The Effect Of Angiotensin Ⅱ On The Action Potentials In Papillary Muscle And Sinoatrial Node Of Guinea Pig And Its Underlying Mechanism

The rennin-angiotensin system (RAS) plays an important role in maintaining the steady-state physiological function of cardiovascular system. The increased activity of RAS contributes to the development of numerous diseases, such as hypertension, cardiac failure, artherosclerosis and so on. Angiotensin II (Ang II) is the most active components in RAS. It is well recognized that Ang II is involved in the initiation of the tissue remodeling during the development of myocardial hypertrophy and congestive heart failure (CHF). Early clinical trials show that angiotensin-converting enzyme inhibitor (ACEI) could reduce the morbidity presumably caused by malignant arrhythmia in the patients with cardiac hypertrophy and heart failure. Recently, the increasing evidence shows that the antagonist of AT1 receptor have similar effectiveness as ACEI in decreasing the incidence of sudden cardiac death (SCD) caused by lethal ventricular arrhythmia in CHF patients. Therefore, ACEI and AT1 receptor antagonist have become the important drugs in the treatment of blood hypertension and CHF. Ang II may be a major risk factor for ventricular arrhythmias and SCD. The prolongation of action potentials derived of delayed repolarization is the most prominent electrophysiological alternation during the development of myocardial hypertrophy and CHF. Recent study (2008) has shown that mice with cardiac-specific overexpression of human AT₁ receptor undergo cardiac remodeling and die prematurely of sudden death. Importantly, the increased incidence of arrhythmia and the repolarization defect also occurred in much younger AT1R mice that do not present signs of hypertrophy, confirming that these arrhythmogenic changes are not secondary to cardiac remodeling. However, it is relatively little is known concerning the role of Ang II in the ventricular repolarization. The aim of the study was to investigate the effects of Ang II on the action potential in multicellular preparations including papillary muscle and sinoatrial node of guinea pig and study its underlying mechanism.Part.1 The effect of angiotensin II on the action potential of papillary muscle in guinea pig and its underlying mechanismAim: To investigate the effect of Ang II on the action potential of papillary muscle in guinea pig and analyse the mechanism.Methods: The conventional glass electrode technique was used to record the action potential of papillary muscle in guinea pig. The action potential profile was collected before and 30 min later in the presence of Ang II under different stimulation frequency (0.5,1.0,4.0Hz). The following action potential parameters were calculated by using a self-edited software: action potential altitude (APA), rest potential (RP), maximal rate of 0 phase depolarization (V_max), duration of 30% depolarization (APD₃₀), duration of 50% depolarization (APD₅₀), duration of 90% depolarization (APD₉₀). Then, in the presence of L-type calcium channel blocker, nimodipine (Nimo) or/and rapid delayed rectifier potassium current (I_kr) blocker, E-4031, the effect of Ang II on action potential was examined again.Results: (1) The APD₉₀ at the stimulatory frequencies of 0.5, 1.0, 4.0 Hz were respectively shorted by 17%, 25%, 20% in the presence of Ang II (1μmol/L), which have no significant difference with the control preparations (P>0.05), in which APD₉₀ was got shorter at the different stimulatory frequencies to 18%, 22%, and 20%, respectively. The change of APD₃₀ and APD₅₀ were identical to that in APD₉₀. (2) Nimo (1μmol/L) significantly shorted APD₃₀, APD₅₀, and APD₉₀ by 28%, 32%, 30% (P<0.05). In the presence of Nimo, Ang II didn't produce any effect on APD. (3) In the presence of Nimo (1μmol/L) plus E-4031(1μmol/L), Ang II had no effect on the APD₃₀, APD₅₀, APD₉₀ (P>0.05), suggesting that Ang II have no potential effect on repolarization through modulation of both inward and outward currents.Conclusion: Compared to parallel control, Ang II(1μmol/L) in different stimulation frequencies have no significant effect on APD₃₀, APD₅₀ and APD₉₀. In the presence of antaonist of ICa-L or/and I_kr, Ang II don not produce any effect on APD. We conclude that Ang II don not affect the repolarization of action potential in guinea pig papillary muscle.Part 2 The effect of angiotensin II on the action potential of sinoatrial node in guinea pig and its underlying mechanismAim: To investigate the electrophysiological effect of Ang II on the sinoatrial node in guinea pig and analyze the potential mechanism.Methods: 1 The spontaneous action potentials of sinoatrial node in guinea pig were recorded in the absence and presence of Ang II (10 nmol/L²Î¼mol/L). The change of the following action potential parameters were calculated: action potential multitude (APA), most diastolic potential (MDP), velocity of 4 phase repolarization (V₄), maximal rate of 0 phase repolarization (V_max) , duration of 30% depolarization (APD₃₀), duration of 50% depolarization(APD₅₀), duration of 90% depolarization (APD₉₀), rate of pacemaker firing (RPF).2 In the presence of Losartan (1μmol/L) or PD123319 (1μmol/L), the effect of Ang II was observed to identify the subtype of receptor, by which Ang II evoked effcct.3 Action potential parameters were calculated in the presence of Nimo (1μmol/L) and compared to those in the presence of Ang II.4 In the presence of Cs+ (If current bloker, 0.1mmol/L, 1mmol/L), the effect of Ang II was observed to investigate the potential effect of Ang II on If . Results: (1) Ang II reduced the firing rate of sinoatrial node in a concentration-dependent manner in the rage of 10 nmol/L²Î¼mol/L, with IC₅₀ of 375nmol/L. Ang II significantly delayed velocity of 4 phase repolarization. (2) Losartan (1μmol/L) reduced the firing rate of sinoatrial node from 148 beats/min to 116 beats/min. Losartan abolished the effect of Ang II (375nmol/L) on the firing rate of sinoatrial node. In contrast, PD1233 19 had no effect of Ang II. (3) Nimo respectively reduced the APA, V_max, RPF by 20%, 60%, and 47% (P<0.05). The changes of action potential profile caused by Nimo (1μmol/L) were obviously different from those produced by Ang II, suggesting that the effect of Ang II on electrial activity of sinoatrial node is not through L-type calcium channel. (4) CsCl (0.1mmol/L) could reduce V₄ by 18% significantly (P<0.05). In the presence of CsCl (0.1mmol/L), Ang II (375nmol/L) could reduce V₄ by 30%(P<0.05).CsCl (1mmol/L) could reduce V₄ by 70% signi ficant ly(P<0.05). In the presence of CsCl (0.1mmol/L), Ang II(375nmol/L) has no effect on V₄ continuely.Conclusion: 1 Ang II reduces the firing rate of sinoatrial node of guinea pig in a concentration-dependent manner, with IC₅₀ is 375nmol/L.2 AT1 receptor mediates the inhibitory effect of Ang II on the firing rate of sinoatrial node.3 Ang II may decrease the firing rate of sinoatrial node through the ibhibition of If.