Protein kinase C and calcium channel activation as determinants of renal vasoconstriction by angiotensin II and endothelin

Uteroplacental Circulation in Normal Pregnancy and Preeclampsia: Functional Adaptation and Maladaptation

Uteroplacental blood flow increases as pregnancy advances. Adequate supply of nutrients and oxygen carried by uteroplacental blood flow is essential for the well-being of the mother and growth/development of the fetus. The uteroplacental hemodynamic change is accomplished primarily through uterine vascular adaptation, involving hormonal regulation of myogenic tone, vasoreactivity, release of vasoactive factors and others, in addition to the remodeling of spiral arteries. In preeclampsia, hormonal and angiogenic imbalance, proinflammatory cytokines and autoantibodies cause dysfunction of both endothelium and vascular smooth muscle cells of the uteroplacental vasculature. Consequently, the vascular dysfunction leads to increased vascular resistance and reduced blood flow in the uteroplacental circulation. In this article, the (mal)adaptation of uteroplacental vascular function in normal pregnancy and preeclampsia and underlying mechanisms are reviewed.

Flavonoids and Their Metabolites: Prevention in Cardiovascular Diseases and Diabetes

The occurrence of atherosclerosis and diabetes is expanding rapidly worldwide. These two metabolic disorders often co-occur, and are part of what is often referred to as the metabolic syndrome. In order to determine future therapies, we propose that molecular mechanisms should be investigated. Once the aetiology of the metabolic syndrome is clear, a nutritional intervention should be assessed. Here we focus on the protective effects of some dietary flavonoids, and their metabolites. Further studies may also pave the way for development of novel drug candidates.

Endothelin and the renal microcirculation

Endothelin (ET) is one of the most potent renal vasoconstrictors. Endothelin plays an essential role in the regulation of renal blood flow, glomerular filtration, sodium and water transport, and acid-base balance. ET-1, ET-2, and ET-3 are the three distinct endothelin isoforms comprising the endothelin family. ET-1 is the major physiologically relevant peptide and exerts its biological activity through two G-protein-coupled receptors: ET(A) and ET(B). Both ET(A) and ET(B) are expressed by the renal vasculature. Although ET(A) are expressed mainly by vascular smooth muscle cells, ET(B) are expressed by both renal endothelial and vascular smooth muscle cells. Activation of the endothelin system, or overexpression of downstream endothelin signaling pathways, has been implicated in several pathophysiological conditions including hypertension, acute kidney injury, diabetic nephropathy, and immune nephritis. In this review, we focus on the effects of endothelin on the renal microvasculature, and update recent findings on endothelin in the regulation of renal hemodynamics.

Endothelin-1, but not angiotensin II, induces afferent arteriolar myosin diphosphorylation as a potential contributor to prolonged vasoconstriction

Bolus administration of endothelin-1 elicits long-lasting renal afferent arteriolar vasoconstriction, in contrast to transient constriction induced by angiotensin II. Vasoconstriction is generally evoked by myosin regulatory light chain (LC20) phosphorylation at Ser19 by myosin light chain kinase (MLCK), which is enhanced by Rho-associated kinase (ROCK)-mediated inhibition of myosin light chain phosphatase (MLCP). LC20 can be diphosphorylated at Ser19 and Thr18, resulting in reduced rates of dephosphorylation and relaxation. Here we tested whether LC20 diphosphorylation contributes to sustained endothelin-1 but not transient angiotensin II-induced vasoconstriction. Endothelin-1 treatment of isolated arterioles elicited a concentration- and time-dependent increase in LC20 diphosphorylation at Thr18 and Ser19. Inhibition of MLCK or ROCK reduced endothelin-1-evoked LC20 mono- and diphosphorylation. Pretreatment with an ETB but not an ETA receptor antagonist abolished LC20 diphosphorylation, and an ETB receptor agonist induced LC20 diphosphorylation. In contrast, angiotensin II caused phosphorylation exclusively at Ser19. Thus, endothelin-1 and angiotensin II induce afferent arteriolar constriction via LC20 phosphorylation at Ser19 due to calcium activation of MLCK and ROCK-mediated inhibition of MLCP. Endothelin-1, but not angiotensin II, induces phosphorylation of LC20 at Thr18. This could contribute to the prolonged vasoconstrictor response to endothelin-1.

ET-1 actions in the kidney: evidence for sex differences

Hypertension and chronic kidney disease are more common in men than in premenopausal women at the same age. In animal models, females are relatively protected against genetic or pharmacological procedures that produce high blood pressure and renal injury. Overactivation or dysfunction of the endothelin (ET) system modulates the progression of hypertension or kidney diseases with the ET(A) receptor primarily mediating vasoconstriction, injury and anti-natriuresis, and ET(B) receptors having opposite effects. The purpose of this review is to examine the role of the ET system in the kidney with a focus on the inequality between the sexes associated with the susceptibility to and progression of hypertension and kidney diseases. In most animal models, males have higher renal ET-1 mRNA expression, greater ET(A) -mediated responses, including renal medullary vasoconstriction, and increased renal injury. These differences are reduced following gonadectomy suggesting a role for sex hormones, mainly testosterone. In contrast, females are relatively protected from high blood pressure and kidney damage via increased ET(B) versus ET(A) receptor function. Furthermore, ET(A) receptors may have a favourable effect on sodium excretion and reducing renal damage in females. In human studies, the genetic polymorphisms of the ET system are more associated with hypertension and renal injury in women. However, the knowledge of sex differences in the efficacy or adverse events of ET(A) antagonists in the treatment of hypertension and kidney disease is poorly described. Increased understanding how the ET system acts differently in the kidneys between sexes, especially with regard to receptor subtype function, could lead to better treatments for hypertension and renal disease. LINKED ARTICLES This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Vascular superoxide production by endothelin-1 requires Src non-receptor protein tyrosine kinase and MAPK activation

ET-1 induces vascular O(2)(*-) production via activation of NADPH oxidase. We have investigated whether c-Src and MAPKs activation are involved in ET-1-induced vascular oxidative response. At 2 h, ET-1 induced an increase in NADPH oxidase-driven O(2)(*-) production in rat isolated aortic rings, which was completely suppressed in PP2 (c-Src inhibitor)-pretreated rings, whereas PP3 (inactive analogue of PP2) was without effect. ET-1 increased the levels of phospho-c-Src, the active form of c-Src, and the phosphorylation of cortactin, a Src-specific substrate. Both c-Src and cortactin phosphorylation induced by ET-1 were prevented by PP2. The increased expression of p47(phox), the main cytosolic subunit of NADPH oxidase, induced by ET-1 was also prevented by PP2. The increased vascular O(2)(*-) production and p47(phox) up-regulation induced by ET-1 was only inhibited in aortic rings coincubated with the ERK1/2 inhibitor, PD98059; being without effects both the p38 MAPK inhibitor, SB203580, and JNK inhibitor, SP600125. Aortic rings incubation with ET-1 increased the phosphorylation of ERK1/2. This effect was suppressed by coincubation with PP2 showing that this event is down-stream of c-Src activation. In conclusion, ET-1 induces NADPH oxidase-driven O(2)(*-) generation through increase of p47(phox) protein expression. The signalling pathway for this effect involves c-Src activation and ERK1/2 phosphorylation.

Rho-kinase inhibition reduces pressure-mediated autoregulatory adjustments in afferent arteriolar diameter

Preglomerular resistance is regulated by calcium influx- and mobilization-dependent mechanisms; however, the role of Rho-kinase in calcium sensitization in the intact kidney has not been carefully examined. Experiments were performed to test the hypothesis that Rho-kinase inhibition blunts pressure-mediated afferent arteriolar autoregulatory behavior and vasoconstrictor responses evoked by angiotensin II and P2X1 receptor activation. Rat kidneys were studied in vitro using the blood-perfused juxtamedullary nephron technique. Autoregulatory behavior was assessed before and during Rho-kinase inhibition with Y-27632 (1.0 microM; n = 5). Control diameter averaged 14.3 +/- 0.8 microm and increased to 18.1 +/- 0.9 microm (P < 0.05) during Y-27632 treatment. In the continued presence of Y-27632, reducing perfusion pressure to 65 mmHg slightly increased diameter to 18.7 +/- 1.0 microm. Subsequent pressure increases to 130 and 160 mmHg yielded afferent arteriolar diameters of 17.5 +/- 0.8 and 16.6 +/- 0.6 microm (P < 0.05). This 11% decline in diameter is significantly smaller than the 40% decrease obtained in untreated kidneys. The inhibitory effects of Y-27632 on autoregulatory behavior were concentration dependent. Angiotensin II responses were blunted by Y-27632. Angiotensin II (1.0 nM) reduced afferent diameter by 17 +/- 1% in untreated arterioles and by 6 +/- 2% during exposure to Y-27632. The P2X1 receptor agonist, alpha, beta-methylene ATP, reduced afferent arteriolar diameter by 8 +/- 1% but this response was eliminated during exposure to Y-27632. Western blot analysis confirms expression of the Rho-kinase signaling pathway. Thus, Rho-kinase may be important in pressure-mediated autoregulatory adjustments in preglomerular resistance and responsiveness to angiotensin II and autoregulatory P2X1 receptor agonists.

Quercetin inhibits vascular superoxide production induced by endothelin-1: Role of NADPH oxidase, uncoupled eNOS and PKC

Chronic administration of the most abundant dietary flavonoid quercetin exerts antihypertensive effects and improves endothelial function. We have investigated the effects of quercetin and its methylated metabolite isorhamnetin (1-10microM) on endothelial dysfunction and superoxide (O(2*)(-)) production induced by endothelin-1 (ET-1, 10nM). ET-1 increased the contractile response induced by phenylephrine and reduced the relaxant responses to acetylcholine in phenylephrine contracted intact aorta, and these effects were prevented by co-incubation with quercetin, isorhamnetin or chelerythrine (protein kinase C (PKC) inhibitor). This endothelial dysfunction was also improved by superoxide dismutase (SOD), apocynin (NADPH oxidase inhibitor) and sepiapterin (tetrahydrobiopterin synthesis substrate). Furthermore, ET-1 increased intracellular O(2*)(-) production in all layers of the vessel, protein expression of NADPH oxidase subunit p47(phox) without affecting p22(phox) expression and lucigenin-enhanced chemiluminescence signal stimulated by calcium ionophore A23187. All these changes were prevented by both quercetin and isorhamnetin. Moreover, apocynin, endothelium denudation and N(G)-nitro-l-arginine methylester (l-NAME, nitric oxide synthase inhibitor) suppressed the ET-1-induced increase in A23187-stimulated O(2*)(-) generation. Moreover, quercetin but not isorhamnetin, inhibited the increased PKC activity induced by ET-1. Taken together these results indicate that ET-1-induced NADPH oxidase up-regulation and eNOS uncoupling via PKC leading to endothelial dysfunction and these effects were prevented by quercetin and isorhamnetin.

L-type calcium channels in the renal microcirculatory response to endothelin

The signaling pathways of endothelin (ET)-1-mediated vasoconstriction in the renal circulation have not been elucidated but appear to be distinct between ETAand ETBreceptors. The purpose of this study was to determine the role of L-type Ca2+channels in the vasoconstrictor response to ET-1 and the ETBreceptor agonist sarafotoxin 6c (S6c) in the rat kidney. Renal blood flow (RBF) was measured with an ultrasonic flow probe in anesthetized rats, and a microcatheter was inserted into the renal artery for drug infusion. All rats were given vehicle (0.9% NaCl) or three successive bolus injections (1, 10, and 100 pmol) of ET-1 or S6c at 30-min intervals ( n = 6 in each group). ET-1 and S6c produced dose-dependent decreases in RBF. The Ca2+channel blocker nifedipine (1.5 μg) significantly attenuated the RBF response only at the highest doses of ET-1 and S6c. In the isolated blood-perfused juxtamedullary nephron preparation, Ca2+channel blockade with diltiazem had a very small inhibitory effect on ET-1-induced decreases in afferent arteriolar diameter only at the lowest concentrations of ET-1. In vascular smooth muscle cells isolated from preglomerular vessels, ET-1 produced a typical biphasic Ca2+response, whereas S6c had no effect on cytosolic Ca2+. Furthermore, Ca2+channel blockade (diltiazem or Ni2+) had no effect on the peak or sustained increase in cytosolic Ca2+produced by ET-1. These results support the hypothesis that L-type Ca2+channels play only a minor role in the constrictor responses to ET-1 in the renal microcirculation.

Modulation of Cardiac Ca2+ Channel by Gq-activating Neurotransmitters Reconstituted in Xenopus Oocytes

L-type dihydropyridine-sensitive voltage dependent Ca(2+) channels (L-VDCCs; alpha(1C)) are crucial in cardiovascular physiology. Currents via L-VDCCs are enhanced by hormones and transmitters operating via G(q), such as angiotensin II (AngII) and acetylcholine (ACh). It has been proposed that these modulations are mediated by protein kinase C (PKC). However, reports on effects of PKC activators on L-type channels are contradictory; inhibitory and/or enhancing effects have been observed. Attempts to reproduce the enhancing effect of AngII in heterologous expression systems failed. We previously found that PKC modulation of the channel depends on alpha(1C) isoform used; only a long N-terminal (NT) isoform was up-regulated. Here we report the reconstitution of the AngII- and ACh-induced enhancement of the long-NT isoform of L-VDCC expressed in Xenopus oocytes. The current initially increased over several minutes but later declined to below baseline levels. Using different NT deletion mutants and human short- and long-NT isoforms of the channel, we found the initial segment of the NT to be crucial for the enhancing, but not for the inhibitory, effect. Using blockers of PKC and of phospholipase C (PLC) and a mutated AngII receptor lacking G(q) coupling, we demonstrate that the signaling pathway of the enhancing effect includes the activation of G(q), PLC, and PKC. The inhibitory modulation, present in both alpha(1C) isoforms, was G(q)- and PLC-independent and Ca(2+)-dependent, but not Ca(2+)-mediated, as only basal levels of Ca(2+) were essential. Reconstitution of AngII and ACh effects in Xenopus oocytes will advance the study of molecular mechanisms of these physiologically important modulations.

Olmesartan Improves Endothelin-Induced Hypertension and Oxidative Stress in Rats

Recent studies have indicated that both endothelin (ET) and angiotensin (Ang) II stimulate oxidative stress, which contributes to the development of hypertension. Here, we examined the effects of Ang II type 1 (AT1) receptor blockade on reactive oxygen species (ROS) formation in ET-dependent hypertension. Chronic ET-1 infusion (2.5 pmol/kg/min, i.v., n=7) into rats for 14 days increased systolic blood pressure from 113+/-1 to 141+/-2 mmHg. ET-1-infused rats showed greater plasma renin activity (8.1+/-0.8 Ang I/ml/h), and greater Ang I (122+/-28 fmol/ml) and Ang II levels (94+/-13 fmol/ml) than vehicle (0.9% NaCl)-infused rats (3.1+/-0.6 Ang I/ml/h, 45+/-8 and 47+/-7 fmol/ml, respectively, n=6). Angiotensin converting enzyme and AT1 receptor expression in aortic tissues were similar between the vehicle- and ET-1-infused rats. Vascular superoxide anion (O2-) production and plasma thiobarbituric acid-reactive substance (TBARS) levels were greater in ET-1-infused rats (27+/-1 counts per minutes [CPM]/mg dry tissue weight and 8.9+/-0.8 micromol/l, respectively) than vehicle-infused rats (16+/-1 CPM/mg and 5.1+/-0.1 micromol/l, respectively). The ET-1-induced hypertension was prevented by simultaneous treatment with a new AT1 receptor antagonist, olmesartan (0.01% in chow, 117+/-5 mmHg, n =7), or hydralazine (15 mg/kg/day in drinking water, 118+/-4 mmHg, n=6). Olmesartan prevented ET-1-induced increases in vascular O2- production (15+/-1 CPM/mg) and plasma TBARS (5.0+/-0.1 micromol/l). Vascular O2- production and plasma TBARS were also decreased by hydralazine (21+/-1 CPM/mg and 7.0+/-0.3 micromol/l, respectively), but these levels were significantly higher than in vehicle-infused rats. These data suggest that ET-dependent hypertension is associated with augmentation of Ang II levels and ROS formation. The combined effects of the elevations in circulating ET-1 and Ang II, as well as the associated ROS production, may contribute to the development of hypertension induced by chronic ET-1 infusion.

Governance of arteriolar oscillation by ryanodine receptors

To investigate the role of ryanodine receptors in glomerular arterioles, experiments were performed using an isolated perfused hydronephrotic kidney model. In the first series of studies, BAYK-8644 (300 nM), a calcium agonist, constricted afferent (19.6 +/- 0.6 to 17.6 +/- 0.5 microm, n = 6, P < 0.01) but not efferent arterioles. Furthermore, BAYK-8644 elicited afferent arteriolar oscillatory movements. Subsequent administration of nifedipine (1 microM) inhibited both afferent arteriolar oscillation and constriction by BAYK-8644 (to 19.4 +/- 0.5 microm). In the second group, although BAYK-8644 constricted afferent arterioles treated with 1 microM of thapsigargin (19.7 +/- 0.6 to 16.8 +/- 0.6 microm, n = 5, P < 0.05), it failed to induce rhythmic contraction. Removal of extracellular calcium with EGTA (2 mM) reversed BAYK-8644-induced afferent arteriolar constriction (to 20.0 +/- 0.5 microm). In the third series of investigations, ryanodine (10 microM) but not 2-aminoethoxyphenyl borate (100 microM) abolished afferent arteriolar vasomotion by BAYK-8644. In the fourth series of experiments, in the presence of caffeine (1 mM), the stronger activation of voltage-dependent calcium channels by higher potassium media resulted in greater afferent arteriolar constriction and faster oscillation. Our results indicate that L-type calcium channels are rich in preglomerular but not postglomerular microvessels. Furthermore, the present findings suggest that either prolonged calcium influx through voltage-dependent calcium channels (BAYK-8644) or sensitized ryanodine receptors (caffeine) is required to trigger periodic calcium release through ryanodine receptors in afferent arterioles.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

Electrophysiological effects of endothelin-1 and their relationship to contraction in rat renal arterial smooth muscle

The electophysiological effects of endothelin-1 (ET-1) and their relationship to contraction remain unclear in the renal circulation. Using endotheliumdenuded arteries from the main branch of the renal artery proximal to the kidney of the rat, we have examined its effects on tension and conducted parallel patch-clamp measurements using freshly isolated smooth muscle cells from this tissue. Pharmacological experiments revealed that ET-1 produced constriction of renal arteries dependent on the influx of extracellular Ca(2+), mediated solely through ET(A) receptor stimulation. Current-clamp experiments revealed that renal arterial myocytes had a resting membrane potential of approximately 32 mV, with the majority of cells exhibiting spontaneous transient hyperpolarizations (STHPs). Application of ET-1 produced depolarization and in those cells exhibiting STHPs, either caused their inhibition or made them occur regularly. Under voltage-clamp conditions cells were observed to exhibit spontaneous transient outward currents (STOCs) inhibited by iberiotoxin. Application of voltage-ramps revealed an outward current activated at approximately -30 mV, sensitive to both 4-AP and TEA. Taken together these results suggest that renal arterial myocytes possess both delayed rectifying K(+) (K(V)) and Ca(2+)-activated K(+) (BK(Ca)) channels. Under voltage-clamp, ET-1 attenuated the outward current and reduced the magnitude and incidence of STOCs: effects mediated solely as a consequence of ET(A) receptor stimulation. Thus, in conclusion, activation of ET(A) receptors by ET-1 causes inhibition of K(V) and BK(Ca) channel activity, which could promote and/or maintain membrane depolarization. This effect is likely to favour L-type Ca(2+) channel activity providing an influx pathway for extracellular Ca(2+) essential for contraction.

Role of protein kinase C in angiotensin II-induced constriction of renal microvessels

Role of protein kinase C in angiotensin II-induced constriction of renal microvessels.

BACKGROUND

Although angiotensin II (Ang II) exerts its action through multiple vasomotor mechanisms, the contribution of phosphoinositol hydrolysis products to Ang II-induced renal vasoconstriction remains undetermined.

METHODS

The role of protein kinase C (PKC) in Ang II-induced afferent (AFF) and efferent (EFF) arteriolar constriction was examined using the isolated perfused hydronephrotic rat kidney.

RESULTS

Ang II (0.3 nmol/L)-induced EFF constriction was refractory to inhibition of voltage-dependent calcium channels by pranidipine (1 micromol/L, 19 +/- 2% reversal) but was completely reversed by a PKC inhibitor, chelerythrine (1 micromol/L, 96 +/- 2% reversal). Furthermore, direct PKC activation by phorbol myristate acetate (PMA; 1 micromol/L) caused prominent EFF constriction, and this constriction was inhibited by manganese and free calcium medium. In contrast, Ang II-induced AFF constriction was completely abolished by pranidipine (98 +/- 4% reversal) and was partially inhibited by chelerythrine (55 +/- 3% reversal). Although PMA elicited marked AFF constriction, this constriction was insensitive to the calcium antagonist, but was totally inhibited by manganese or free calcium medium.

CONCLUSIONS

PKC plays an obligatory role in Ang II-induced EFF constriction that requires extracellular calcium entry through nonselective cation channels. In contrast, in concert with our recent findings demonstrating a complete dilation by thapsigargin, Ang II-induced AFF constriction is mainly mediated by inositol trisphosphate (IP3) and voltage-dependent calcium channel pathways, but could not be attributed to the PKC-activated calcium entry pathway (for example, nonselective cation channels). Rather, Ang II-stimulated PKC may cross-talk to the IP3/voltage-dependent calcium channel pathway and could modulate the vasoconstrictor mechanism of the AFF. Thus, the role of PKC during Ang II stimulation differs in AFF and EFF, which may constitute segmental heterogeneity in the renal microvasculature.

Current concepts. Concussion in sports

This is a special report of the findings of the Concussion Workshop, sponsored by the AOSSM in Chicago in December 1997. Here follows a listing of the members of the workshop: Julian Bailes, MD, American Association of Neurological Surgeons; Arthur Boland, MD, AOSSM; Charles Burke III, MD, National Hockey League; Robert Cantu, MD, American College of Sports Medicine; Letha “Etty” Griffin, MD, National Collegiate Athletic Association; David Hovda, PhD, Neuroscientist, UCLA School of Medicine; Mary Lloyd Ireland, MD, American Academy of Orthopaedic Surgeons; James Kelly, MD, American Academy of Neurology; Greg Landry, MD, American Academy of Pediatrics; Mark Lovell, PhD, Neuropsychology Specialist, Henry Ford Health Systems; James Mathews, MD, American College of Emergency Physicians; Michael McCrea, PhD, Neuropsychology Specialist, Waukesha Memorial Hospital; Douglas McKeag, MD, American Medical Society for Sports Medicine; Dennis Miller, ATC, National Athletic Trainers Association; Jeffrey Minkoff, MD, AOSSM; Stephen Papadopoulus, MD, Congress of Neurological Surgeons; Elliott Pellman, MD, National Football League; Richard Quincy, MS, PT, ATC, Sports Physical Therapy, El Pomar Sports Center; Herbert Ross, DO, American Osteopathic Academy of Sports Medicine; Bryan Smith, MD, National Collegiate Athletic Association; and Edward Wojtys, MD, Workshop Chairman, AOSSM.

The views in this report do not necessarily represent the views of the entire group comprising the Concussion Workshop Group.

Regulation of protein kinase C in the muscular layer of human placental stem villi vessels

Protein kinase C (PKC) activity in the muscular layer of stem villi vessels from the human term placenta was studied. Resting state PKC activity was distributed evenly between the cytosol and the particulate fractions. Upon stimulation by three different activators, phorbol 12-myristate 13-acetate, fluoride and endothelin-1, a translocation of PKC activity from the cytosolic to the particulate fraction was observed. The expression and distribution of PKC isoforms were then examined by Western blot analysis using specific antibodies to PKC isoforms. At least four PKC isoforms, PKCalpha, PKCbeta1, PKCbeta2, PKCzeta, and trace amounts of PKCepsilon were detected in both fractions. Their relative responses to the different agonists were examined by quantifying their subcellular redistribution. No significant differential activation of the four mainly expressed PKC isoforms were observed in response to stimulation with any of the stimuli. Moreover, our results show that endothelin-1 induced translocation/activation of PKC in this vascular smooth muscle.

Endothelin impairs ATP-sensitive K+ channel function after brain injury

In piglets, pial arteries constrict, ATP-sensitive K+ (KATP) channel function is impaired, and cerebrospinal fluid endothelin-1 (ET-1) increases to 10(-10) M after brain injury [fluid percussion injury (FPI)]. Nitric oxide (NO) elicits dilation via guanosine 3',5'-cyclic monophosphate (cGMP) and KATP channel activation. This study was designed to characterize the relationship between ET-1 and impaired function of KATP channels after FPI. Injury was produced via the lateral FPI technique in piglets equipped with a closed cranial window. Cromakalim, a KATP agonist, produced dilation that was attenuated by FPI and partially restored by BQ-123, an ET-1 antagonist (11 +/- 1 and 23 +/- 2 vs. 2 +/- 1 and 4 +/- 1 vs. 8 +/- 1 and 17 +/- 2% for responses to 10(-8) and 10(-6) M cromakalim before FPI, after FPI, and after FPI with BQ-123, respectively). Because ET-1 constriction may antagonize dilation, separate experiments were conducted under conditions of equivalent baseline diameter in the absence and presence of ET-1 (10(-10) M). Cromakalim dilation was attenuated by ET-1 and partially restored by the protein kinase C (PKC) inhibitor staurosporine (12 +/- 1 and 28 +/- 1 vs. 2 +/- 1 and 21 +/- 3 vs. 9 +/- 1 and 29 +/- 2% for 10(-8) and 10(-6) M cromakalim, cromakalim with ET-1, and cromakalim with ET-1 + staurosporine, respectively). Similar interactions were observed with calcitonin gene-related peptide, 8-bromoguanosine 3',5'-cyclic monophosphate, and the NO releasers sodium nitroprusside and S-nitroso-N-acetylpenicillamine. These data show that ET-1 blunts KATP channel-, NO-, and cGMP-mediated dilation. These data suggest that ET-1 contributes to altered cerebral hemodynamics after FPI through impairment of KATP channel function via PKC activation.

Cellular mechanisms mediating rat renal microvascular constriction by angiotensin II

To assess cellular mechanisms mediating afferent (AA) and efferent arteriolar (EA) constriction by angiotensin II (AngII), experiments were performed using isolated perfused hydronephrotic kidneys. In the first series of studies, AngII (0.3 nM) constricted AAs and EAs by 29+/-3 (n = 8, P < 0.01) and 27+/-3% (n = 8, P < 0.01), respectively. Subsequent addition of nifedipine restored AA but not EA diameter. Manganese (8 mM) reversed EA constriction by 65+/-9% (P < 0.01). In the second group, the addition of N-ethylmaleimide (10 microM), a Gi/Go protein antagonist, abolished AngII- induced EA (n = 6) but not AA constriction (n = 6). In the third series of experiments, treatment with 2-nitro-4-carboxyphenyl-N, N-diphenyl-carbamate (200 microM), a phospholipase C inhibitor, blocked both AA and EA constriction by AngII (n = 6 for each). In the fourth group, thapsigargin (1 microM) prevented AngII-induced AA constriction (n = 8) and attenuated EA constriction (8+/-2% decrease in EA diameter at 0.3 nM AngII, n = 8, P < 0.05). Subsequent addition of manganese (8 mM) reversed EA constriction. Our data provide evidence that in AAs, AngII stimulates phospholipase C with subsequent calcium mobilization that is required to activate voltage-dependent calcium channels. Our results suggest that AngII constricts EAs by activating phospholipase C via the Gi protein family, thereby eliciting both calcium mobilization and calcium entry.

Endothelins in the normal and diseased kidney

Endothelin-1 (ET-1) is a 21-amino acid peptide that potently modulates renal function. ET-1 is produced by, and binds to, most renal cell types. ET-1 exerts a wide range of biologic effects in the kidney, including constriction of most renal vessels, mesangial cell contraction, inhibition of sodium and water reabsorption by the nephron, enhancement of glomerular cell proliferation, and stimulation of extracellular matrix accumulation. ET-1 functions primarily as an autocrine or paracrine factor; its renal effects must be viewed in the context of its local production and actions. This is particularly important when comparing ET-1 biology in the nephron, where it promotes relative hypotension through increased salt and water excretion, with ET-1 effects in the vasculature, where it promotes relative hypertension through vasoconstriction. Numerous studies indicate that ET-1 is involved in the pathogenesis of a broad spectrum of renal diseases. These include those characterized by excessive renal vascular resistance, such as ischemic renal failure, cyclosporine (CyA) nephrotoxicity, radiocontrast nephropathy, endotoxemia, rhabdomyolysis, acute liver rejection, and others. ET-1 appears to play a role in cell proliferation in the setting of inflammatory glomerulonephritides. The peptide also may mediate, at least in part, excessive extracellular matrix accumulation and fibrosis occurring in chronic renal failure, diabetes mellitus, and other disorders. Deranged ET-1 production in the nephron may cause inappropriate sodium and water retention, thereby contributing to the development and/or maintenance of hypertension. Finally, impaired renal clearance of ET-1 may cause hypertension in patients with end-stage renal disease. Many ET-1 antagonists have been developed; however, their clinical usefulness has not yet been determined. Despite this, these agents hold great promise for the treatment of renal diseases; it is hoped that the next decade will witness their introduction into clinical practice.

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

Role of chloride channels in afferent arteriolar constriction

The effects of IAA-94, a chloride channel blocker and/or low chloride perfusate on afferent arteriolar (AA) constriction by angiotensin II (Ang II), norepinephrine (NE) and increasing pressure (80 to 160 mm Hg) were assessed using isolated perfused hydronephrotic kidneys. In the first series of experiments, Ang II (0.3 nM) constricted AAs by 33 +/- 3% (N = 5, P < 0.01). Subsequent addition of diltiazem (10 microM) restored the decrements in the AA diameters. In the presence of diltiazem (10 microM), increasing pressure did not constrict AAs. In the second series of experiments. elevation of pressure constricted AAs by 20 +/- 2% (N = 7. P < 0.01). Subsequent addition of IAA-94 (30 microM) failed to alter the basal AA diameter and myogenic responsiveness. However, Ang II-induced AA constriction was abolished by IAA-94. In the third series of experiments, decreasing extracellular chloride exaggerated AA constriction by 0.1 nM of Ang II (from 13 +/- 2 to 20 +/- 3%, N = 6, P < 0.05). Similarly, low chloride perfusate enhanced NE (0.1 microM)-induced AA constriction (from 14 +/- 2 to 19 +/- 2%, N = 6, P < 0.05). In contrast, myogenic responsiveness was not influenced by reducing chloride concentrations. The present data provide evidence that both Ang II and NE induce AA constriction by opening chloride channels and subsequent activation of voltage-dependent calcium channels, and suggest that the myogenic response is mediated by activating voltage-dependent calcium channels independently of chloride channels.

Role of large Ca(2+)-activated K+ channels in regulation of mesangial contraction by nitroprusside and ANP

The patch-clamp method, in conjunction with measurements of cell contraction, was employed to investigate activation by guanosine 3',5'-cyclic monophosphate (cGMP) and guanylyl cyclase-stimulating vasodilators of large Ca(2+)-activated K+ channels (BKCa) in human glomerular mesangial cells (MC). In cell-attached patches, with physiological NaCl bathing solutions, BKCa was activated transiently by nitroprusside [NP; a nitric oxide (NO) donor], atrial natriuretic peptide (ANP), and dibutyryl cGMP (DBcGMP), reaching peak responses between 10 and 60 s and decreasing to near baseline activity within the next 120 s. In the presence of LY-83583, a specific inhibitor of guanylyl cyclase, BKCa was activated on cell by DBcGMP but not by NP or ANP. In all cases, the increase in channel activity coincided with a decrease in channel amplitude, indicating that the membrane potential was approaching equilibrium potential as BKCa was activated. If membrane potential was maintained depolarized with 140 mM KCl in the bathing solution, DBcGMP induced a sustained activation of BKCa. In the continued presence of DBcGMP, BKCa was further activated when 100 nM angiotensin II (ANG II) was added to the bathing solution. Experiments were performed to determine the role of BKCa in the regulation by vasorelaxants of mesangial contraction measured as percent maximal and defined by reduction in length induced by replacing 135 mM bath NaCl with KCl. Contraction by ANG II (100 nM = 60.5%) was attenuated by NP (100 microM), ANP (1.0 microM), and DBcGMP (10 microM) in the absence, but not the presence, of iberiotoxin, a specific inhibitor of BKCa. These results indicate that guanylyl cyclase-stimulating vasorelaxants counteract ANG II-induced contraction of MC, in part, by repolarizing the membrane through activation of BKCa channels.

MCI-154-induced relaxation in vascular smooth muscles of guinea pig

We investigated the vasorelaxant effects of MCI-154, a cardiotonic agent designed to target thin filaments in cardiac muscles in intact and skinned vessels from guinea pigs. In normal Krebs-Henseleit solution, MCI-154 (10(-7)-10(-4) M) inhibited the contractions induced by angiotensin II, (Ang II), endothelin-1 (ET-1), phenylephrine, and phorbol 12-myristate 13-acetate (PMA) in a concentration-dependent manner in guinea pig aorta. In Ca(2+)-free solutions, ET-1 and PMA caused slowly developing and sustained contractions in guinea pig aorta, whereas phenylephrine and caffeine induced transient contractions due to Ca2+ release from the sarcoplasmic reticulum (SR). MCI-154 (10(-7)-10(-4) M) inhibited the contractile responses to ET-1 and PMA. MCI-154 also reduced the contraction induced by Ca2+ release from phenylehrine- and caffeine-sensitive Ca2+ store sites. On the other hand, the relaxation response to MCI-154 was not affected by the presence of methylene blue, a guanylate cyclase inhibitor or by the removal of endothelial cells. MCI-154 decreased the Ca(2+)-activated tension development in saponin-treated skinned fibers from guinea pig femoral arteries. The effects of MCI-154 were not potentiated in the presence of protein kinase A (PKA), whereas those of cyclic AMP were potentiated, possibly because of lack of protein kinase A. The present experiments demonstrate that MCI-154 inhibits vascular contraction when the contractions are produced by any of three mechanisms: protein kinase C (PKC) activation, Ca2+ mobilization from store sites, or sensitization of contractile elements by Ca2+.

Mechanisms of angiotensin II chronotropic effect in anaesthetized dogs

1. The chronotropic effect of angiotensin II (5 micrograms in 1 ml of Tyrode solution), injected directly into the sinus node artery of 24 anaesthestized and vagotomized dogs pretreated with a beta-adrenoceptor antagonist, was evaluated before and after the administration of: (a) an angiotensin II AT1 receptor antagonist (losartan, 50 micrograms kg-1 min-1 infused i.v. for 120 min), (b) an alpha-adrenoceptor antagonist (prazosin, 1 mg kg-1 i.v. bolus injected), (c) a Ca2+ channel blocker (nifedipine 50, 100 and 200 micrograms kg-1 i.v. bolus injected) and (d) a protein kinase inhibitor (staurosporine, 800 nM infused via the sinus node artery at 0.6 ml min-1 for 15 min). 2. Losartan and staurosporine by themselves had no effect on basal systemic arterial pressure and heart rate, whereas prazosin and nifedipine caused significant diminutions of both parameters. 3. Angiotensin II induced significant increases in heart rate, the mean augmentations being 29 +/- 2 beats min-1. Losartan, nifedipine and staurosporine significantly decreased the chronotropic effect of angiotensin II, the mean respective diminutions being 65 +/- 8, 40 +/- 9 and 64 +/- 10%, whereas prazosin had no effect. 4. This work has demonstrated that angiotensin II exerts in vivo a significant positive chronotropic effect that is mediated via AT1 receptors located in the region of the sinoatrial node. This effect is independent of the adrenergic system. It is decreased by the inhibition of the production of protein kinases, most probably of protein kinase C, and by the blockade of the voltage-sensitive L-type Ca2+ channels. Other studies are obviously needed to ascertain the role of angiotensin II in the control of heart rate and/or the genesis of arrhythmias.