Aerobic and anaerobic metabolism in smooth muscle cells of taenia coli in relation to active ion transport

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Effects of glucose deprivation on the contractile response of the rabbit bladder to repetitive stimulation

The urinary bladder requires an adequate energy supply to maintain contractile function. The primary metabolic fuel is glucose. Through glycolysis and oxidative phosphorylation, high energy phosphates are generated, which in turn supply the metabolic energy for the contractile activities of the urinary bladder. The aim of this study was to determine the effects of glucose deprivation and recovery from glucose deprivation on the phasic and tonic components of the contractile responses of rabbit bladder strips to field stimulation, bethanechol, and KCl. The results can be summarized as follow: In response to glucose deprivation, (1) the tonic responses to field stimulation, bethanechol, and KCl all decreased at a significantly greater rate than the phasic responses; (2) the phasic and tonic responses to field stimulation were both reduced to less than 10% of control within 70 minutes of initiating glucose deprivation; (3) the tonic responses to bethanechol and KCI were reduced to approximately 10% of control within 180 minutes whereas the phasic responses remained stable at 40 and 30%, respectively; and (4) glucose replacement stimulated a rapid and nearly complete recovery of the phasic and tonic components of the responses to field stimulation, bethanechol, and KCI. These results indicate that the tonic responses to all forms of stimulation are more sensitive to glucose deprivation than the phasic responses.

Contractile and metabolic properties of longitudinal smooth muscle from rat urinary bladder and the effects of aging

Longitudinal smooth muscle strips taken from the urinary bladders of Sprague-Dawley rats, aged approximately 6 months and 24 months, were examined under a variety of conditions. Force development in response to electrical field stimulation and to cumulative addition of Ca2+ in the continual presence of 64 mM. KCl was the same in both adult and aged preparations. In response to cumulative additions of carbachol, however, it was observed that there was a shift to the right of the dose-response curve and a decrease in maximal force in the aged muscle strips. The maximal velocity of shortening was significantly lower in muscles from aged animals than in those from young adult animals. The metabolic tension cost during isometric contraction was, however, the same in both groups suggesting that the decline in Vus is largely due to factors not influencing energetic cost. The aged muscles also exhibited a lower basal metabolic rate and a reduced contribution of aerobic glycolysis to total metabolic energy production during both quiescence and stimulation under normoxic conditions than did muscles from young adult animals. They were, however, able to increase their rate of lactate production to the same levels as young adult muscles when stimulated under anoxic conditions.

The nature of fuel provision for the Na+,K(+)-ATPase in porcine vascular smooth muscle

1. The specific contributions of aerobic glycolysis and oxidative metabolism to Na+ pump activity were quantitated in porcine carotid arteries under aerobic conditions. 2. Active reaccumulation of potassium by potassium-depleted tissues could be supported by oxidative metabolism alone, anaerobic metabolism in the presence of glucose, or a combination of oxidative metabolism and aerobic glycolysis, but not under anaerobic conditions in the absence of glucose. 3. Increasing levels of potassium added to potassium-depleted arteries under aerobic conditions resulted first in stimulation of aerobic lactate release which saturated at 0.028-0.036 mumol min-1 g-1, which was then followed by a stimulation of oxidative metabolism. This behaviour is opposite to the classic Pasteur effect. 4. The dependence of potassium uptake and lactate release on the concentration of potassium added to potassium-depleted arteries ('potassium re-entry concentration') under aerobic conditions were qualitatively similar. The K0.5 (concentration at which the velocity is half-maximally activated) and Vmax (the maximum velocity) for lactate release were 1.2 +/- 0.3 mM and 0.037 mumol min-1 g-1, respectively; those for K+ uptake were 4.3 +/- 0.4 mM and 0.399 mumol min-1 g-1. 5. The stoichiometric ratio between potassium uptake and ATP as calculated from lactate release approximated theoretical values of 2:1 (assuming 1 ATP per lactate) when potassium re-entry concentrations were less than 2 mM; higher concentrations of potassium produced ratios up to 9:1. 6. Physiological pump rates, as determined by potassium efflux studies, corresponded to potassium re-entry concentrations of less than or equal to 2 mM, the same potassium re-entry concentrations where the stoichiometry between potassium transport and aerobic glycolysis approximated the theoretical ratio of 2:1. Increases in oxidative metabolism were not detected in this range, but were detected at potassium re-entry concentrations of greater than or equal to 4 mM. 7. It was concluded that at physiological Na+ pump rates, aerobic glycolytic metabolism supported the N+,K(+)-ATPase; at higher pump rates, oxidative metabolism was required for pump support as well.

Comparison of endogenous and exogenous sources of ATP in fueling Ca2+ uptake in smooth muscle plasma membrane vesicles

A smooth muscle plasma membrane vesicular fraction (PMV) purified for the (Ca2+/Mg2+)-ATPase has endogenous glycolytic enzyme activity. In the presence of glycolytic substrate (fructose 1,6-diphosphate) and cofactors, PMV produced ATP and lactate and supported calcium uptake. The endogenous glycolytic cascade supports calcium uptake independent of bath [ATP]. A 10-fold dilution of PMV, with the resultant 10-fold dilution of glycolytically produced bath [ATP] did not change glycolytically fueled calcium uptake (nanomoles per milligram protein). Furthermore, the calcium uptake fueled by the endogenous glycolytic cascade persisted in the presence of a hexokinase-based ATP trap which eliminated calcium uptake fueled by exogenously added ATP. Thus, it appears that the endogenous glycolytic cascade fuels calcium uptake in PMV via a membrane-associated pool of ATP and not via an exchange of ATP with the bulk solution. To determine whether ATP produced endogenously was utilized preferentially by the calcium pump, the ATP production rates of the endogenous creatine kinase and pyruvate kinase were matched to that of glycolysis and the calcium uptake fueled by the endogenous sources was compared with that fueled by exogenous ATP added at the same rate. The rate of calcium uptake fueled by endogenous sources of ATP was approximately twice that supported by exogenously added ATP, indicating that the calcium pump preferentially utilizes ATP produced by membrane-bound enzymes.

Metabolic correlates to pacemaker activity in the smooth muscle of guinea-pig mesotubarium

Oxygen consumption (FO2) and lactate production (Flac) were measured during spontaneous activity in the guinea-pig mesotubarium. During spontaneous contractions FO2 increased to maximally 0.270 +/- 0.025 mumol min-1g-1 (n = 23), followed by a rapid fall immediately upon relaxation. In the relaxed period (5-15 min) between spontaneous contractions FO2 continued to slowly decrease by about 25% towards a final value of 0.150 +/- 0.01 mumol min-1g-1. Flac showed no consistent variation during the relaxed period. Ouabain (10(-6)M) produced a contracture, which was abolished by the Ca2(+)-antagonist felodipine (10(-6)M). In the presence of felodipine, addition of ouabain caused depolarization and a decrease of oxygen consumption by 21% and of lactate production by 31%. Exchange of glucose in the physiological Krebs solution for beta-hydroxybutyrate did not influence spontaneous activity, while subsequent addition of cyanide (2 mM) abolished contractions and caused a hyperpolarization of 15 mV. Blockade of ATP-dependent K+ channels by addition of glibenclamide (10(-7)M) to the relaxed muscle in this situation caused spontaneous contractile activity to reappear. In glucose-containing Krebs solution glibenclamide had no effect on the spontaneous contractile and electrical activity and contractions persisted after addition of cyanide. The relaxing and hyperpolarizing effect of pinacidil could be counteracted by addition of glibenclamide. The results suggest that a decrease in electrogenic Na+/K(+)-pump activity in the course of the relaxed period between contractions could contribute to the pacemaker behaviour. ATP-dependent K+ channels, while having little influence on the spontaneous contractile activity under normal metabolic conditions, could be activated during blockade of aerobic and anaerobic metabolism, leading to inhibition of pacemaker activity.

Metabolism and force in hypertrophic smooth muscle from rat urinary bladder

Ten days of urinary outlet obstruction in the rat induced a threefold increase in bladder weight. Active force of control and hypertrophic bladder muscle strips was measured at varying PO2 levels after high-K+, carbachol, or electrical field stimulation. Highest force output was obtained with carbachol. Force per muscle area was lower in the hypertrophic muscles. The basal rates of oxygen consumption and lactate formation were similar in the two groups. The metabolic tension cost (ATP turnover/active force) was similar in the two groups for activation with high K+ and carbachol. In anoxia the active force decreased, but this was less pronounced in the hypertrophied muscle. Hypertrophied muscle could, in contrast to the controls, maintain a sustained K+ contracture in anoxia. Basal metabolic rates and tension cost were markedly reduced in anoxia for both groups. The lower force per area with unaltered tension cost, in hypertrophic muscles under all experimental conditions, may reflect unaltered intrinsic properties of the contractile system, although the amount of contractile material has decreased relative to cell volume. The increased resistance to anoxia may reflect a metabolic adaptation to impaired oxygen supply to the hypertrophied tissue.

Glycolysis-induced discordance between glucose metabolic rates measured with radiolabeled fluorodeoxyglucose and glucose

We have developed an autoradiographic method for estimating the oxidative and glycolytic components of local CMRglc (LCMRglc), using sequentially administered [18F]fluorodeoxyglucose (FDG) and [14C]-6-glucose (GLC). FDG-6-phosphate accumulation is proportional to the rate of glucose phosphorylation, which occurs before the divergence of glycolytic (GMg) and oxidative (GMo) glucose metabolism and is therefore related to total cerebral glucose metabolism GMt: GMg + GMo = GMt. With oxidative metabolism, the 14C label of GLC is temporarily retained in Krebs cycle-related substrate pools. We hypothesize that with glycolytic metabolism, however, a significant fraction of the 14C label is lost from the brain via lactate production and efflux from the brain. Thus, cerebral GLC metabolite concentration may be more closely related to GMo than to GMt. If true, the glycolytic metabolic rate will be related to the difference between FDG- and GLC-derived LCMRglc. Thus far, we have studied normal awake rats, rats with limbic activation induced by kainic acid (KA), and rats visually stimulated with 16-Hz flashes. In KA-treated rats, significant discordance between FDG and GLC accumulation, which we attribute to glycolysis, occurred only in activated limbic structures. In visually stimulated rats, significant discordance occurred only in the optic tectum.

Effects of substrate and hypoxia on smooth muscle metabolism and contraction

Suprabasal heat production, oxygen consumption, and lactate production were measured, together with force, in 30-s isometric contractions of longitudinal smooth muscle from rabbit urinary bladder at 27 degrees C. Either glucose or pyruvate was provided as exogenous substrate. Under aerobic conditions with glucose as substrate, force averaged 95 mN/mm2 and heat production 121 mJ/g. Oxygen consumption (0.18 mumol/g) could account for only two-thirds of the total energy expenditure represented as heat production. The remaining one-third was accounted for by aerobic lactate production (0.36 mumol/g). When pyruvate replaced glucose as substrate, both the force developed and the total heat liberated were unchanged. Oxygen consumption, however, increased by approximately 40% (to 0.25 mumol/g) and was able to fully account for the measured heat production. The frequency of spontaneous contractions under aerobic conditions was always reduced in the presence of pyruvate. Under anaerobic conditions force was essentially unaltered, and heat production was only slightly reduced (101 mJ/g) with glucose present. Lactate production increased threefold over that under aerobic conditions. With pyruvate as substrate both force and heat production declined markedly (to less than 5% of the aerobic values). The results indicate that under aerobic conditions and with glucose as substrate, smooth muscle of rabbit urinary bladder generates about one-third of its suprabasal energy requirements through glycolysis and that glycolysis can be further accelerated under anaerobic conditions to provide sufficient energy to sustain contraction. If pyruvate replaces glucose as substrate, the metabolism shifts to being virtually all oxidative, and contraction can no longer be sustained in the absence of oxygen.

Glycolysis and oxidative phosphorylation during activation of the sodium pump in the taenia from guinea-pig caecum

1. In the taenia from the guinea-pig caecum, the relative contribution of glycolysis and oxidative phosphorylation to activation of the Na pump was estimated by measuring changes in the rate of O2 consumption (QO2) and lactate production (Q1act). The Na pump was activated by K readmission following prior treatment with K-free solution. The QO2 and Q1act were both increased significantly by K readmission 60 min after exposure to K-free solution. These changes and the reaccumulation of tissue K were abolished by ouabain (10 microM). Spontaneous mechanical activity ceased in K-free solution and the K-induced increase in QO2 and Q1act was produced before the mechanical activity was restarted. 2. Similar changes were obtained when ouabain (10 microM) was removed after 60 min treatment. It was estimated from the changes in QO2 and Q1act that the oxidative and glycolytic metabolism each supplied about 50% of the total ATP demand for pump activation. 3. In Ca-free solution, the increase in QO2 caused by K readmission was transient and QO2 returned to the previous value in K-free solution in 10-20 min. However, the increase in QO2 was maintained in the absence of glucose. When glucose was replaced with beta-hydroxybutyrate (beta-HB, 11.8 mM), which is metabolized only through oxidative phosphorylation, K readmission also produced a sustained increase in QO2. 4. In glycogen-depleted preparations, K readmission produced little or no increase in QO2 in the absence of substrate or in the presence of glucose. On the other hand, in the presence of beta-HB (11.8 mM), a typical increase in QO2 (about 0.1 mumol min-1 g-1) was observed in response to K readmission. Lactate production was negligible in the absence of substrate or in the presence of beta-HB, but it was significantly increased after K readmission in the presence of glucose. The increase in tissue K content following K readmission was the same in the presence of glucose or beta-HB. 5. In the glycogen-depleted preparations the increase in QO2 caused by beta-HB was dependent on the presence of both Na and K in the medium, and sensitive to ouabain. Furthermore, this response was reversibly suppressed by glucose.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic differences between norepinephrine- and K+-stimulated anococcygeus muscle

The effects of norepinephrine (NE) and K+ stimulation on aerobic glycolysis (JLac), O2 utilization (JO2), and isometric force of isolated rat anococcygeus smooth muscle were investigated to determine the relationship between metabolism and contractility. A large sustained increase in JLac was measured during NE stimulation, but only a small rise in JLac occurred during high-KCl depolarization. JO2, in both cases, correlated with the force response. The difference in JLac could not be explained solely on the lower force output of K+-depolarized muscles or on increased glycogenolysis during NE stimulation, but was perhaps due to activation of energy-requiring processes associated with receptor- rather than depolarization-mediated Ca2+ mobilization. When muscles were stimulated with a K2SO4 (substituted for NaCl) Krebs-Ringer bicarbonate (KRB) rather than a N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered medium, JLac was significantly elevated due to an increased intracellular pH as a result of the low external Cl-. Lowering the pH of KRB or adding the anion exchange inhibitor 4-4'-diisothiocyanostilbene-2,2'disulfonic acid prevented the rise in JLac and also restored the initial fast component of the force response to NE in Cl(-)-free solution.

Energy metabolism and transduction in smooth muscle

Early investigations into the nature of the coupling between energy transduction and metabolism in smooth muscle, particularly from the laboratories of Bülbring and Lundholm, suggested that specific metabolic pathways could independently supply energy for ion transport and actin-myosin interactions. Subsequent work has solidified the concept that oxidative phosphorylation is specifically coupled to tension generation and maintenance, whereas, aerobic glycolysis is not only a vital characteristic of smooth muscle metabolism, but also is likely to be independently coupled to Na-K transport at the plasmalemma. The independence of oxidative and glycolytic metabolism is reflected as a compartmentation of carbohydrate metabolism in the porcine carotid artery. The coupling of these independent metabolic pathways with specific energy utilizing processes, indicates a means by which energy production and transduction can be closely and efficiently regulated. The coupling of glycogenolysis to mitochondrial respiration may have evolved as a direct response to the energetic needs of VSM. That is, the large glycogenolytic response in the initial minutes of stimulation may be necessary to maximize the cellular production of ATP during the presteady state. Likewise, the coupling between aerobic glycolysis and Na-K transport indicates a sensitive and efficient means of coordinating energy metabolism with ion transport at the membrane level. Additionally, the regulation of substrate supply, i.e. glucose transport, also may be closely coordinated with changes in ion transport. One may speculate that alterations in the microenvironment of each compartment can independently regulate intermediary metabolism and therefore allow the cell to quickly and efficiently respond to localized stimuli. Thus, stimulation of Na-K transport could effectively regulate energy production at the membrane level without mobilizing or competing with the energy transduction of other cellular processes. This compartmentation of energy utilization may be highly advantageous, since oxidative metabolism is closely coordinated with mechanical activity and therefore regulation of blood flow. Future investigations will attempt to elucidate which intracellular signals which are responsible for the regulation of these functionally independent compartments of energy metabolism and transduction in VSM. In more general terms, our findings provide a basis from which future questions concerning the regulation of cellular metabolism must be directed. The cellular cytoplasm can no longer be envisioned as a homogeneous compartment, but rather a complex array of functional subcompartments which may be individual

Possibility of metabolic control of membrane excitation

In the guinea pig taenia coli, when glycogen is depleted by repeating Ca-induced contracture in excess K solution containing no glucose, the tension cannot be maintained. The decrease in tension is accompanied by reduction of high energy phosphate compounds and oxygen consumption. When substrate is readmitted to the glycogen-depleted preparation in the presence of 2.4 mM Ca and 20 mM K, the first response is hyperpolarization of the membrane and relaxation, and this is followed by depolarization and development of contracture. The latter response is blocked by verapamil, suggesting that energy supply increases the Ca conductance of the plasma membrane. The early response is considered to be due to activation of electrogenic Ca pump, since this is not affected by ouabain as well as removal of Na and K. ATP produced by substrate readmission is probably preferentially utilized for Ca pump activation to reduce the intracellular Ca. The recovery of tension is likely to be brought about by ATP supply not only to the contractile machinery but also to the plasma membrane to remove inactivation of Ca conductance. It is postulated that as the energy source is depleted, energy consumption is automatically limited by suppressing Ca influx, as a self-defence mechanism. Since beta HB is as effective as glucose in the recovery of these processes, and also in the activation of electrogenic Na pump, the metabolic pathway of oxidative phosphorylation alone can support these functions without a contribution of the glycolytic pathway.

Separation of aerobic glycolysis from oxidative metabolism and contractility in rat anococcygeus muscle

The relationship of aerobic glycolysis (JLac) and O2 utilization (JO2) to contractility and the activity of the Na+-K+ pump was investigated in rat anococcygeus smooth muscle. Removal of extracellular potassium concentration ([K+]o) caused an initial increase followed by a decrease in JLac below the resting rate. Restoration of [K+]o caused JLac to increase, and this response, but not the initial one observed in zero [K+]o, was prevented by ouabain. These changes in JLac were not related to contractility, since a similar response was observed in both phentolamine-treated and chemically denervated muscles that exhibited minimal force and no significant changes in JO2. Tension development of innervated muscles incubated in K+-free medium was related to changes in JO2. A logarithmic relationship between JLac and [K+]o suggested that changes in JLac resulted from effects on glucose transport or utilization secondary to changes in the membrane potential. In stimulated muscles JLac and JO2 were both positively correlated with isometric force and were not affected by ouabain. Separation of JLac from JO2 and force production under some conditions supports the hypothesis that a functional compartmentalization of metabolic pathways exists in anococcygeus muscle.

Effects of glucose removal and readmission on potassium contracture in the guinea-pig taenia coli

The effects of removal and readmission of substrates on the K contracture were investigated in the guinea-pig taenia coli. When, after exposure to excess K in Ca-free and glucose-free medium, the readmission and removal of 2.4 mM-Ca were repeated at regular intervals, the Ca-induced contractions decreased progressively. The decrease was more marked in the late than in the early part of the tension response. The rate of O2 consumption decreased when the normal medium was replaced by glucose-free, Ca-free, excess-K solution, but substantially recovered following Ca readmission. ATP and creatine phosphate contents decreased during the Ca-induced contraction, but recovered partially during the subsequent relaxation in Ca-free solution. The effects of glucose removal were rapidly reversed when glucose or beta-hydroxybutyrate (beta-HB) were readmitted to the bathing solution. In the absence of Ca, readmission and removal of the substrates produced an insignificant change in O2 consumption, but the next Ca contraction was potentiated, the effect being stronger with glucose than beta-HB. When the tonic contraction evoked by 2.4 mM-Ca readmission had been abolished in glucose-free, high-K solution, a rise of the external Ca concentration to 10 mM, or 5 microM-carbachol, still produced a transient contraction. This suggests that the tonic contraction has disappeared partially because of diminished Ca influx. In glycogen-depleted preparations, the depolarization caused by carbachol, or by 20 mM-K, was increased and spike discharge initiated when glucose was readmitted.(ABSTRACT TRUNCATED AT 250 WORDS)

O2 consumption, aerobic glycolysis and tissue phosphagen content during activation of the Na+/K+ pump in rat portal vein

Oxygen consumption, lactate production and tissue contents of ATP, phosphocreatine (PCr) and lactate were measured following readdition of K+ to K+-depleted rat portal veins, in order to study the energy turnover associated with Na+/K+ pumping. During incubation in K+-free medium at 37 degrees C spontaneous contractions disappeared in 10-20 min. Readdition of K+ (5.9 mM) after 40 min K+-free incubation caused hyperpolarization of the cell membrane for the first 5-10 min and then gradual depolarization with return of spontaneous action potentials and contractions by 10-20 min. During the first 4-6 min after K+ readdition aerobic lactate production was about doubled and then gradually returned to the original level (0.17 mumol/min g) at about 20 min. The increase in glycolytic rate was prevented by 1 mM ouabain. In contrast, O2 consumption (in K+-free medium, 0.38 mumol/min g) rose by about 10% when K+ was added and this increase lasted about 5 min. By 8 min after K+ addition the increased glycolysis and oxidative phosphorylation had accounted for each about the same amount of extra ATP generation over that extrapolated from the steady rate before K+ addition. The average total increase in ATP turnover in the first 8 min was 15%. During this period there was no change in the cellular content of ATP, PCr, or extractable ADP. The results indicate that Na+/K+ pumping utilizes a relatively small share of the total energy turnover in the vascular smooth muscle but is to a large extent dependent on aerobic glycolysis and therefore a major site of carbohydrate usage.

Functional compartmentalization of oxidative and glycolytic metabolism in vascular smooth muscle

The metabolism of vascular smooth muscle is characterized by an unusual component of aerobic glycolysis. Lactate production, even under fully oxygenated conditions, is of similar magnitude to the rate of oxygen consumption when compared on a molar basis. Although the underlying mechanisms are unknown, the ratio of glycolytic to oxidative metabolism has been suggested to be an index of vascular myopathy. Measurements of the rate of O2 consumption (JO2), lactate production (Jlac), and isometric force in porcine coronary arteries were made under conditions known to alter both active force (delta Po) and Na+-K+ transport. As previously reported, JO2 was strongly correlated with delta Po; Jlac, however, was correlated with conditions that alter Na+-K+ transport. Under conditions known to inhibit Na+-K+ transport (10(-5) M ouabain, absence of extracellular K+ or Na+), Jlac was inhibited even though delta Po and JO2 were increased. The coupling of Na+-K+ transport with aerobic glycolysis was not dependent on tonicity or the major anion species, nor was it an effect simply on tissue lactate permeability as indicated by studies of tissue lactate content. Metabolic measurements made at similar levels of delta Po indicate that Jlac is markedly inhibited by ouabain, whereas JO2 shows little effect. Thus the unusual aerobic glycolysis observed in vascular smooth muscle appears to be linked to Na+-K+ transport processes and not to some nonspecific metabolic deficiency. Experiments on both systemic and pulmonary arteries from rat, rabbit, dog, and pig indicate that these results are not limited solely to porcine coronary vessels.

The effect of hypoxia on mechanical and electrical properties of smooth muscle from the rat portal vein

The effect of hypoxia on the electrical and mechanical activity of rat portal vein smooth muscle were investigated using intracellular microelectrode technique in combination with contraction force measurements. In control conditions the "resting" potential of the muscle cells was -58.0 +/- 0.1 mV (mean +/- S.E.). Bursts of action potentials, 5-10s long, appeared at regular intervals (2-3/min) in association with phasic contractions. In hypoxia (PO2 at about 10 mm Hg) there was a marked decrease in force and, often, a moderate increase in rate of the spontaneous contractions. Electrically, these changes corresponded to a decrease in length and an increase in frequency of the bursts of action potentials. The general level of membrane polarization and individual action potentials were not affected. In prolonged hypoxia there was a tendency towards dissociation of the electrical and mechanical activities. Increasing [K+]0 or reducing [Na+]0 produced a restoration of the hypoxically depressed force development. Qualitatively similar results with respect to the depression of myogenic activity by hypoxia, and its alleviation in decreased [K+]0 and reduced [Na+]0, were obtained in low Ca (1 mM) solutions. The effects of hypoxia, and their variation with [K+]0 and [Na+]0, could be explained on the basis of pH dependent electro-mechanical uncoupling.

Oxygen consumption and lactate production of the rat portal vein in relation to its contractile activity

Energy turnover in the isolated rat portal vein was investigated by measurement of oxygen consumption (JO2) and lactate production (JLA) under simultaneous recording of mechanical activity. In spontaneous activity under aerobic conditions and at optimal muscle length JO2 and JLA were 0.55 and 0.62 micromol/min X g, respectively, corresponding to an ATP-production of 4.3 micromol/min X G. When muscle length was changed, an approximately linear relation was found between energy turnover and mean isometric tension. The tension-indpendent part of ATP-production was 3.0 micromol/min X g. In Ca2+-free solution the metabolic rate was 20% lower still. JO2 was nearly equal in isometric contractions and in afterloaded isotonic contractions from the same initial muscle length. During a maximal tonic contracture in 5+-depolarized portal vein JO2 increased to about twice that in spontaneous activity. Changes in contracture force by variations in muscle length or in [Ca2+]0 were associated with identical linear relations between JO2 and active tension. This relation was less steep than the corresponding relation for spontaneous activity. The anaerobic lactate production of the portal vein was 2.7 times theaerobic leve. The accelerated glycolysis did not compensate for eliminated oxidative metabolism. Under substrate-free aerobic conditions no lactate was produced by the muscle and compared to the control situation JO2 declined more than could be accounted for by reduced mechanical activity alone. The metabolic turnover rate in relation to isometric tension is high in the rat portal vein compared to that of tonic vascular smooth muscle from larger vessels. This correlates with differences in dyanmic mechanical properties. At comaparable tension levels in the portal vein, the rate of cross-bridge turnover may be higher in spontaneous phasic activity than in sustained contracture.

Microcalorimetric determination of energy expenditure due to active sodium-potassium transport in the soleus muscle and brown adipose tissue of the rat

1. The resting heat production rate (E) of soleus muscles from young rats and brown adipose tissue from adult rats was measured by means of a perfusable heat flux microcalorimeter in the absence and presence of ouabain. In the soleus muscle, the acute response of E to ouabain was compared with the ouabain-suppressible components of 22Na-efflux and 42K-influx. 2. In standard Krebs-Ringer bicarbonate buffer, ouabain (10(-3)M) induced an immediate but transient decrease in E of around 5%. Both in muscle and adipose tissue this was followed by a progressive rise in heat production rate. 3. When the medium was enriched with Mg (10 mM), ouabain produced a sustained decrease in E of the same magnitude as in the standard medium and the secondary rise was less marked or abolished. Under these conditions, in the soleus muscle, ouabain inhibited E by 5% (i.e. by 1-76 +/- 0-22 mcal.g wet wt.-1.min-1), 22Na-efflux by 58% (0-187 +/- 0-013 micronmole. g wet wt.-1.min-1) and 42K-influx by 34% (0-132 +/- 0-028 micronmole. g wet wt.-1.min-1). 4. When the muscles were loaded with Na by pre-incubation in K-free Mg-enriched medium, the addition of K (3mM) induced an immediate ouabain-suppressible increase in E of 2-98 +/- 0-33 mcal. g wet wt.-1.min-1 and a concomitant stimulation of 22Na-efflux of 0-388 +/- 0-136 micronmole. g wet wt.-1.min-1. 5. Maximum Na/ATP ratios for the active Na-K transport process were computed, with no assumption as to the in vivo free energy of ATP hydrolysis. These were 2-1, 1-9 and 2-3 under the conditions described in paragraphs (2), (3) and (4) respectively. 6. The calculated reversible thermodynamic work associated with active Na-K transport corresponded to 34% of the measured ouabain-induced decrease in E. On the premise that the maximum efficiency of the cellular energy conservation processes is 65%, this estimate indicates that the minimum energetic efficiency of ATP utilization by the active Na-K transport process in mammalian muscle is 52%.

Active and passive Ca2+ fluxes across cell membranes of the guinea-pig taenia coli

The exchange of Ca between the extracellular fluid and the cellular compartment has been investigated in smooth muscle cells of taenia coli. It was found that during the initial phase of metabolic depletion by DNP + IAA, the net inwards flux of Ca amounts to 0.02 pmol cm(-2)-sec(-1). This increase might be proportional to the physiological calcium leak. The study of the relation between the inwardly directed Na gradient and the cellular Ca content has revealed that this Na gradient exerts no effect during prolonged exposure to K-free solution and a very limited effect during exposure to Na-deficient solutions. The cellular 45Ca release induced by metabolic inhibition is not affected by substituting Li or choline for Na. The supplementary calcium which enters the cells during exposure to a solution at low temperature is extruded on returning to a solution at 35 degrees C, even if the Na gradient is reversed. This finding and the effects of metabolic inhibition indicate that Ca extrusion in smooth muscle cells is a process which depends on metabolism and which is not affected by the inwardly directed Na gradient.