronic administration of caffeine in the hamster may slightly increase bone turnover as evidenced by the BMC decrease, bone morphometry was not altered. Thus the osteoporotic changes were not proved in this study.
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PMID:Effects of long-term treatment with caffeine on the ultrastructure of the golden hamster parathyroid gland and tibia. 1086 Apr

The pharmacokinetics of caffeine were determined in 10 camels after an intravenous dose of 2.35 mg kg(-1). The data obtained (median and range) were as follows. The elimination half-life (t(1/2)) was 31.4 (21.2 to 58.9) hours, the steady state volume of distribution (V(SS)) was 0.62 (0.51 to 0.74) litre kg(-1)and the total body clearance (Cl(T)) was 14.7 (8.70 to 19.7) ml kg(-1)per hour. Renal clearance estimated in two camels was 0.62 and 0.34 ml kg(-1)per hour. In vitro plasma protein binding (mean +/-SEM, n = 10) to a concentration of 2 and 8 microg ml(-1)was 36.0 +/- 0.24 and 39.2 +/- 0.36 per cent respectively. Theophylline and theobromine were identified as caffeine metabolites in serum and urine. The terminal elimination half-life of the former, estimated in two camels, was 70. 4 and 124.4 hours. Caffeine could be detected in the urine for 14 days.
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PMID:The pharmacokinetics, metabolism and urinary detection time of caffeine in camels. 1092 97

Anthracyclines, such as daunorubicin (Daun), and other quinone-containing compounds can stimulate the formation of toxic free radicals. The present study tests the hypothesis that the quinone moiety of Daun, by increasing free-radical production, disrupts sarcoplasmic reticulum (SR) function and thereby inhibits myocardial contractility in vitro. We compared Daun with its quinone-deficient analogue, 5-iminodaunorubicin (5-ID), using experimental interventions to produce various contractile states that depend on SR function. At concentrations of Daun or 5-ID that did not alter contractility (dF/dt) of steady-state contractions (1 Hz) in electrically paced atria isolated from adult rabbits, only Daun significantly attenuated the positive inotropic effects on dF/dt of increased rest intervals (PRP; post-rest potentiation) or increased stimulation frequencies. Attenuation was to 98+/-6% at 1 Hz, and 73+/-8 and 67+/-8% for 30 and 60 sec PRP, respectively, and 73+/-3 and 63 +/-3% at 2 and 3 Hz, respectively, for 88 microM Daun (P<0.05, vs pre-drug baseline values, mean +/- SEM). These effects of Daun were similar to those of caffeine (2 mM), an agent well known to deplete cardiac SR calcium. We also examined the effect of Daun in isolated neonatal rabbit atria, which lack mature, functional SR; Daun did not alter the force-frequency relationship or PRP contractions. Additional studies in Ca(2+)-loaded SR microsomes indicated that both Daun and 5-ID opened Ca(2+) release channels, with Daun being 20-fold more potent than 5-ID in this respect. Neither anthracycline, however, induced free-radical formation in SR preparations (assayed via nicking of supercoiled DNA) prior to stimulating Ca(2+) release. Thus, our results indicate that Daun impairs myocardial contractility in vitro by selectively interfering with SR function; the quinone moiety of Daun appears to mediate this cardiotoxic effect, acting through a mechanism that does not involve free radicals.
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PMID:Daunorubicin cardiotoxicity: evidence for the importance of the quinone moiety in a free-radical-independent mechanism. 1102 Apr 45

Synaptosomes from the optic lobes of squid (Loligo forbesi) were prepared by homogenization and allowed to settle onto glass coverslips. Synaptosomes were loaded with Ca(2+) sensitive dyes (Fura-2 AM, Calcium Green-1 AM and Calcium Green-5N AM), visualized by light microscopy and Ca(2+) sensitive fluorescence signals recorded and analyzed. With Fura-2, resting Ca(2+) was found to be 80 nM (n = 10, SEM 5.7). Addition of K(+) (30 mM), caffeine (3 mM) and thapsigargin (10 microM) evoked transient increases in cytoplasmic Ca(2+). Addition of BAPTA-AM (20 microM) decreased intrasynaptosomal free Ca(2+). Similar results were obtained with Calcium Green-1 AM but not with Calcium Green-5N AM. We conclude that synaptosomes from the squid optic lobe posses intact membranes and mechanisms to regulate intrasynaptosomal free [Ca(2+)], as well as caffeine sensitive Ca(2+) stores. The results of this study are discussed with respect to the role of Ca(2+) in presynaptic protein synthesis.
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PMID:Ca(2+) dynamics in synaptosomes isolated from the squid optic lobe. 1110 69


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