Three-dimensional structure of the synaptic contact of the neuromuscular junction in the rat lumbrical muscle.
(25/1361)
This study examined the three-dimensional structures of the synaptic contact in rat lumbrical muscles by scanning electron microscopy using three different methods: the aldehyde prefix-osmium-dimethyl sulfoxide-osmium method (A-O-D-O method), the cell-extraction method, and the NaOH-digestion method. These three methods visualized the motor nerve endings, subneural basal lamina and postsynaptic sarcolemma, respectively. The motor nerve endings were composed of a cluster of spherical and cylindrical terminals. Pores on the presynaptic membrane were considered openings of exocytotic vesicles. The postsynaptic side of the subneural basal lamina showed numerous ridges, corresponding to junctional folds. Most of the ridges rose vertically from their base. The ridges showed widening, narrowing, and branching. The subneural basal lamina appeared to be composed of small granular substances. The basal lamina of the primary synaptic clefts had pores 25-30 nm in diameter, which may facilitate the transport of acetylcholine (ACh) without being hydrolyzed by ACh esterase in the lamina. On the outer surface of the postsynaptic sarcolemma in a sole plate, the primary synaptic clefts were composed of a mixture of depressions and gutters; so far as we know, this represents the only example of such a phenomenon. These depressions and gutters seem to fit respectively into the spherical and cylindrical terminals of the motor nerve endings. The openings of the junctional folds consisted of a mixture of many slits and a few pits in the primary synaptic clefts. (+info)
Formation of planar and spiral Ca2+ waves in isolated cardiac myocytes.
(26/1361)
A novel Nipkow-type confocal microscope was applied to image spontaneously propagating Ca2+ waves in isolated rat ventricular myocytes by means of fluo-3. The sarcolemma was imaged with di-8-ANEPPS and the nucleus with SYTO 11. Full frame images in different vertical sections were obtained at video frame rate by means of an intensified CCD camera. Three types of Ca2+ waves were identified: spherical waves, planar waves, and spiral waves. Both spherical waves and spiral waves could initiate a planar wave, and planar waves were not influenced by the presence of a nucleus. Spiral waves, however, were consistently found adjacent to a nucleus and displayed a slower propagation rate and slower rate of increase in Ca2+ concentration in the wave front than did spherical and planar waves. The planar waves were apparent throughout the vertical axis of the cell, whereas spiral waves appeared to have a vertical height of approximately 3 microm, less than the maximum thickness of the nucleus (5.0 +/- 0.3 microm). These results provide experimental confirmation of previous modeling studies which predicted an influence of the nucleus on spiral-type Ca2+ waves. When a spontaneous Ca2+ wave is small relative to the size of the nucleus, it appears that the Ca2+ buffering by the nucleus is sufficient to slow the rate of spontaneous propagation of the Ca2+ wave in close proximity to the nucleus. These findings thus support the idea that the nucleus can influence complex behavior of Ca2+ waves in isolated cardiac myocytes. (+info)
Electrical remodeling in ischemia and infarction.
(27/1361)
This is a review of the electrophysiologic changes occurring at different times following myocardial infarction, both in the infarcted region (substrate) and in areas remote from the infarct. Regulators of channel function which might contribute to re-modeling, including autocrine/paracrine factors involved in ion channel gene regulation, are discussed. (+info)
Ca channels in cardiac myocytes: structure and function in Ca influx and intracellular Ca release.
(28/1361)
There are Ca channels in the plasma membrane and also the sarcoplasmic reticulum (SR) membrane in cardiac myocytes. The relationship between channel structure, associated proteins and function of these Ca channels is discussed. The sarcolemmal Ca channels are crucial both to the basic cellular electrophysiological properties and control of cardiac contractility (via excitation-contraction coupling). The intracellular Ca release channels (or ryanodine receptors) respond to triggering events mediated by sarcolemmal ion currents and are largely responsible for releasing Ca which activates the myofilaments to produce contraction. Several possible mechanisms of excitation-contraction coupling are discussed. The Ca released from the SR can also feedback on several sarcolemmal ion currents and alter action potential configuration as well as contribute to arrhythmogenesis. (+info)
Dihydropyridine and beta adrenergic receptor binding in dogs with tachycardia-induced atrial fibrillation.
(29/1361)
BACKGROUND: We have shown that rapid atrial activation, as occurs during atrial fibrillation (AF), reduces L-type Ca2+ current (ICa) and that this is the principal mechanism of the action potential duration and refractoriness changes that characterize tachycardia-induced atrial remodeling. The present study was designed to determine whether atrial tachycardia alters biochemical indices of the number of L-type Ca2+ channels and/or of the number and binding affinity of beta-adrenergic receptors. METHODS: In canine atrial sarcolemmal preparations, the number and binding affinity of dihydropyridine receptors were determined with the use of 3H-nitrendipine and that of beta-adrenergic receptors with 125I-iodocyanopindolol. Results were obtained with preparations from dogs paced at 400/min for 1 (P1, n = 20), 7 (P7, n = 9), and 42 (P42, n = 9) days, and compared with observations in sham-operated controls (P0, n = 14). RESULTS: Pacing reduced the Bmax of dihydropyridine receptors, from 157 +/- 18 fmol/mg (P0) to 116 +/- 9 fmol/mg (P1, P < 0.05), 100 +/- 14 fmol/mg (P7, P < 0.05) and 94 +/- 9 fmol/mg (P42, P < 0.01). The affinity of dihydropyridine receptors was unchanged, with the Kd averaging 711 +/- 102 pM. 656 +/- 74 pM, 633 +/- 155 pM and 585 +/- 92 pM in P0, P1, P7 and P42 dogs. Neither Bmax nor Kd of beta-adrenergic receptors was altered by rapid pacing. Values of Bmax of dihydropyridine receptors correlated with atrial ICa current density (r2 = 0.95) and ERP (r2 = 0.99). CONCLUSIONS: Rapid atrial activation results in downregulation in the number of dihydropyridine receptors without altering the number or affinity of beta-adrenergic receptors. The reductions in ICa that play an important role in the atrial electrical remodeling by which 'AF begets AF' appear to be due at least in part to a decrease in the number of L-type Ca2+ channels in cardiac cell membranes. (+info)
Beneficial effects of propionyl L-carnitine on sarcolemmal changes in congestive heart failure due to myocardial infarction.
(30/1361)
OBJECTIVE: Earlier studies have revealed sarcolemmal (SL) defects in congestive heart failure due to myocardial infarction; however, the mechanisms of SL changes in the failing heart are poorly understood. Since congestive heart failure is associated with various metabolic abnormalities including a deficiency of carnitine, we examined the effects of propionyl L-carnitine, a carnitine derivative, in animals with congestive heart failure. METHODS: For this purpose, heart failure in rats was induced by occluding the coronary artery and 3 weeks later the animals were treated with 100 mg/kg (i.p. daily) propionyl L-carnitine for 4 weeks. The sham control group received saline injections. The animals were assessed for their left ventricular function. SL membranes were examined for Na(+)-K+ ATPase, Na(+)-Ca2+ exchange and adenylate cyclase activities. RESULTS: A marked improvement in the attenuated left ventricular function of the experimental animals was seen upon treatment with propionyl L-carnitine. The SL adenylyl cyclase activities in control, untreated failing hearts and treated failing hearts were 590 +/- 36, 190 +/- 22 and 320 +/- 21 pmol cAMP/mg/10 min, whereas the SL Na(+)-K+ ATPase activities were 35.7 +/- 2.8, 22.5 +/- 2.4 and 30.1 +/- 2.8 mumol Pi/mg/h, respectively. Furthermore, the SL Na(+)-dependent Ca(2+)-uptake activity, which decreased in the failing hearts (4.6 +/- 0.4 vs. 9.3 +/- 0.7 nmol Ca2+/mg/2 s for control), was improved (6.8 +/- 0.5 nmol Ca2+/mg/2 s) significantly following treatment with propionyl L-carnitine. CONCLUSION: These results indicate that metabolic therapy with propionyl L-carnitine may attenuate defects in the SL membrane and thus may improve heart function in congestive heart failure due to myocardial infarction. (+info)
Regulation of sarcolemmal Na(+)/H(+) exchanger activity by angiotensin II in adult rat ventricular myocytes: opposing actions via AT(1) versus AT(2) receptors.
(31/1361)
Increased sarcolemmal Na(+)/H(+) exchanger activity has been implicated as a mediator of the cardiac actions of angiotensin II. We studied the receptor subtypes and signaling pathways involved in the regulation of sarcolemmal Na(+)/H(+) exchanger activity by angiotensin II in adult rat ventricular myocytes. Cells were loaded with the pH-sensitive fluoroprobe carboxy-seminaphthorhodafluor-1, and acid efflux rates estimated during recovery from intracellular acidosis were used to quantify exchanger activity. Sarcolemmal Na(+)/H(+) exchanger activity was not affected by angiotensin II alone but was increased by angiotensin II plus PD123319 (AT(2) antagonist). In contrast, angiotensin II plus losartan (AT(1) antagonist) or CGP42112A (AT(2) agonist) did not affect exchanger activity. The increase in Na(+)/H(+) exchanger activity induced by angiotensin II plus PD123319 was blocked by losartan, PD98059 (extracellular signal-regulated kinase inhibitor), GF109203X (protein kinase C inhibitor), and tyrphostin AG1478 (epidermal growth factor receptor kinase inhibitor). Extracellular signal-regulated kinase phosphorylation and activity, measured by immunoblot analysis and an immune-complex kinase assay, respectively, were increased significantly by angiotensin II plus PD123319; these increases were blocked by losartan and PD98059. The increase in extracellular signal-regulated kinase phosphorylation induced by angiotensin II plus PD123319 was blocked also by GF109203X and tyrphostin AG1478. These data show that AT(1) stimulation increases sarcolemmal Na(+)/H(+) exchanger activity in adult rat ventricular myocytes and that this response requires extracellular signal-regulated kinase activation through a protein kinase C- and epidermal growth factor receptor-mediated mechanism. The positive effect of AT(1) stimulation on Na(+)/H(+) exchanger activity is counteracted by simultaneous AT(2) stimulation through a mechanism that does not involve direct inhibition of the exchanger or attenuation of extracellular signal-regulated kinase activation. (+info)
Cardiac and skeletal muscle mitochondria have a monocarboxylate transporter MCT1.
(32/1361)
To evaluate the potential role of monocarboxylate transporter-1 (MCT1) in tissue lactate oxidation, isolated rat subsarcolemmal and interfibrillar cardiac and skeletal muscle mitochondria were probed with an antibody to MCT1. Western blots indicated presence of MCT1 in sarcolemmal membranes and in subsarcolemmal and interfibrillar mitochondria. Minimal cross-contamination of mitochondria by cell membrane fragments was verified by probing for the sarcolemmal protein GLUT-1. In agreement, immunolabeling and electron microscopy showed mitochondrial MCT1 in situ. Along with lactic dehydrogenase, the presence of MCT1 in striated muscle mitochondria permits mitochondrial lactate oxidation and facilitates function of the "intracellular lactate shuttle." (+info)