Library

Notes: Class III Antiarrhythmics and β-Adrenergic Stimulation. Introduction: Blockade of the rapid delayed rectifier potassium current (IKr) as an important mechanism for current Class III antiarrhythmics is less effective in action potential prolongation during β-adrenergic activation. We hypothesized that blockade of the increased slow IK (IKs) current during β-adrenergic stimulation could improve action potential prolongation and tested this hypothesis by comparison of three different IK blockers: dofetilide, a selective blocker of IKr; ambasilide, a nonselective blocker of IK; and chromanol 293B, a selective blocker of IKs. Methods and Results: Transmembrane action potential duration was determined in guinea pig papillary muscles. After equilibration with the potassium channel blockers (dofetilide 10 nM, amhasilide 10 μM, chromanol 293B 10 μM), isoproterenol (10 and 100 nM) was added. The action potential prolonging effect of dofetilide was reduced in the presence of increasing concentrations of isoproterenol whereas the effect of ambasilide was much less reduced. In contrast, the effect of chromanol 293B clearly was increased in the presence of both concentrations of isoproterenol. No afterdepolarizations were observed after application of isoproterenol in control. Following isoproterenol, but not before, dofetilide and chromanol 293B induced early afterdepolarizations in 20% and 17% of the papillary muscles, whereas ambasilide and chromanol 293B induced delayed afterdepolarizations in 27% and 17%, respectively. Conclusion: In contrast to dofetilide, the Class III effect of ambasilide is less reduced and the effect of chromanol 293B is enhanced during β-adrenergic stimulation. Our data support the hypothesis that IKs blockade improves the efficacy of antiarrhythmics in action potential prolongation during β-adrenergic activation; however, this effect may increase the risk of afterdepolarizations.

Notes: [Auszug] Calcium release from the endoplasmic reticulum controls a number of cellular processes, including proliferation and contraction of smooth muscle and other cells. Calcium release from inositol 1,4,5-trisphosphate (IP3)-sensitive stores is negatively regulated by binding of ...

Notes: Summary 1. The inotropic effects of benzyl-and substituted benzyl triesters of cyclic AMP and cyclic GMP were studied in isometrically contracting guineapig papillary muscles driven at a rate of 0.2 Hz. 2. The triesters of cyclic AMP produced marked concentration-dependent positive inotropic effects. The maximum increase of contraction force (ΔF c) was 2.2 g (n=5) for p-MeBncAMP and 1.8 g for BncAMP and o-NO2BncAMP, respectively (n=5 for each ester). Total contraction time (t 1+t 2) was thereby shortened by 44 ms (n=15). Threshold and maximally effective concentrations for the positive inotropic effects were 1×10−4 M and 6×10−4M (BncAMP), 3×10−4 M and 3×10−3 M (p-MeBncAMP) and 1×10−5 M and 2×10−4 M (o-NO2BncAMP), respectively. 3. The phosphodiesterase inhibitor 1-methyl,3-isobutyxanthine (IBMX) (2×10−5 M) shifted the concentration-effect curves of all 3 triesters to the left, decreasing the concentrations required to produce a half-maximal increase of the force of contraction by a factor of 6. 4. The time necessary to produce a half-maximum inotropic effect was 2 min for p-MeBncAMP, 10 min for BncAMP and 66 min for o-NO2BncAMP. The inotropic effects could not be reversed upon washing the preparations for 1 h with a drug-free solution. 5. The concentration-effect curves of the triesters of cyclic AMP were not affected by previous blockade of the β-adrenoceptors with 5×10−6 M (±)-propranolol. 6. BncGMP at concentrations ranging from 1×10−5 to 2×10−3 M, produced no inotropic effects on 3 guinea-pig papillary muscles, while a decrease of the contraction force was detectable 30 min after the addition of 2×10−3 M p-MeBncGMP, reaching 75% of the control value within 1 h (n=3). At 3×10−3 M, both esters induced a positive inotropic effect (ΔF c) of 0.2 g which was completed in 3–4 min. The negative inotropic effect of 2×10−3 M p-MeBncAMP was immediately reversed upon washing the preparation while the positive inotropic effect was not affected. 7. Spontaneous hydrolysis of the benzyl triesters of cyclic AMP or cyclic GMP in Krebs-Henseleit buffer gave rise to cyclic AMP or cyclic GMP and to the respective benzyl alcohols. The electron donating substituent p-CH3 of the p-MeBncAMP or the p-MeBncGMP increased the hydrolytic reactivity by a factor of 14 when compared to the unsubstituted benzyl esters which in turn were 5 times more reactive than the triesters which contained the electron withdrawing substituent o-NO2. 8. Triesters of cyclic AMP and cyclic GMP (3×10−3 M) inhibited beef heart phosphodiesterase (PDE) by 17–29% and by 58–65%, respectively (n=4 for each ester). The specific PDE-inhibitor IBMX (2×10−5 M) blocked PDE by 52% (n=4). 9. The data presented in this study are in agreement with the hypothesis that benzyl triesters of cyclic AMP are simply a transport form of cyclic AMP across the cell membrane and, once inside, they are hydrolyzed with a characteristic time course to cyclic AMP which is responsible for the positive inotropic effects. Assuming that the benzyl triesters of cyclic GMP are also intracellularly converted to cyclic GMP, its postulated mediator role of a negative inotropic effect is not in accordance with the present results.

Notes: Abstract The aim of the present study was to determine the mechanism of the positive inotropic effect of carbachol on ventricular myocardium. Carbachol produced a concentration-dependent (0.1 to 300 μmol/l) increase in contraction force on the catecholamine-depleted papillary muscle of the guinea pig without affecting the normal action potential or the slow action potential evoked in 24 mmol/l K+. Since atropine prevented the inotropic effect of carbachol, muscarinic receptors were involved. Carbachol (300 μmol/l) produced an increase in intracellular Na+-ion-activity,a Na i , by about 3 mmol/l in the quiescent muscle, and the time course of thea Na i change corresponded with the development of the positive inotropic effect as determined in the stimulated preparation (0.2 Hz). The effect of carbachol on force of contraction and ona Na i was diminished by reducing [Ca2+]0. The positive inotropic effect of carbachol was dependent on repetitive activity and was markedly enhanced in the presence of dihydro-ouabain. The results are consistent with the hypothesis, that carbachol increases the Na+ permeability of the sarcolemma via muscarinic receptors, and enhances force of contraction by stimulating the Na+−Ca2+-exchange.

Notes: Abstract  The cloned BK channel α subunit from human myometrium was stably expressed in Chinese hamster ovary cells, either alone (CHOα cells) or in combination with the auxiliary β subunit (CHOα+β cells). We studied basic channel properties and the effects of cGMP- and cAMP-dependent protein kinases on the BK channel activity. Coexpression of α and β subunits enhanced the Ca2+ and voltage sensitivity of the BK channel, and decreased the inhibitory potency of iberiotoxin. Blocking and stimulating effects on BK channel activity by charybdotoxin and nitric oxide, respectively, were independent of the β subunit. The cGMP kinase Iα and cAMP kinase failed to affect BK channel activity in CHOα and CHOα+β cells at different [Ca2+]i and voltages. In contrast, BK channels in freshly isolated myometrial cells from postmenopausal women responded to cAMP kinase and cGMP kinase with a fourfold and twofold decrease in their open probability (NP o), respectively. These effects could be reversed by alkaline phosphatase and remained unaffected by the phosphatase inhibitor okadaic acid (100 nM). In 28% of myometrial cells, however, cAMP and cGMP kinases increased NP o 2-fold and 3.5-fold, respectively. This stimulation was enhanced rather than reversed by alkaline phosphatase and was abolished by 100 nM okadaic acid. The results suggest that in stably transfected CHO cells the expressed BK channel is not regulated by cAMP kinase and cGMP kinase. However, in native myometrial cells stimulatory and inhibitory regulation of BK channels by cAMP kinase and cGMP kinase was observed, suggesting that channel regulation by the protein kinases requires factors that are not provided by CHO cells. Alternatively, failure of regulation may have been due to the primary structure of the myometrial BK channel protein used in this study.

Notes: Summary 1. The inotropic effects of (±)-isoprenaline and several phosphodiesterase (PDE)-inhibitors were studied in isometrically contracting guinea-pig papillary muscles driven at a rate of 0.2 Hz. The maximal positive inotropic effect of isoprenaline (ΔF c=2.2 g;n=7) corresponded to that of 1-methyl, 3-isobutylxanthine (IBMX) and theophylline (ΔF c=2.1 and 2.2 g, respectively;n=7 each). Papaverine produced only a slight increase in contractile force (ΔF c=0.38 g;n=7), which in some preparations was completely absent. 2. The positive inotropic effects of IBMX, theophylline, and papaverine were not due to release of noradrenaline from adrenergic nerve endings. Blockade of the β-adrenoceptors with 5×10−6 M (±)-propranolol did not prevent the positive inotropic effect of IMBX. 3. The relaxation time,t 2, was shortened by isoprenaline (36 ms) more than by IBMX (28 ms) while the time to peak force,t 1, was reduced by 12 ms by isoprenaline and by 24 ms by IBMX. Isoprenaline and IBMX abbreviated the total contraction time,t 1+t 2, to nearly the same extent (48 and 52 ms, respectively). Theopylline and papaverine prolongedt 1+t 2 by 68 and 30 ms, respectively, due to a lengthening oft 2. 4. IBMX, theophylline, and papaverine shifted the isoprenaline concentration-effect curve to the left, decreasing the concentration of isoprenaline required to produce a half-maximal increase in the force of contraction from 1×10−8 M to 2×10−9 M for IBMX and papaverine and to 3.2×10−9 M for theophylline. IBMX enhanced the inotropic effect of dibutyryl cyclic AMP by a factor of 7. 5. Among several PDE-inhibitors, IBMX was found to mimic the effects of isoprenaline with regard to its maximum inotropic effect and its shortening oft 1+t 2. Isoprenaline-like effects of theophylline and papaverine were probably masked by the involvement of additional factors discussed in this study.