Negative inotropic effect of carbachol and interaction between acetylcholine receptor-operated potassium channel (K.ACh channel) and GTP binding protein in mouse isolated atrium--a novel methodological trial

Eukaryotic elongation factor 2 kinase inhibitor, A484954 potentiates β-adrenergic receptor agonist-induced acute decrease in diastolic blood pressure in rats

Eukaryotic elongation factor 2 (eEF2) kinase (eEF2K) acts to inhibit protein translation through phosphorylating a specific substrate, eEF2. We previously found that the increased eEF2K expression in mesenteric artery mediates hypertension development in spontaneously hypertensive rats. More recently, we have revealed that a selective eEF2K inhibitor, A484954 induced vasorelaxation via opening inward rectifier K⁺ channel and activating β₂-adrenergic receptor in smooth muscle of rat isolated mesenteric artery, which contributes to prevent noradrenaline-induced acute increase in blood pressure (BP). In this study, we further explored acute effects of A484954 on BP in rats, especially focusing the action on β-adrenergic receptor. We also examined whether A484954 affects contraction and heart rate (HR) of isolated heart. BP and HR were measured by a carotid cannulation method in rats. Isometric contraction and HR in rat isolated atria were also measured pharmacologically. A484954 potentiated adrenaline-induced decrease in diastolic BP (DBP) but not increase in systolic BP (SBP). A484954 potentiated isoproterenol-induced decrease in DBP but not SBP. Contrastingly, A484954 prevented a non-β-adrenergic receptor agonist, angiotensin II-induced increase in both SBP and DBP. In isolated left atria, A484954 caused contraction, which was prevented by a β-adrenergic receptor antagonist, propranolol. In isolated right atria, A484954 increased HR. In conclusion, we for the first time demonstrated that A484954 potentiates β-adrenergic receptor agonist-induced decrease in DBP possibly through vasorelaxation mediated via activating β₂-adrenergic receptor. It was also demonstrated that A484954 causes contraction of rat isolated heart via activating β₁-adrenergic receptor.

Sinoatrial Beat to Beat Variability Assessed by Contraction Strength in Addition to the Interbeat Interval

Beat to beat variability of cardiac tissue or isolated cells is frequently investigated by determining time intervals from electrode measurements in order to compute scale dependent or scale independent parameters. In this study, we utilize high-speed video camera recordings to investigate the variability of intervals as well as mechanical contraction strengths and relative contraction strengths with nonlinear analyses. Additionally, the video setup allowed us simultaneous electrode registrations of extracellular potentials. Sinoatrial node tissue under control and acetylcholine treated conditions was used to perform variability analyses by computing sample entropies and Higuchi dimensions. Beat to beat interval variabilities measured by the two recording techniques correlated very well, and therefore, validated the video analyses for this purpose. Acetylcholine treatment induced a reduction of beating rate and contraction strength, but the impact on interval variability was negligible. Nevertheless, the variability analyses of contraction strengths revealed significant differences in sample entropies and Higuchi dimensions between control and acetylcholine treated tissue. Therefore, the proposed high-speed video camera technique might represent a non-invasive tool that allows long-lasting recordings for detecting variations in beating behavior over a large range of scales.

Endostatin inhibits bradykinin-induced cardiac contraction

.Endogenous fragments of extracellular matrix are known to possess various biological effects. Levels of endostatin, a fragment of collagen type XVIII, increase in certain cardiac diseases, such as cardiac hypertrophy and myocardial infarction. However, the influence of endostatin on cardiac contraction has not been clarified. In the present study, we investigated the effects of endostatin on bradykinin-induced atrial contraction. Isometric contractile force of mouse isolated left atria induced by electrical current pulse was measured. Voltage-dependent calcium current of guinea pig ventricular myocytes was measured by a whole-cell patch-clamp technique. Endostatin (100–1,000 ng/ml) alone treatment had no influence on left atrial contraction. On the other hand, pretreatment with endostatin (300 ng/ml) significantly inhibited bradykinin (1 µM)-induced contraction and voltage-dependent calcium current. These data suggest that endostatin may decrease bradykinin-induced cardiac contraction perhaps through the inhibition of voltage-dependent calcium channel.