Effect of hyperoxic resuscitation on propensity of germinal matrix haemorrhage and cerebral injury.

AIMS: Intraventricular haemorrhage (IVH) and cerebral injury are major neurological disorders of premature infants. The effect of hyperoxic resuscitation on the occurrence of IVH and cerebral injury is elusive. Therefore, we asked whether hyperoxia during neonatal resuscitation increased the incidence and severity of IVH and cerebral injury in premature newborns. METHODS: Premature rabbit pups, delivered by C-section, were sequentially assigned to receive 100%, 40% or 21% oxygen for 15 or 60 min at birth. The pups were treated with intraperitoneal glycerol at 24-h postnatal age to determine the incidence and severity of glycerol-induced IVH. Vascular endothelial growth factor and angiopoietin-2 genes and protein expression, endothelial proliferation as well as free radical levels and antioxidants were assessed in the germinal matrix, white matter and cortex of pups exposed to 100% oxygen and to 21% oxygen. RESULTS: Exposure with 100% oxygen for 1 h did not adversely exacerbate the incidence of glycerol-induced IVH in premature rabbit pups. Compared with room air, 100% oxygen enhanced mRNA expression of both vascular endothelial growth factor and angiopoietin-2 as well as reactive oxygen species levels in the germinal matrix. Hyperoxia did not affect endothelial proliferation, apoptosis or neuronal degeneration in the forebrain. CONCLUSION: Our data suggest that 100% oxygen exposure for 1 h does not increase the risk of IVH or cerebral injury in premature rabbit pups. Although extrapolating rabbit neural developmental data into humans has obvious limitations, we speculate that hyperoxia of short duration at birth in premature infants may not result in major neurological adverse effects.

Advances and challenges in geroscience research: An update.

Aging remains the most pervasive risk factor for a wide range of chronic diseases that afflict modern societies. In the United States alone, incidence of age-related diseases (e.g., cardiovascular disease, stroke, Alzheimer's disease, vascular cognitive impairment and dementia, cancer, hypertension, type-2 diabetes, chronic obstructive pulmonary disease, and osteoarthritis) is on the rise, posing an unsustainable socioeconomic burden even for the most developed countries. Tackling each and every age-related disease alone is proving to be costly and ineffective. The emerging field of geroscience has posed itself as an interdisciplinary approach that aims to understand the relationship between the biology of aging and the pathophysiology of chronic age-related diseases. According to the geroscience concept, aging is the single major risk factor that underlies several age-related chronic diseases, and manipulation of cellular and systemic aging processes can delay the manifestation and/or severity of these age-related chronic pathologies. The goal of this endeavor is to achieve health improvements by preventing/delaying the pathogenesis of several age-related diseases simultaneously in the elderly population by targeting key cellular and molecular processes of aging instead of managing diseases of aging as they arise individually. In this review, we discuss recent advances in the field of geroscience, highlighting their implications for potential future therapeutic targets and the associated scientific challenges and opportunities that lay ahead.

Flow-induced constriction in arterioles of hyperhomocysteinemic rats is due to impaired nitric oxide and enhanced thromboxane A(2) mediation.

Hyperhomocysteinemia (HHcy) is thought to promote arteriosclerosis and peripheral arterial disease, in part by impairing the function of endothelium. Because flow-induced dilation is mediated by the endothelium, we hypothesized that HHcy alters this response by interfering with the synthesis/action of NO and prostaglandins. Thus, changes in the diameter of isolated, pressurized (at 80 mm Hg) gracilis skeletal muscle arterioles (diameter approximately 170 microm) from control and methionine diet-induced HHcy rats were investigated with videomicroscopy. Increases in intraluminal flow (from 0 to 25 microL/min) resulted in dilations of control arterioles (maximum, 34+/-4 microm). In contrast, increases in flow elicited constrictions of HHcy arterioles (-36+/-3 microm). In control arterioles, the NO synthase inhibitor N:(omega)-nitro-L-arginine-methyl ester significantly attenuated (approximately 50%) dilation, whereas the additional administration of indomethacin, an inhibitor of cyclooxygenase, eliminated flow-induced dilation. In the arterioles of HHcy rats, flow-induced constriction was not affected by N:(omega)-nitro-L-arginine-methyl ester, whereas it was abolished by indomethacin or the prostaglandin H(2)/thromboxane A(2) (TXA(2)) receptor antagonist SQ 29,548 or the TXA(2) synthase inhibitor CGS 13,080. Thus, in HHcy, increases in intraluminal flow elicit constrictions of skeletal muscle arterioles due to the impaired NO and enhanced TXA(2) mediation of the response, alterations that likely contribute to the development of peripheral arterial disease.

Increased myogenic tone in skeletal muscle arterioles of diabetic rats. Possible role of increased activity of smooth muscle Ca2+ channels and protein kinase C.

OBJECTIVE: The diabetes mellitus-induced microangiopathy is still not clearly characterized. In this study we aimed to elucidate the effect of streptozotocin (STZ)-induced diabetes on myogenic response of isolated rat skeletal muscle arterioles and the mechanisms responsible for its alterations. METHODS: Male rats were divided into two groups: (1) control rats (C, plasma glucose: 6.4 +/- 0.5 mmol/l, n = 40) (2) diabetic rats (DM, 65 mg/kg STZ i.v., plasma glucose: 25.7 +/- 0.7 mmol/l, n = 40). Changes in diameter of isolated, cannulated gracilis skeletal muscle arterioles (approximately 130 microns in diameter) were measured by video-microscopy. RESULTS: Step increases in perfusion pressure (PP; from 10 to 140 mmHg) elicited significantly greater constrictions in DM than in C gracilis arterioles, in the presence of the endothelium (E). Also, a step increase in PP (from 40 to 100 mmHg) elicited greater and faster constrictions in DM vs. C arterioles. There were no significant differences in the pressure-passive diameter (in Ca2+ free solution) curves of arterioles. Dilations to acetylcholine were impaired in arterioles of DM as compared to those of C rats (EC50, C: 4.0 +/- 0.9 x 10(-9) mol/l, DM: 4.8 +/- 2.0 x 10(-8) mol/l (p < 0.01), and unaffected by inhibition of nitric oxide synthesis with L-NNA (10(-4) mol/l). Arteriolar constrictions to norepinephrine (NE) were significantly greater in DM compared to those of C rats (EC50, C: 6.2 +/- 0.6 x 10(-7) mol/l, DM: 8.0 +/- 2.0 x 10(-8) mol/l, p < 0.01) both in the presence and absence of E. In the absence of the E, constrictions to increases in pressure, or Ca2+ (0.25-7.5 mmol/l), or the voltage-dependent Ca(2+)-channel agonist Bay K 8644 (EC50; DM: 4.2 +/- 1.5 x 10(-10) mol/l, C: 1.7 +/- 0.8 x 10(-9) mol/l, p < 0.05) or the protein kinase C activator phorbol 12-myristate 13-acetate (PMA, EC50; DM: 6 +/- 2 x 10(-9) mol/l, C: 2 +/- 1 x 10(-8) mol/l, p < 0.05) were significantly greater in arterioles of DM compared to those of C rats. CONCLUSION: The novel findings of our study are that in diabetes mellitus the myogenic response of rat skeletal muscle arterioles is enhanced, which seems to be independent from the impaired endothelial function present simultaneously, and likely due to the increased activity of voltage-dependent Ca2+ channels and/or upregulation of protein kinase C in arteriolar smooth muscle.

Review of cardiovascular effects of fluoxetine, a selective serotonin reuptake inhibitor, compared to tricyclic antidepressants.

Fluoxetine is an antidepressant drug, a potent and specific inhibitor of serotonin reuptake (SSRI). Evidence suggests that being compared with tricyclic antidepressants, fluoxetine may cause significantly fewer anticholinergic, antihistaminergic and cardiotoxic side effects in the treatment of major depressive disorders. Chronic treatment with fluoxetine was not reported to affect the electrocardiogram (ECG). There is no clinical evidence of conduction delay and very little evidence of orthostatic hypotension. In the overdosed patients fewer cardiac symptoms were reported than with tricyclic antidepressants. However, dysrhythmia (atrial fibrillation and bradycardia) and syncope associated with fluoxetine treatment and overdose were reported. Although such reports have not been common, they do raise concerns. Thus we investigated the direct cardiovascular effects of the fluoxetine in isolated heart preparations and vessels of rats and rabbits. From 10(-6)M to 10(-4)M concentrations fluoxetine showed cardiodepressant and vasodilatory effects. These effects were similar to those of previously reported on tricyclic compounds. This review is a brief summary of possible cardiovascular effects of fluoxetine and other new SSRIs antidepressants from the literature based on experience of clinical studies and our experiments with fluoxetine on isolated rat and rabbit cardiac preparations and vessels. Possible explanations of the lower incidence of cardiovascular complications with fluoxetine in humans and cardiodepressant effects in vitro are discussed.

Speculations on difference between tricyclic and selective serotonin reuptake inhibitor antidepressants on their cardiac effects. Is there any?

The cardiovascular effects and toxicity of tricyclic antidepressants (TCAs) have been well documented in medical literature. The most common manifestation of such effects is slowing of intraventricular conduction, manifested by prolonged PR, QRS and QT intervals on the standard electrocardiogram (ECG) and postural hypotension. In contrast to TCAs, selective serotonin reuptake inhibitors (SSRIs), including fluoxetine and citalopram, are considered to cause less effect on cardiac impulse conduction. In addition, these compounds induced significantly less anticholinergic, antihistaminergic and cardiotoxic side-effects than TCAs. However, there is an increasing number of case reports on dysrhythmias, like atrial fibrillation or bradycardia and syncope associated with fluoxetine and another SSRI treatment and overdose. Although such reports have not been common, they do raise concerns. In cardiac tissues isolated from canine, rabbit, rat and guinea pig hearts we have found that fluoxetine and citalopram inhibited cardiac Na+ and Ca2+ channels. These direct cardiac electrophysiological effects were similar to those of observed for tricyclic antidepressants clomipramine and imipramine. The inhibition of cardiac Ca2+ and Na+ channels by fluoxetine may explain most cardiac side-effects observed occasionally with the drug and mild but significant bradycardia reported during chronic treatment. Our results suggest that fluoxetine and citalopram may have antiarrhythmic (class I + IV type), as well as proarrhythmic properties (due to impairment of atrioventricular or intraventricular conduction and shortening of repolarization). Taking all these into consideration, in depressed patients having also severe cardiac disorders, ECG control may be suggested during fluoxetine and probable another SSRI therapy. The primary goal of this review is to compare these direct cardiac effects of fluoxetine and citalopram to those of previously reported for TCAs. This paper also summarizes the recently observed effects of fluoxetine apparently not related to the blockage of 5-HT transporter based on literature.

Serotonin reuptake inhibitor, fluoxetine, dilates isolated skeletal muscle arterioles. Possible role of altered Ca2+ sensitivity.

1. Inhibitors of serotonin reuptake in the central nervous system, such as fluoxetine, may also affect the function of vascular tissues. Thus, we investigated the effect of fluoxetine on the vasomotor responses of isolated, pressurized arterioles of rat gracilis muscle (98 +/- 4 microns in diameter at 80 mmHg perfusion pressure). 2. We have found that increasing concentrations of fluoxetine dilated arterioles up to 155 +/- 5 microns with an EC50 of 2.5 +/- 0.5 x 10(-6) M. 3. Removal of the endothelium, application of 4-aminopyridine (4-AP, an inhibitor of aminopyridine sensitive K+ channels), or use of glibenclamide (an inhibitor of ATP-sensitive K+ channels) did not affect the vasodilator response to fluoxetine. 4. In the presence of 10(-6), 2 x 10(-6) or 10(-5) M fluoxetine noradrenaline (NA, 10(-9)-10(-5) M) and 5-hydroxytryptamine (5-HT, 10(-9)-10(-5)M)-induced constrictions were significantly attenuated resulting in concentration-dependent parallel rightward shifts of their dose-response curves (pA2 = 6.1 +/- 0.1 and 6.9 +/- 0.1, respectively). 5. Increasing concentrations of Ca2+ (10(-4) 3 x 10(-2) M) elicited arteriolar constrictions (up to approximately 30%), which were markedly reduced by 2 x 10(-6)M fluoxetine, whereas 10(-5)M fluoxetine practically abolished these responses. 6. In conclusion, fluoxetine, elicits substantial dilations of isolated skeletal muscle arterioles, a response which is not mediated by 4-AP- and ATP-sensitive K+ channels or endothelium-derived dilator factors. The findings that fluoxetine had a greater inhibitory effect on Ca2+ elicited constrictions than on responses to NA and 5-HT suggest that fluoxetine may inhibit Ca2+ channel(s) or interfere with the signal transduction by Ca2+ in the vascular smooth muscle cells.

The vasoconstrictor effects of L-NAME, a nitric oxide synthase inhibitor, in pregnant rabbits.

1. We have used anaesthetized, acutely instrumented non-pregnant (NP) and late pregnant (P) New Zealand white rabbits to examine the possible role of nitric oxide (NO) in the pregnancy-induced fall of vascular tone and arterial pressure. Systemic, renal and pulmonary vascular resistance, as well as plasma concentrations of cyclic GMP (PcGMP) were compared before and after the inhibition of NO synthesis by N(G)-nitro-L-arginine methyl ester (L-NAME). 2. P rabbits had lower baseline total peripheral resistance (TPR), mean arterial pressure (MAP) and higher PcGMP than NP controls (all P < 0.05 or less). L-NAME (1, 10, 50 mg kg1, i.v.) resulted in dose-dependent elevation of TPR in both groups. However, the absolute, as well as percentage increases in TPR were greater (P < 0.05) in NP than in P rabbits. 3. Cardiac output (CO) was reduced more (P < 0.01) by NO inhibition in NP than P rabbits. Therefore, despite the smaller increase in TPR, the elevation of MAP was greater (P < 0.001) in P than NP rabbits. After L-NAME, NP rabbits developed more severe bradycardia and a greater increase of pulmonary vascular resistance which might have contributed to the more pronounced reduction of CO. 4. PcGMP increased in both groups following L-NAME, but more (P < 0.01) in NP than P rabbits. 5. Infusion of acetylcholine (ACh, 0.02 micromol l-1 kg-1) reduced MAP and TPR more (both P < 0.05) in NP than P rabbits and L-NAME reduced the ACh-induced depressor response only in NP rabbits. 6. These results suggest that the low vascular tone and arterial pressure in pregnant rabbits is not mediated by NO.

Selected contribution: NO released to flow reduces myogenic tone of skeletal muscle arterioles by decreasing smooth muscle Ca(2+) sensitivity.

To clarify the contribution of intracellular Ca(2+) concentration ([Ca(2+)](i))-dependent and -independent signaling mechanisms in arteriolar smooth muscle (aSM) to modulation of arteriolar myogenic tone by nitric oxide (NO), released in response to increases in intraluminal flow from the endothelium, changes in aSM [Ca(2+)](i) and diameter of isolated rat gracilis muscle arterioles (pretreated with indomethacin) were studied by fluorescent videomicroscopy. At an intraluminal pressure of 80 mmHg, [Ca(2+)](i) significantly increased and myogenic tone developed in response to elevations of extracellular Ca(2+) concentration. The Ca(2+) channel inhibitor nimodipine substantially decreased [Ca(2+)](i) and completely inhibited myogenic tone. Dilations to intraluminal flow (that were inhibited by N(omega)-nitro-L-arginine methyl ester) or dilations to the NO donor S-nitroso-N-acetyl-DL-penicillamine (that were inhibited by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) were not accompanied by substantial decreases in aSM [Ca(2+)](i). 8-Bromoguanosine cGMP and the cGMP-specific phosphodiesterase inhibitor zaprinast significantly dilated arterioles yet elicited only minimal decreases in [Ca(2+)](i). Thus flow-induced endothelial release of NO elicits relaxation of arteriolar smooth muscle by a cGMP-dependent decrease of the Ca(2+) sensitivity of the contractile apparatus without substantial changes in the pressure-induced level of [Ca(2+)](i).

Oxidative stress-induced isoprostane formation may contribute to aspirin resistance in platelets.

Recent studies suggest that aggregation of platelets from patients with coronary artery and cerebrovascular disease may be resistant to low-dose aspirin (ASA) treatment, which may promote plaque-associated thrombus formation. However, the underlying mechanisms of platelet ASA resistance are poorly understood. ASA is thought to inhibit platelet aggregation primarily by inactivating the cyclooxygenase (COX), thus decreasing the synthesis of the pro-aggregatory arachidonic acid metabolite thromboxane A(2) (TxA(2)). However, recent studies also identified a non-enzymatic, oxidation-dependent pathway for the synthesis of the arachidonic acid derivative isoprostanes, which exhibit potent vasoconstrictor and pro-aggregatory effects similar to that of TxA(2). Because the pathophysiological conditions that promote arteriosclerotic vascular diseases (e.g. hypercholesterolemia, diabetes, hyperhomocysteinemia) are thought to be associated with an increased formation of reactive oxygen species and increased plasma isoprostane levels, it can be hypothesized that increased COX-independent isoprostane formation in platelets contribute to ASA resistance.

Electrophysiological effects of fluoxetine in mammalian cardiac tissues.

Fluoxetine is a widely used antidepressant compound having selective serotonin reuptake inhibitor properties. In this study, the actions of fluoxetine were analyzed in guinea pig, rat, rabbit and canine ventricular myocardiac preparations using conventional microelectrode and whole cell voltage clamp techniques. Low concentrations of fluoxetine (1-10 micromol/l) caused significant shortening of action potential duration (APD) and depression of the plateau potential in guinea pig and rabbit papillary muscles and single canine ventricular myocytes. In rat papillary muscle, APD was not affected by fluoxetine (up to 100 micromol/l), however, the drug decreased the force of contraction with EC50 of 10 micromol/l. Fluoxetine (10 micromol/l) also decreased the maximum velocity of depolarization and action potential overshoot in each species studied. At this concentration no effect was observed on the resting membrane potential; high concentration (100 micromol/l), however, caused depolarization. In voltage clamped canine ventricular myocytes, fluoxetine caused concentration-dependent block of the peak Ca2+ current at 0 mV with EC50 of 5.4+/-0.94 micromol/l and Hill coefficient of 1.1+/-0.14 (n=6). In addition, 10 micromol/l fluoxetine shifted the midpoint of the steady-state inactivation curve of the Ca2+ current from -20.7+/-0.65 to -26.7+/-1 mV (P<0.001, n=8) without changing its slope factor. These effects of fluoxetine developed rapidly and were fully reversible. Fluoxetine did not alter voltage-dependence of activation or time constant for inactivation of I(Ca). Fluoxetine had no effect on the amplitude of K+ currents (I(K1) and I(to)). The inhibition of cardiac Ca2+ and Na+ channels by fluoxetine may explain most cardiac side effects observed occasionally with the drug. Our results suggest that fluoxetine may have antiarrhythmic (class I + IV type), as well as proarrhythmic properties (due to impairment of atrioventricular or intraventricular conduction and shortening of repolarization). Therefore, in depressed patients with cardiac disorders, ECG control may be suggested during fluoxetine therapy.

Selective serotonin-reuptake inhibitor antidepressants increase the risk of falls and hip fractures in elderly people by inhibiting cardiovascular ion channels.

Surprising results from recently published retrospective studies show that the use of new selective serotonin-reuptake inhibitor antidepressants (SSRIs), similarly to the older tricyclic antidepressants (TCAs), increases the risk of falls and hip fractures among elderly people.The mechanism whereby antidepressants increase this risk is complex and may include orthostatic hypotension, arrhythmias, sedation and confusion. The increased risk of falls and hip fractures with the use of TCAs is not surprising considering their well-known cardiovascular, anticholinergic and antihistaminergic side-effects. But the increased risk of falls with SSRIs is highly unexpected since these drugs are believed to be free from the disadvantages of TCAs. We hypothesized that the new SSRI antidepressants may also have cardiovascular effects similarly to the older TCA compounds, which may be an explanation for the increased rate of falls and hip fractures. The experimental and clinical evidence in support of this hypothesis are discussed.

Can simvastatin promote tumor growth by inducing angiogenesis similar to VEGF?

Recent studies suggest that the HMG-CoA reductase inhibitor simvastatin--similar to vascular endothelial growth factor (VEGF)--may promote angiogenesis by activation of a protein kinase Akt-nitric oxide synthase dependent pathway in endothelial cells, an effect that may be beneficial in the treatment of ischemic heart disease. However, induction of angiogenesis by VEGF contributes importantly to the blood supply of developing tumors and tumor metastases as well. Thus, it can be hypothesized that chronic systemic treatment of elderly patients with a drug that induces angiogenesis by a VEGF-like manner will also promote tumor growth.

Long-term deficiency of circulating and hippocampal insulin-like growth factor I induces depressive behavior in adult mice: a potential model of geriatric depression.

Numerous studies support the hypothesis that deficiency of insulin-like growth factor I (IGF-1) in adults contributes to depression, but direct evidence is limited. Many psychological and pro-cognitive effects have been attributed to IGF-1, but appropriate animal models of adult-onset IGF-1 deficiency are lacking. In this study, we use a viral-mediated Cre-loxP system to knockout the Igf1 gene in either the liver, neurons of the CA1 region of the hippocampus, or both. Knockout of liver Igf1 reduced serum IGF-1 levels by 40% and hippocampal IGF-1 levels by 26%. Knockout of Igf1 in CA1 reduced hippocampal IGF-1 levels by 13%. The most severe reduction in hippocampal IGF-1 occurred in the group with knockouts in both liver and CA1 (36% reduction), and was associated with a 3.5-fold increase in immobility in the forced swim test. Reduction of either circulating or hippocampal IGF-1 levels did not alter anxiety measured in an open field and elevated plus maze, nor locomotion in the open field. Furthermore, local compensation for deficiencies in circulating IGF-1 did not occur in the hippocampus, nor were serum levels of IGF-1 upregulated in response to the moderate decline of hippocampal IGF-1 caused by the knockouts in CA1. We conclude that adult-onset IGF-1 deficiency alone is sufficient to induce a depressive phenotype in mice. Furthermore, our results suggest that individuals with low brain levels of IGF-1 are at increased risk for depression and these behavioral effects are not ameliorated by increased local IGF-1 production or transport. Our study supports the hypothesis that the natural IGF-1 decline in aging humans may contribute to geriatric depression.

The aging kidney: role of endothelial oxidative stress and inflammation.

The population in the Western world is aging. In 1996 those aged 60 years and over formed 21% of the EU population, by 2022 this proportion will have risen to 27%. Based on current trends a third of the EU population could be 60 years of age and over by the age 2050. Epidemiological studies suggest that even in the absence of other risk factors (e.g. diabetes, hypertension, hypercholesterolemia), advanced age itself significantly increases cardiovascular morbidity by promoting the development of atherosclerosis and by impairing normal cellular functions. One of the most prominent organs affected by aging is the kidney. There is evidence that age-associated phenotypic changes may be an important cause of renal failure. We propose that vascular oxidative stress and inflammation are generalized phenomena during senescence, which importantly contribute to the morphological and functional changes in the aging kidney. The present review focuses on some of the mechanisms by which advanced age may promote vascular oxidative and nitrosative stress and the possible downstream mechanisms by which reactive oxygen and nitrogen species may impair vascular and renal function in aging.

Endothelin and prostaglandin H(2)/thromboxane A(2) enhance myogenic constriction in hypertension by increasing Ca(2+) sensitivity of arteriolar smooth muscle.

The myogenic response of skeletal muscle arterioles is enhanced in hypertension because of the release of endothelin (ET) and prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) from the endothelium. We hypothesized that ET and PGH(2)/TxA(2) modulate Ca(2+) signaling in arteriolar smooth muscle and thereby enhance myogenic constriction. Thus, simultaneous changes in intracellular Ca(2+) concentration in smooth muscle ([Ca(2+)](i)), measured by fura 2 microfluorometry (expressed as Ca(2+) fluorescence ratio [R(Ca)]), and diameter were obtained as a function of intraluminal pressure (P(i)) in isolated cannulated gracilis muscle arterioles (diameter approximately 120 micrometer) of normotensive Wistar rats (WR) and spontaneously hypertensive rats (SHR). In the absence of extracellular Ca(2+), increases in P(i) from 20 to 160 mm Hg increased the passive diameter of arterioles without changes in R(Ca). In the presence of extracellular Ca(2+) and endothelium, increases in P(i) elicited similar increases in R(Ca) (30+/-7% for control and 33+/-8% for SHR at 160 mm Hg) but a significantly (P<0.05) greater constriction of SHR arterioles compared with WR arterioles (at 160 mm Hg, 55+/-4% versus 38+/-2%, respectively, of passive diameter). In the absence of the endothelium, P(i)-induced changes in the R(Ca) and diameter of SHR and WR arterioles did not differ significantly. Also, a step increase in P(i) (from 80 to 140 mm Hg) elicited a similar increase in R(Ca) but greater constrictions in SHR versus WR arterioles. In the presence of the TxA(2) receptor inhibitor SQ29,548 and the ET(A) receptor inhibitor BQ123, there was no difference between responses of SHR and WR arterioles. In WR arterioles, increasing concentrations of KCl elicited a significant increase in R(Ca) (38+/-7% at 80 mmol/L) and completely constricted the arterioles. In contrast, constrictions to ET (52+/-7% at 3x10(-12) mol/L) and the TxA(2) agonist U46619 (40+/-8% at 3x10(-9) mol/L) were not accompanied by increases in R(Ca) at submaximal concentrations. Collectively, these findings suggest that in hypertension, endothelium-derived ET and PGH(2)/TxA(2) increase the Ca(2+) sensitivity of the contractile apparatus of arteriolar smooth muscle; thus, the similar increases in [Ca(2+)](i) in response to the elevation of intraluminal pressure elicit greater myogenic constriction.

Mediation of EDHF-induced reduction of smooth muscle [Ca(2+)](i) and arteriolar dilation by K(+) channels, 5,6-EET, and gap junctions.

OBJECTIVE: To characterize the role of K(+) channels, the cytochrome P-450 (CYP) metabolite 5,6-EET, and gap junctions in modulation of arteriolar myogenic tone by a non-nitric oxide nonprostaglandin mediator, termed "endothelium-dependent hyperpolarizing factor" (EDHF), released to acetylcholine (ACh) in skeletal muscle arterioles. METHODS: In isolated rat gracilis arterioles, simultaneous changes in smooth muscle (aSM) [Ca(2+)](i) (assessed by changes in fura-2 ratiometric signal, R(Ca)) and diameter were measured in response to ACh in the presence of indomethacin and L-NAME. RESULTS: ACh, the K(ATP) channel opener pinacidil, and the Ca(2+) channel inhibitor verapamil elicited comparable decreases in aSM [Ca(2+)](i) (max.: -32 +/- 3%, 29 +/- 3%, and -30 +/- 3%, respectively) and arteriolar dilations (max.: 90 +/- 4%, 96 +/- 2%, and 95 +/- 2%, respectively). ACh-induced responses were inhibited by KCl-depolarization, K(Ca) channel blockers (TEA, charybdotoxin), or gap junction inhibitors (18alpha-glycyrrhetinic acid, hyperosmolar sucrose). The K(ATP) channel inhibitor glibenclamide, the K(IR) channel inhibitor barium chloride, or the CYP inhibitor 17-octadecynoic acid (ODYA) were without effect. The putative EDHF analogue 5,6-EET elicited constrictions in the presence of the endothelium that could be prevented by indomethacin or a TxA(2) receptor antagonist, whereas in the absence of the endothelium, EDHF elicited only small, charybdotoxin-insensitive decreases in aSM R(Ca) and dilations (max.: -8 +/- 2% and 27 +/- 4%, respectively). CONCLUSIONS: In skeletal muscle arterioles, EDHF 1) substantially and rapidly reduces myogenic tone by decreasing aSM [Ca(2+)](i) via opening K(Ca) channels, 2) it is unlikely to be 5,6-EET or other CYP metabolites, but 3) requires functional gap junctions.

Homocysteine reduces smooth muscle [Ca2+]i and constrictor responses of isolated arterioles.

Chronic elevation of plasma homocysteine concentration has been shown to be associated with impaired vascular function. The acute direct effect of homocysteine on the tone and vasoactive responses of arterioles and the possible underlying mechanisms, however, have not yet been elucidated. Thus arterioles were isolated from gracilis muscle of rats (d: approximately 130 microm) and their diameter was measured by videomicroscopy. Homocysteine (10(-6)-10(-4) M) elicited dose-dependent dilation of arterioles (maximum: 44+/-6% at 10(-4) M). The dilation was not affected by the presence of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester or by removal of the endothelium, or the free radical scavenger catalase and superoxide dismutase, or the K+ channel inhibitors glibenclamide, 4-aminopyridine, or tetraethyl ammonium. Incubation of vessels with homocysteine (10(-4) M, 20 min) did not affect dilations to acetylcholine or sodium nitroprusside, whereas it significantly decreased constrictions to norepinephrine (at 10(-6) M; control: 57+/-7%, homocysteine: 21+/-5%) and to the thromboxane A2 analogue U46619 (at 10(-8) M: control: 44+/-3%, homocysteine: 20+/-4%). Homocysteine (10(-4) M), similar to the voltage-operated Ca2+ channel inhibitor nitrendipine (10(-8) M), significantly decreased the arteriolar smooth muscle [Ca2+]i as assessed by changes in the fura-2 ratiometric signal (R(Ca), -6+/-1% and -24+/-3%, respectively). These data suggest that in isolated arterioles homocysteine decreases pressure-induced tone and responses to vasoconstrictor agents, likely by altering Ca2+ signaling of arteriolar smooth muscle.

Serotonin reuptake inhibitor fluoxetine decreases arteriolar myogenic tone by reducing smooth muscle [Ca2+]i.

Previous studies showed that the serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine (Prozac) dilates skeletal muscle and cerebral arterioles independent of the endothelium. We hypothesized that fluoxetine affects the contractile activity of arteriolar smooth muscle by interfering with Ca2+ signaling pathways. The effects of fluoxetine on pressure-induced tone of isolated rat skeletal muscle arterioles (approximately 110 microm) were investigated by videomicroscopy. Changes in smooth muscle [Ca2+]i were measured simultaneously by the fura-2 ratiometric method. Elevation of intraluminal pressure (from 20 to 120 mm Hg) increased (by approximately 20%) the smooth muscle calcium fluorescence ratio (R(Ca)) and resulted in a significant myogenic constriction (approximately 40%). Fluoxetine and nifedipine significantly decreased R(Ca) (by approximately 30%) and abolished pressure-induced arteriolar tone (EC50, 3.1 x 10(-6) and 6.0 x 10(-9) M, respectively). Constrictions to the L-type Ca2+ channel opener Bay K 8644 also were inhibited and abolished by increasing doses of fluoxetine (3 x 10(-6) and 10(-5) M, respectively). In the presence of 10(-5) M fluoxetine, a concentration that elicited submaximal (approximately 80%) dilation, elevation of extracellular Ca2+ concentration (from 2.5 to 15 mM) normalized R(Ca) and restored arteriolar myogenic tone. Thus, fluoxetine reduces [Ca2+]i and tone of arteriolar smooth muscle, likely by interfering with Ca2+ entry. We speculate that the "calcium antagonist" effect of fluoxetine may be an additional element in the therapeutic actions of this drug.

Electrophysiological effects of homocysteine in isolated rat right ventricular papillary muscles and left atria.

There is clinical and epidemiological evidence that elevated plasma homocysteine (Hcy) levels are associated with increased myocardial infarction mortality; however, very little is known about Hcy's direct cardiac effects. Thus, we aimed to characterize the cellular electrophysiologic effects of Hcy, a sulfur-containing amino acid in isolated rat hearts. A conventional microelectrode technique was used in left atria and right ventricular papillary muscles. At concentrations higher than 10(6) M, Hcy significantly decreased the maximum rate of rise of the depolarization phase (Vmax) in both cardiac preparations in a dose-dependent manner. Hcy at 10(-4)-5 x 10(-4) M concentrations increased the action potential duration (APD) at late stages of repolarization (at 75% and 90% of APD) both in atria and in ventricles. There was a slight decrease in action potential amplitude in ventricular papillary muscles and atria at concentrations higher that 10(-5) M. The resting membrane potential and the early repolarization phase (APD25 and APD50) remained unchanged in every preparation studied at all concentrations of Hcy administered. The present data suggest that homocysteine may decrease the Na+ channel activity in in vitro cardiac preparations. reserved.