External potassium and action potential propagation in rat fast and slow twitch muscles.

The role of extracellular K+ concentration in the propagation velocity of action potential was tested in isolated rat skeletal muscles. Different K+ concentrations were produced by KCl additions to extracellular solution. Action potentials were measured extracellularly by means of two annular platinum electrodes. Fibre bundles of m. soleus (SOL), m. extensor digitorum longus (EDL), red (SMR) and white (SMW) part of m. sternomastoideus were maximum stimulated. The conduction velocity (c.v.) was calculated from the distance between the electrodes and the time delay of the potentials measured at 22 degrees C. In Tyrode solution containing 5 mmol/l K+, the c.v. was close to 1 m.s-1. Bundles of the fast muscle type seemed to have a somewhat higher c.v. The differences observed in these studies were not significant. At higher temperatures, the c.v. increased (Q10 of approx. 2) and a dissociation between SMR and SMW muscles appeared. An elevation of K+ concentration to 10 mmol/l induced a drop of the c.v. by approx. 25% and 15% in EDL and SOL muscles, respectively. After return to normal solution, the recovery was not complete within 30 min. In K+ free solution the c.v. of EDL and SM muscles rose by a factor of 1.5, but less in SOL muscles. The weaker response of SOL to K+ modification was related to the higher resistance of this muscle to fatigue. This suggestion was supported by experiments on fatigued fibre bundles. Immediately after a tetanic stimulation producing fatigue, the c.v. of EDL and SOL muscles dropped similarly as in 10 mmol/l K+; again, the drop was less for SOL muscles. Adrenaline (0.5-10.0 mumol/l) enhanced both the c.v. and the twitch amplitude. The results support the suggestion that extracellular K+ accumulation during activity is an essential factor of muscle fatigue.     

ATP synthesis driven by a potassium diffusion potential in Methanobacterium thermoautotrophicum is stimulated by sodium

Abstract ATP synthesis driven by a potassium diffusion potential was studied in cell suspensions of Methanobacterium thermoautotrophicum (Marburg). This transient increase in the intracellular ATP content was stimulated five-fold by the addition of sodium ions, from about 2 nmol ATP/min × mg cells (dry weight) at 0.07 mM Na⁺ to about 10 nmol ATP/min × mg cells at 25 mM Na⁺.     

Resting and stimulated respiration in vitro in the central nervous system. Regional distribution and relation to cell density

1. The respiratory rates in vitro of ten structures of the rabbit brain were measured in the presence of a normal (5mm) and an elevated (50mm) concentration of K(+) ions. The results were expressed on a dry-weight basis and in terms of cell density. 2. On a dry-weight basis, with a normal concentration of K(+), there was a steady decrease in respiratory rate on passing from the cerebral cortex through successively lower centres to the brain-stem, i.e. respiration was negatively correlated with phylogenetic age. 3. When the resting respiratory rates were expressed in terms of cell density there was no correlation with phylogenetic age. The neuron-containing structures of the cerebrum and brain-stem had identical respiratory rates with the exception of the cerebral cortex which had a higher rate. 4. The K(+)-stimulated respiratory rates/cell also showed a negative correlation with phylogenetic age; the higher centres responded with a greater percentage increase than the lower. 5. The results from the cerebellar cortex were anomalous. 6. There were variations of respiratory rate within the diencephalon. The thalamus had a relatively high respiratory rate, the anterior and ventral regions relatively low and the hypothalamus intermediate. 7. The oxygen consumption of white matter was not increased by a high external concentration of K(+).     

Differences in cellular contractile protein contents among porcine smooth muscles: evidence for variation in the contractile system

Cellular myosin, actin, and tropomyosin contents and ratios were determined for arterial (carotid, aorta, and coronary), intestinal (circular and longitudinal), esophageal, uterine, and tracheal smooth muscles inthe pig. Tissue protein contents were estimated by densitometry of polyacrylamide gels after electrophoresis of sodium dodecyl sulfate-treated tissue homogenates. Cellular contractile protein contents were estimated by correction for extracellular spaces. Cellular myosin contents were similar in each tissue (average +/- 1 SEM = 19.6 +/- 0.8 mg/g cell wet wt). However, the cellular contents of the thin filament proteins, actin and tropomyosin, were significantly higher in the arteries than in the nonarterial tissues. The calculated weight ratios of actin: myosin averaged 2.6 +/- 0.2 in the three arterial tissues and 1.5 +/- 0.1 in the nonarterial tissues, which may be compared with 0.36 in vertebrate striated muscles. The actin:tropomyosin weight ratios for all tissues were 3.7 +/- 0.1, a value comparable to the skeletal muscle ratio. The physiological implications of variations in the cellular thin filament protein contents are unknown, but these variations probably contribute to the observed differences in contractile function among various smooth muscles.     

Time dependence of transmembrane potential changes and intracellular calcium flux in stimulated human monocytes

An important characteristic of the functional differentiation of the blood monocyte is the development of its capacity to recognize and respond to stimuli. This ability is mediated to a large extent by specific receptor glycoproteins located on the cell surface. Stimulation of mononuclear phagocytes via these receptors results in a rapid rise in intracellular Ca++ concentration, accompanied or followed by a change in membrane potential, generation of oxidative products, degranulation, and effector functions such as phagocytosis, aggregation, or locomotion. While the development of these characteristics is difficult to characterize in vivo, several investigators have demonstrated in vitro changes in these cells that correlate with the development of effector function. To examine the mechanisms of specific membrane-stimulus interactions of monocytes as they differentiate into macrophage-like cells, we studied the responses of human monocytes and of monocytes incubated in serum-containing medium for up to 96 hr to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP). Freshly isolated monocytes exhibited little change in transmembrane potential following stimulation with an optimal concentration of peptide and underwent a significant increase only after 48 hr in culture. While constant resting intracellular Ca++ concentrations were maintained during the culture period, intracellular Ca++ levels following fMLP stimulation increased with with incubation in serum, for up to 96 hr. In contrast, fMLP-induced respiratory burst activity increased from 0 to 24 hr in culture; it remained elevated at 48 hr but declined again by 96 hr. Incubation of the cells for 24 hr increased their random (unstimulated) motility in modified Boyden chambers but did not alter the cells' directed (chemotactic) response to fMLP in comparison to the response of freshly isolated monocytes. Peptide binding to the cells did not increase during the incubation period, indicating that an increase in receptor number or in affinity for fMLP was not responsible for the enhanced responsiveness to fMLP as incubation time increased. These studies indicate that incubation of monocytes in serum-containing medium leads to a complex, altered series of responses to fMLP that correlate with the differentiation of the original monocytes in vitro and may relate to the in vivo differentiation of monocytes to macrophages.     

Bacterial growth in ground beef prepared from electrically stimulated and nonstimulated muscles.

Ground beef samples prepared from electrically stimulated and nonstimulated biceps femoris and infraspinatus muscles were inoculated with Lactobacillus sp., Pseudomonas sp., Acinetobacter sp., or a mixture of Lactobacillus spp., Pseudomonas spp., Acinetobacter spp., Moraxella sp., Microbacterium thermosphactum, and Erwinia herbicola. There were no significant differences in growth of various bacteria in ground beef made from electrically stimulated and nonstimulated muscles.     

State of the contractile apparatus during the development of a pathological process in the muscles. III. The nature and sequence of the structural changes in the contractile apparatus in Zenker's necrosis]

Using polarized ultraviolet (UV) fluorescence microscopy, it was shown that the local damage of muscle fibres causes in their morphologically unchanged parts the alternation of regions, being in different functional states referred to as "pseudocontraction" and "superrelaxation". The pattern of UV fluorescence anisotropy suggests that conformation of contractile proteins by "pseudocontraction" is similar to that at contraction, though changes in sarcomere length do not occur. The "superrelaxation" is characterized by a desorganization of myofilaments. During the spreading of Zenker's necrosis, the "pseudocontraction" is seen transferred first into "superrelaxation", and then into irreversible contracture and rigor. "The boundary of Zenker's necrosis" overlaps with the boundary of the self-propagating irreversible contracture. There is no proper boundary of Zenker's necrosis, because the destructive changes are observed over all the muscle fibre. Contraction nodules arise in the regions of "superrelaxation" and follow the changes of the contractile system, peculiar of contracture. The study of the influence of medium ionic composition on the development of Zenker's necrosis suggests that the arising and spreading of destruction are inseparably associated with the irreverrsible changes of intracellular membrane structures, and with the possibility of propogation of the damage signal by these structures along muscle fibres.     

Resting membrane potential and action potential of Nitella expansa protoplasts

Inside negative membrane potentials were observed for protoplastsobtained from Nitella expansa leaf internodal cells in mediacontaining 1 to 100 mM CaCl₂ using the microelectrode technique.The potential values were less negative than the membrane potentialof intact N. expansa leaf internodal cells. In addition, anaction potential consisting of two components—a fast componentand a slow component—was induced by electrical stimulationfor the protoplasts as well as the intact cells. (Received December 18, 1979; )     

Dependence upon external calcium for contractile activity in two molluscan proboscis muscles.

1. Both the radular sac and odontophore retractor muscles of Buccinum undatum depend upon [Ca]0 to raise the [Ca]i concentration of the contractile system to activation level. 2. The K-induced responses of the muscles depend mainly upon [Ca]0 for activator Ca while the ACh responses depend upon [Ca]0 to raise stored intracellular Ca to activation levels. 3. In the radular sac muscle, it is probable that the inward current is carried by Na+ or is Na(+)-dependent and this current may release [Ca]i for contraction since the muscle became spontaneously active during ACh- and K-contractures in Ca-free seawater containing 2 mM EGTA as a calcium chelator. 4. It is proposed that since calcium antagonists are more inhibitory on ACh responses than on K-contractures, ACh releases the activator calcium for the contractile system through a slow-type Ca channel while high K releases Ca through a fast-type calcium channel in these muscles.     

Gadolinium prevents stretch-mediated contractile dysfunction in isolated papillary muscles

We tested the hypothesis that overstretching the myocardium could induce and/or exacerbate contractile dysfunction via stretch-activated (SA) ion channels. Maximum developed tension (T(max)), normalized to a control value, was compared in guinea pig papillary muscles held at one of three resting lengths (physiological stretch, overstretch, and unloaded) for 85 min. Overstretched muscles exhibited decreased contractile force (T(max) = 0.77 +/- 0.03) compared with physiological and unloaded muscles (T(max) = 0.93 +/- 0.05 and 1.03 +/- 0.07, respectively). Gd(3+), an SA channel antagonist, eliminated the adverse effect of overstretching (T(max) = 0.98 +/- 0.06), but nifedipine, a dihydropyridine (DHP) antagonist of L-type calcium channels, did not (T(max) = 0.82 +/- 0.04). Exposure to modified hypoxia-reoxygenation (MHR) during physiological stretch resulted in decreased contractility (T(max) = 0.63 +/- 0.07), an effect that was exacerbated by overstretching (T(max) = 0.44 +/- 0.04). Gd(3+) mitigated the effects of overstretch during MHR (T(max) = 0.64 +/- 0.05), but DHP did not (T(max) = 0.48 +/- 0.04). These data suggest that overstretching of the myocardium contributes to contractile abnormalities via SA channels that are distinct from L-type calcium channels.     

Metabolic and contractile differentiation of rabbit muscles during growth

• 1.1. A study was carried out of post-natal evolution of the oxidative, glycolytic and contractile capacities in various types of rabbit muscle.
• 2.2. At birth, muscles are non-differentiated and present very limited metabolic and contractile activity, metabolism is mainly oxidative in all muscles.
• 3.3. Although muscular discrimination is manifest from the sixth week after birth, the glycolytic metabolism reaches its maximum capacity only after six to eight weeks.
• 4.4. Subsequently, oxidative metabolic capacity steadily decreases until adulthood.

     

The epithelial potassium channel Kir7.1 is stimulated by progesterone

The choroid plexus (CP) epithelium secretes cerebrospinal fluid and plays an important role in healthy homeostasis of the brain. CP function can be influenced by sex steroid hormones; however, the precise molecular mechanism of such regulation is not well understood. Here, using whole-cell patch-clamp recordings from male and female murine CP cells, we show that application of progesterone resulted in specific and strong potentiation of the inwardly rectifying potassium channel Kir7.1, an essential protein that is expressed in CP and is required for survival. The potentiation was progesterone specific and independent of other known progesterone receptors expressed in CP. This effect was recapitulated with recombinant Kir7.1, as well as with endogenous Kir7.1 expressed in the retinal pigment epithelium. Current-clamp studies further showed a progesterone-induced hyperpolarization of CP cells. Our results provide evidence of a progesterone-driven control of tissues in which Kir7.1 is present.     

Z-line formins promote contractile lattice growth and maintenance in striated muscles of C. elegans

Muscle contraction depends on interactions between actin and myosin filaments organized into sarcomeres, but the mechanism by which actin filaments incorporate into sarcomeres remains unclear. We have found that, during larval development in Caenorhabditis elegans, two members of the actin-assembling formin family, CYK-1 and FHOD-1, are present in striated body wall muscles near or on sarcomere Z lines, where barbed ends of actin filaments are anchored. Depletion of either formin during this period stunted growth of the striated contractile lattice, whereas their simultaneous reduction profoundly diminished lattice size and number of striations per muscle cell. CYK-1 persisted at Z lines in adulthood, and its near complete depletion from adults triggered phenotypes ranging from partial loss of Z line-associated filamentous actin to collapse of the contractile lattice. These results are, to our knowledge, the first genetic evidence implicating sarcomere-associated formins in the in vivo organization of the muscle cytoskeleton.     

Changes in tropomyosin subunit pattern in chronic electrically stimulated rabbit fast muscles.

The tropomyosin subunit ratio of rabbit fast muscle (α:β = 80:20) changes to that characteristic of skeletal slow muscles (α:β = 55:45) on continuous (10 Hz) stimulation for 3 weeks. The altered myosin light chain pattern and histochemical ATPase stain also show clear changes of fast → slow transformation. However, the rate of changes in the light chain patterns of myosin are slower than those of tropomyosin subunits. These results do not support the previous finding (Amphlett et al., Nature $\text{257}$, 602, 1975) that the tropomyosin subunit pattern remains unaltered during transformation of skeletal muscles and the conclusion that the genetic expression of tropomyosin is regulated under separate control from other myofibrillar proteins. Rather, our results suggest that the polymorphic patterns of all myofibrillar proteins in skeletal muscles undergo changes in a temporal manner during skeletal muscle transformation.