Uptake and metabolism of putrescine in confluent LLC-PK1 cells

Modeling to link regional myocardial work, metabolism and blood flows

Given the mono-functional, highly coordinated processes of cardiac excitation and contraction, the observations that regional myocardial blood flows, rMBF, are broadly heterogeneous has provoked much attention, but a clear explanation has not emerged. In isolated and in vivo heart studies the total coronary flow is found to be proportional to the rate-pressure product (systolic mean blood pressure times heart rate), a measure of external cardiac work. The same relationship might be expected on a local basis: more work requires more flow. The validity of this expectation has never been demonstrated experimentally. In this article we review the concepts linking cellular excitation and contractile work to cellular energetics and ATP demand, substrate utilization, oxygen demand, vasoregulation, and local blood flow. Mathematical models of these processes are now rather well developed. We propose that the construction of an integrated model encompassing the biophysics, biochemistry and physiology of cardiomyocyte contraction, then combined with a detailed three-dimensional structuring of the fiber bundle and sheet arrangements of the heart as a whole will frame an hypothesis that can be quantitatively evaluated to settle the prime issue: Does local work drive local flow in a predictable fashion that explains the heterogeneity? While in one sense one can feel content that work drives flow is irrefutable, the are no cardiac contractile models that demonstrate the required heterogeneity in local strain-stress-work; quite the contrary, cardiac contraction models have tended toward trying to show that work should be uniform. The object of this review is to argue that uniformity of work does not occur, and is impossible in any case, and that further experimentation and analysis are necessary to test the hypothesis.

Aortocaval fistula in rat: a unique model of volume-overload congestive heart failure and cardiac hypertrophy

Despite continuous progress in our understanding of the pathogenesis of congestive heart failure (CHF) and its management, mortality remains high. Therefore, development of reliable experimental models of CHF and cardiac hypertrophy is essential to better understand disease progression and allow new therapy developement. The aortocaval fistula (ACF) model, first described in dogs almost a century ago, has been adopted in rodents by several groups including ours. Although considered to be a model of high-output heart failure, its long-term renal and cardiac manifestations are similar to those seen in patients with low-output CHF. These include Na⁺-retention, cardiac hypertrophy and increased activity of both vasoconstrictor/antinatriureticneurohormonal systems and compensatory vasodilating/natriuretic systems. Previous data from our group and others suggest that progression of cardiorenal pathophysiology in this model is largely determined by balance between opposing hormonal forces, as reflected in states of CHF decompensation that are characterized by overactivation of vasoconstrictive/Na⁺-retaining systems. Thus, ACF serves as a simple, cheap, and reproducible platform to investigate the pathogenesis of CHF and to examine efficacy of new therapeutic approaches. Hereby, we will focus on the neurohormonal, renal, and cardiac manifestations of the ACF model in rats, with special emphasis on our own experience.

Modulation of neuronal voltage-activated calcium and sodium channels by polyamines and pH

The endogenous polyamines spermine, spermidine and putrescine are present at high concentrations inside neurons and can be released into the extracellular space where they have been shown to modulate ion channels. Here, we have examined polyamine modulation of voltage-activated Ca(2+) channels (VACCs) and voltage-activated Na(+) channels (VANCs) in rat superior cervical ganglion neurons using whole-cell voltage-clamp at physiological divalent concentrations. Polyamines inhibited VACCs in a concentration-dependent manner with IC(50)s for spermine, spermidine, and putrescine of 4.7 +/- 0.7, 11.2 +/- 1.4 and 90 +/- 36 mM, respectively. Polyamines caused inhibition by shifting the VACC half-activation voltage (V(0.5)) to depolarized potentials and by reducing total VACC permeability. The shift was described by Gouy-Chapman-Stern theory with a surface charge density of 0.120 +/- 0.005 e(-) nm(-2) and a surface potential of -19 mV. Attenuation of spermidine and spermine inhibition of VACC at decreased pH was explained by H(+) titration of surface charge. Polyamine-mediated effects also decreased at elevated pH due to the inhibitors having lower valence and being less effective at screening surface charge. Polyamines affected VANC currents indirectly by reducing TTX inhibition of VANCs at high pH. This may reflect surface charge induced decreases in the local TTX concentration or polyamine-TTX interactions. In conclusion, polyamines inhibit neuronal VACCs via complex interactions with extracellular H(+) and Ca. Many of the observed effects can be explained by a model incorporating polyamine binding, H(+) binding and surface charge screening.

Interaction of N1,N12-diacetylspermine with polyamine transport systems of polarized porcine renal cell line LLC-PK1

LLC-PK(1) cells grown on porous membrane filters were employed as a model system to explore the renal transport of polyamines. The polarity of LLC-PK(1) monolayers was confirmed by the exclusive appearance of a Na(+)-dependent alpha-methylglucoside transport system on the apical surface. The uptake of free polyamines from the basolateral side of monolayers was consistent with the existence of a single class of transport system, while the existence of two kinetically distinct polyamine transport systems with higher and lower affinities on apical membranes was suggested. The results of competition studies indicated that each of these transporters was able to interact with putrescine, spermidine and spermine. LLC-PK(1) cells incorporated monoacetylspermine from the apical surface of monolayers at about half the rate of spermine uptake. Monoacetylspermine inhibited spermidine uptake, indicating that free polyamine transport systems also recognized the monoacetylated derivative. In contrast, N(1),N(12)-diacetylspermine did not inhibit spermidine uptake, nor was it incorporated into the cells, indicating the absence of transport systems that recognize N(1),N(12)-diacetylspermine on the apical membranes of LLC-PK(1) cells. These results may be relevant as to our previous observation that the content of diacetylpolyamines in urine is relatively constant, and may explain the excellence of N(1),N(12)-diacetylspermine as a tumor marker.

Natriuretic peptides and salt sensitivity: endocrine cardiorenal integration in heart failure

Mammalian hearts contain a family of peptides with potent natriuretic, diuretic, and vasorelaxant actions. In addition to atrial natruretic peptide (ANP) and brain natriuretic peptide, recent studies in humans and animals have suggested that the N-terminal ANP prohormone fragment 31-67 may represent another adaptive mechanism to achieve body fluid homeostasis. Furthermore, these investigations have also suggested that via different mechanisms of action on target organisms, the C-terminal hormone ANP 99-126 and pro-ANP 31-67 may coordinate and contribute to the regulation of hemodynamic and renal function in pathophysiologic situations, such as heart failure.

Vasopeptidase inhibitors, neutral endopeptidase inhibitors, and dual inhibitors of angiotensin-converting enzyme and neutral endopeptidase

Vasopeptidase inhibitors represent a new class of cardiovascular drugs. They function as a combined angiotensin-converting enzyme (ACE) inhibitor and neutral endopeptidase (NEP) inhibitor, the latter of which potentiates the actions of atrial natriuretic peptide (ANP) by minimizing its degradation in the circulation. The consequence of such dual inhibition is a synergistic reduction of vasoconstriction and enhancement of vasodilation, thereby serving to more effectively reduce blood pressure. Furthermore, inhibition of the renin-angiotensin-aldosterone system (RAAS) prevents physiologic compensatory responses in vivo seen with NEP inhibition alone. Vasopeptidase inhibitors have also shown to potentiate bradykinin and adrenomedullin, which additionally contribute to cardiovascular regulation. The most extensively researched and promising agents within the class of VP inhibitors is omapatrilat, a mercaptoacyl derivative of a bicyclic thiazepinone dipeptide. It is a single molecule with equal potency and affinity for ACE and NEP inhibition. Although ACE inhibition tends to more selectively benefit high-renin models of hypertension, vasopeptidase inhibition has been shown to be equally efficacious in low-, normal-, and high-renin models. Contrary to NEP inhibition alone, omapatrilat has also demonstrated the ability to significantly reduce blood pressure in spontaneously hypertensive rats, the equivalent of essential hypertension in humans. Studies also suggest that omapatrilat has cardioprotective properties, especially in the setting of congestive heart failure. More specifically, animal models have demonstrated omapatrilat to be more effective than ACE inhibition alone in remodeling the heart and improving its contractile function. Human studies have documented the efficacy of omapatrilat in the treatment of both hypertension and, to a lesser extent, heart failure. Safety concerns (specifically angioedema) are currently being addressed before the widespread utilization of this promising new agent.

Role of endothelium-derived nitric oxide in the regulation of cardiac oxygen metabolism: implications in health and disease

Endothelium-derived NO is considered to be primarily an important determinant of vascular tone and platelet activity; however, the modulation of myocardial metabolism by NO may be one of its most important roles. This modulation may be critical for the regulation of tissue metabolism. Several physiological processes act in concert to make endothelial NO synthase-derived NO potentially important in the regulation of mitochondrial respiration in cardiac tissue, including (1) the nature of the capillary network in the myocardium, (2) the diffusion distance for NO, (3) the low toxicity of NO at physiological (nanomolar) concentrations, (4) the fact that low PO(2) in tissue facilitates the action of NO on cytochrome oxidase, and (5) the formation of oxygen free radicals. A decrease in NO production is involved in the pathophysiological modifications that occur in heart failure and diabetes, disease states associated with altered cardiac metabolism that contributes to the evolution of the disease process. In contrast, several drugs (eg, angiotensin-converting enzyme inhibitors, amlodipine, and statins) can restore or maintain endogenous production of NO by endothelial cells, and this mechanism may explain part of their therapeutic efficiency. Thus, the purpose of this review is to critically evaluate the role of NO in the control of mitochondrial respiration, with special emphasis on its effect on cardiac metabolism.

Natriuretic peptides in the pathophysiology of congestive heart failure

A hallmark of congestive heart failure (CHF) is the activation of the cardiac endocrine system, in particular atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). The natriuretic peptides are a group of structurally similar but genetically distinct peptides that have diverse actions in cardiovascular, renal, and endocrine homeostasis. ANP and BNP are of myocardial cell origin and C-type natriuretic peptide (CNP) is of endothelial origin. ANP and BNP bind to the natriuretic peptide-A receptor (NPR-A), which, via 3',5'-cyclic guanosine monophosphate (cGMP), mediates natriuresis, vasodilatation, renin inhibition, antimitogenesis, and lusitropic properties. CNP lacks natriuretic actions but possesses vasodilating and growth inhibiting actions via the guanylyl cyclase-linked natriuretic peptide-B receptor. All three peptides are cleared by the natriuretic peptide-C receptor and degraded by the ectoenzyme neutral endopeptidase 24.11, both of which are widely expressed in kidney, lung, and vascular wall. Recently, a fourth member of the natriuretic peptide, Dendroaspis natriuretic peptide (DNP) has been reported to be present in human plasma and atrial myocardium and is elevated in plasma of human CHF.

Renal response to acute neutral endopeptidase inhibition in mild and severe experimental heart failure

BACKGROUND

Neutral endopeptidase 24.11 (NEP) is a metalloprotease that is localized in the greatest abundance in the kidney and degrades natriuretic peptides, such as atrial natriuretic peptide (ANP). Mild congestive heart failure (CHF) is characterized by increases in circulating ANP without activation of the renin-angiotensin-aldosterone system (RAAS) or sodium retention. In contrast, severe CHF is characterized by sodium retention and coactivation of both ANP and the RAAS.

METHODS AND RESULTS

We defined the acute cardiorenal actions of the NEP inhibitor candoxatrilat (8 microg. kg(-1). min(-1)) in 4 groups of anesthetized dogs (normal, n=8; mild CHF, n=6; severe CHF, n=5; and severe CHF with chronic AT(1) receptor antagonism, n=5). Mild CHF was produced by rapid ventricular pacing at 180 bpm for 10 days and severe CHF at 245 bpm for 10 days. In mild CHF, urinary sodium excretion and glomerular filtration rate were greatest in response to acute NEP inhibition compared with the response in either control animals or those with severe CHF. Furthermore, an increase in glomerular filtration rate was observed only in mild CHF in association with increases in renal blood flow and decreases in renal vascular resistance and distal tubular sodium reabsorption. Urinary ANP and cGMP excretion, markers for renal biological actions of ANP, were greatest in mild CHF. The renal actions observed in mild CHF were attenuated in severe CHF and not restored by chronic AT(1) receptor antagonism.

CONCLUSIONS

The results of the present study demonstrate that acute NEP inhibition in mild CHF results in marked increases in renal hemodynamics and sodium excretion that exceed that observed in control animals and severe CHF. These studies underscore the potential therapeutic role for NEP inhibition to enhance renal function in mild CHF, an important phase of CHF that is marked by selective activation of endogenous ANP in the absence of an activated RAAS.

Effects of eprosartan on renal function and cardiac hypertrophy in rats with experimental heart failure

Activation of the renin-angiotensin system may contribute to the derangement in renal and cardiac function in congestive heart failure. The present study evaluated the effects of eprosartan, a selective angiotensin II receptor antagonist, on renal hemodynamic and excretory parameters and on the development of cardiac hypertrophy in rats with aortocaval fistula, an experimental model of congestive heart failure. Infusion of eprosartan (1.0 mg/kg) in rats with aortocaval fistula produced a significant increase (+34%) in total renal blood flow and a sustained decrease (-33%) in the calculated renal vascular resistance. These effects on renal hemodynamics were more pronounced than those observed in sham-operated control rats and occurred despite a significant fall (-12%) in mean arterial blood pressure. Moreover, eprosartan caused a preferential increase in renal cortical blood perfusion and significantly increased glomerular filtration in rats with congestive heart failure. Chronic administration of eprosartan (5.0 mg/kg per day for 7 days through osmotic minipumps inserted intraperitoneally on the day of operation) resulted in a significant enhancement of urinary sodium excretion compared with nontreated rats with heart failure. Moreover, administration of eprosartan to salt-retaining rats with congestive heart failure resulted in a progressive increase and ultimate recovery in urinary sodium excretion. Finally, early treatment with eprosartan blocked the development of cardiac hypertrophy in rats with aortocaval fistula to a larger extent than the angiotensin-converting enzyme inhibitor enalapril. These findings emphasize the importance of angiotensin II in mediating the impairment in renal function and induction of cardiac hypertrophy in heart failure and further suggest that angiotensin II receptor blockade may be a useful treatment of these consequences in severe cardiac failure.

Impact of diffusional oxygen transport on oxidative metabolism in the heart

The resistance for the oxygen molecule to diffuse from the capillary blood to the cell surface produces remarkably large gradients of oxygen partial pressure (PO2) in the extracellular space. In addition, the intracellular radial gradients of PO2 may not be ignored particularly when the cellular oxygen consumption is increased. These PO2 gradients together result in a quite low intracellular PO2 in the cardiomyocyte in vivo. Thus, the cellular oxidative metabolism may be limited by diffusional transport of oxygen from the capillary blood to mitochondria. In this review, quantitative aspects and physiological relevances of the PO2 gradient in the myocardium are discussed.

Protective effects of CGS 30440, a combined angiotensin-converting enzyme inhibitor and neutral endopeptidase inhibitor, in a model of chronic renal failure

The purpose of these studies was to compare the effects of CGS 30440 (CGS), a dual angiotensin-converting enzyme inhibitor (ACEI)/neutral endopeptidase inhibitor (NEPI) to benazepril (BZ), an ACEI, in a model of five-sixths nephrectomy. The doses of BZ and CGS 30440 tested were 6.5 micromol/kg/day and 2.2 micromol/kg/day. Drugs or vehicle (V) were administered subcutaneously for 6 weeks with dosing initiated 1 week after renal mass reduction. At 6 weeks of receiving drug (7 weeks after five-sixths nephrectomy), CGS/6.5 and BZ/6.5 and CGS/2.2 maintained systolic blood pressures (SBP) at presurgical values. BZ/2.2 did not reduce SBP and was similar to the V group. Urinary protein excretion increased >10-fold in the V-treated group. BZ, at either dose, reduced the proteinuria slightly. CGS/6.5 and CGS/2.2 caused significant (p < 0.05) reductions in proteinuria. Creatinine clearance (Cr(cl)), was reduced by 82% in V, 65 and 61% in the CGS/6.5 and CGS/2.2 groups, and by 69 and 74% in the BZ/6.5 and BZ/2.2 groups, respectively. Both CGS treatments improved the fractional excretion of Na+ (%FE(Na)) significantly from the BZ and V groups. The %FE(Na) for BZ at either dose did not differ from that of V. Elevated urinary cyclic guanosine monophosphate (cGMP), an indicator suggesting increased intrarenal levels of atrial natriuretic peptide (ANP), was observed only in the CGS groups. Histologic examination indicated that BZ/6.5 reduced glomerular sclerosis and the extent of tubular dilation, whereas BZ/2.2 had little effect. CGS, especially at the high dose, virtually normalized the glomerular and tubular pathology. Compared with BZ, CGS 30440 treatment further diminished tubular dilation and proteinaceous cast formation. These tubular effects are consistent with some of the renal actions of ANP. The results from these studies indicate that CGS 30440, a combined ACEI/NEPI, conferred a greater renal protective effect than did ACE inhibition alone.

Impaired nitric oxide-mediated renal vasodilation in rats with experimental heart failure: role of angiotensin II

BACKGROUND

Congestive heart failure (CHF) is associated with a decrease in renal perfusion. Because endothelium-derived NO is important in the regulation of renal blood flow (RBF), we tested the hypothesis that an impairment in the NO system may contribute to the decrease in RBF in rats with experimental CHF.

METHODS AND RESULTS

Studies were performed in rats with experimental high-output CHF induced by aortocaval (AV) fistula and sham-operated controls. In controls, incremental doses of acetylcholine (ACh, 1 to 100 microg x kg(-1) x min(-1)) increased RBF and caused a dose-related decrease in renal vascular resistance (RVR). However, the increase in RBF and decrease in RVR were markedly attenuated in rats with CHF. Likewise, the effects of ACh on urinary sodium and cGMP excretion were also diminished in CHF rats, as was the renal vasodilatory effect of the NO donor S-nitroso-N-acetylpenicillamine (SNAP). These attenuated responses to endothelium-dependent and -independent renal vasodilators in CHF rats occurred despite a normal baseline and stimulated NO2+NO3 excretion and normal expression of renal endothelial NO synthase (eNOS), as determined by eNOS mRNA levels and immunoreactive protein. Infusion of the NO precursor L-arginine did not affect baseline RBF or the response to ACh in rats with CHF. However, administration of the nonpeptide angiotensin II receptor antagonist A81988 before ACh completely restored the renal vasodilatory response to ACh in CHF rats.

CONCLUSIONS

This study demonstrates that despite a significant attenuation in the NO-related renal vasodilatory responses, the integrity of the renal NO system is preserved in rats with chronic AV fistula. This impairment in NO-mediated renal vasodilation in experimental CHF appears to be related to increased activity of the renin-angiotensin system and may contribute further to the decrease in renal perfusion seen in CHF.

The effects of pathophysiological increments in brain natriuretic peptide in left ventricular systolic dysfunction

Plasma levels of brain natriuretic peptide (BNP) are raised in patients with left ventricular impairment and may play a role in the adaptation to left ventricular impairment. Manipulation of BNP levels may have therapeutic potential. The effects of BNP have not been well studied in patients with left ventricular impairment. We studied the effects of low-dose BNP infusion, reproducing the increment in plasma BNP seen with progression from mild to severe heart failure in patients with impaired left ventricular systolic function. BNP was infused in a placebo-controlled, single-blind, crossover design at a rate of 3.3 pmol x kg(-1) x min(-1) over 4 hours to 8 patients with a history of congestive heart failure and persistent impairment of left ventricular systolic function (left ventricular ejection fraction <35%). Endocrine, renal, and hemodynamic effects were measured. Compared with time-matched placebo-control, BNP infusion decreased mean systemic arterial pressure (peak decrease, 17.1 mm Hg; P=.04), mean pulmonary artery pressure (peak decrease, 6.1 mm Hg; P=.007), mean pulmonary capillary wedge pressure (peak decrease, 5.5 mm Hg; P=.04), and systemic vascular resistance (peak decrease, 1400 dyne s(-1) cm(-5); P=.015), but cardiac output and heart rate were unchanged. Urinary volume and urinary excretion of sodium and potassium were not altered. BNP infusion increased plasma cGMP (2.3-fold change, P=.002). Plasma atrial natriuretic peptide levels were increased for the first hour of BNP infusion (peak increase, 11.5 pmol/L; P=.005). Plasma aldosterone levels were unchanged during but increased over time-matched control levels after the end of the BNP infusion (peak increase, 90 pmol/L; P=.02). Plasma renin activity and cortisol and catecholamine levels were unchanged. Low-dose infusion of BNP causes favorable hemodynamic changes and relative neurohormonal suppression but has attenuated renal effects in patients with impaired left ventricular systolic function.

Diaphragm structure and function in health and disease

The diaphragm is the primary muscle of inspiration, and as such uncompromised function is essential to support the ventilatory and gas exchange demands associated with physical activity. The normal healthy diaphragm may fatigue during intense exercise, and diaphragm function is compromised with aging and obesity. However, more insidiously, respiratory diseases such as emphysema mechanically disadvantage the diaphragm, sometimes leading to muscle failure and death. Based on metabolic considerations, recent evidence suggests that specific regions of the diaphragm may be or may become more susceptible to failure than others. This paper reviews the regional differences in mechanical and metabolic activity within the diaphragm and how such heterogeneities might influence diaphragm function in health and disease. Our objective is to address five principal areas: 1) Regional diaphragm structure and mechanics (GAF). 2) Regional differences in blood flow within the diaphragm (WLS). 3) Structural and functional interrelationships within the diaphragm microcirculation (DCP). 4) Nitric oxide and its vasoactive and contractile influences within the diaphragm (MBR). 5) Metabolic and contractile protein plasticity in the diaphragm (SKP). These topics have been incorporated into three discrete sections: Functional Anatomy and Morphology, Physiology, and Plasticity in Health and Disease. Where pertinent, limitations in our understanding of diaphragm function are addressed along with potential avenues for future research.

Polyamine transport in mammalian cells. An update

The uptake and release of the natural polyamines putrescine, spermidine and spermine by mammalian cells are integral parts of the systems that regulate the intracellular concentrations of these biogenic amines according to needs. Although a general feature of all tissues, polyamine uptake into intestinal mucosa cells is perhaps the most obvious polyamine transport pathway of physiological and pathophysiological importance. Mutant cell lines lacking the ability to take up polyamines from the environment are capable of releasing polyamines. This indicates that uptake and release are functions of two different transport systems. The isolation of a transporter gene from a mammalian cell line is still lacking. Overaccumulation of polyamines is controlled by release and by a feedback regulation system that involves de novo synthesis of antizyme, a well known protein that also regulates the activity of ornithine decarboxylase. Recent work has demonstrated that Ca(2+)-signalling pathways are also involved. Although there is consensus about the importance of polyamine uptake inhibitors in the treatment of neoplastic disorders, a practically useful uptake inhibitor is still missing. However, the attempts to target tumours, and to increase the selectivity of cytotoxic agents by combining them with the polyamine structure, are promising. New, less toxic and more selective anticancer drugs can be expected from this approach.

Simulation of intraluminal gas transport processes in the microcirculation

Intraluminal resistance to gas transport between the microcirculation and tissue was neglected for a half-century following the early work of Krogh. In recent years it has come to be understood that this neglect is seriously in error. This paper reviews the background for the long period of misdirection, and progress in placing the simulation of gas transport processes on a more accurate, quantitative basis.

Diffusion of myoglobin in skeletal muscle cells--dependence on fibre type, contraction and temperature

We measured the diffusion coefficient of myoglobin (DMb) inside mammalian skeletal muscle cells with a microinjection technique. A small bolus of horse Mb was injected into a single muscle fibre and the subsequent time-dependent changes of the Mb profiles along the fibre axis were measured with a microscope-photometer. For fibres of the rat soleus muscle at 22 degrees C, a DMb of 1.3.10(-7) cm2/s was found, confirming a result obtained previously by us for rat diaphragm muscle with a photo-oxidation technique. In the extensor digitorum longus muscle of the rat, a higher value of 1.9.10(-7) cm2/s was measured. Auxotonic muscle contractions did not change the apparent DMb. For the temperature range between 22 degrees C and 37 degrees C, a temperature coefficient. Q10, of 1.5 was calculated. The implication of this result for the role of Mb in the facilitation of oxygen transport was examined. Model calculations show that with this relatively low DMb value, the intracellular oxygen supply can be improved only slightly.

Pulmonary and renal neutral endopeptidase EC 3.4.24.11 in rats with experimental heart failure

Congestive heart failure is characterized by avid sodium retention and a blunted renal response to exogenous and endogenous atrial natriuretic peptide. Inhibition of neutral endopeptidase EC 3.4.24.11, the main enzyme that degrades natriuretic peptides, produces a natriuretic response in different models of congestive heart failure. This raises the possibility that an increase in either the expression or activity of neutral endopeptidase is responsible for these phenomena. In the present study, we examined (1) the renal effects of SQ-28,603, a neutral endopeptidase inhibitor, in rats with moderate and severe congestive heart failure induced by an aortocaval fistula compared with sham controls, and (2) neutral endopeptidase expression and activity in the lungs and kidneys of these rats. Infusion of SQ-28,603 (40 mg/kg IV) induced a significant natriuretic response in normal rats and rats with moderate congestive heart failure. This response was blunted in rats with severe congestive heart failure. Surprisingly, renal neutral endopeptidase mRNA levels, assessed by quantitative reverse transcriptase-polymerase chain reaction; protein levels, assessed by Western blotting; and activity, assessed by gelatin gels, were comparable in all groups. Pulmonary neutral endopeptidase mRNA levels decreased by 45% in rats with severe congestive heart failure but not in rats with mild congestive heart failure. In addition, pulmonary neutral endopeptidase immunoreactivity levels and activity were significantly decreased in congestive heart failure in correlation with the severity of the disorder.(ABSTRACT TRUNCATED AT 250 WORDS)

Natriuretic response to neutral endopeptidase inhibition is blunted by enalapril in healthy men

We studied six healthy male subjects in a randomized, placebo-controlled, single-blind fashion to determine the comparative effects on renal hemodynamics and natriuresis of the angiotensin-converting enzyme inhibitor enalapril (5 mg on each of 5 days preceding the study), the neutral endopeptidase inhibitor candoxatrilat (200 mg IV), and the combination of enalapril and candoxatrilat. Enalapril pretreatment alone, compared with placebo, produced slight nonsignificant increments in absolute and fractional sodium excretions and a marked increase in effective renal plasma flow but no change in glomerular filtration rate. Candoxatrilat alone produced marked augmentation of both absolute and fractional sodium excretions. The candoxatrilat-mediated increment in absolute sodium excretion was significantly correlated with increases in urinary cGMP and plasma atrial natriuretic peptide in response to this drug, but neither effective renal plasma flow nor glomerular filtration rate was altered compared with placebo. Combining enalapril pretreatment with candoxatrilat significantly attenuated the increments in absolute and fractional sodium excretions in response to the neutral endopeptidase inhibitor. Blood pressure was reduced by enalapril alone compared with placebo, whereas candoxatrilat treatment alone led to a marginal but significant enhancement of blood pressure. The combination of enalapril and candoxatrilat abolished any significant blood pressure change compared with placebo. Thus, candoxatrilat-mediated natriuresis occurs via a renal tubular rather than glomerular mechanism and is blunted by enalapril. This attenuation by enalapril may occur by interference with angiotensin II-dependent effects on the renal tubule or on systemic blood pressure.

Inorganic cation dependence of putrescine and spermidine transport in human breast cancer cells

The mechanism of polyamine uptake in mammalian cells is still poorly understood. The role of inorganic cations in polyamine transport was investigated in ZR-75-1 human breast cancer cells. Although strongly temperature dependent, neither putrescine nor spermidine uptake was mediated by a Na+ cotransport mechanism. In fact, Na+ and cholinium competitively inhibited putrescine uptake relative to that measured in a sucrose-based medium. On the other hand, ouabain, H+, Na+, and Ca2+ ionophores, as well as dissipation of the K+ diffusion potential, strongly inhibited polyamine uptake in keeping with a major role of membrane potential in that process. Polyamine transport was inversely dependent on ambient osmolality at near physiological values. Putrescine transport was inhibited by 70% by decreasing extracellular pH from 7.2 to 6.2, whereas spermidine uptake had a more acidic optimum. Deletion of extracellular Ca2+ inhibited putrescine uptake more strongly than chelation of intracellular Ca2+. In fact, bound divalent cations were absolutely required for polyamine transport, as shown after brief chelation of the cell monolayers with EDTA. Either Mn2+, Ca2+, or Mg2+ sustained putrescine uptake activity with high potency (Km = 50-300 microM). Mn2+ was a much stronger activator of spermidine than putrescine uptake, suggesting a specific role for this metal in polyamine transport. Other transition metals (Co2+, Ni2+, Cu2+, and Zn2+) were mixed activators/antagonists of carrier activity, while Sr2+ and Ba2+ were very weak agonists, while not interfering with Ca2+/Mg(2+)-dependent transport. Thus, polyamine uptake in human breast tumor cells is negatively affected by ionic strength and osmolality, and is driven, at least in part, by the membrane potential, but not by the Na+ electrochemical gradient. Moreover, the polyamine carrier, or a tightly coupled accessory component, appears to have a high-affinity binding site for divalent cations, which is essential for the uptake mechanism.

Polyamine regulation of Na+/glucose symporter expression in LLC-PK1 cells

Addition of polyamines or their analogs to newly confluent LLC-PK1 cells resulted in down-regulation of Na(+)-dependent glucose transport (symport) activity. Polyamines prevented the induction of this symporter by the differentiation inducer hexamethylene bisacetamide (HMBA) but did not influence induction by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Partial depletion of endogenous polyamines after addition of alpha-difluoromethylornithine (DFMO) resulted in a 4 to 5-fold increase in symporter expression. Symporter induction by either HMBA or DFMO was inhibited by the protein kinase inhibitor H-7 but H-7 did not affect symporter induction by IBMX. Changes in symporter activity were accompanied by changes in levels of the 75 kD symporter subunit detected by Western blot. Cultures exposed to HMBA exhibited reduced levels of ornithine decarboxylase activity. Our results suggest that induction of symporter expression by HMBA may be mediated in part by its effects on polyamine metabolism, and point to parallel roles of polyamines and cyclic AMP in regulating the expression of this physiologically important renal transport system.