Carvedilol blocks the repolarizing K+ currents and the L-type Ca2+ current in rabbit ventricular myocytes

Activation of Capsaicin-Sensitive Sensory Neurons by Carvedilol, a Nonselective β-Blocker, in Spontaneous Hypertensive Rats

We performed a study in spontaneous hypertensive rats (SHR) to determine whether carvedilol, a nonselective beta-adrenoceptor antagonist, activates capsaicin-sensitive sensory neurons (CSSNs), thereby promoting the release of calcitonin gene-related peptide (CGRP), a neuropeptide with an important role in maintenance of cardiovascular homeostasis. Carvedilol given intravenously at a dose of 0.3 mg/kg transiently decreased the mean arterial blood pressure (MABP) and increased renal tissue blood flow with increases in CGRP levels in plasma and kidney. These effects induced by carvedilol were not seen in animals pretreated with capsazepine, an antagonist of capsaicin. Although 1.0 mg/kg cavedilol markedly decreased MABP, it neither increased renal tissue blood flow nor CGRP levels in plasma and kidney. Prazosin, a selective alpha(1)-adrenoceptor antagonist, and bisoprolol, a selective beta(1)-adrenoceptor antagonist, decreased MABP with capsazepine, showing no antagonistic action in either cases, and these agents increased neither renal tissue blood flow nor levels of CGRP in plasma and kidney. Both ICI 118,551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol], a selective beta(2)-adrenoceptor antagonist, at a dose of 0.25 mg/kg and capsaicin mimicked effects induced by 0.3 mg/kg carvedilol. Administration of 1.0 mg/kg ICI 118,551 produced effects similar to those induced by 1.0 mg/kg carvedilol. These observations strongly suggested that the low dose of carvedilol might activate CSSNs in SHR to increase the release of CGRP, thereby decreasing blood pressure with an increase in renal tissue blood flow. The effects induced by carvedilol seemed to be mediated by its beta(2)-adrenoceptor blockade activity.

Inhibitory effects of carvedilol on calcium channels in vascular smooth muscle cells

Carvedilol has hypotensive effects and inhibits agonist-induced cell proliferation of vascular smooth muscle and then prevents vascular remodeling. However, the basic mechanisms have not been clarified. We examined the effects of carvedilol on [Ca2+]i mobilization and voltage-dependent L-type Ca2+ current (ICa.L) in vascular smooth muscle cells, and compared them with metoprolol. [Ca2+]i was measured using fura-2 AM and patch clamp techniques in rat embryonic aortic smooth muscle cells (A7r5). In the presence of extracellular Ca2+, vasopressin and endothelin-1 increased [Ca2+]i due first to the Ca2+ release from store sites, and subsequently Ca2+ entry. Carvedilol did not inhibit the Ca2+ release, but significantly suppressed the sustained rise due to Ca2+ entry concentration-dependently. Nilfedipine and nicardipine (10 microM) partly inhibited the sustained rise, but carvedilol inhibited it more effectively than the Ca2+ channel blockers. Under voltage clamp conditions, carvedilol (0.2-10 microM) reversibly inhibited the ICa.L concentration-dependently without any changes in the current-voltage relationships of ICa.L. Carvedilol shifted the steady-state inactivation for ICa.L to more negative potentials and inhibited ICa.L in a voltage-dependent manner. In addition, carvedilol did not inhibit Ca2+ release from store sites induced by thapsigargin, but significantly inhibited the sustained rise due to capacitative Ca2+ entry unrelated to ICa.L. In contrast, metoprolol did not mimic these effects of carvedilol. These results provide evidence that carvedilol inhibits ICa.L and may also inhibit the channels for agonist (vasopressin and endothelin-1)-induced Ca2+ entry in vascular smooth muscle cells, which might contribute to the vasorelaxing and antiproliferative effects of carvedilol.

Protective effect of carvedilol on abnormality of L-type calcium current induced by oxygen free radical in cardiomyocytes

The protective effect of carvedilol on abnormality of L-type calcium current induced by oxygen free radical in single guinea pig ventricular myocytes was studied. Whole-cell patch clamp technique was used to study the effect of H2O2 (0.5 mmol/L) on L-type calcium current in single guinea pig ventricular myocytes and the action of pretreatment with carvedilol (0.5 micromol/L). 0.5 micromol/L carvedilol had no significant effect on ICa,L and its channel dynamics. In the presence of 0.5 mmol/L H2O2, peak current of ICa,L was reduced significantly (P<0.001), the I-V curve of ICa,L was shifted upward, steady-state activation curve and steady-state deactivation curve of ICa,L were shifted left and recovery time of ICa,L was delayed significantly (P<0.001). 0.5 micromol/L carvedilol significantly alleviated the inhibitory effect of H2O2 on ICa,L as compared with that in H2O2 group (P<0.01). In addition, carvedilol reversed the changes of dynamics of ICa,L induced by H2O2. It was concluded that carvedilol could alleviate the abnormality of L-type calcium current induced by oxygen free radical in cardiomyocytes. It shows partly the possible mechanism of the special availability of carvedilol in chronic heart failure.

Characteristic effects of alpha1-beta1,2-adrenergic blocking agent, carvedilol, on [Ca2+]i in ventricular myocytes compared with those of timolol and atenolol

Beta-adrenergic stimulation and the resultant Ca(2+) load both seem to be associated with progression of heart failure as well as hypertrophy. Because the alpha(1)-, beta(1,2)-blocker, carvedilol, has been shown to be outstandingly beneficial in the treatment of heart failure, its direct effects on intracellular calcium ion concentration ([Ca(2+)](i)), including antagonism to isoproterenol, in ventricular myocytes were investigated and then compared with a selective beta(1)-blocker, atenolol, and a non-selective beta(1,2)-blocker, timolol. At 1-300 nmol/L, carvedilol decreased the amplitude of [Ca(2+)] (i) by approximately 20% independently of its concentration, which was a similar effect to timolol. All the beta-blockers at 10 nmol/L decreased the amount of cAMP, but atenolol had the least effect. Carvedilol in the micromol/L order further diminished the amplitude of [Ca(2+)](i) transients, and at 10 micromol/L increased the voltage threshold for pacing myocytes. These effects were not observed with timolol or atenolol. L-type Ca2+ currents (I(Ca)) were decreased by carvedilol in the micromol/L order in a concentration dependent manner. As for the beta-antagonizing effect, the concentrations of carvedilol, timolol, and atenolol needed to prevent the effect of isoproterenol by 50% (IC(50)) were 1.32, 2.01, and 612 nmol/L, respectively. Furthermore, the antagonizing effect of carvedilol was dramatically sustained even after removal of the drug from the perfusate. Carvedilol exerts negative effects on [Ca(2+)](i), including inhibition of the intrinsic beta-activity, reduction of I(Ca) in the micromol/L order, and an increase in the threshold for pacing at > or =10 micromol/L. Data on the IC(50) for the isoproterenol effect suggest that carvedilol could effectively inhibit the [Ca(2+)](i) load induced by catecholamines under clinical conditions.

Novel effect of carvedilol on Ca2+ movement in renal tubular cells

The effect of carvedilol on intracellular free Ca(2+) levels ([Ca(2+)](i)) has not been explored previously. This study was aimed to examine the effect of carvedilol on Ca(2+) handling in renal tubular cells. Madin-Darby canine kidney cells were used as a model for renal tubular cells and fura-2 was used as a fluorescent Ca(2+) probe. Carvedilol increased [Ca(2+)](i) in a concentration-dependent manner with an EC(50) value of 5 microM. Extracellular Ca(2+) removal partly inhibited the [Ca(2+)](i) signals. Carvedilol-induced Ca(2+) influx was verified by measuring Mn(2+)-induced quench of fura-2 fluorescence. Carvedilol-induced store Ca(2+) release was reduced by pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor) but not with 5 microM ryanodine or 2 microM carbonylcyanide m-chlorophenylhydrazone (a mitochondrial uncoupler). Carvedilol (30 microM)-induced Ca(2+) release was not affected by inhibiting phospholipase C with 1-(6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-l)amino)hexyl)-1H-pyrrole-2,5-dione (U73122; 2 microM), but was potentiated by increasing cAMP levels or inhibiting protein kinase C. The carvedilol-induced Ca(2+) mobilization was not significantly sequestered by the endoplasmic reticulum or mitochondria. This study shows that carvedilol increased [Ca(2+)](i) in renal tubular cells by causing Ca(2+) release from the endoplasmic reticulum and other unknown stores in an inositol-1,4,5-trisphosphate-independent manner, and by inducing Ca(2+) influx. The Ca(2+) release was modulated by cAMP and protein kinase C.

Carvedilol: molecular and cellular basis for its multifaceted therapeutic potential

Carvedilol is a unique cardiovascular drug of multifaceted therapeutic potential. Its major molecular targets recognized to date are membrane adrenoceptors (beta 1, beta 2, and alpha 1), reactive oxygen species, and ion channels (K+ and Ca2+). Carvedilol provides prominent hemodynamic benefits mainly through a balanced adrenoceptor blockade, which causes a reduction in cardiac work in association with peripheral vasodilation. This drug assures remarkable cardiovascular protection through its antiproliferative/atherogenic, antiischemic, antihypertrophic, and antiarrhythmic actions. These actions are a consequence of its potent antioxidant effects, amelioration of glucose/lipid metabolism, modulation of neurohumoral factors, and modulation of cardiac electrophysiologic properties. The usefulness of carvedilol in the treatment of hypertension, ischemic heart disease, and congestive heart failure is based on a combination of hemodynamic benefits and cardiovascular protection.

Short- and long-term effects of amiodarone on the two components of cardiac delayed rectifier K(+) current

BACKGROUND

Amiodarone is the most promising drug for the treatment of life-threatening tachyarrhythmias in patients with structural heart disease. The pharmacological effects of amiodarone on cardiac ion channels are complex and may differ for short-term and long-term administration.

METHODS AND RESULTS

The delayed rectifier K(+) current (I(K)) of ventricular myocytes isolated from rabbit hearts was recorded with the whole-cell voltage-clamp technique. I(K) was separated into 2 components by use of specific blockers for either I(Ks) (chromanol 293B, 30 micromol/L) or I(Kr) (E-4031, 10 micromol/L). Short-term application of amiodarone caused a concentration-dependent decrease in I(Kr) with an IC(50) of 2.8 micromol/L (n=8) but only a minimal reduction in I(Ks). The short-term effects of amiodarone were also determined in Xenopus oocytes expressing the cloned human channels that conduct I(Kr) and I(Ks) (HERG and KvLQT1/minK). HERG current in oocytes was reduced by amiodarone (IC(50)=38 micromol/L), whereas KvLQT1/minK current was unaffected by 300 micromol/L amiodarone. To study the effects of long-term drug administration, rabbits were treated for 4 weeks with oral amiodarone (100 mg. kg(-1). d(-1)) before cell isolation. Long-term administration of amiodarone decreased I(K) to 55% (n=10) in control rabbits and altered the relative density of I(Kr) and I(Ks). The majority (92%) of current was I(Kr). mRNA levels of rabbit ERG,KVLQT1, and minK in left ventricular myocardium did not differ between control and long-term amiodarone.

CONCLUSIONS

Amiodarone has differential effects on the 2 components of I(K), depending on the application period; short-term treatment inhibits primarily I(Kr), whereas long-term treatment reduces I(Ks).

Species-specific difference in distribution of voltage-gated L-type Ca(2+) channels of cardiac myocytes

Ca(2+) influx via sarcolemmal voltage-dependent Ca(2+) channels (L-type Ca(2+) channels) is the fundamental step in excitation-contraction (E-C) coupling in cardiac myocytes. Physiological and pharmacological studies reveal species-specific differences in E-C coupling resulting from a difference in the contribution of Ca(2+) influx and intracellular Ca(2+) release to activation of contraction. We investigated the distribution of L-type Ca(2+) channels in isolated cardiac myocytes from rabbit and rat ventricle by correlative immunoconfocal and immunogold electron microscopy. Immunofluorescence labeling revealed discrete spots in the surface plasma membrane and transverse (T) tubules in rabbit myocytes. In rat myocytes, labeling appeared more intense in T tubules than in the surface sarcolemma. Immunogold electron microscopy extended these findings, showing that the number of gold particles in the surface plasma membrane was significantly higher in rabbit than rat myocytes. In rabbit myocyte plasma membrane, the gold particles were distributed as clusters in both regions that were associated with junctional sarcoplasmic reticulum and those that were not. The findings are consistent with the idea that influx of Ca(2+) via surface sarcolemmal Ca(2+) channels contributes to intracellular Ca(2+) to a greater degree in rabbit than in rat myocytes.

Low dose carvedilol inhibits progression of heart failure in rats with dilated cardiomyopathy

The cardioprotective properties of carvedilol (a vasodilating beta-adrenoceptor blocking agent) were studied in a rat model of dilated cardiomyopathy induced by autoimmune myocarditis. Twenty-eight days after immunization, surviving Lewis rats (32/43=74%) were divided into three groups to be given 2 mg kg(-1) day(-1) (Group-C2, n=10) or 20 mg kg(-1) day(-1) (Group-C20, n=10) of carvedilol, or vehicle (0.5% methylcellulose, Group-V, n=12). After oral administration for 2 months, body weight, heart weight (HW), heart rate (HR), rat alpha-atrial natriuretic peptide (r-ANP) in blood, central venous pressure (CVP), mean blood pressure (mean BP), peak left ventricular pressure (LVP), left ventricular end-diastolic pressure (LVEDP), +/-dP dt(-1) and area of myocardial fibrosis were measured. Values were compared with those for normal Lewis rats (Group-N, n=10). Two out of 12 (17%) rats in Group-V died from day 28 to day 42 after immunization. No rat died in Groups-C2, -C20 and -N. Although the CVP, mean BP, LVP and +/-dP dt(-1) did not differ among the three groups, the HW, HR and r-ANP in Group-C2 (1.14+/-0.03, 339+/-16 and 135+/-31) and Group-C20 (1.23+/-0.04, 305+/-8 and 156+/-24) were significantly lower than those in Group-V (1.36+/-0.04 g, 389+/-9 beats min(-1) and 375+/-31 pg ml(-1), respectively). The LVEDP in Group-C2 was significantly lower than that in Group-V (7.4+/-1.4 and 12.2+/-1.2 mmHg, respectively, P<0. 05). The area of myocardial fibrosis in Group-C2 was smaller than that in Group-V (12+/-1 and 31+/-2%, P<0.01). These results indicate that a low dose of carvedilol has beneficial effects on dilated cardiomyopathy.