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PubMed1998

Regulation of spontaneous rhythmic contractions in rat pregnant myometrium by corticotrophin-releasing factor.

To elucidate the role of corticotrophin-releasing factor (CRF) in the spontaneous rhythmic contractions of pregnant myometrium, isometric contractions of longitudinal myometrial preparations taken from Sprague-Dawley rats (18-21 days gestation) were measured. CRF (0.03-30 nM) significantly increased the amplitude (117 +/- 4% of control at 30 nM, mean +/- S.E.M.) and significantly decreased the frequency (74 +/- 5%) in a concentration-dependent manner. The area under the tension curve did not increase (89 +/- 8%). Such effects of CRF were not observed in preparations pretreated with the RP diastereomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS, 10 microM), an inhibitor of protein kinase A. In preparations pretreated with indomethacin (1 microM), CRF caused little change in the amplitude but significantly decreased the frequency of the contractions. Intracellular cAMP levels in the preparations, measured by enzyme immunoassay, rose when CRF was applied at 30 nM. These results suggest that the slight positive inotropic action of CRF is due to activation of both cAMP and prostaglandin pathways, whereas the negative chronotropic action is due solely to the cAMP signalling pathway.

Animals↗
PubMed1983

Force-length relations in cardiac muscle segments.

Using a new technique that measures the length of a segment in the central region of isolated papillaries, we have determined the force-segment length relation for ferret papillary muscles at 27 degrees C. The muscles contracted under muscle length isometric (auxotonic) and segment isometric conditions in physiological solutions containing 9.0, 4.5, 2.25, and 1.125 mM Ca2+. Force-segment length relations obtained from auxotonic and segment isometric contractions were identical in a given Ca2+ concentration. Calcium variations, however, changed the position, shape, and segment length intercept of the force-segment length relation. Force, at a given segment length, increased with increasing Ca2+ up to 9.0 mM Ca2+. The force-segment length relation changed shape from linear, in 4.5 mM Ca2+, to concave in 1.125 mM Ca2+. The segment length intercept was found by extrapolation to be 68, 69, and 74% SLmax in 4.5, 2.25, and 1.125 mM Ca2+, respectively. Two passive force corrections were used to calculate the developed force-segment length relations. Assuming passive force to be related primarily to segment length yields curves that change shape with Ca2+ concentration, suggesting length-dependent activation. On the other hand, assuming passive force to be related to muscle length results in curves for different Ca2+ concentrations that are nearly vertically shifted versions of each other, suggesting the influence of internal loads.

Animals↗
PubMed2003

Strength training counteracts motor performance losses during bed rest.

The purpose of the study was to determine the effect of bed rest with or without strength training on torque fluctuations and activation strategy of the muscles. Twelve young men participated in a 20-day bed rest study. Subjects were divided into a non-training group (BRCon) and a strength-training group (BRTr). The training comprised dynamic calf-raise and leg-press exercises. Before and after bed rest, subjects performed maximal contractions and steady submaximal isometric contractions of the ankle extensor muscles and of the knee extensor muscles (2.5-10% of maximal torque). Maximal torque decreased for both the ankle extensors (9%, P < 0.05) and knee extensors (16%, P < 0.05) in BRCon but not in BRTr. For the ankle extensors, the coefficient of variation (CV) for torque increased in both groups (P < 0.05), with a greater amount (P < 0.05) in BRCon (88%) compared with BRTr (41%). For the knee extensors, an increase in the CV for torque was observed only in BRCon (22%). The increase in the CV for torque in BRCon accompanied the greater changes in electromyogram amplitude of medial gastrocnemius (122%) and vastus lateralis (59%) compared with BRTr (P < 0.05). The results indicate that fluctuations in torque during submaximal contractions of the extensor muscles in the leg increase after bed rest and that strength training counteracted the decline in performance. The response varied across muscle groups. Alterations in muscle activation may lead to an increase in fluctuations in motor output after bed rest.

Adult↗
PubMed1981

The first heart sound in left bundle branch block: an echophonocardiographic study.

To examine the nature of the first heart sound in left bundle branch block (LBBB), 26 patients with LBBB were studied using echophonocardiography. Apexcardiograms and systolic time intervals were also obtained from these patients and the results were compared with 20 normal subjects. Mitral closure preceded tricuspid closure by 0.02-0.04 second in 19 of the 20 normal subjects. In contrast, in subjects with LBBB there was a variable delay in mitral closure so that in seven subjects the sequence of mitral and tricuspid closure was normal, in 11 it was reversed compared wih the normal group, and in eight closure of both valves was simultaneous. The interval from the onset of ventricular depolarization to mitral closure was 0.074 +/- 0.019 (mean +/- SD) for the patients with LBBB and 0.057 +/- 0.008 second for the normal group (p < 0.001). With LBBB the first sound tended to be of reduced intensity and separate mitral and tricuspid components could not be identified. Systolic time intervals and the apexcardiograms demonstrated a delayed onset of left ventricular contraction and an increased isometric contraction time in the LBBB group, both of which would contribute to a delayed mitral valve closure and decreased first heart sound intensity.

Adult↗
PubMed1993

Mechanisms of oxygen-induced contraction of ductus arteriosus isolated from the fetal rabbit.

The present study was designed to investigate the effect of O2 on intracellular Ca concentration ([Ca]i) in the ductus arteriosus and the mechanisms for O2-induced ductal contraction. The force of isometric contraction of the ring of the ductus arteriosus isolated from fetal rabbits at 30 days of gestation (term, 31 days) was measured. The ductus arteriosus was loaded with fura 2, a calcium-sensitive dye, and [Ca]i was determined from the ratio of fluorescence intensity at 340 and 380 nm excitation wavelengths. The ductus arteriosus was initially superfused with hypoxic control solutions and contraction was induced by application of oxygenated solutions. The O2-induced contraction of the ductus arteriosus was associated with increases in [Ca]i and was eliminated in the absence of extracellular calcium. An increase in [K]o from 5 to 50 mM, which causes membrane depolarization, induced ductal contraction. The calcium channel blockers verapamil, diltiazem, and nickel caused a similar inhibition of O2-induced contraction as well as KCl-induced contraction. The role of intracellular calcium stores in O2-induced ductal contraction was examined using ryanodine, an inhibitor of calcium uptake and release from the sarcoplasmic reticulum. The inhibition of O2-induced contraction by ryanodine was minimal. Infusion of glibenclamide, an inhibitor for opening the ATP-sensitive potassium channel, caused contraction of the ductus arteriosus in the hypoxic solution. Cromakalim, an opener of ATP-sensitive potassium channels, completely relaxed the contraction induced by O2. These data suggest that O2 increases [Ca]i and causes contraction in the ductus arteriosus. Application of O2 may change from anaerobic to aerobic metabolism and depolarize membrane potential by closing the ATP-sensitive potassium channel, which in turn increases calcium influx via the voltage-dependent calcium channel. Mechanisms other than the ATP-sensitive potassium channel may also be involved in the O2-induced contraction and remain to be studied.

Animals↗
PubMed2002

Age-related EMG variables during maximum voluntary contraction.

We studied the neuromuscular adaptation that occurs with aging by comparing changes in surface electromyography (EMG) variables from the tibialis anterior muscle in 12 young (21.4 +/- 1.7 yr. old) and 13 older subjects (70.8 +/- 3.1 yr. old). EMG variables such as the muscle fiber conduction velocity, median frequency, and averaged rectified value were calculated during maximum voluntary contraction for 5-sec. isometric contractions. The dorsiflexion force, muscle fiber conduction velocity, median frequency, and average rectified value during maximum voluntary contraction were significantly smaller in the older than in the younger group (p < .05). These results suggested that the neuromuscular system in older subjects is affected by the selective atrophy of fast twitch fibers and differences in motor unit firing statistics. Our results suggest the utility of applying the EMG observed during maximum voluntary contraction to the noninvasive evaluation of neuromuscular function in elderly persons.

Adult↗