Endothelin-1 inhibits L-type Ca2+ current enhanced by isoprenaline in rat atrial myocytes

Endothelin-1 (ET-1) was shown to exert direct cardiac effects by complex signaling pathways and to interact with neurotransmitter regulation of cardiac activity. The effect of ET-1 was investigated on the beta-adrenergic stimulation of cardiac L-type Ca2+ current (ICaL) on isolated rat atrial myocytes by using the patch-clamp technique. ET-1 (5 x 10(-8) M) reversed the increase in ICaL induced by isoprenaline (10(-6) M) but had no effect on basal ICaL and on (-) Bay K 8644-increased ICaL (10(-6) M); so ET-1 might exert an effect only when the Ca2+ channels are phosphorylated. The antiadrenergic action of ET-1, blocked by BQ-123 (10(-6) M) and unaffected by IRL 1038 (3.5 x 10(-8) M) should be mediated by ET-A receptors. The inhibitory action of ET-1 was still observed when ICaL was previously increased by forskolin (3 x 10(-6) M), 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP; 200 microM), or cAMP (100 microM) in presence of isobutyl methyl xanthine (IBMX; 10(-6) M), suggesting that the antiadrenergic action of ET-1 on ICaL was exerted independent of the cAMP-dependent phosphorylation pathway. ET-1 is known to be an activator of phosphoinositide hydrolysis, resulting in an increased production of IP3 and diacylglycerol (DAG). A Ca(2+)-dependent inhibition of ICaL consequently to an elevation of the intracellular Ca2+ pool via IP3 might be excluded in the action of ET-1, because of the presence of EGTA in the intrapipette medium. ET-1 reversed the isoprenaline-induced increase in ICaL in the presence of protein kinase C inhibitor [PKC(19-31); 100 microM), making unlikely the involvement of a DAG-dependent activation of PKC. Therefore the antiadrenergic action of ET-1 might also be independent on the phosphoinositide pathway.

Antagonism of beta-adrenergic stimulation of L-type Ca2+ current by endothelin in guinea-pig atrial cells

Experiments carried out with isolated guinea pig atrial cells, using the patch clamp technique, demonstrated that endothelin-1 reversed the increase in L-type Ca2+ current (ICaL) induced by isoprenaline. Similar effects of endothelin-1 were observed when ICaL was previously increased by forskolin or 8-bromo-cAMP. These results suggested that the endothelin antagonism of beta-adrenergic stimulation of ICaL was exerted independently of the cAMP-dependent phosphorylation pathway.

Cyclic nucleotide phosphodiesterases from frog atrial fibers: isolation and drug sensitivities

The cyclic nucleotide phosphodiesterase (PDE) forms present in frog atrial fibers were isolated and characterized by their drug sensitivities. DEAE-sephacel chromatography of cytosolic PDE activity resolved three major PDE forms: peak A hydrolyzed both adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) and was activated by calcium-calmodulin (PDE I); peak B also hydrolyzed both cAMP and cGMP but was activated by 5 microM cGMP (PDE II); peak C specifically hydrolyzed cAMP (PDE IV). Rolipram specifically inhibited PDE IV (Ki = 1.1 microM), whereas dipyridamole potently inhibited both PDE II (Ki = 4.6 microM) and PDE IV (Ki = 0.8 microM). Atrial fiber PDE I was preferentially inhibited by zaprinast (Ki = 10 microM). 3-Isobutyl-1-methyl xanthine (IBMX) and theophylline inhibited nonspecifically all three different enzymes. The positive inotropic drug CI 930 only inhibited the different isolated atrial PDE forms at concentrations greater than 200 microM. However, under assay conditions for which PDE IV was specifically inhibited (presence of 100 microM rolipram), an IC50 of 17 microM for CI 930 was observed on the remaining 26% cAMP hydrolytic activity of peak C (which could represent a cGMP-inhibited PDE form: PDE III). The same PDE forms were also found in frog ventricle. The major difference between frog atrial fiber (and ventricular tissue) PDEs and mammalian cardiac PDEs is that the main cytosolic cAMP-specific hydrolytic activity in frog heart is due to PDE IV rather than PDE III. Rolipram, dipyridamole, and zaprinast might be useful tools to investigate the participation of cAMP in frog atrial contraction (unpublished observations).

Forskolin effects on slow inward current and phasic tension of frog atrial fibres: modulation by adenosine and phosphodiesterase inhibitors

The effects of forskolin, which is known as a direct activator of adenylate cyclase were studied on the slow inward calcium current (Isi) and phasic tension of frog atrial fibres. Forskolin induced a dose-dependent positive inotropic effect related to an increase in the slow inward calcium current. These effects, which were not reproduced by 1,9-dideoxyforskolin, seemed to result from an activation of adenylate cyclase. The action of forskolin was antagonized by adenosine and potentiated by phosphodiesterase inhibitors with the following order of potency: rolipram greater than theophylline greater than dipyridamole; M & B 22,948 was without influence. This study suggests that adenosine and rolipram might be suitable tools for studying the implication of cAMP in the modulation of contraction in frog atrium.

Theophylline effects on phasic tension and slow inward current of isolated frog atrial fibres: influence on the action of isoprenaline

The action of theophylline on phasic tension and slow inward current (Isi) was investigated on isolated voltaged-clamped frog atrial fibres. The action of theophylline was both dose- and time-dependent; the most effective dose was 2.10(-3)M. Theophylline induced a rapid positive inotropic effect which cannot be attributed to a beta-adrenoceptor stimulation or to an adenosine antagonism; this effect was closely related to an increase in slow inward current (Isi), probably due to an inhibition of phosphodiesterase activity and a subsequent increase in cAMP content. The decline of the positive inotropic effect of theophylline was discussed in terms of additional effects of the drug. Theophylline, consistently with its phosphodiesterase inhibitory action, was shown to potentiate the effects of isoprenaline on phasic tension and Isi; however no potentiation was observed after reserpine-induced depletion of endogenous catecholamines. It might be thought that non-specific effects of reserpine on some steps of cardiac excitation-contraction coupling counteract the effects of theophylline.

Effects of isoprenaline on tonic tension and Na-Ca exchange in frog atrial fibres

The action of isoprenaline, a purely beta-agonist, was investigated on frog atrial fibres under voltage clamp conditions; tonic tension was induced by long depolarizing pulses and the outward delayed current simultaneously developed. The cumulative dose-response curves showed that isoprenaline increased the peak of tonic tension in the concentration range 10(-8) to 3. 10(-6) mol.l-1, with a maximum effect for 10(-6) mol.l-1. The positive inotropic action of isoprenaline was associated with an increase in the rates of tension rise and of relaxation. Isoprenaline also increased the amplitude of the outward delayed current in a dose-dependent manner. The effects of isoprenaline (10(-6) mol.l-1) on tonic tension and outward delayed current were not observed in the presence of propranolol (10(-7) mol.l-1). Experiments carried out in low-sodium solution demonstrated that the action of isoprenaline on tonic tension can be explained by activation of Na-Ca exchange; the enhanced relaxation might result from the same process. These results suggested that the positive inotropic action of isoprenaline is mediated not only by the well-known increase in the slow inward current but also by activation of the Na-Ca exchange mechanism.

Effects of chloride replacement and chloride transport blockade on the tonic tension of frog atrial trabeculae

The effects of a chloride-poor medium (methanesulfonate substituted) and a chloride transport inhibitor (SITS) on the outward delayed current and the tonic tension were studied on frog atrial trabeculae under voltage-clamp conditions. The outward delayed current decreased in low-chloride medium (10.5 mmol/l) or in the presence of SITS (2 mmol/l). The tonic tension increased in chloride-poor solution and decreased following SITS. The replacement of chloride by methanesulfonate enhanced the transient increase of tonic tension induced by low external sodium concentration while SITS reduced it. In the same conditions, the effect of the chloride-poor medium was abolished in the presence of SITS. The results showing an increase in Na-Ca exchange in low-chloride medium and a decrease by SITS are discussed assuming that changes in the inner negative charge density influenced the Na-Ca exchange mechanism; the influence of pHi variation are also considered.

Electrophysiological and mechanical studies of frog heart adrenoceptor stimulation by epinine

Electrophysiological and mechanical effects of epinine, an alpha- and beta-agonist, were investigated in frog atrial fibres. From dose-response curves it was demonstrated that epinine produced an increase in amplitude and duration of the plateau, and in duration of the action potential. Corresponding increases in peak tension, rate of tension development, time to peak tension and rate of relaxation were noted. The positive inotropic effect of epinine was mediated only by myocardial beta-adrenoceptor stimulation; over the whole range of concentrations, the inotropic action of epinine was not influenced by phentolamine (3.10(-6) mol . litre-1), but was inhibited by propranolol (3.10(-8) mol . litre-1). The mechanism of epinine action involved an important increase in slow calcium current Isi responsible for the changes in the action potential, while the positive inotropic effect of the drug was attributed to the resulting increase in intracellular calcium concentration. The action of epinine seemed independent of cAMP concentration, because the effects of epinine on contraction were not significantly increased by papaverine (10(-5) mol . litre-1). In frog heart, a beta-stimulation induced specific effects on action potential and tension. The positive inotropic effect of beta-catecholamines resulted directly from electrical changes induced by an increase in slow calcium current.