Calcium removal kinetics of the sarcoplasmic reticulum ATPase in skeletal muscle

A simple recipe for setting up the flux equations of cyclic and linear reaction schemes of ion transport with a high number of states: The arrow scheme

The calculation of flux equations or current-voltage relationships in reaction kinetic models with a high number of states can be very cumbersome. Here, a recipe based on an arrow scheme is presented, which yields a straightforward access to the minimum form of the flux equations and the occupation probability of the involved states in cyclic and linear reaction schemes. This is extremely simple for cyclic schemes without branches. If branches are involved, the effort of setting up the equations is a little bit higher. However, also here a straightforward recipe making use of so-called reserve factors is provided for implementing the branches into the cyclic scheme, thus enabling also a simple treatment of such cases.

Quantifying Ca2+ release and inactivation of Ca2+ release in fast- and slow-twitch muscles

The aims of this study were to quantify the Ca(2+) release underlying twitch contractions of mammalian fast- and slow-twitch muscle and to comprehensively describe the transient inactivation of Ca(2+) release following a stimulus. Experiments were performed using bundles of fibres from mouse extensor digitorum longus (EDL) and soleus muscles. Ca(2+) release was quantified from the amount of ATP used to remove Ca(2+) from the myoplasm following stimulation. ATP turnover by crossbridges was blocked pharmacologically (N-benzyl-p-toluenesulphonamide for EDL, blebbistatin for soleus) and muscle heat production was used as an index of Ca(2+) pump ATP turnover. At 20°C, Ca(2+) release in response to a single stimulus was 34 and 84 μmol (kg muscle)(-1) for soleus and EDL, respectively, and increased with temperature (30°C: soleus, 61 μmol kg(-1); EDL, 168 μmol kg(-1)). Delivery of another stimulus within 100 ms of the first produced a smaller Ca(2+) release. The maximum magnitude of the decrease in Ca(2+) release was greater in EDL than soleus. Ca(2+) release recovered with an exponential time course which was faster in EDL (mean time constant at 20°C, 32.1 ms) than soleus (65.6 ms) and faster at 30°C than at 20°C. The amounts of Ca(2+) released and crossbridge cycles performed are consistent with a scheme in which Ca(2+) binding to troponin-C allowed an average of ∼1.7 crossbridge cycles in the two muscles.

Vasopressin: mechanisms of action on the vasculature in health and in septic shock

BACKGROUND

Vasopressin is essential for cardiovascular homeostasis, acting via the kidney to regulate water resorption, on the vasculature to regulate smooth muscle tone, and as a central neurotransmitter, modulating brainstem autonomic function. Although it is released in response to stress or shock states, a relative deficiency of vasopressin has been found in prolonged vasodilatory shock, such as is seen in severe sepsis. In this circumstance, exogenous vasopressin has marked vasopressor effects, even at doses that would not affect blood pressure in healthy individuals. These two findings provide the rationale for the use of vasopressin in the treatment of septic shock. However, despite considerable research attention, the mechanisms for vasopressin deficiency and hypersensitivity in vasodilatory shock remain unclear.

OBJECTIVE

To summarize vasopressin's synthesis, physiologic roles, and regulation and then review the literature describing its vascular receptors and downstream signaling pathways. A discussion of potential mechanisms underlying vasopressin hypersensitivity in septic shock follows, with reference to relevant clinical, in vivo, and in vitro experimental evidence.

DATA SOURCE

Search of the PubMed database (keywords: vasopressin and receptors and/or sepsis or septic shock) for articles published in English before May 2006 and manual review of article bibliographies.

DATA SYNTHESIS AND CONCLUSIONS

The pathophysiologic mechanism underlying vasopressin hypersensitivity in septic shock is probably multifactorial. It is doubtful that this phenomenon is merely the consequence of replacing a deficiency. Changes in vascular receptors or their signaling and/or interactions between vasopressin, nitric oxide, and adenosine triphosphate-dependent potassium channels are likely to be relevant. Further translational research is required to improve our understanding and direct appropriate educated clinical use of vasopressin.

Marked synergism between vasopressin and angiotensin II in a human isolated artery

OBJECTIVE

To determine the direct contractile effects of angiotensin II (AII) and vasopressin (VP), and the effects of combinations of these agonists, in human isolated gastroepiploic arteries in vitro.

DESIGN

Laboratory and clinical investigation.

SETTING

University laboratory and hospital.

SUBJECTS

Ring segments were prepared from gastroepiploic arteries obtained from 57 patients undergoing gastrectomy. Blood samples were obtained from ten patients after severe hemorrhage and from five healthy volunteers.

INTERVENTIONS

Mechanical activity in the rings was assessed using a strain gauge. Plasma concentrations of AII and VP in the blood samples were measured using radioimmunoassay kits.

MEASUREMENTS AND MAIN RESULTS

Both AII (1 or 10 ng/mL) and VP (100 or 500 pg/mL) produced concentration-dependent contractions in the rings. However, whereas VP produced reproducible sustained contractions, the contractile responses induced by multiple applications of AII showed marked desensitization (i.e., tachyphylaxis). Indeed, by the sixth application of either 1 ng/mL AII or 10 ng/mL AII, the contractile responses were <20% of the initial (control) response. During applications of AII after the sixth, the co-application of a low concentration of VP (100 pg/mL) fully restored the contractile response to AII in a clearly more-than-additive fashion. Similarly, the tachyphylaxis seen on AII application alone did not occur with repeated applications of an AII + norepinephrine mixture. In patients who had experienced hemorrhage, there were marked elevations of both AII and VP plasma concentrations, with values as high as 2.2 ng/mL and 550 pg/mL, respectively.

CONCLUSION

These results indicate that there is a powerful synergism between the contractile effects of low-dose VP and AII in this human isolated artery. Moreover, the elevations of plasma AII and VP levels during hemorrhage suggest that this synergism may be both physiologically and clinically important in optimizing vasoconstriction and maintaining blood pressure in such critical states.

Comparison between the predictions of diffusion-reaction models and localized Ca2+ transients in amphibian skeletal muscle fibers

We developed a three-dimensional cylindrical diffusion-reaction model of a single amphibian myofibril in which Ca(2+) release occurred only at the Z-line. The model incorporated diffusion of Ca(2+), Mg(2+), and all relevant buffer species, as well as the kinetic binding reactions between the buffers and appropriate ions. Model data was blurred according to a Gaussian approximation of the point spread function of the microscope and directly compared with experimental data obtained using the confocal spot methodology. The flux parameters were adjusted until the simulated Z-line transient matched the experimental one. This model could not simultaneously predict key parameters of the experimental M- and Z-line transients, even when model parameters were adjusted to unreasonably extreme values. Even though the model was accurate in predicting the Z-line transient under conditions of high [EGTA], it predicted a significantly narrower Ca(2+) domain than observed experimentally. We modified the model to incorporate a broader band of release centered at the Z-line. This extended release model was superior both in simultaneously predicting critical features of the Z- and M-line transients as well as the domain profile under conditions of high [EGTA]. We conclude that a model of release occurring exclusively at the Z-line cannot explain our experimental data and suggest that Ca(2+) may be released from a broader region of the sarcoplasmic reticulum than just the T-tubule-sarcoplasmic reticulum junction.

Effect of vasopressin on sublingual microcirculation in a patient with distributive shock

OBJECTIVE

To assess the sublingual microcirculation in a patient during vasopressin administration for a distributive shock after cardiopulmonary bypass.

DESIGN AND SETTING

Case-report in the Department of Intensive Care of a university hospital.

PATIENT

A 53 year-old man developed severe distributive shock after cardiac transplant, requiring massive doses of vasopressor agents.

METHODS

Vasopressin administered twice at a dose of 0.02 U/min increased mean blood pressure and allowed partial weaning of other vasopressor drugs. Microcirculatory alterations were assessed by orthogonal polarization spectral technique: 50% and 60% of capillaries were perfused at baseline, and these proportions did not worsen when vasopressin was administered.

CONCLUSIONS

Despite its strong vasopressor effects vasopressin infusion did not worsen microcirculatory alterations in this patient with distributive shock following cardiac surgery.

Vasopressin-induced facilitation of adrenergic responses in the rat mesenteric artery is V1-receptor dependent

1. The present study was designed to analyse the possible involvement of V1- and V2-receptors in vasopressin (AVP)-induced facilitation of the sympathetic nervous system. Furthermore, we aimed to determine whether the site of facilitation by AVP is located pre- or postsynaptically. 2. Electrical field stimulation (EFS) was applied on the rat mesteric artery to activate the sympathetic nervous system. In addition, we evaluated the direct vascular effects of AVP. The postsynaptic effect of AVP on the sympathetic nervous system was investigated by exposing the vessels to exogenous noradrenaline. These experiments were performed in the absence or presence of selective V1 and V2 receptor antagonists SR 49059 and SR 121463, respectively. Desmopressin was applied as a selective V2 agonist. 3. The direct vasoconstrictor effect of AVP was antagonized by SR 49059 and not by SR 121463. Desmopressin neither showed any direct vasoconstrictor effect nor produced vasodilatation after a precontraction induced by noradrenaline (10 microM). The EFS-induced rise in vascular tone could be increased by a sub-pressor concentration of AVP. This fascilitation could be antagonized by SR 49059, but not by SR 121463. Desmopressin did not influence the increase in vascular tone during EFS. Vasoconstriction induced by exogenous noradrenaline could be facilitated by a sub-pressor concentration of AVP and this selective postsynaptic effect could be antagonized by V1-receptor blockade. 4. In conclusion, the AVP-induced facilitation of the sympathetic nervous system is completely V1-receptor dependent and at least partly postsynaptically mediated.

Vasopressin-induced presynaptic facilitation of sympathetic neurotransmission in the pithed rat

OBJECTIVE

Several studies have shown that arginine vasopressin (AVP) potentiates the sympathetic nervous transmission in isolated vessels. The present study investigates such a potentiation in the pithed rat model.

METHODS

Male Wistar rats weighing 270-310 g were used. Spinal-cord stimulation was applied, with frequencies of 0.25-4 Hz, in the presence or absence of a subpressor dose of intravenous (i.v.) AVP (1 pmol/kg per min). In addition, the effect of AVP on postsynaptic alpha-adrenoceptor-mediated responses was studied using exogenously administered noradrenaline (NA). For this purpose dose-response curves (DRCs) for NA (i.v.) were constructed.

RESULTS

In the pithed rat model endogenously generated angiotensin II facilitates neurally mediated increments in vascular resistance. Without the administration of the angiotensin II type 1 (AT1) antagonist, irbesartan, the facilitating effect of AVP was not visible. However, after the administration of the AT1 antagonist, irbesartan, the facilitating effect of AVP became apparent. The stimulation-induced rise in diastolic blood pressure (DBP) was enhanced in the presence of AVP from 63.7 +/- 4.5 to 78.6 +/- 4.2 mmHg, at a stimulation frequency of 4 Hz. The vasopressin receptor V1 antagonist, SR-49059, completely inhibited this AVP-induced facilitation, whereas the V2 antagonist, SR-121463B, or the V2 agonist, desmopressin, did not. The DRC of exogenously administered NA was not influenced by AVP.

CONCLUSION

The stimulating effect of AVP on sympathetic neurotransmission is completely dependent on the stimulation of presynaptically located V1 receptors. The facilitating effect of angiotensin II on the sympathetic nervous system (SNS) in the pithed rat model masks the facilitating effect of AVP in this preparation.

Hemodynamic and metabolic effects of low-dose vasopressin infusions in vasodilatory septic shock

OBJECTIVE

To investigate the physiologic effects of exogenous vasopressin as a potential alternative to traditional high-dose catecholamine therapy for septic patients with vascular hyporeactivity to catecholamines.

DESIGN

Prospective, case-controlled study.

SETTING

Intensive care unit of a university hospital.

PATIENTS

Vasopressin was infused in 16 critically ill septic patients who remained persistently hypotensive despite infusions of pharmacologic doses of catecholamines.

INTERVENTION

Continuous intravenous infusion of vasopressin at 0.04 units/min for 16 hrs, in place of escalating the amount of catecholamines being infused.

MEASUREMENTS AND MAIN RESULTS

After administration of vasopressin, systemic vascular resistance and mean arterial pressure were immediately and significantly increased in comparison with the values obtained just before vasopressin. When the vasopressin infusions were discontinued, mean arterial pressure decreased immediately and dramatically. We did not detect any obvious adverse cardiac effects during the vasopressin infusions. Vasopressin had no effect on other hemodynamic parameters or any of the metabolic parameters studied, including measures of oxygenation, plasma glucose, or electrolytes. Urine output increased significantly during the administration of vasopressin, although this effect may be nonspecific. Lactate concentrations decreased, particularly in the survival group, but the decreases were not significant. Overall survival was 56%.

CONCLUSIONS

Low-dose vasopressin infusions increased mean arterial pressure, systemic vascular resistance, and urine output in patients with vasodilatory septic shock and hyporesponsiveness to catecholamines. The data indicate that low-dose vasopressin infusions may be useful in treating hypotension in these patients.

A metabolic control analysis of kinetic controls in ATP free energy metabolism in contracting skeletal muscle

A system analysis of ATP free energy metabolism in skeletal muscle was made using the principles of metabolic control theory. We developed a network model of ATP free energy metabolism in muscle consisting of actomyosin ATPase, sarcoplasmic reticulum (SR) Ca(2+)-ATPase, and mitochondria. These components were sufficient to capture the major aspects of the regulation of the cytosolic ATP-to-ADP concentration ratio (ATP/ADP) in muscle contraction and had inherent homeostatic properties regulating this free energy potential. As input for the analysis, we used ATP metabolic flux and the cytosolic ATP/ADP at steady state at six contraction frequencies between 0 and 2 Hz measured in human forearm flexor muscle by (31)P-NMR spectroscopy. We used the mathematical formalism of metabolic control theory to analyze the distribution of fractional kinetic control of ATPase flux and the ATP/ADP in the network at steady state among the components over this experimental range and an extrapolated range of stimulation frequencies (up to 10 Hz). The control analysis showed that the contractile actomyosin ATPase has dominant kinetic control of ATP flux in forearm flexor muscle over the 0- to 1.6-Hz range of contraction frequencies that resulted in steady states, as determined by (31)P-NMR. However, flux control begins to shift toward mitochondria at >1 Hz. This inversion of flux control from ATP demand to ATP supply control hierarchy progressed as the contraction frequency increased past 2 Hz and was nearly complete at 10 Hz. The functional significance of this result is that, at steady state, ATP free energy consumption cannot outstrip the ATP free energy supply. Therefore, this reduced, three-component muscle ATPase system is inherently homeostatic.

The effects of vasopressin on endotoxin-induced attenuation of contractile responses in human gastroepiploic arteries in vitro

We studied the effects of vasopressin on contraction in normal and endotoxin-treated human gastroepiploic arterial rings in vitro. In this tissue, vasopressin (50-500 pg/mL) produced concentration-dependent, endothelium-independent contractions. Vasopressin also potentiated the contraction elicited by 1.0 micromol/L norepinephrine (NE) in both the presence and absence of endothelium. Endotoxin (10 microg/mL) attenuated the 1.0 micromol/L NE-induced contractions, and this attenuation was reversed by 300 micromol/L N(G)-nitro-L-arginine-methyl ester (L-NAME) and by 300 micromol/L N(G)-nitro-L-arginine (L-NoArg). After 12 h endotoxin treatment, the vasopressin-induced contraction was attenuated, and the enhancing effect of vasopressin was diminished. However, both before and after endotoxin, the enhancement produced by vasopressin was larger than the vasopressin-contraction itself. An antagonist of the vasopressin V1 receptor, 1.0 micromol/L beta-mercapto-[beta,beta-cyclopentamethylenpropionyl1,O-MeTyr2+ ++,Arg8]-vasopressin, and an antagonist of V1 + V2 receptor receptor, 1.0 micromol/L des-Gly9-[beta-mercapto-beta,beta-cyclopentamethylenepropionyl1 ,O-Et-Tyr2,Val,Arg8]-vasopressin, each diminished the vasopressin-induced enhancement of the NE contraction.

IMPLICATIONS

The results of our study suggest that, in addition to its direct vasoconstrictor effect, vasopressin strongly enhances the responses to norepinephrine through V1-receptor stimulation and that vasopressin could find a role in the management of endotoxin-induced vasodilation.

Arginine vasopressin enhances sympathetic constriction through the V1 vasopressin receptor in human saphenous vein

BACKGROUND

Arginine vasopressin (AVP) not only acts directly on blood vessels through V1 receptor stimulation but also may modulate adrenergic-mediated responses in animal experiments in vivo and in vitro. The aim of the present study was to investigate whether AVP can contribute to an abnormal adrenergic constrictor response of human saphenous veins.

METHODS AND RESULTS

Saphenous vein rings were obtained from 32 patients undergoing coronary artery bypass surgery. The vein rings were suspended in organ bath chambers for isometric recording of tension. AVP (3x10[-9] mol/L) enhanced the contractions elicited by electrical field stimulation at 1, 2, and 4 Hz (by 80%, 70%, and 60%, respectively) and produced a leftward shift of the concentration-response curve to norepinephrine (half-maximal effective concentration decreased from 6.87x10[-7] to 1.04x10[-7] mol/L; P<.05). The V1 vasopressin receptor antagonist d(CH2)5Tyr(Me)AVP (10[-6] mol/L) prevented the potentiation evoked by AVP. The selective V1 receptor agonist [Phe,2 Orn8]-vasotocin (3x[-10]-9 mol/L) induced potentiation of electrical stimulation-evoked responses, which was also inhibited in the presence of the V1 receptor antagonist (10[-6] mol/L). In contrast, the V2 receptor agonist desmopressin (10[-9] to 10[-7] mol/L) did not modify neurogenic responses, and the V2 receptor antagonist [d(CH2)5, D-Ile,2 Ile,4 Arg8]-vasopressin (10[-8] to 10[-6] mol/L) did not prevent the potentiation induced by AVP. The dihydropyridine calcium antagonist nifedipine (10[-6] mol/L) did not affect the potentiating effect of AVP.

CONCLUSIONS

The results suggest that low concentrations of AVP facilitate sympathetic neurotransmission and potentiate constrictor effects of norepinephrine in human saphenous veins. These effects appear to be mediated by V1 receptor stimulation and are independent of calcium entry through dihydropyridine calcium channels. Thus, AVP may contribute to vascular mechanisms involved in acute ischemic syndromes associated with venous grafts, particularly if the sympathetic nervous system is activated.