Calcium uptake in mitochondria and vesicles of heart and skeletal muscle in presence of potassium, sodium, k-strophanthin and pentobarbital

Regulatory volume decrease in Ehrlich ascites tumor cells is not mediated by a rise in intracellular calcium

Ehrlich ascites tumor cells suspended in hyposmotic solution initially swell and then shrink back towards normal volume, a process known as regulatory volume decrease (RVD). RVD is characterized by a specific loss of KCl, although the mechanism for this is currently unknown. The hypothesis that a rise in intracellular calcium ([Ca2+]i) activates calcium-sensitive ion conductances to initiate RVD was investigated. The results indicate that in the Ehrlich cell no rise in [Ca2+]i occurs when the extracellular osmolality is reduced from 300 mosM to 180 mosM. These findings were substantiated by the lack of sensitivity of RVD to the Ca(2+)-sensitive K+ channel blockers charybdotoxin (CTX) and nifedipine. In contrast, the ionophore ionomycin induced a cell shrinkage that was sensitive to CTX and nifedipine indicating that a rise in [Ca2+]i could play a role in cell volume reduction but that this occurred by a mechanism different from that observed in RVD. The conclusion from these experiments is that Ca2+ does not act as a second messenger for RVD in the Ehrlich cell.

Distribution of (Na+ + K+)ATPase and sodium channels in skeletal muscle and electroplax

The distributions of (Na+ + K+)ATPase and sodium channels in skeletal muscle fibres and electrocytes were determined by immunofluorescent and immunoelectron microscopic techniques using antibodies against rat and eel (Na+ + K+)ATPase and the eel electric organ sodium channel. The extrajunctional sarcolemma of skeletal muscle was uniformly stained by polyclonal antibodies against (Na+ + K+)ATPase and the sodium channel. The T-tubule system of skeletal muscle was also labelled heavily for both (Na+ + K+)ATPase and the sodium channel. The terminal cisternae of the sarcoplasmic reticulum was stained for (Na+ + K+)ATPase but not sodium channels. At the motor endplate, (Na+ + K+)ATPase-like immunoreactivity was present along the plasmalemma of motor nerve terminals but not along the postsynaptic junctional sarcolemma. Paradoxically, a monoclonal antibody that binds to the alpha form of the catalytic subunit of (Na+ + K+)ATPase from rat hepatocytes and renal tubule cells did not label the enzyme in rat skeletal muscle. In electrocytes, (Na+ + K+)ATPase-like immunoreactivity was concentrated primarily along the plasmalemma and calveolae of the non-innervated face. In contrast, sodium channel-like immunoreactivity was concentrated along the plasmalemma of the innervated face except in the clefts of the postsynaptic membrane. Thus, we conclude that at endplates both the (Na+ + K+)ATPase of rat skeletal muscle and sodium channels of eel electrocytes are not concentrated in the juxtaneuronal postsynaptic membrane. We also interpret the failure of the monoclonal anti-alpha (Na+ + K+)ATPase antibodies to bind to the enzyme in muscle to indicate that the catalytic subunit of skeletal muscle (Na+ + K+)ATPase displays different epitopes than does the alpha subunit of kidney and liver.

Interference of pentobarbital and thiopental with the vascular contraction and noradrenaline release in human cerebral arteries

Thiopental and pentobarbital induced dose-dependent vasodilations in human cerebral arteries previously contracted with noradrenaline (NA), serotonin (5-HT) and KCl. Preincubation with both barbiturates decreased the contractions evoked by the three agents. Pentobarbital and thiopental reduced the Ca2+-induced contractile effects in K+-depolarized arteries and 5-HT-Ca2+ and NA-Ca2+ contractions dose-dependently. The tritium release evoked by K+ from these vessels prelabelled with [3H]NA was significantly reduced by both barbiturates at 10(-3) M and by Ca2+ removal. These results indicate that pentobarbital and thiopental essentially produce a similar interference with Ca2+ influx inhibiting the contractile responses induced by the three vasoactive agents and the exocytotic NA release evoked by K+.

Cardiac glycosides and sodium/potassium-ATPase

The sodium/potassium-ATPase complex is, according to modern research, the binding site for cardiac glycosides on the outer surface of the cell membrane and their receptor. Inhibition of this enzyme by cardiac glycosides leads for instance in the heart to a decrease or a delay in membrane sodium/potassium-ion transport, and indirectly to an increase in the intracellular ionized calcium-concentration and an increase in cardiac contractile force. According to recent observations the activity of the sodium/potassium-ATPase or its concentration, and therefore the concentration of binding sites can increase in some tissues after long term treatment with cardiac glycosides. This might explain the occasionally observed tolerance to digitalis glycosides.

Effect of pentobarbital on the noradrenaline release induced by drugs and field electrical stimulation from cerebral and femoral arteries of the cat

The present studies showed that field electrical stimulation, high potassium (K+), tyramine and ionophore X537A induced tritium release from cerebral and femoral arteries of cat prelabelled with [3H]noradrenaline. The secretion caused by K+ or field stimulation was CA2+-dependent and was antagonized by high concentrations of pentobarbital (10(-4) and 10(-3) M), whereas that induced by the rest of the drugs was unchanged in the same situations. The noradrenaline uptake by these arteries was reduced by pentobarbital (10(-3) M and 10(-4) M). These results suggest that this barbiturate interferes with Ca2+ entry to the adrenergic nerve endings, and therefore antagonizes the noradrenaline release by Ca2+-dependent processes (exocytosis).

Inotropic effects of digitoxin in isolated guinea-pig heart under conditions which alter contraction

In order to examine which calcium pool(s) might contribute to the therapeutic action of digitalis, the positive inotropic effect of digitoxin was quantified under conditions in which different calcium pools were either predominant or suppressed. Isolated left atrial muscle of guinea-pig heart was stimulated at 0.5 Hz at 30 degrees C. After a brief rest period, the first contraction (post-rest contraction) observed when electrical stimulation was resumed was markedly greater than the contraction observed under 0.5 Hz stimulation. Post-rest contraction was apparently dependent on the rest period and related to a ryanodine-sensitive, verapamil-insensitive calcium pool. Post-rest contraction was moderately enhanced by 0.2 microM digitoxin, either in the absence or presence of verapamil. A step-wise increase in the frequency of stimulation following the rest period caused a typical staircase phenomenon, which was markedly suppressed by verapamil but not by ryanodine. Digitoxin markedly augmented the staircase in the absence or presence of ryanodine. Paired-pulse stimulation markedly increased the developed tension, which was slightly reduced by verapamil but not by ryanodine. Digitoxin substantially increased the developed tension evoked by paired-pulse stimulation. In left atrial preparations of rat heart, an increase in stimulation frequency decreased the force of contraction; however, paired-pulse stimulation increased the force, indicating that the enhancement of developed tension by an increase in stimulation frequency and that by paired-pulse stimulation have different mechanisms. Thus, the positive inotropic action of digitoxin does not appear to be restricted to a specific calcium pool; however, the inotropic effect was greater under the conditions in which superficial calcium pool plays a predominant role.

Inotropic effects of potassium rich solutions of frog cardiac muscles

1. Inotropic effects of potassium rich solutions on frog cardiac muscle have been studied bith in current clamp and voltage clamp conditions, using a double sucrose gap apparatus. 2. Potassium rich solutions cause either a positive or a negative inotropic effect, together with an increase or a decrease in the duration of the action potential, according to the preparation. 3. The phasic phase of contraction and the slow inward current are decreased in amplitude; the reversal potential of the slow inward current is shifted towards more negative values. 4. The tonic phase of contraction is first decreased, the increased; the effects are correlated with modifications of the background current, initially in the inward, then in an outward direction. 5. The tension level obtained in contracture experiments is increased or decreased, according to the direction of the changes in the background current. 6. The effects of potassium rich solutions are still observed in he presence of ouabain, suggesting that they are independent of any effect on the sodium-potassium pump. 7. The effects of potassium rich solutions are still observed when external sodium is replaced by sucrose; they disappear (except the effect on the background current) when external sodium is replaced by lithium. 8. The results, which indicate that potassium ions play a role in the regulation of the intracellular concentration of calcium ions, are discussed in relation to a possible K/Ca exchange mechanism, to the Na/Ca exchange and to the role of intracellular calcium stores.

Interference of pentobarbitone with the contraction of vascular smooth muscle in goat middle cerebral artery

Pentobarbitone (10(-5) to 10(-3) M) decreased the basal tone of vascular smooth muscle of goat middle cerebral artery in a dose-dependent manner as well as relaxing established contractions induced by noradrenaline (NA) (10(-5) M), 5-hydroxytryptamine (5-HT) (10(-5) M) and KCl(120 mM). Preincubations with pentobarbitone reduced the contractions evoked by these three agents in a dose-dependent way. It also decreased Ca2+ -induced contractile responses in K+ -depolarized arteries and 5-HT-Ca2+ and NA-Ca2+ contractions dose-dependently. Contractions induced by K+ were more sensitive to the depressant actions of the drug than those produced by NA and 5-HT. The small contractions evoked by K+ and 5-HT in Ca2+ -free medium were also reduced in its presence. The antagonism Ca2+ -pentobarbitone was insurmountable. These results suggest that the drug interferes with Ca2+ entry and Ca2+ release from cells stores, and therefore with the smooth muscle contractions.

Pentobarbital sodium inhibits calcium uptake in vascular smooth muscle

This report demonstrates that the commonly used anesthetic agent, pentobarbital sodium, in concentrations of 1 . 10(-4) to 2 . 10(-3) M inhibits calcium (Ca2+) uptake in both rat aortic and portal venous smooth muscle. The data indicate that total exchangeable Ca2+ in portal vein is reduced by about 15% in 1 . 10(-4) M pentobarbital sodium, while the intracellular exchangeable Ca2+ is reduced by 24%. On the other hand, in aortic smooth muscle, while 5--20 . 10(-4) M pentobarbital sodium reduces total exchangeable Ca2+ by about 15%, intracellular Ca2+ is reduced by 22% in 5 . 10(-4) M pentobarbital sodium and by 38% in 2 . 10(-3) M pentobarbital sodium. The present studies thus reveal that concentrations of pentobarbital sodium known to be present during induction of surgical anesthesia can exert significant inhibitory effects on exchangeability and transmembrane movement of Ca2+ in at least two different types of blood vessels.

Pentobarbitone and skeletal muscle contractions: on the interaction with the effect elicited by the beta-adrenoceptor agonist, terbutaline

The soleus, a slow-contracting muscle, and the extensor digitorum longus (EDL), a fast-contracting muscle from guinea-pig were prepared for isometric recording in vitro. Subtetanic contractions were evoked by transmural field-stimulation. Pentobarbitone increased the force of contraction in both muscles. In the soleus it shifted the stimulation frequency-response curve to the left. Terbutaline caused a decrease in the force of subtetanic contractions of the soleus, an effect which was dependent on the stimulation frequency. In the presence of pentobarbitone, the stimulation frequency had to be lowered by about 2 HZ in order to maintain the optimum response to terbutaline. The EDL responded to terbutaline with an increased force of contraction. In this case the stimulation frequency was less critical and the effects were the same in the presence and in the absence of pentobarbitone. Experiments with alpha-chloralose yielded results similar to those obtained with pentobarbitone.

Calcium and pancreatic beta-cell function. I. Stimulatory effects of pentobarbital on insulin release

Islets microdissected from ob/ob-mice were exposed to 3mM pentobarbital in media which were normal or deficient in Ca2+. This treatment resulted in marked decrease of the islet content of cyclic AMP recorded in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Pentobarbital had a dual effect on insulin release. In addition to being a potent inhibitor of glucose-stimulated insulin release in media containing 2.56 mM Ca2+ it increased the amounts of insulin released in high glucose media deficient in Ca2+. There was a transient stimulation with ordinary concentrations of Ca2+ and 3mM glucose whtn the media also contained 3-isobutyl-1-methylxanthine. The stimulatory effect of pentobarbital persisted after replacing part of the Ca2+ in the beta-cell membrane with lanthanum ions and it could not be mimicked by lowering the oxygen tension of the incubation medium. It is suggested that pentobarbital stimulation of insulin release is the result of a specific action of the drug on the distribution of Ca2+ within the pancreatic beta-cells.

The effect of the internal sodium concentration on calcium fluxes in isolated guinea-pig auricles

1. Calcium efflux from guinea-pig auricles followed saturation kinetics when [Ca](o) and [Na](o) were changed while the ratio [Ca](o)/[Na](o) (2) was kept constant. The Michaelis constant, K(m) (Ca+Na) = 40 mM, suggests that a hypothetical carrier system, responsible for sodium-calcium exchange, is far from saturation with the inside concentrations of these ions.2. [Na](i) was altered in the auricles between 12.5 and 60 mM/kg fibre water while total cellular calcium concentration ([Ca](t)) at the beginning of the influx period was not significantly different in the various groups of preparations.3. (45)Ca influx increased appreciably with increasing [Na](i). (45)Ca influx from sodium-poor solution corresponded to an almost equal increase in [Ca](t), while [Ca](t) did not change much in preparations loaded with (45)Ca in Tyrode solution. When the sodium-activated fraction of calcium influx was plotted against [Na](i) (2) the resulting curve indicated saturation with K(m) (Na) = 3500 (mM [Na](i))(2) and maximal influx rate, J(i, max) (Ca') = 1.35 mM/kg wet weight x 10 min.4. When the preparations were re-equilibrated for various times in normal Tyrode solution after [Na](i) had been increased, both the sodium-activated component of calcium influx and [Na](i) (2) decreased with approximately the same rate constants.5. Calcium efflux from auricles with high [Na](i) was increased when it was measured in Tyrode solution while the efflux in sodium-poor solution was inhibited.6. Auricles with increased [Na](i) showed a positive inotropic contractile response.7. The main conclusion reached by these experiments is that calcium influx is affected by [Na](i) in a way which is compatible with a carrier-mediated sodium-calcium exchange system.