Smooth muscle energetics and theories of cross-bridge regulation

The energetics of smooth muscle is characterized by low tension cost (rate of ATP utilization per isometric force/cross-section area), ranging from 100- to 500-fold less than skeletal muscle. The efficiency (ATP usage per work) of smooth muscle, although less well documented, is also somewhat (4-fold) less than skeletal muscle. Another well-known characteristic of smooth muscle is the linear relation between the steady-state of ATP utilization (JATP) and isometric force. Recently, Murphy and colleagues [C.-M. Hai and R. A. Murphy. Am. J. Physiol. 254 (Cell Physiol. 23) C99-C106, 1988] have put forth a kinetic model of cross-bridge regulation that predicts the time course of stress and myosin light chain phosphorylation (MLC-Pi). The energetics consequences of this model, in brief, are that the low tension cost is partly attributed to a slow detachment rate of the myosin cross bridge when dephosphorylated when attached to actin ("latch state"), whereas the lower efficiency is ascribed to a high rate of myosin phosphorylation-dephosphorylation inherent to a fit of data to this kinetic scheme. This latter corollary is somewhat controversial in light of current interpretations of smooth muscle energetics data. Using SCoP software (National Biomedical Simulation Resource, Duke University), we tested this model in terms of fitting existing data with respect to 1) is a high myosin-dephosphorylation adenosine triphosphatase (ATPase) necessary to fit the available data on the time course of stress and MLC-Pi?; and 2) can this model predict the observed linear relation between the steady-state rate of ATP hydrolysis (JATP) and isometric force?(ABSTRACT TRUNCATED AT 250 WORDS)

Glycogen metabolism during tension generation and maintenance in vascular smooth muscle

To study the regulation of glycogen utilization in vascular smooth muscle, we measured the content of glycogen and glucose 6-phosphate and the activity of the glycogen phosphorylase and glycogen debrancher enzymes in porcine carotid artery. During active contraction, the rates of glycogen phosphorylase and glycogenolysis were as high as expected. Despite this, glycogen content did not decrease to less than approximately 50% of control levels even after sustained contractions. The activity of glycogen debrancher enzyme was found to be limiting glycogen utilization at this point. Although glycogenolysis is closely coordinated with increases in oxidative metabolism concomitant with active contraction, the maximal level of tension obtained after stimulation was not substantially reduced under conditions where glycogen debrancher enzyme was limiting glycogen utilization. On the other hand, the rate of tension generation was increased in these tissues. Thus glycogen utilization is not necessary for maximal force generation per se, but may influence other muscle contractile properties. Finally, during steady-state tension maintenance, glycogen utilization is likely to be regulated by the intracellular concentrations of metabolic intermediates (glucose, glucose 6-phosphate), as it is in skeletal muscle.

Preferential support of Ca2+ uptake in smooth muscle plasma membrane vesicles by an endogenous glycolytic cascade

Studies of intact smooth muscle have suggested that its anomalous aerobic lactate production may reflect an intracellular compartmentation of glycolytic enzyme cascades designed to support specific exergonic processes. In particular, we have postulated a membrane-associated glycolytic cascade that preferentially supports the ATP requirements of membrane functions. We tested this hypothesis by using a smooth muscle plasma membrane fraction (PMV) purified for calcium pump activity. We show that glycolytic enzymes are endogenous in PMV and can produce NADH, ATP, and lactate from fructose 1,6-diphosphate in the presence of glycolytic cofactors. This glycolytic cascade can fuel the calcium pump despite the presence of an ATP trap that eliminated calcium uptake fueled by exogenously added ATP. This plasma membrane glycolytic cascade is coupled to calcium pump function in a tissue with both oxidative and glycolytic metabolism. Thus coupling of metabolic cascades with the specific processes they subserve may be a more general feature of cellular organization than was previously thought.

A calmodulin-binding peptide relaxes skinned muscle from guinea-pig taenia coli

During smooth muscle activation the calcium calmodulin complex interacts with myosin light chain kinase (MLCK) whereby activating it. A synthetic peptide analogue (RS20) corresponding to the calmodulin recognition sequence of MLCK has been synthesized and previously found to inhibit the calmodulin stimulated light chain kinase activity. Here we studied the effect of this peptide on skinned fibers from guinea pig taenia coli. Maximal contractions induced by 30 microM Ca2+ at 0.1 microM calmodulin could be completely relaxed by the peptide at 1 microM. The inhibitory effect was accompanied by partial dephosphorylation only of the regulatory myosin light chain. Relaxation could be reversed by addition of calmodulin which also increased the extent of light chain phosphorylation. The calmodulin concentration required for reversing the inhibition depended on the concentration of the inhibitory peptide suggesting that the peptide competed with MLCK for the calmodulin binding site. As the calcium-calmodulin-peptide mixture constitutes a calmodulin buffer, our results suggest, that the peptide is a calmodulin antagonist unique in terms of its potency and that less than nanomolar concentrations of free calmodulin may be required for inducing smooth muscle contractions.

Role of magnesium in activation of smooth muscle

We studied the effects of Mg2+-free solutions on isometric force (F0) and unloaded shortening velocity (Vus) in contractions elicited by Ca2+ or by ATP after thiophosphorylation by adenosine 5'-O-(3-thiotriphosphate (ATP gamma S) in chemically skinned guinea pig taenia coli smooth muscle. In Mg2+-free solutions, increasing Ca2+ did not increase Fo above resting levels. At the peak of a control contraction elicited by Ca2+, transfer to Mg2+-free (but Ca2+-containing) solutions resulted in a rapid relaxation and concomitant dephosphorylation of myosin. After ATP gamma S, a contracture required neither Mg2+ nor Ca2+ in the solutions for control levels of Fo. Vus in the Mg2+-free solutions after ATP gamma S was approximately 50% of control and could be restored to near control levels by addition of Mg2+ but not Ca2+. After ATP gamma S, pretreatment with 4 mM EDTA and contracture in 0.1 mM EDTA-containing solutions decreased Fo to 70-80% of control and Vus to 50-60% of control. Our results suggest that the relatively high requirement for Mg2+ for contraction in skinned smooth muscle largely reflects the Mg2+ dependence of myosin kinase and not for actin-myosin interaction. The dependence of Fo on Mg2+ (in the presence of excess ATP) in taenia coli is less than that reported for skeletal muscle. Appreciable force can be maintained with no added Mg2+ in the presence of 4 mMEDTA, and thus it appears that ATP4- can be a substrate for contraction after ATP gamma S treatment. In addition, our data imply that any Ca2+-dependent regulatory mechanism that does not involve myosin phosphorylation/dephosphorylation, if present, requires Mg2+ for expression.

Dependence of force, velocity, and O2 consumption on [Ca2+]o in porcine carotid artery

The relationship between extracellular calcium concentration ([Ca2+]o), isometric force (Fo), unloaded shortening velocity (Vus), and the rate of ATP utilization (JATP) were studied in vascular smooth muscle (VSM) during the steady state of an isometric contraction at 37 degrees C. Experiments were conducted on porcine carotid artery media strip and ring preparations that were stimulated with 109 mM KCl substituted for NaCl. Unloaded shortening velocity was estimated by the "slack" test method. Both Fo and Vus were dependent on [Ca2+]o. Vus at 7.5 mM [Ca2+]o was 1.7 times greater than Vus at 1.6 mM [Ca2+]o. The difference in force at these two Ca2+ concentrations was more variable than Vus, but in general was less than the change in Vus. The rate of ATP utilization was assessed from steady-state measurements of tissue O2 consumption. Increasing [Ca2+]o in the range of 0.15-7.5 mM resulted in an increase in both JATP and Fo. The relation between JATP and Fo was nonlinear with JATP increasing proportionately more than Fo between 1.6 and 7.5 mM Ca2+. The calcium-dependent increase in JATP appears to be primarily related to contractile protein interaction, since the effect of [Ca2+]o on JATP was substantially reduced during stimulation at short muscle lengths (Lmin), where tension development is absent. The economy of tension maintenance was 3.3. times greater when measured in the [Ca2+]o range of 0.15 and 1.6 mM than when measured at 1.6 to 7.5 mM [Ca2+]o. These data indicate that [Ca2+]o may regulate both the number and the rate of cycling of cross bridges in porcine carotid artery.

Compartmentation of carbohydrate metabolism in vascular smooth muscle

In vascular smooth muscle, oxidative phosphorylation and glycolysis are independently regulated. Previous studies indicated that the independent regulation of these pathways was related to a compartmentation of carbohydrate metabolism. To further study carbohydrate metabolism, glucose transport and the incorporation of radiolabel from glucose into glycogen and lactate were measured after the oxidative and glycolytic pathways were independently altered. Ouabain stimulated mechanical activity, oxygen consumption, and glycogenolysis, whereas lactate production was decreased. Although glycogenolysis was substantial, glucose was the only substrate for lactate, indicating that intermediates derived from glycogen do not mix with those from glucose uptake. Thus glycogenolysis and glycolysis are carried out by independent enzymatic pathways. Insulin-stimulated lactate production and glucose transport without affecting the other parameters. Again, lactate was produced only from glucose. Phenytoin decreased isometric tension and oxygen consumption, whereas stimulating lactate production and glycogenolysis. Glycogen was the primary substrate for the lactate produced. Our findings indicate that the compartmentation of substrate utilization is ascribable to the coordination of glycogenolysis with increases in oxygen consumption and the coupling of glycolysis to the Na-K-adenosine triphosphatase. The coupling of independent energy providing pathways to specific endergonic processes indicates a mechanism by which cellular energetic efficiency may be optimized.

Evaluation of the Microbial Quality of Raw Milk 1

Commercial evaluation of the microbial quality of raw milk presents a major challenge, and new methods are burdened by being compared to imprecise presently used standard methods. Extensive comparisons in commercial and research laboratory environments were made using a method that involved direct enumeration of single cells in comparison to colony forming units. The correlations were from 0.50 to 0.99 depending on treatment of the data. Repetition of all tests on milk from individual farms indicated that inherent variation in quality at the farm, sampling, testing, and evaluating the results showed the extreme inadequacy of the presently established methods of grading raw milk. More frequent tests with appropriate averaging would improve the likelihood of correct decisions on quality grade.

Compartmentation of carbohydrate metabolism in vascular smooth muscle: evidence for at least two functionally independent pools of glucose 6-phosphate

Previous work has indicated that there are at least two functionally independent Embden-Meyerhof pathways within the vascular smooth muscle of porcine carotid artery. We tested this hypothesis by analyzing the isotopic equilibrium between medium glucose and intracellular glucose 6-phosphate under basal conditions and after 30 min of mechanical activation, during which time the rate of glycogenolysis has been found to be substantial. Under basal conditions, the specific activity of glucose 6-phosphate equilibrated to a level which was not in isotopic equilibrium with medium glucose suggesting that there is a significant pool of glucose 6-phosphate which is not readily accessible to medium glucose. After 15 min of mechanical activation, the specific activity of intracellular glucose 6-phosphate was found to decrease significantly from its apparent steady-state distribution, indicating that glycogen was likely to be a significant source for glucose 6-phosphate. Since the specific activity of lactate was unaltered from its equilibrium distribution under similar stimulus conditions, these findings substantiate the existence of at least two independent pools of glucose 6-phosphate.

Effects of forskolin and cyclic nucleotides on isometric force in rat aorta

The present study was undertaken to determine the extent to which cyclic nucleotide-induced relaxation in the intact rat aorta is mediated at the level of the contractile system. The relaxant effects of the cyclic nucleotide analogues [8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) and dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP)] and forskolin were examined in both the intact vessel and a Triton X-100-skinned preparation of rat thoracic aorta. Relaxation of a norepinephrine-induced contraction was essentially complete 30 min after the addition of 50 microM 8-BrcGMP [% relaxation = 87.2 +/- 4.4% (n = 4)], 100 microM DBcAMP [98.2 +/- 1.2% (n = 4)], and 1 microM forskolin [107.0 +/- 3.3% (n = 5)]. These same doses were ineffective in relaxing precontracted skinned rat aortic rings compared with the relaxation achieved in the intact vessel. The largest relaxation in the skinned aortas was achieved with the addition of 1 microM forskolin [17.4 +/- 1.5% (n = 4)]. The addition of catalytic subunit of cAMP-dependent protein kinase had no effect on isometric tension in the precontracted skinned aorta. Preincubation with the cyclic nucleotide analogues or forskolin in a low-Ca2+ solution (pCa less than 8) was also ineffective in inhibiting subsequent isometric tension development. Our results suggest that only a very small fraction of the relaxation with cyclic nucleotides and forskolin in the intact rat aorta is due to the action of these agents at the level of the contractile system.

Eicosonoid metabolism and beta-adrenergic mechanisms in coronary arterial smooth muscle: potential compartmentation of cAMP

Beta-Adrenergic relaxation in bovine coronary arteries is enhanced by inhibition of eicosonoid metabolism and inhibited by its stimulation. We investigated the interaction between eicosonoid metabolism and beta-adrenergic mechanisms by studying the effect of perturbations of eicosonoid metabolism on vascular adenosine 3',5'-monophosphate (cAMP) content and the cAMP-dependent relaxation of isometric force and activation of glycogen phosphorylase. KCl (35 mM) elicited a contraction, activated phosphorylase, and slightly decreased cAMP content. Isoproterenol (10(-7) M) relaxed the KCl contraction, further increased phosphorylase activity, and increased cAMP. Neither indomethacin (5 X 10(-6) M) nor arachidonic acid (3 X 10(-5) M) affected the KCl contraction, but arachidonic acid increased both cAMP and phosphorylase activity and indomethacin decreased cAMP. Indomethacin potentiated the relaxation induced by isoproterenol but inhibited the activation of phosphorylase and had no effect on the isoproterenol-induced increase in cAMP. Arachidonic acid, on the other hand, inhibited the isoproterenol-induced relaxation but potentiated both the increases of phosphorylase activity and cAMP. Thus neither relaxation nor phosphorylase activity was related in a straightforward manner to the total cAMP content. A direct relation between cAMP, relaxation, and phosphorylase can be reconciled with the antiparallel effects of alterations of eicosonoid metabolism observed in this study by a proposed model in which the effects of cAMP are assumed to be functionally compartmentalized.

Isomyosin transitions in ventricles of aldosterone-salt hypertensive rats

The isomyosin composition in left and right ventricles from aldosterone-salt-treated hypertensive rats and from vehicle-infused and aldosterone-infused normotensive control rats was compared. A significant incremental increase (20%) in the percentage of V3 isomyosin and parallel decrease in the percentage of V1 isomyosin occurred in both left and right ventricles from aldosterone-salt-treated animals compared with those in normotensive vehicle-infused controls. No change in the ventricular isomyosin distribution was observed in animals infused with aldosterone without salt, which indicates that aldosterone does not directly affect the ventricular isomyosin composition. The changes in left ventricular isomyosin composition were accompanied by significant left ventricular hypertrophy (38%; p less than 0.05), whereas no hypertrophy was observed in the right ventricle. Plasma thyroxine levels were significantly lower in aldosterone-salt-treated rats (3.7 +/- 0.6 micrograms/dl; p less than 0.05) than in normotensive vehicle-infused (6.0 +/- 0.7 micrograms/dl) or aldosterone-infused (6.7 +/- 0.3 micrograms/dl) controls. These results indicate that factors such as alterations in thyroid status or a volume overload component of this hypertensive model, in addition to increased systolic blood pressure, may contribute to a biventricular shift in isomyosin composition in the aldosterone-salt model of hypertension.

Modulation of beta adrenergic responsiveness by arachidonic acid metabolites in isolated bovine coronary arteries

The present study was designed to determine whether interference with endogenous arachidonic acid metabolism or exogenously administered cyclooxygenase and lipoxygenase products affects the relaxation of bovine coronary artery in response to isoproterenol. Rings of bovine coronary artery were suspended for isometric tension recordings in organ chambers filled with Krebs-Ringer bicarbonate solution (37 degrees C) gassed with 95% O2-5% CO2 (pH 7.4). In depolarized coronary artery rings (35 mM KCI) isoproterenol induced a dose-dependent relaxation, which was significantly augmented by the cyclooxygenase inhibitor indomethacin and depressed by arachidonic acid. The mixed lipoxygenase/cyclooxygenase inhibitor phenidone or the lipoxygenase products leukotriene D4 and C4 did not affect beta adrenergic responsiveness. Phenidone antagonized the facilitatory action of indomethacin. Exogenous arachidonic acid in the presence of indomethacin and phenidone depressed the relaxation induced by isoproterenol. Prostacyclin and prostaglandin E2 reduced beta adrenergic responsiveness, which was not affected by indomethacin. The data suggest that arachidonic acid depresses beta adrenergic responsiveness in the bovine coronary artery via cyclooxygenase and some noncyclooxygenase, nonlipoxygenase metabolites. Lipoxygenase products, other than leukotrienes D and C, may have a facilitatory action.

Mechanical and biochemical characterization of the contraction elicited by a calcium-independent myosin light chain kinase in chemically skinned smooth muscle

The contraction induced by a Ca2+-independent myosin light chain kinase (MLCK-) was characterized in terms of isometric force (Fo), immediate elastic recoil (SE), unloaded shortening velocity (Vus), shortening under a constant load and ATPase activity of chemically skinned smooth muscle preparations. These parameters were compared to those measured in a Ca2+ -induced contraction to assess the nature of cross bridge interaction in the MLCK-induced contraction. Fo developed in chicken gizzard fibers as well as SE were similar in contractions elicited by either agent. Vus in the contraction induced by MLCK-(0.36 mg/ml) was similar though averaged 39.3 +/- 8.9% less than Vus induced by Ca2+ (1.6 X 10(-6) M) in the control fibers. Addition of Ca2+ (1.6 X 10(-6) M) to a contraction induced by MLCK-resulted in small increases in both Fo and Vus. Shortening under a constant load was similar for both types of contractions. The contraction induced by MLCK-was accompanied by an increased rate of ATP hydrolysis. The MLCK-induced contraction is thus kinetically similar though not identical to a contraction induced by Ca2+. We conclude that with respect to actin-myosin interaction, MLCK-and Ca2+ -induced contractions are similar.

Characterization of a coronary vasoconstrictor produced by cultured endothelial cells

The vasoactive effects of media obtained from bovine aortic endothelial cells (EC) in culture were directly tested on isolated rings of the porcine left anterior descending coronary artery. Increasing concentrations of EC-conditioned culture media resulted in progressive dose-dependent increments in isometric tension in porcine, bovine, and canine coronary arteries; the response did not require an intact endothelium. Control (nonconditioned) media and that conditioned by fibroblasts or vascular smooth muscle cells in culture had negligible effects on vessel tone. The vasoconstriction required extracellular Ca2+ and was unaffected by inhibitors of cyclooxygenase and lipoxygenase or by antagonists to the alpha- or beta-adrenergic, serotonergic, histaminergic, or cholinergic receptor systems. Calibrated gel filtration of the media indicated a molecular weight of 8,500 for the vasoactive factor; treatment of the EC-conditioned media with either sodium dodecyl sulfate, trypsin, alkali, or with acid hydrolysis completely abolished the vasoconstrictive effect. These findings and others now provide evidence for the existence of an EC-derived polypeptide vasoconstrictor that may be important in the regulation of vascular smooth muscle contractility.

Calcium sensitivity of isometric force in intact and chemically skinned aortas during the development of aldosterone-salt hypertension in the rat

We investigated the role of altered vascular calcium handling in the development of aldosterone-salt hypertension in the rat. The calcium sensitivity of isometric force in response to 50 mM KCl was compared in aortic rings from control and aldosterone-hypertensive rats. Over the entire range of calcium concentrations studied, responses in aortas from the hypertensives were significantly depressed compared to controls [ED50: aldosterone-hypertensive rats (n = 6), 0.739 +/- 0.137; controls (n = 7), 0.141 +/- 0.021 mM; P less than 0.001]. However, calcium sensitivity in response to 1 microM norepinephrine was similar in aortas from both hypertensives and controls [ED50: aldosterone-hypertensive rats (n = 7), 0.196 +/- 0.022; controls (n = 7), 0.180 +/- 0.024 mM]. The calcium sensitivity of Triton X-100 skinned aortic rings from aldosterone-hypertensive rats was likewise not significantly different from sensitivity in controls [ED50: aldosterone-hypertensive rats (n = 9), 3.61 X 10(-7) +/- 0.57; controls (n = 8), 3.89 X 10(-7) +/- 0.64 M]. Therefore, the observed decrease in calcium sensitivity in response to membrane depolarization in aortas from aldosterone-hypertensive rats probably is not due to a change in calcium sensitivity of the contractile system itself. The time course for development of changes in calcium handling in vessels from the aldosterone-hypertensive rats was found to be quite different from the time course for changes in monovalent ion metabolism. Whereas increases in monovalent ion permeability reportedly appear as early as one week after the start of aldosterone-salt treatment, significant alterations in calcium handling were not apparent until after four weeks of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal relationships between isometric force, phosphorylase, and protein kinase activities in vascular smooth muscle

Vascular smooth muscle contractility is tightly coupled to ATP production by intermediary metabolism. To elucidate mechanisms underlying coordination of metabolism and contractility we studied the time course of isometric force, and the activation of phosphorylase and cAMP-dependent protein kinases during stimulation of bovine coronary arterial strips with KCl. Isometric force reached a maximum after 10 min of exposure to 30 mM KCl (ED90) and was sustained throughout the subsequent 20-min period of contraction. In contrast, activation of phosphorylase was biphasic: enzymic activity reached a maximum (176 +/- 10% of control) after 3 min of contraction and then, though remaining above control, activity declined to a lower level (135 +/- 7% of control). However, no change occurred in the activity ratios for cAMP-dependent protein kinase assessed in either the presence (type II isozyme) or absence (type I isozyme) of 0.5 M NaCl. These data suggest that the activation of phosphorylase during K+-induced contraction is independent of the cAMP system. The biphasic activation of phosphorylase may reflect transient changes in the intracellular concentration of Ca2+ or the activation of a phosphatase(s) during the response.

Alterations of arachidonic acid metabolism modulate adenosine relaxation in isolated coronary arteries

The possible modulation of adenosine-induced relaxation by endogenous prostaglandins was studied in isolated porcine, bovine and canine coronary arteries. Artery rings were mounted for isometric tension recording in organ bath filled with Krebs-Ringer-bicarbonate solution (37 degrees C). Increasing concentrations (10(-6)-10(-4) M) of adenosine relaxed contractions induced by 35 mM KCl. The cyclo-oxygenase inhibitor indomethacin (5 X 10(-6) M) significantly augmented the relaxations in response to adenosine in coronary arteries isolated from all three species. In the porcine coronary artery (which was most sensitive to exogenous adenosine) the mixed lipoxygenase/cyclo-oxygenase inhibitor phenidone augmented, while exogenous arachidonic acid (3 X 10(-5) M) or a decrease in bath O2-level from 95 to 20% depressed adenosine-induced relaxations. Indomethacin completely prevented the inhibitory effect of decreasing bath O2, but only partially antagonized the suppression caused by arachidonic acid. The data demonstrate that arachidonic acid and its cyclo-oxygenase metabolites modulate (depress) adenosine responsiveness in isolated coronary arteries.

Two distinct effects of oxygen on vascular tone in isolated porcine coronary arteries

The relation between PO2 and vessel tone was studied in isolated porcine left descending coronary artery rings. Porcine left descending coronary artery mounted isometrically and equilibrated in Krebs-bicarbonate solution (37 degrees C, pH 7.4, when gassed with 95% oxygen + 5% carbon dioxide) exhibited spontaneous basal tone. Decreasing bath PO2 to 40%, 20%, and 12% elicited sustained increases in basal tension which were reversible, ranging between 10% and 20% of the contraction induced by 40 mM potassium chloride. Further decreases in PO2 to near zero (anoxia) resulted in relaxation to baseline. Cyclooxygenase inhibitors indomethacin (5.5 X 10(-6) M), aspirin (5 X 10(-5) M), and meclofenamate (10(-5) M) decreased vascular tone and totally prevented coronary vasoconstriction induced by lowering bath PO2 to 12% or 40% but did not affect anoxic vasorelaxation. Neither basal tone nor the vasoconstriction induced by decreases in bath PO2 were influenced by the antihistaminergic drug pyribenzamine (10(5) M) or by the alpha-adrenergic blocker phentolamine (10(-6]. Isoproterenol (10(-9) to 10(-8) M) or an elevation of the bath potassium concentration from 5.9 to 11 mM significantly augmented coronary vasoconstriction induced by lowering bath PO2 from 95% to 40%. Elevation of the bath potassium chloride concentration to 40 mM further increased isometric force but inhibited the vasoconstriction in response to decreasing PO2 from 95% to 40%. Anoxia relaxed contractions induced by 40 mM potassium chloride, histamine, or ouabain. The data suggest the existence of two distinct oxygen-sensitive mechanisms in porcine coronary arteries, both of which regulate vascular tone. One is activated at relative high PO2 values (10-40%), and the vasoconstriction induced by this mechanism is mediated by vascular prostaglandin synthesis. The other is expressed at low PO2 values (near zero), and the depression of mechanical activity by this mechanism may be related to limitation of oxidative energy metabolism. The first mechanism can be augmented by beta-adrenoceptor stimulation indicative of an interaction between vascular prostaglandin synthesis and beta-adrenergic mechanisms in the coronary artery wall.

Glucose uptake in porcine carotid artery: relation to alterations in active Na+-K+ transport

In vascular smooth muscle (VSM), aerobic lactate production can account for as much as 30% of the basal rate of ATP production. Generally, glucose transport is thought to be the rate-limiting step for glycolysis in unstimulated VSM. In this work we provide evidence that the intracellular concentration of glucose is negligible in porcine carotid artery, indicating that glucose transport is rate limiting for its utilization. Since aerobic glycolysis appears to be coupled to active Na+-K+ transport in this tissue, we examined the effects of altering ion transport on glucose transport. Glucose uptake and 3-O-methyl-D-glucose transport were accelerated, though intracellular glucose remained negligible in artery rings that were incubated with 80 mM KCl, which is known to stimulate active Na+-K+ transport, as well as aerobic glycolysis and mechanical activity. On the other hand, inhibitors of active Na+-K+ transport (ouabain, Na+-free media), which also elicit mechanical activity, had little effect on sugar transport but significantly inhibited aerobic glycolysis and caused an intracellular accumulation of glucose. Our results indicate the following: 1) that glucose transport is regulated in VSM; 2) that the intracellular concentration of Ca2+ does not appear to regulate sugar transport, since changes in glucose and 3-O-methyl-D-glucose transport are not always seen in association with increased mechanical activity, and 3) that the decrease in aerobic glycolysis associated with the inhibition of active Na+-K+ transport is not due to a decrease in glucose transport but rather to an inhibition of glucose utilization.

Metabolic and mechanical properties of aortas from aldosterone-salt hypertensive rats

Aortas from aldosterone-salt treated hypertensive rats and vehicle-infused normotensive controls were compared with respect to mechanical properties, metabolism, and energy turnover. Passive wall stress at any given circumference was slightly higher in aortas from the hypertensive rats, whereas active isometric force normalized to cross-sectional area was similar in vessels from the two groups at their respective optimal circumference for active tension developments [hypertensive rats: (n = 14), 19.73 +/- 1.98; controls: (n = 13), 22.57 +/- 2.13 mN/mm2). Metabolic parameters were measured with the aortas held at optimal circumference for active tension development, the optimal length for tension development (n = 6), 5.67 +/- 0.17 and (n = 7), 5.29 +/- 0.18 mm for control and aldosterone-salt groups, respectively. Basal oxygen consumption rate was elevated significantly in aortas from the hypertensive rats [(n = 14), 0.457 +/- 0.026 vs. (n = 13), 0.267 +/- 0.028 mumol/min per g; P less than 0.001]. Under resting conditions, lactate production rate similar in aortas from the two groups [hypertensive rats: (n = 20), 0.129 +/- 0.010; controls: (n = 21), 0.112 +/- 0.008 mumol/min per g]. Upon activation with added KCl, the rate of oxygen consumption and lactate production increased with tension development in both groups, but the stimulated rate of oxygen consumption was higher in hypertensives compared to controls [(n = 14), 0.580 +/- 0.031 vs. (n = 13), 0.441 +/- 0.049 mumol/min per g; P less than 0.025]. The stimulated rates of lactate production were similar in the two groups [hypertensive rats: (n = 20), 0.172 +/- 0.009; controls: (n = 21), 0.118 +/- 0.009].(ABSTRACT TRUNCATED AT 250 WORDS)

O2-sensitivity of beta adrenergic responsiveness in isolated bovine and porcine coronary arteries

The relation between pO2 and beta adrenergic responsiveness was studied in isolated bovine and porcine coronary artery rings. Isoproterenol elicited a concentration-dependent relaxation of bovine and porcine coronary artery rings precontracted with KCI (2 X 10(-2) M) or histamine (10(-6) M); beta adrenergic responsiveness was significantly lower in K+-depolarized coronary arteries. A decrease of bath pO2 from 95 to 40% significantly reduced beta adrenergic responsiveness in both coronary preparations precontracted with either KCI or histamine. Similarly, exogenous arachidonic acid (3 X 10(-6) to 3 X 10(-5) M) depressed isoproterenol-induced relaxations in both tissues. Indomethacin (5 X 10(-6) M) augmented beta adrenergic responsiveness in the presence of 95% O2 and prevented the inhibitory effects of the decrease in bath pO2 and arachidonic acid in both preparations. The experimental data suggest that the demonstrated O2-sensitivity of beta adrenergic responsiveness is mediated by vascular prostaglandin synthesis in isolated large coronary arteries.

Dissociation of phosphorylase a activation and contractile activity in rat portal vein

Isometric force, glycogen phosphorylase activity and lactate production were measured under conditions known to alter intracellular Ca2+ and cAMP to assess the role of these messengers in the coordination of metabolism with contractility in rat portal vein. Total phosphorylase (a + b) activity, was independent of treatment. The activity ratio phosphorylase activity ratio in the presence of isoproterenol and papaverine was dependent on or high-K+ medium, and 0.57 after 20 min treatment with 10(-5) M isoproterenol + 10(-4) M papaverine. Under both of these conditions the muscle was totally relaxed. The phosphorylase activity ratio in the presence of isoproterenol and papaverine was dependent on extracellular Ca2+, both in normal and depolarizing medium. This suggests a lower Ca2+ sensitivity of the contractile than the phosphorylase system under these conditions, known to be associated with raised intracellular cAMP. During spontaneous activity and high-K+ induced contractures phosphorylase activity was increased compared to the relaxed state in Ca2+-free medium. A high level of phosphorylase activity (0.48) was elicited by the addition of 300 mM sucrose, which induces a contracture in Ca2+-free medium. Lactate production was in general parallel to phosphorylase activity, except for a relative increase in anoxia. The results suggest that in the intact cell the Ca2+-mediated linkage of contraction and phosphorylase may be modified by cAMP changing the Ca2+ sensitivities of the two systems in opposite directions.

Energetics of isometric contraction in porcine carotid artery

The rate of ATP utilization ( JATP ) and unloaded shortening velocity ( Vus ) of porcine carotid artery were studied during tension development and maintenance to assess the importance of mechanisms proposed to account for its high economy of tension maintenance. Previous work from our laboratory established that the rate of O2 consumption ( JO2 ) can be used to measure rates of tissue phosphagen utilization as early as 30 s after initiation of contraction. Studies at 37 degrees C of tissue JO2 indicated the following. 1) The basal JO2 was 0.069 +/- 0.017 mumol O2 X min-1 X g-1) (n = 10). During steady-state tension maintenance suprabasal JO2 was 0.079 +/- 0.012 mumol O2 X min-1 X g-1 for tissues stimulated with KCl (n = 11) and 0.213 +/- 0.022 (n = 3) for tissues stimulated with KCl + histamine. 2) For both stimulation conditions an initial elevated (peak) suprabasal JO2 was observed that correlated with the rate of tension development. The peak suprabasal JO2 was approximately twice the steady-state suprabasal JO2 measured 10 min after stimulation. 3) There was no significant change in JO2 during tension redevelopment after a rapid length step at 15-20 min after stimulation (n = 4). Studies of Vus indicated the following. 1) During steady-state tension maintenance Vus was 0.008 +/- 0.001 tissue lengths/s for tissues stimulated with KCl (n = 13) tissue lengths/s for tissues stimulated with KCl + histamine (n = 6). 2) An initial twofold peak in Vus was observed at 0.25-0.5 min in that decreased to steady-state levels by 4 min after KCl stimulation, whereas minor decreases in Vus still occurred until 10-20 min after KCl + histamine stimulation. 3) The measured transient in Vus , although biphasic, does not temporally correlate with the transient in JO2 even after the JO2 time course was corrected for delays resulting from O2 diffusion and the electrode response time. The mechanism underlying the observed maximum change of two- to fourfold in both JATP and Vus is not sufficient to account for the high economy of tension maintenance.

Vascular smooth muscle energetics

ATP utilization (delta approximately P) during an isometric contraction has been studied in terms of both measurements of oxygen consumption and lactate production as well as of the tissue nucleotide and metabolite levels. The contribution of breakdown of preformed ATP and phosphocreatine (PCr) pools to delta approximately P during contraction is minor compared to that made by metabolic synthesis of ATP. For tonic vascular smooth muscle (VSM), in fact, no change in ATP or PCr from resting levels can be measured. In contrast to amphibian skeletal muscle, a P:O of 3 can be demonstrated in VSM. In both tonic and phasic VASM, delta approximately P is biphasic with contraction duration, attaining a maximal value before that of isometric force and declining to a steady-state value approximately 60% of the maximal suprabasal rate during the maintenance of constant isometric force. The steady-state rate of ATP utilization per unit force maintained increases with extracellular Ca2+. Both the pre-steady-state temporal dependence and the steady-state dependence on Ca2+ are consistent with the hypothesis that myosin phosphorylation modulates the cross-bridge cycle rates. VSM metabolism, when viewed in terms of ATP synthesis, is primarily oxidative. However, even under fully oxygenated conditions, lactate is the major end product of glucose catabolism. Recent work has shown that aerobic lactate production is specifically coupled to Na-K transport in many, but not all, vascular tissues. Oxidative metabolism, on the other hand, is strongly related to active isometric force. The biochemical basis of this functional compartmentation was investigated at the level of substrate specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

Compartmentation of glycolytic and glycogenolytic metabolism in vascular smooth muscle

Vascular smooth muscle is characterized by a high rate of aerobic lactate production, which may be altered independently of oxidative phosphorylation. This finding suggested a cytoplasmic compartmentation of metabolism. Exogenous glucose was found to be the sole precursor of aerobic glycolysis under unstimulated conditions. Although tissue depolarization with high K+ resulted in a substantial reduction of endogenous glycogen, exogenous glucose remained the sole precursor of aerobic lactate production. These data showed unequivocally that carbohydrate metabolism is compartmentalized in vascular smooth muscle.

The dependence of unloaded shortening velocity on Ca++, calmodulin, and duration of contraction in "chemically skinned" smooth muscle

Unloaded shortening velocity, a mechanical parameter associated with the rate of cross-bridge cycling, was investigated in chemically skinned guinea pig taenia coli and hog carotid artery. Shortening velocity was measured by the technique described by Edman, whereby large length steps are rapidly imposed on the muscle and the time under unloaded conditions is determined from the isometric myograms. Shortening velocity determined in this manner was similar to Vmax from the Hill force-velocity relations reported for both living and skinned taenia coli, and, in the case of carotid artery, was at least as large as that reported for living muscle. The behavior of shortening velocity was qualitatively similar for both preparations. Shortening velocity was strongly temperature dependent, with a Q10 of approximately 3.6. Shortening velocity was found to be dependent on both the Ca++ and calmodulin concentration. In contrast to the dependence of isometric force on Ca++-calmodulin, shortening velocity could be increased further by the addition of Ca++ and/or calmodulin under conditions when isometric force was maximized. Incubation with ATP-gamma S, which presumably maximizes the phosphorylation of myosin, did not increase shortening velocity beyond the maximum value obtained in the presence of Ca++-calmodulin alone. The development of shortening velocity after exposure to high Ca++ solution was found to precede that of isometric force. The steady state value tended to be slightly lower than the maximum shortening velocity, the largest difference observed being less than 1.5-fold. Thus, whereas both isometric force and shortening velocity are dependent on the Ca++-calmodulin concentration in skinned smooth muscle, the dependencies are not identical, differing with respect to temporal development and concentration. These differences may underlie the decline in velocity with maintained isometric force observed in living smooth muscle.

The effects of isoproterenol and ouabain on oxygen consumption, lactate production, and the activation of phosphorylase in coronary artery smooth muscle

In previous work, oxidative metabolism in vascular smooth muscle was found to be correlated with isometric force; aerobic lactate production, however, was found to be substantial and correlated with Na-K transport processes (Paul et al., 1979; Paul, 1980). In this work, the role of glycogen phosphorylase in this unusual functional compartmentalization of of vascular energy metabolism was investigated. Isometric force, oxygen consumption, lactate production, and the level of phosphorylase activity were measured in porcine coronary arterial segments. For comparison with previous studies, phosphorylase activity was also measured in rabbit aorta. Added potassium chloride induced a contracture in which oxygen consumption, lactate production, and phosphorylase activity all increased. Ouabain also induced a contracture, and an increase in oxygen consumption and phosphorylase activity. However, lactate production was inhibited. Isoproterenol, when added to a potassium chloride-induced contracture, elicited a relaxation in isometric force, and oxygen consumption returned to basal levels; however, lactate production and phosphorylase activity remained at the previously elevated levels. Isoproterenol alone had only marginal effects on all parameters studied. The results indicate that the role of phosphorylase in vascular metabolism is complex and unlikely to be a rate-limiting factor for the observed aerobic glycolysis. The high levels of phosphorylase activity observed in the absence of actomyosin interaction support a mechanism of beta-adrenergic relaxation in which the sensitivity of actin-myosin interaction to calcium ion is decreased.

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.

Phosphagen and metabolite content during contraction in porcine carotid artery

Previous studies (Paul, R. J. Chemical energetics of vascular smooth muscle. In: Handbook of Physiology: The Cardiovascular System. Bethesda, MD: Am. Physiol. Soc., 1980, p. 201-235) have shown that vascular oxygen consumption reaches a steady state at approximately twice the basal rate during maintenance of isometric contraction. The time course of the attainment of a metabolic steady state, the metabolic signal for the observed increase in respiration, and the contribution of endogenous phosphagens to the energetics of isometric contraction are not known with certainty. To this end, the time course of the tissue content of ATP, ADP, AMP, phosphocreatine (PCr), inorganic phosphate (Pi), and lactate were measured during a KCl-induced isometric contraction in porcine carotid artery and compared with values in the basal state. Oxygenated unpoisoned strips were frozen at 0, 0.5, 1, and 15 min of contraction, and tissue extracts were analyzed using analytical isotachophoresis. No statistically significant changes from the basal levels of ATP and PCr were measured. A small but significant increase in ADP was seen at all times. An increase in Pi of 1.25 mumol/g was observed at 0.5 min, which decreased in time. Tissue lactate content increased by 1.79 mumol/g after 1 min of contraction. The calculated range of cellular free ADP (ADPfree), 44-123 microM, may be sufficient to saturate oxidative phosphorylation. This and the apparent lack of change of ADPfree from basal during contraction suggest that it does not play a role in the coordination of metabolism and contractility. From as early as 0.5 min, when less than 40% of peak isometric force is attained, intermediary metabolism provides the total ATP required for contraction.

Physical and biochemical energy balance during an isometric tetanus and steady state recovery in frog sartorius at 0 degree C

Frog sartorius muscle stimulated isometrically for 3 s every 256 s to attain a steady state in which initial heat (QI), recovery heat (QR), rate of O2 consumption (JO2), and isometric force (PO) generated are constant for each cycle. For a 3-s tetanus given every 256 s, JO2 was 0.106 mumol/(min . g blotted weight), approximately 71% of the maximum rate observed, whereas lactate production was negligible under these conditions. QI, QT(= QI + QR), and QT/QI were 88.2, 181.5, 2.06 mJ/g blotted weight, respectively. The high-energy phosphate breakdown (delta approximately P) breakdown during the first 3-s tetanus was not different from that during a contraction in the steady state and averaged 1.1 mumol/g blotted weight. Less than half of the initial heat could be accounted for in terms of the extent of the known chemical reactions occurring during contraction. From the stoichiometry of the theoretical biochemical pathways, the amount of ATP synthesized in the steady state exceeds delta approximately P during contraction by more than twofold, corresponding to an apparent ADP:O ratio of 1.5. If it is assumed that carbohydrate oxidation is the only net chemical reaction in the steady state, the total heat production can be explained on the basis of the measured JO2. Under this assumption, heat production during recovery was less than that expected on the basis of the oxygen consumption and delta approximately P during contraction. These observations support the hypothesis that the unexplained enthalpy production and low apparent ADP:O ratio are causally related, i.e., that the reaction(s) producing the unexplained heat during contraction is reversed during the recovery period.

Vascular smooth muscle. Calmodulin and cyclic AMP-dependent protein kinase after calcium sensitivity in porcine carotid skinned fibers

Recent work on vascular smooth muscle actomyosin has indicated that the Ca2+ sensitivity of both ATPase and superprecipitation are affected by calmodulin (CaM) and cyclic AMP-dependent protein kinase (cPK). Using a "chemically skinned" arterial preparation, we have extended these observations to the intact structured contractile system. Media from hog carotid artery were skinned with 1% Triton X-100 followed by a 50% glycerol-ATP salt solution, in which the strips were stored at -25 degrees C. Small strips (thickness between 0.1 and 0.2 mm) were mounted isometrically and relaxed in a Mg-ATP salt solution, pH 6.7, Ca2+ 10(-8) M, 30 degrees C. Ca2+ elicited a contraction with an ED50 of 10(-6) M. Isometric force was between 1 and 4 mN, consistent with the force observed before skinning. With time, the preparation became less sensitive with an increase in ED50 to 10(-5.7) M. CaM (4 micro M) reverses this loss, stabilizes the preparation, and sharply accelerates the rate of tension development. The ED50 in the presence of 4 micro M CaM shifts to about 10(-7) M. This effect is dose-dependent, with the half maximal effect at about 0.4 micro M CaM. Submaximal Ca2+ contractions can be reversibly depressed by preincubation of relaxed fibers with cPK catalytic subunit (300 U/ml), even in the presence of 4 micro M CaM. An inhibition of about 50% of the contraction at 0.2 micro M Ca2+ was obtained, whereas only 20% inhibition was found at 6 micro M Ca2+. Our findings suggest that changes in vascular contractility cannot be described solely in terms of changes in cytoplasmic Ca2+, and that changes in the sensitivity of the contractile protein to a given Ca2+ concentration are also potential mechanisms for vasodilation.

Comparison of physical and biochemical energy balances: chemical breakdown, heat production, and oxygen consumption in frog sartorius muscle

During the last decade two types of energy balance discrepancies were reported: 1) enthalpy production (heat + work) during contraction is greater than that expected on the basis of the known molar enthalpies and phosphagen breakdown; 2) recovery oxygen consumption is greater than that predicted when using standard biochemical stoichiometry and the phosphagen breakdown during contraction (delta approximately P). To test whether these phenomena were causally related, measurements of delta approximately P, oxygen consumption (Jo2), and enthalpy production during contraction (QI) and recovery (QR) were made on frog sartorius muscle at O C. To achieve equal precision among these diverse measurements, a steady-state protocol involving 3-s isometric tetani at 256-s intervals was employed. delta approximately P during the first 3-s tetanus was not different from that during a contraction in the steady state, and averaged 1.1 mumol/g. Steady-state Jo2 was 0.11 mumol-min-1.g-1, approximately 70% of the maximum rate. QI and QR were 88.2 and 93.3 mJ/g (QR/QI = 1.06). Neither the enthalpy during contraction nor recovery oxygen consumption could be accounted for in terms of delta approximately P. However, a total energy balance was achieved, i.e., the total enthalpy production could be accounted for solely in terms of the measured Jo2 and the molar enthalpy of carbohydrate oxidation. As the oxidation of glycogen is implicated as the only net reaction, the unknown reactions producing the unexplained enthalpy during contraction must be reversed during the recovery period. This reversal would require hydrolysis during recovery of mumol ATP per 35.2 mJ of unexplained enthalpy if the theoretical ADP/O ratio of 3.25 is to be attained. Thus a major portion of the phosphagen breakdown associated with muscle contraction is likely to occur during recovery.

Upper genital tract abnormalities in the Syrian hamster as a result of in utero exposure to diethylstilbestrol. I. Uterine cystadenomatous papilloma and hypoplasia

Prenatal exposure to diethylstilbestrol (DES) causes a significant increase in the carcinogenic response of the hamster's reproductive tract to subsequent DES treatment. Uteri from DES treated females from untreated- and DES treated mothers (C.D & D.D) have abnormal hyperplasia with characteristic finger-like structures projecting into the lumen of the uteri. Inside these papillae along with the rest of the stroma are cystic glands. We found that these glands had no openings into the uterine lumen and that they "begin" and "end" in the stroma. In addition there are two types of cells lining the cystic gland i.e., pale cells and acidophilic cells. Capillary beds surround the cystic glands. We have named these uterine structures "cystadenomatous papilloma". In addition, we found a spectrum of hyperplastic abnormalities in C.D and D.D uteri and carcinoma in situ in D.D uteri. Similar neoplasms have been described in human pathology. Ultrasound observations have demonstrated that in utero exposure to DES may result in uterine hypoplasia and because it appears to be similar to the changes seen in prenatally DES treated (D.C) hamsters, cross sectional areas of C.C (untreated control), D.C, C.D and D.D uteri were compared. Our results show that later in life uterine hypoplasia also occurs in the 100 days old D.C hamsters and since D.C uteri present hyperplasia with cystic structures, our data support the hypothesis that in utero DES-treated human females may later in life develop benign and malignant lesions in their reproductive tract. Much of these data corresponds to what has been found in humans, and consequently warrants further investigation into the use of Syrian hamster as a model to understand the uterine abnormal morphogenesis in regards to hypoplasia, adenocarcinoma and carcinoma development in the human.

Vascular smooth muscle: aerobic glycolysis linked to sodium and potassium transport processes

Under aerobic conditions, glucose is primarily catabolized by vascular smooth muscle to lactate, in spite of an adequate oxidative capacity. Although this is often considered to be indicative of some nonspecific metabolic insufficiency, there is evidence that aerobic glycolysis is specifically coupled to sodium and potassium transport processes, whereas oxidative metabolism is couple to contracticle energy requirements.

The aerobic metabolism of porcine carotid artery and its relationship to isometric force. Energy cost of isometric contraction

1. The O2 consumption and aerobic lactate production of porcine carotid artery have been studied in relation to active isometric tension maintenance in order to estimate the tension cost, i.e., ATP hydrolysis rate per unit force maintained. 2. The relationship between total metabolism and isometric force was measured at several muscle lengths in order to distinguish tension-dependent and -independent components of ATP utilization. 3. At the maximum isometric tension observed, 2 kg wt/wt/cm2, 1.3 mumole ATP/M in per gram wet weight out of a total metabolism of 2 mumole/min is tension de pardent. 4. The absolute levels of force, O2 consumption, lactate production and the ratio of oxidative to glycolytic metabolism varied considerably with the stimulant; however, the tension-dependent ATP utilization was invariant with respect to the mode of stimulation.

Chemical energetics in repeated contractions of frog sartorius muscles at 0 degrees C

1. Recovery oxygen consumption (DeltaO(2)) was measured in aerobic, unpoisoned frog sartorius muscles at 0 degrees C following a series of repeated tetani.2. For a series of tetani having nearly identical tension-time integrals, DeltaO(2) was dependent on the interval between tetani and reached a maximal value for intervals equal to or greater than 200 sec.3. For a series of tetani at short (5 sec) intervals, DeltaO(2) was not distinguishable from DeltaO(2) following a single long tetanus having a tension-time integral similar to that of the sum of the series.4. A model is proposed to account for these observations in terms of the initial chemical reactions (Delta approximately P) including a saturable non-steady state reaction which utilizes about 1 mumole Delta approximately P/g and which is superimposed on an energy utilization proportional to the tension-time integral. Measurements of DeltaO(2) and Delta approximately P as functions of tetanic duration and interval between tetani are consistent with such a model.5. These experiments show that, in addition to parameters usually considered, the previous contractile history of a muscle plays a significant role in determining the chemical energy cost of an isometric contraction.

Cross bridge ATP utilization in arterial smooth muscle

Measurements of oxygen consumption and lactate production were used to estimate the ATP production in porcine carotid arteries. Changes in the total energy metabolism were correlated with changes in force which was altered by changing the muscle length under maximum stimulation. This tension-dependent metabolism could be used to distinguish between cross bridge and other ATP utilization. The tension-dependent metabolism was equal under K+ -depolarization or histamine stimulation. Tension cost was 300x smaller than skeletal muscle while the cross bridge cycle duration 30x longer.

Relationship between initial chemical reactions and oxidative recovery metabolism for single isometric contractions of frog sartorius at 0 degrees C

1. Measurements of intitial chemical changes and recovery oxygen consumption (deltaO2) were made in unpoisoned aerobic frog sartorius muscles at 0 degrees C to provide independent measures of the chemical energy used for isometric tetani of various durations. 2. DeltaO2 was measured polarographically and increased in a curvilinear fashion with stimulus duration. For stimulations longer than 4 sec deltaO2 was a linear function of the tension-time integral. 3. Measurements of the changes in the content of phosphorylcreatine, 'free' creatine and inorganic phosphate were made in muscles rapidly frozen during a tetanus. The average of these quantities, delta approximately P, was used to measure the initial 'high energy' phosphate utilization. No break-down of 'high-energy' phosphate compounds was detected up to 200 sec after relaxation of tension. Changes in the content of ATP were not observed except for a small decrease (-0-25+/-0-1 mumole/g) in muscles tetanized for 1 sec. 4. Delta approximately P also increase curvilinearly with tetanus duration and, for tetanic durations greater than 4 sec, was a linear function of the tension-time integral. 5. Both deltaO2 and delta approximately P were quantitatively related by a constant scaling factor of about 4-3 (delta approximately P/deltaO2) throughout the range of tetanic durations studied. The constancy of this ratio.provides evidence against the hypothesis that a significant 'missing reaction' provides energy during any one portion of the tetanus. Several hypothesis may account for the numerical value of the ration delta approximately P/deltaO2.

Aerobic recovery metabolism following a single isometric tetanus in frog sartorius muscle at 0 degrees C

1. Basal and recovery O2 consumption, delatO2, in frog sartorius muscles at 0 degrees C were measured with a polarographic electrode. Reproducible observations were made with the same muscle over many hours. 2. The experimental records had an exponential form except for the early phases of recovery following a single isometric tetanus. Diffusion of O2 within the muscle was adequate to account for this deviation from an exponential time course of recovery. The time constant of the recovery O2 consumption increased with the duration of tetanic stimulation from 5 to 20 sec. 3. Lactate synthesis was measureable in unstimulated aerobic muscles and increased in proportion to total O2 consumption as long as the muscle did not lack O2. The contribution of glycolysis to the total chemical energy production during recovery was 6-9%; for hypoxic muscles it was greater. 4. The resynthesis of phosphorylcreatine and the decrease in inorganic phosphate and free creatine following a tetanus showed an exponential time course similar to recovery O2. Initial concentrations were re-attained within 60 min following a 20 sec tetanus. 5. We conclude that recovery O2 consumpation is a useful and accurate measure of the net chemical energy utilization for a single contraction.

Relation between length, isometric force, and O2 consumption rate in vascular smooth muscle

The length-tension and length-oxygen consumption rate relationships were studied in bovine mesenteric vein at 37 degrees C. The absence of spontaneous mechanical activity permits straightforward interpretation in terms of active (smooth muscle) and passive components of the vein wall. Longitudinal loops, the predominant smooth muscle component being oriented in the longitudinal (axial) direction, were maximally stimulated using epinephrine (2-5 mug-ml-1). An optimum length for isometric tension development was exhibited at which the passive tension was 25% of the total tension. The population regression indicated that tension was developed at lengths which ranged from 0.33 to 1.41 times the length at which maximum tension was developed. Oxygen consumption was measured using a Clark-type polarographic electrode. Basal oxygen consumption was 0.432 plus or minus 0.014 (n equal to 121) mumol-min-1 (g dry wt)-1. The basal rate was found to be independent of the passive tension. Under conditions of maximal stimulation, the oxygen consumption rate at L-o, the resting length at which the tissue maintained 1 g-wt passive tension, was approximately twice the basal rate. The length dependence of the suprabasal oxygen consumption was parallel to that of the active isometric force. This parallel relation reflected a linear relation between active isometric force (deltaP-o) and suprabasal oxygen consumption rate (deltaJ-o2). The slope of the deltaJ-o2-deltaP-o linear regression was 0.142 plus or minus 0.013 nmol O2-MIN-1 (G-WT-CM)-1. DeltaJ-o2 at the minimum contracted length, at which no active force was developed, was 15-20% of the deltaJ-o2 measured when maximum isometric force was developed. This provides an upper bound to the rate of chemical energy utilization required for activation processes. The length dependence of active isometric force and chemical energy utilization is most simply interpreted in terms of a sliding-filament model.