山莨菪碱对家兔心室肌细胞L-型钙通道的影响

目的　研究山莨菪碱 (Ani)对家兔心室肌细胞L 型钙通道的电生理作用。方法　采用全细胞膜片钳 (Wholecellpatchclamp)技术 ,当维持电位为 - 40mV ,刺激频率为0 5Hz ,钳制时间为 2 5 0ms,步进电压为 10mV ,去极到 6 0mV ,观察不同浓度的Ani对L 型钙通道电流 (L ICa)的影响。结果　 0 1mmol·L-1Ani对L ICa无影响 (P >0 0 5 ) ;0 2、0 4和 0 8mmol·L-1Ani对L ICa的抑制为 36 3% ,5 1 1%和 72 8% (P均 <0 0 1) ,且浓度愈大 ,其抑制作用愈强 ,但都不使I U曲线的峰值发生偏移。结论　Ani能浓度依赖性阻滞L ICa,具有钙拮抗作用。     

Uncoupling between beta-adrenoceptors and adenylate cyclase in dog ischemic myocardium

Summary We evaluated the effects of ischemic injury on the myocardial adenylate cyclase system, 5 h after ligation of the left anterior descending coronary in 5 anesthetized dogs. Crude cardiac membrane preparations were isolated from control and ischemic areas of ventricular myocardium and tested for: 1. L-(¹²⁵I)iodocyanopindolol binding, in the absence and presence of ±-isoprenaline and GTP, and 2. adenylate cyclase activity. The density of beta-adrenoceptors increased by 35% in membranes from ischemic areas while the proportion of receptors in a high affinity state for ±-isoprenaline decreased from 43% to 20%. Adenylate cyclase activities in the basal state and under stimulation with NaF, forskolin, Gpp(NH)p, ±-isoprenaline and VIP were all markedly and similarly reduced, being only about 30% of comparable activities in membranes from control areas. The ±-isoprenaline subsensitivity of cardiac adenylate cyclase can, thus, be attributed to a defective enzymatic system and not to a reduction in the number of beta-adrenoceptors implying that the internal components of the system were more sensitive to acute ischemia than the outward oriented hormone receptors. It is tempting to ascribe this uncoupling to a functional depletion in the guanine nucleotide-binding regulatory protein Ns that might reflect a loss of high energy phosphate stores including GTP.Abbreviations CYP cyanopindolol - Gpp(NH)p 5-guanyl imidodiphosphate - VIP vasoactive intestinal peptide - Ns guanine nucleotide-binding regulatory protein     

Effectiveness of cardiac glycosides in human myocardium with and without "downregulated" beta-adrenoceptors.

We investigated the "receptor-effector-coupling" in the beta-adrenoceptor- and the Na+, K(+)-ATPase-mediated systems in nonfailing hearts and terminally failing human myocardium from patients with cardiomyopathy. The density of beta-adrenoceptors in the failing human myocardium was significantly (p less than 0.01) lower as compared with nonfailing hearts, whereas the receptor density and affinity measured by [3H]ouabain binding (cardiac glycoside receptor) was not different in either group. The maximal inotropic response to isoprenaline was significantly reduced in papillary muscle strips from failing human hearts (2.1 +/- 0.5 mN) as compared with control hearts (8.0 +/- 1.0 mN; p less than 0.05). Ouabain remained effective in both groups (6.8 +/- 1.0 vs. 5.5 +/- 0.6 mN; NS). The positive inotropic response due to extracellular Ca2+ elevation (1.8-15 mM) was studied for comparison. Maximal Ca2+ effects were reduced by 30% in failing human myocardium (7.2 +/- 0.5 mN vs. 5.1 +/- 0.8 mN, p less than 0.05). Ouabain had effectiveness (95%) similar to that of Ca2+ in nonfailing and failing human cardiac muscle. It is concluded that treatment with cardiac glycosides may still be effective in end-stage heart failure with "downregulated" beta-adrenoceptors, as judged from these in vitro studies.     

Developmental changes in beta-adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in rat ventricular muscles.

1. In an attempt to explain the previous electrophysiological data on the ontogeny of beta-adrenergic and muscarinic cholinergic interactions on cardiac Ca2+ current, biochemical studies were performed on the ontogeny of beta-adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in cardiac muscle of developing rats: 16-20 days old foetuses, 0-20 days old neonates, and 2-3 months old adults. 2. Developmental changes in cardiac beta-adrenoceptors, muscarinic cholinoceptors, and Ca2+ channels were determined with the use of specific radioligands, [3H]-dihydroalprenolol (DNA), [3H]-quinuclidinyl benzilate (QNB), and [3H]-nitrendipine (NTD), respectively. 3. The Bmax value (fmol mg-1 tissue) for [3H]-DNA binding started to increase on post-gestation day 20, reached almost its maximum level on neonatal day 6, kept almost the same level until neonatal day 20, and then decreased slightly to its adult level. 4. The Bmax value (fmol mg-1 tissue) for [3H]-QNB binding started to increase on post-gestation day 16, reached almost its maximum level on neonatal day 0, remained almost constant until neonatal day 15, and then decreased to its adult level. 5. The Bmax value (fmol mg-1 tissue) for [3H]-NTD binding increased with age between post-gestation day 18 and neonatal day 15, stayed almost constant until neonatal day 20, and then decreased to its adult level. 6. The Kd values for [3H]-DHA, [3H]-QNB, and [3H]-NTD bindings remained almost constant during the developmental period examined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of type 1 protein phosphatase activity by activation of beta-adrenoceptors in ventricular myocardium

The regulation of protein phosphatase (PP) activity by cardiac beta-adrenergic receptor stimulation with isoproterenol (ISO) was studied in four groups of guinea pigs consisting of seven animals each. Group 1 received the vehicle solution only intraperitoneally; group 2, 6 microg/kg of ISO; group 3, 60 microg/kg of ISO; and group 4, 600 microg/kg of ISO. Total PP activity (consisting of both type 1 and type 2A PP), activity of each PP subtype, the cAMP-dependent protein kinase activity ratio (-cAMP/+cAMP), the phosphorylation of PP inhibitor 1, and the phosphorylation of phospholamban were measured in ventricular tissue. PP activity was also studied in ventricular cardiomyocytes isolated from guinea pigs treated with and without 1 microM ISO or 1 microM ISO plus 10 microM propranolol, an antagonist of the beta-adrenoceptor. PP activity decreased significantly in membrane vesicles, but not in cytosolic fractions, of guinea pigs treated with 60 and 600 microg/kg of ISO compared with untreated animals. The PKA activity ratio, PLB phosphorylation, and PP inhibitor 1 phosphorylation increased in ventricles of guinea pigs treated with 60 and 600 microg/kg of ISO compared with vehicle-treated animals. The decrease in overall PP activity was due primarily to a reduction in type 1 but not type 2A PP activity. In isolated ventricular cardiomyocytes, PP activity was decreased significantly after treatment with 1 microM ISO, and this inhibition was reversed by treatment with 10 microM propranolol. The membrane vesicles of group 1 animals did not release any catalytic subunit of type 1 PP upon phosphorylation by exogenous PKA. These results indicate that activation of cardiac beta-adrenoceptors inhibits type 1 PP activity via phosphorylation of PP inhibitor 1 in the ventricles. This effect is associated with the well-known effect of ISO on increases in the PKA activity ratio and PLB phosphorylation. Inhibition of type 1 PP activity could be one possible mechanism, in addition to activation of adenylate cyclase, by which ISO mediates enhanced contractility of the heart.     

Relationship between the stereoselective negative inotropic effects of verapamil enantiomers and their binding to putative calcium channels in human heart.

Ventricular preparations from patients with mitral disease and hypertrophic obstructive cardiomyopathy (HOCM) were set up to contract isometrically. Ventricular membrane particles were also prepared and putative calcium channels were labelled with [3H]-nimodipine. Positive staircase was induced by varying the rate of stimulation of isolated strips from 6 min-1 to 120 min-1 in the presence of 6-60 microM (-)-adrenaline or (-)-noradrenaline. (-)-Verapamil 3-5 microM or (+)-verapamil 20-30 microM reversed the force-frequency relationship (i.e. caused negative staircase) in preparations from patients with mitral disease or HOCM. In subendocardial strips of ventricular septum from 5 patients with HOCM paced at 60 min-1, both (-)-verapamil and (+)-verapamil caused cardiodepression. Half-maximal cardiodepression was observed with 0.4 microM (-)-verapamil and with 3 microM (+)-verapamil. [3H]-nimodipine bound to ventricular membrane particles in a saturable, reversible fashion to a high affinity site with an equilibrium dissociation constant of 0.23 nM. The density of these sites was 95 fmol mg-1 of membrane protein. Binding of the tritiated 1,4-dihydropyridine was stereoselectively inhibited by 1,4-dihydropyridine enantiomers and nifedipine. (-)-Verapamil and (+)-verapamil inhibited high affinity [3H]-nimodipine binding in a negative heterotropic allosteric manner with (-)-verapamil being 5 times more potent than (+)-verapamil on an IC50 basis. At a given [3H]-nimodipine concentration, (+)-verapamil inhibited a greater fraction of specific [3H]-nimodipine binding. The allosteric mode of (+)-verapamil inhibition of [3H]-nimodipine binding was confirmed by kinetic studies. (-)-Verapamil shifted (+)-verapamil-binding inhibition curves to the right in an apparently competitive fashion. The inversion of staircase caused by both verapamil enantiomers suggests that they cause a use-dependent channel blockade. The similar potency ratios for binding and for cardiodepression are indicative of a common locus of action for both verapamil enantiomers within the calcium channel.     

Effect of theophylline on calcium exchangeability in ventricular myocardium

Summary The effects of theophylline on contractile force and myocardial calcium exchangeability were studied in isolated, electrically driven Langendorff perfused guinea-pig hearts. Following a 30-min exposure to ⁴⁵Ca, total cellular calcium and ⁴⁵Ca activity were measured in right ventricular samples. Non-toxic theophylline concentrations (5×10^–5–10^–3 g/ml) which augmented contractile force without producing arrhythmias or contractures had no effect on total tissue calcium and did not alter the size of the fraction of cellular calcium exchangeable under steadystate conditions. A toxic concentration of theophylline (2×10^–3 g/ml) induced contractures and increased the amount of exchangeable cellular calcium. The latter effect was due to an increase in total calcium; the unlabelled cellular calcium fraction remained unchanged under the influence of all theophylline concentrations studied. The results suggest that theophylline increases the steady-state calcium exchangeability in ventricular myocardium only when the total calcium concentration is also increased.     

Two subtypes of dihydropyridine-sensitive calcium channels in rat ventricular muscle.

Two opposite inotropic effects of the dihydropyridine activators, CGP 28392 and Bay K 8644, given at the same concentration (1-2 microM) were found in rat papillary muscles: a positive effect in polarized tissue (4 mM KCl) and a negative one during partial depolarization. The depressive effect found at a low rate or after a short rest was associated with marked prolongation of the Ca2(+)-mediated action potential, indicating that the drugs behave as Ca channel stimulators. The depressive effect of the activation on the resting state contraction was antagonized by nifedipine (2 microM) and high Mg2+ (5 mM). It was suggested that at least two subtypes of the L-type, dihydropyridine-sensitive channels underlie the opposite inotropic responses of the activators. The positive effect is apparently caused by conventional stimulation of Ca2+ entry through the cell membrane, whereas the negative effect is probably due to the stimulation of Ca2+ efflux from the sarcoplasmic reticulum, leading to depletion of intracellular stores. The effect was proposed to be mediated by activation of junctional channels linked to sarcoplasmic reticulum Ca2+ release. An important role for these channels in triggering the sarcoplasmic reticulum Ca2+ release and regulation of force-frequency relation is proposed.     

Mechanism of action of bucindolol in human ventricular myocardium.

The mechanism of action of the beta-receptor antagonist bucindolol was examined in human ventricular myocardium. Bucindolol was found to be a high-affinity competitive beta-blocking agent as determined by bucindolol-[125I]iodocyanopindolol (ICYP) competition curves (KI = 3.7 +/- 1.3 x 10(-9) M, n = 10). This value was in general agreement with bucindolol KB's, determined by antagonism of isoproterenol-stimulated adenylate cyclase activity (KB = 2.8 +/- 0.55 x 10(-9) M, n = 5) or isoproterenol-augmented contraction of right ventricular trabeculae (KB = 2.9 +/- 1.9 x 10(-9) M, n = 3). In contrast, the alpha 1-receptor KI, determined at bucindolol-125IBE2254 (IBE) competition binding in rat cardiac membranes, was 1.2 x 10(-7) M. Bucindolol exhibited no beta 1- or beta 2-receptor subtype selectivity as deduced from blockade of the beta-agonist-coupled adenylate cyclase system, receptor-binding studies with preparations of human ventricular myocardium with predominantly beta 1 or beta 2 receptors, or receptor-binding studies in model systems consisting of beta 1 (guinea pig myocardial membranes) or beta 2 receptors (human mononuclear and frog myocardial membranes). In membranes derived from human ventricular myocardium and human lymphocytes, bucindolol recognized a high-affinity agonist-binding site as determined by guanine nucleotide modulation of competition-binding curves. Although bucindolol has measurable intrinsic sympathomimetic activity (ISA) in some animal systems, no increase in adenylate cyclase activity or muscle contraction was detected in preparations of human heart. In conclusion, bucindolol is a high-affinity nonselective beta-receptor antagonist with no evidence of intrinsic sympathomimetic activity in human ventricular myocardium.     

Relationship between lipolysis and cyclic AMP generation mediated by atypical beta-adrenoceptors in rat adipocytes.

1. The nature of the beta-adrenoceptor(s) mediating adenylyl cyclase activation in rat adipocyte ghosts by (-)-isoprenaline and the lipolytically selective beta-adrenoceptor agonist, BRL 37344, was investigated by use of the beta 1-selective antagonist, CGP 20712A. The results were compared with lipolysis in adipocytes. 2. While in lipolysis BRL 37344 was a full and 10 times more potent agonist than (-)-isoprenaline, in adenylyl cyclase activation similar pD2 values for both agonists were found. BRL 37344 was only a partial agonist on rat adipocyte adenylyl cyclase, with an intrinsic activity of 0.62. 3. With CGP 20712A small rightward shifts of the (-)-isoprenaline concentration-response curve (CRC) were observed at concentrations up to 10 microM, while at 100 microM and 1 mM clear rightward shifts occurred. The BRL 37344 CRC was not shifted with antagonist concentrations up to 10 microM. Only at 100 microM and 1 mM CGP 20712A were rightward shifts observed. 4. CGP 20712A concentrations of 10 microM and 100 microM depressed the maximum of the (-)-isoprenaline CRC to 89 and 60%, while the BRL 37344 CRCs retained the control maximum effect (62% of (-)-isoprenaline). Only at 1 mM CGP 20712A, was the CRC of BRL 37344 depressed, while the (-)-isoprenaline maximum was diminished further. 5. It was concluded that as with lipolysis, (-)-isoprenaline acts both through typical beta 1- and atypical beta 3-adrenoceptors for activation of adenylyl cyclase, while BRL 37344 acts solely through atypical beta 3-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Both cell surface and internalized beta-adrenoceptors are reduced in the failing myocardium.

To study myocardial beta-adrenoceptor internalization in heart failure, we measured beta-adrenoceptor density in the particulate, light vesicle and supernatant fractions of ventricular tissue of dogs with experimental right ventricular failure and sham-operated dogs. Tissue was fractionated by centrifugation, and beta-adrenoceptors were measured by [125I]iodocyanopindolol binding. Compared to sham-operated controls, beta-adrenoceptors were reduced in all fractions of right ventricular tissue from heart failure animals. Thus, the decreased surface (particulate fraction) receptors observed cannot be explained by internalization alone, and must be associated with altered receptor synthesis or degradation.     

Biphasic effect of bimoclomol on calcium handling in mammalian ventricular myocardium

1. Concentration-dependent effects of bimoclomol, the novel heat shock protein coinducer, on intracellular calcium transients and contractility were studied in Langendorff-perfused guinea-pig hearts loaded with the fluorescent calcium indicator dye Fura-2. Bimoclomol had a biphasic effect on contractility: both peak left ventricular pressure and the rate of force development significantly increased at a concentration of 10 nM or higher. The maximal effect was observed between 0.1 and 1 microM, and the positive inotropic action disappeared by further increasing the concentration of bimoclomol. The drug increased systolic calcium concentration with a similar concentration-dependence. In contrast, diastolic calcium concentration increased monotonically in the presence of bimoclomol. Thus low concentrations of the drug (10 - 100 nM) increased, whereas high concentrations (10 microM) decreased the amplitude of intracellular calcium transients. 2. Effects of bimoclomol on action potential configuration was studied in isolated canine ventricular myocytes. Action potential duration was increased at low (10 nM), unaffected at intermediate (0.1 - 1 microM) and decreased at high (10 - 100 microM) concentrations of the drug. 3. In single canine sarcoplasmic calcium release channels (ryanodine receptor), incorporated into artificial lipid bilayer, bimoclomol significantly increased the open probability of the channel in the concentration range of 1 - 10 microM. The increased open probability was associated with increased mean open time. The effect of bimoclomol was again biphasic: the open probability decreased below the control level in the presence of 1 mM bimoclomol. 4. Bimoclomol (10 microM - 1 mM) had no significant effect on the rate of calcium uptake into sarcoplasmic reticulum vesicles of the dog, indicating that in vivo calcium reuptake might not substantially be affected by the drug. 5. In conclusion, the positive inotropic action of bimoclomol is likely due to the activation of the sarcoplasmic reticulum calcium release channel in mammalian ventricular myocardium.     

Effects of berberine of L- and T-type calcium channels in guinea pig ventricular myocytes.

AIM: To study the effects of berberine (Ber) on L-(ICa,L) and T-type (ICa,T) channels in isolated guinea pig ventricular myocytes. METHODS: Using whole cell patch-clamp recording technique. RESULTS: Ber 10, 30 mumol.L-1 inhibited the ICa, L from 1400 +/- 247 pA to 978 +/- 204 pA (n = 5 cells of 5 guinea pigs, P < 0.05), and to 664 +/- 179 pA (n = 5, P < 0.01), respectively. The inhibitory effect was concentration-dependent and non-frequency-dependent. The peak value of ICa,L in the current-voltage relationship was decreased. Ber affected the inactivation kinetics of ICa,L. The half activation potential (V0.5) was shifted from -27.8 mV to -34.2 mV and the slope factor (kappa) was changed from 9.22 into 13.03. Ber did not affect the activation kinetics. Ber 10 and 30 mumol.L-1 also inhibited ICa,T (from 154 +/- 80 pA to 101 +/- 78 pA, and to 48 +/- 45 pA, n = 8 cells of 5 guinea pigs, P < 0.05). CONCLUSION: Ber possessed blocking effects on both L- and T-type calcium channels.     

Classification of beta-adrenoceptors in human isolated bronchus.

(+/-)-Isoprenaline (Iso), (-)-adrenaline (Ad), (-)-noradrenaline (NA), (+/-)-phenylephrine (Phe) and the beta 2-selective adrenoceptor agonist (+/-)-fenoterol (Fen) caused a concentration-dependent relaxation of human isolated bronchial preparations. Iso, Ad and NA caused complete relaxation of both spontaneous and carbachol-induced bronchial tone. Fen, which was only tested in preparations where tone was induced with carbachol, also caused complete relaxation. However, Phe was a partial agonist in all preparations tested. When relaxation responses to these amines were calculated as a % of their maximal effects, comparison of EC50 values showed that the order of potency was Iso greater than Ad = Fen greater than NA greater than Phe (92:27:25:1:0.2) in preparations with carbachol-induced tone and Iso greater than Ad greater than NA greater than Phe (112:38:1:0.3) in preparations with spontaneous tone. pA2 values determined for the beta-adrenoceptor antagonists propranolol (non-selective), atenolol (beta-selective) and ICI-118, 551 (beta 2-selective), using Iso as an agonist were, 9.3, 5.3 and 9.1 respectively. These results indicate that beta 2-adrenoceptors mediate relaxation of human isolated bronchus to sympathomimetic amines in preparations obtained 4-14 h post-mortem from non-diseased lung. alpha-Adrenoceptors were apparently sparse or absent in this tissue.     

Comparison of the beta-adrenoceptors in the myocardium and coronary vasculature of the kitten heart.

The relative potencies of (minus) noradrenaline, (minus)-adrenaline, (plus or minus)-isoprenaline and (plus or minus)-salbutamol have been assessed for their positive inotropic and chronotropic actions on kitten isolated atria. These relative potencies have been compared with those obtained for the relative coronary dilator potencies in two preparations. These were intact hearts perfused by Langendorff's method and isolated perfused coronary arteries from the kitten. The relative molar potencies for inotropic effects were noradrenaline 1: adrenaline1: iso-prenaline 7: salbutamol 0.6. The observed ratios for chronotropic effects were not significantly different from those for inotropic effects. The relative potencies of noradrenaline, adrenaline and isoprenaline as myocardial stimulants were similar to their relative potencies as coronary dilators in the intact heart. Similar relative potencies for noradrenaline and isoprenaline were also found in the isolated coronary artery but adrenaline was only one third as active as noradrenaline on this preparation. In both the intact heart and in the isolated coronary artery salbutamol was about one tenth as active as noradrenaline. It was therefore less active relative to noradrenaline as a coronary dilator than as a myocardial stimulant. In spite of these differences in relative potency ratios for myocardial and coronary dilator effects, similar dissociation constants (Kb values) for practolol against isoprenaline were found in driven atria and in isolated coronary arteries. Myocardial and coronary vascular beta-adrenoceptors thus can both be placed in the general classification of beta1-adrenoceptors.     

Adenosine: A natural modulator of L-type calcium channels in atrial myocardium?

The mechanism of action of adenosine at the level of atrial myocardium has been a matter of debate. Electrophysiological studies showed that adenosine increases K+ efflux which may reduce Ca2+ influx, indirectly, by shortening the myocardial action potential. Recently some authors proposed that adenosine also depresses Ca2+ influx by a direct action on the L calcium channel, but, this effect being lower than that on voltage-dependent K+ channels, it was considered of minor importance. The effect of adenosine and its stable analogues was studied in the presence of the dihydropyridine Bay K 8644, a highly specific L-type calcium channel agonist, on isolated guinea-pig atria. The inotropic effect of the calcium channel activator was found to be antagonized by adenosine A1-receptor agonists. Binding studies showed that the effect on Bay K 8644 was not due to the interaction between adenosine analogues and dihydropyridines at the level of a common receptor site on L-type Ca2+ channels. Inhibitors of K+ channels did not antagonize the effect of adenosine analogues against Bay K 8644. Experimental conditions aimed to unmask an effect on slow Ca2+ currents (i.e. K+ depolarized paced atria), further supported that adenosine analogues may act in atria as negative modulators on L-type Ca2+ channels. Finally, the use of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a highly specific A1-receptor antagonist, demonstrated that the antagonism of Bay K 8644 by adenosine analogues is strictly dependent on A1 receptors. The above data support the possibility of a dual signal transduction pathway to ion channels (K+ and Ca2+) linked to A1 receptors in atrial myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Absence of mitochondrial beta-adrenoceptors in guinea pig myocardium: evidence for tissue disparity.

1. Binding sites in guinea pig myocardial tissue labelled by (-)-[125I] cyanopindolol (CYP) were investigated using differential centrifugation and autoradiographic techniques. Autoradiographs of myocardial sections (0.1 microns) indicated (-)-[125I]CYP binding to sarcolemmal membrane. A low density of binding sites was observed to mitochondria. 2. Binding studies were performed in subcellular fractions. The density of binding sites in the mitochondrial fraction (36.1 +/- 9.4 fmol/mg protein) was less than 10% that in the sarcolemmal membrane (371.7 +/- 38.2 fmol/mg protein). The beta 1/beta 2-adrenoceptor subtype ratio in the mitochondrial fraction (83.3/16.7) was similar to that in the sarcolemmal fraction (87.1/12.9). 3. Ouabain (100 microM), in the presence of sodium azide (0.4 mM), inhibited a Na+K+ stimulated ATPase activity (1.0 +/- 0.2 mumol Pi/mg protein/hr reduction), indicating a low but significant level of sarcolemmal contamination of the mitochondrial fraction. 4. The study showed beta-adrenoceptors in guinea pig heart are located primarily on the sarcolemmal membrane of myocardium. No evidence was obtained for beta-adrenoceptors over mitochondria, as has been suggested in other tissues and species, but that this binding was to sarcolemmal inclusions in the mitochondrial fraction.     

The role of beta-adrenoceptors in coronary blood flow distribution in normal and ischemic canine myocardium.

beta-Adrenoceptor agonists increase myocardial ischemic injury, mainly by elevating myocardial oxygen consumption. Moreover, it has been shown that isoprenaline may "steal" regional myocardial blood flow (RMBF) from ischemic to non ischemic areas and from epicardium to endocardium. The mechanisms of these two isoprenaline-induced redistributions of RMBF have been investigated by the use of radioactive microspheres in an experimental model of canine myocardial ischemia with simultaneous measurement of ST-segment elevation. Isoprenaline increased RMBF in both epi- and endocardial non ischemic areas and in epicardial ischemic areas, leading to a significant decrease in the endo/epi ratio. After atenolol, isoprenaline still increased RMBF but to a lesser extent and the endo/epi ratio was still decreased. Salbutamol, in doses inducing no significant changes in cardiac parameters or myocardial oxygen consumption, produced effects similar to those of isoprenaline. These results indicate a non-homogeneous beta2-stimulation-induced vasodilation in endo- and epicardium, which might be due either to the higher epicardial coronary vasocilatory reserve or to a heterogeneous distribution of transmural beta2-adrenoceptors. Isoprenaline also decreased the ischemic/non ischemic total blood flow ratio (I/NI) and caused further increases in ST-segment elevation. These effects were abolished by atenolol pretreatment, indicating the deleterious effects of isoprenaline-induced tachycardia in this I/NI decrease and in the ischemic injury.     

Calcium antagonist binding sites in failing and nonfailing human ventricular myocardium

Studies in myopathic hamsters have described an increase in calcium antagonist binding sites, which is presumably associated with an increase in the number of calcium channels. Such an abnormality might predispose the heart to further myocardial damage from calcium overload. We tested the hypothesis that calcium antagonist binding sites are increased in human idiopathic dilated cardiomyopathy by examining [3H]PN 200-110 and [3H]nitrendipine binding in membranes prepared from nonfailing controls and severely failing ventricles with idiopathic dilated cardiomyopathy. Despite the fact that beta receptor density was decreased by 50% in failing hearts (iodocyanopindolol Bmax 84.4 +/- 8.9 fmol/mg protein in nonfailing hearts vs 42.9 +/- 3.2 fmol/mg in failing hearts, P less than 0.01), dihydropyridine calcium antagonist binding sites were not reduced significantly by heart failure. Maximum binding of [3H]PN 200-110 was 92.9 +/- 19.4 fmol/mg protein in membranes derived from failing ventricles, and 93.5 +/- 17.4 fmol/mg in membranes derived from nonfailing ventricles (P = NS); values for [3H]nitrendipine maximum binding were similar to those for [3H]PN 200-110 and also were not reduced significantly in failing ventricles. Additionally, the dissociation constants (KD) for [3H]nitrendipine and [3H]PN 200-110 were not significantly different in failing and nonfailing heart. We conclude that dihydropyridine calcium antagonist binding sites are not altered significantly in the failing human left ventricle with idiopathic dilated cardiomyopathy.     

Fendiline inhibits L-type calcium channels in guinea-pig ventricular myocytes: a whole-cell patch-clamp study.

1. Fendiline, a diphenylalkylamine type of antianginal drug, was examined for its effects on L-type calcium channels in guinea-pig ventricular myocytes by the whole-cell patch-clamp technique. 2. Fendiline (0.3-100 microM) applied extracellularly inhibited the calcium channel current (ICa) in a concentration- and time-dependent manner. The IC50 of fendiline was 17.0 +/- 2.43 microM and the Hill slope was 1.39 +/- 0.23. 3. Inhibition of ICa by fendiline appeared with an onset of less than 3 s. 4. Fendiline inhibited ICa at all the membrane potentials tested and shifted the current-voltage curve upwards. The overall calcium channel conductance (gCa) of the cell was reduced and conductance-voltage curve was shifted to the left in the presence of fendiline. 5. Isoprenaline (0.5-1 microM), a beta-adrenoceptor agonist, partially reversed the inhibitory effect of fendiline on ICa. 6. It is suggested that fendiline applied extracellularly blocks L-type calcium channels and reduces calcium channel conductance of the cell. The calcium channels thus inhibited are, nevertheless, still available for beta-adrenoceptor stimulation.