摘要:

The Ca2+-independent portion of transient outward K+current (Ito) exhibits a transmural gradient in ventricle. To investigate control mechanisms for this gradient, we studied canine epicardial and endocardial ventricular myocytes with use of the whole-cell patch-clamp technique.Itowas larger in amplitude, had a more negative voltage threshold for activation, and had a more negative midpoint of inactivation in epicardium. Recovery from inactivation was >10-fold slower in endocardium. Incubation of epicardial myocytes with angiotensin II for 2 to 52 hours alteredItoto resemble unincubated endocardium and reduced the amplitude of the phase 1 notch of the action potential. In contrast, incubation of endocardial myocytes with losartan for 2 to 52 hours alteredItoto resemble unincubated epicardium and induced a phase 1 notch in the action potential. With RNase protection assays, we determined that incubations with angiotensin II or losartan did not alter mRNA levels for either Kv4.3 or Kv1.4; thus, a change in the &agr; subunit forItois unlikely to be responsible. To test whether posttranslational modification produced the effects of angiotensin II, we coexpressed Kv4.3 and the angiotensin II type 1a receptor inXenopusoocytes. Incubation with angiotensin II increased the time constant for recovery from inactivation of the expressed current by 2-fold with an incubation time constant of 3.7 hours. No effect on activation or inactivation voltage dependence was observed. These results demonstrate that the properties ofItoin endocardium and epicardium are plastic and likely under the tonic-differing influence of the renin-angiotensin system.

ISSN:0009-7330    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Tuberous sclerosis complex (TSC) is a rare genetic disorder characterized by the appearance of benign tumors in multiple organs, including the heart. Disease progression is accompanied by homozygous mutation at 1 of 2 loci (designated TSC1 or TSC2), leading to the suggestion that these genes function as tumor suppressors. In this study, we generated a series of TSC2 cDNAs in which one or more structural motifs were deleted, with the hope that expression of the modified gene product would override the growth-inhibitory activity of the endogenous TSC2 gene product. Several of the modified cDNAs enhanced growth rate, increased endocytosis, and promoted aberrant protein trafficking when expressed in NIH-3T3 cells, thereby mimicking phenotypes typical of TSC2-deficient cells. Surprisingly, targeted expression of the most potent TSC2 cDNA to the heart did not perturb cardiac development. However, the level of cardiomyocyte DNA synthesis in adult transgenic mice was elevated >35-fold during isoproterenol-induced hypertrophy compared with their nontransgenic siblings. These results suggest that alteration of TSC2 gene activity in combination with &bgr;-adrenergic stimulation can reactivate the cell cycle in a limited number of terminally differentiated adult cardiomyocytes.

ISSN:0009-7330    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Lipids, monocytes, and arterial wall cells are primary components involved in atherogenesis. Using the inbred mouse strains C57BL/6J (B6) and C3H/HeJ (C3H), which have been extensively studied as models of the genetic control of diet-induced atherosclerosis, we examined which of these components determine genetic susceptibility. To test whether dietary responsiveness is involved, a congenic strain of C3H carrying an apoE-null allele (apoE−/−) was constructed. Although C3H.apoE−/−mice had higher plasma cholesterol levels, they developed much smaller lesions than their B6.apoE−/−counterpart on either chow or Western diets. Reciprocal bone marrow transplantation between the strains, with congenics carrying the same H-2 haplotype, was performed to examine the role of monocytes. The atherosclerosis susceptibility was not altered in the recipient mice, indicating that variations in monocyte function were not involved. Endothelial cells isolated from the aorta of B6 mice exhibited a dramatic induction of monocyte chemotactic protein-1, macrophage colony–stimulating factor, vascular cell adhesion molecule-1, and heme oxygenase-1 in response to minimally modified LDL, whereas endothelial cells from C3H mice showed little or no induction. In a set of recombinant inbred strains derived from the B6 and C3H parental strains, endothelial responses to minimally modified LDL cosegregated with aortic lesion size. These data provide strong evidence that endothelial cells, but not monocytes or plasma lipid levels, account for the difference in susceptibility to atherosclerosis between the 2 mouse strains.

ISSN:0009-7330    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Myocardial NO signaling appears elevated in heart failure (HF). Whether this results from increased NO production, induction of the high-output NO synthase (NOS)2 isoform, or changes in NOS regulatory pathways (such as caveolae) remains controversial. We tested the hypothesis that increased abundance of caveolin-3 and/or sarcolemmal caveolae contribute to increased NO signaling in pacing-induced HF. Abundance of caveolin-3 (0.59±0.08 versus 0.29±0.08 arbitrary units,P=0.01) but not caveolin-1 was increased in HF compared with control conditions, assessed by Western blot. Additionally, transmission electron microscopy revealed increased caveolae (2.7±0.4 versus 1.3±0.3 per micrometer myocyte membrane,P<0.005). The association between caveolin-3 and NOS3 at the sarcolemma and T tubules was unchanged in HF compared with control myocytes. The impact of NOS inhibition with l-NG-methylarginine hydrochloride (L-NMMA) on &bgr;-adrenergic inotropy was assessed in conscious dogs before and after HF. In control dogs, dobutamine (5 &mgr;g · kg−1· min−1) increased +dP/dt by 36±7%, and this was augmented to 66±24% by 20 mg/kg L-NMMA (P=0.04 versus without L-NMMA, n=8) but not affected by 10 mg/kg L-NMMA (34±10%,P=NS; n=8). In HF, dobutamine +dP/dt response was depressed (P<0.001 versus control), and increased concentrations were required to match control inotropic responses (10 to 15 &mgr;g · kg−1· min−1, 48±7%). L-NMMA enhanced +dP/dt responses similarly at 10 mg/kg (61±17%,P=0.02; n=4) and 20 mg/kg (54±7%,P=0.04; n=7). Caveolin-3 abundance positively correlated with L-NMMA augmentation of dobutamine inotropic responses in HF (r=0.9,P=0.03; n=4). Thus, in canine pacing-induced HF, expression of caveolin-3 and of sarcolemmal caveolae is increased. This increase is associated with augmented agonist-stimulated NO signaling, likely via a compartmentation effect.

ISSN:0009-7330    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Although Ca2+waves in cardiac myocytes are regarded as arrhythmogenic substrates, their properties in the heart in situ are poorly understood. On the hypothesis that Ca2+waves in the heart behave diversely and some of them influence the cardiac function, we analyzed their incidence, propagation velocity, and intercellular propagation at the subepicardial myocardium of fluo 3–loaded rat whole hearts using real-time laser scanning confocal microscopy. We classified Ca2+waves into 3 types. In intact regions showing homogeneous Ca2+transients under sinus rhythm (2 mmol/L [Ca2+]o), Ca2+waves did not occur. Under quiescence, the waves occurred sporadically (3.8 waves · min−1· cell−1), with a velocity of 84 &mgr;m/s, a decline half-time (t1/2) of 0.16 seconds, and rare intercellular propagation (propagation ratio <0.06) (sporadic wave). In contrast, in presumably Ca2+-overloaded regions showing higher fluorescent intensity (113% versus the intact regions), Ca2+waves occurred at 28 waves · min−1· cell−1under quiescence with a higher velocity (116 &mgr;m/s), longer decline time (t1/2=0.41 second), and occasional intercellular propagation (propagation ratio=0.23) (Ca2+-overloaded wave). In regions with much higher fluorescent intensity (124% versus the intact region), Ca2+waves occurred with a high incidence (133 waves · min−1· cell−1) and little intercellular propagation (agonal wave). We conclude that the spatiotemporal properties of Ca2+waves in the heart are diverse and modulated by the Ca2+-loading state. The sporadic waves would not affect cardiac function, but prevalent Ca2+-overloaded and agonal waves may induce contractile failure and arrhythmias.

ISSN:0009-7330    

出版商:OVID    

年代:2000

数据来源: OVID