Effects of candesartan, an angiotensin II receptor type I blocker, on atrial remodeling in spontaneously hypertensive rats

Hypertension-induced structural remodeling of the left atrium (LA) has been suggested to involve the renin–angiotensin system. This study investigated whether treatment with an angiotensin receptor blocker, candesartan, regresses atrial remodeling in spontaneously hypertensive rats (SHR). Effects of treatment with candesartan were compared to treatment with a nonspecific vasodilatator, hydralazine. Thirty to 32-week-old adult male SHR were either untreated (n = 15) or received one of either candesartan cilexetil (n = 9; 3 mg/kg/day) or hydralazine (n = 10; 14 mg/kg/day) via their drinking water for 14 weeks prior to experiments. Untreated age- and sex-matched Wistar- Kyoto rats (WKY; n = 13) represented a normotensive control group. Untreated SHR were hypertensive, with left ventricular hypertrophy (LVH) compared to WKY, but there were no differences in systolic pressures in excised, perfused hearts. LA from SHR were hypertrophied and showed increased fibrosis compared to those from WKY, but there was no change in connexin-43 expression or phosphorylation. Treatment with candesartan reduced systolic tail artery pressures of conscious SHR below those of normotensive WKY and caused regression of both LVH and LA hypertrophy. Although hydralazine reduced SHR arterial pressures to those of WKY and led to regression of LA hypertrophy, it had no significant effect on LVH. Notably, LA fibrosis was unaffected by treatment with either agent. These data show that candesartan, at a dose sufficient to reduce blood pressure and LVH, did not cause regression of LA fibrosis in hypertensive rats. On the other hand, the data also suggest that normalization of arterial pressure can lead to the regression of LA hypertrophy

Electrophysiological properties of myocytes isolated from the mouse atrioventricular node:L-type ICa, IKr, If, and Na-Ca exchange

The atrioventricular node (AVN) is a key component of the cardiac pacemaker-conduction system. This study investigated the electrophysiology of cells isolated from the AVN region of adult mouse hearts, and compared murine ionic current magnitude with that of cells from the more extensively studied rabbit AVN. Whole-cell patch-clamp recordings of ionic currents, and perforated-patch recordings of action potentials (APs), were made at 35–37°C. Hyperpolarizing voltage commands from −40 mV elicited a Ba(2+)-sensitive inward rectifier current that was small at diastolic potentials. Some cells (Type 1; 33.4 ± 2.2 pF; n = 19) lacked the pacemaker current, I(f), whilst others (Type 2; 34.2 ± 1.5 pF; n = 21) exhibited a clear I(f), which was larger than in rabbit AVN cells. On depolarization from −40 mV L-type Ca(2+) current, I(C)(a,L), was elicited with a half maximal activation voltage (V(0.5)) of −7.6 ± 1.2 mV (n = 24). I(C)(a,L) density was smaller than in rabbit AVN cells. Rapid delayed rectifier (I(K)(r)) tail currents sensitive to E-4031 (5 μmol/L) were observed on repolarization to −40 mV, with an activation V(0.5) of −10.7 ± 4.7 mV (n = 8). The I(K)(r) magnitude was similar in mouse and rabbit AVN. Under Na-Ca exchange selective conditions, mouse AVN cells exhibited 5 mmol/L Ni-sensitive exchange current that was inwardly directed negative to the holding potential (−40 mV). Spontaneous APs (5.2 ± 0.5 sec(−1); n = 6) exhibited an upstroke velocity of 37.7 ± 16.2 V/s and ceased following inhibition of sarcoplasmic reticulum Ca(2+) release by 1 μmol/L ryanodine, implicating intracellular Ca(2+) cycling in murine AVN cell electrogenesis

Inhibition of a TREK-like K⁺ channel current by noradrenaline requires both β₁- and β₂-adrenoceptors in rat atrial myocytes

AIMS: Noradrenaline plays an important role in the modulation of atrial electrophysiology. However, the identity of the modulated channels, their mechanisms of modulation, and their role in the action potential remain unclear. This study aimed to investigate the noradrenergic modulation of an atrial steady-state outward current (I(Kss)). METHODS AND RESULTS: Rat atrial myocyte whole-cell currents were recorded at 36°C. Noradrenaline potently inhibited I(Kss) (IC(50) = 0.90 nM, 42.1 ± 4.3% at 1 µM, n = 7) and potentiated the L-type Ca(2+) current (I(CaL), EC(50) = 136 nM, 205 ± 40% at 1 µM, n = 6). Noradrenaline-sensitive I(Kss) was weakly voltage-dependent, time-independent, and potentiated by the arachidonic acid analogue, 5,8,11,14-eicosatetraynoic acid (EYTA; 10 µM), or by osmotically induced membrane stretch. Noise analysis revealed a unitary conductance of 8.4 ± 0.42 pS (n = 8). The biophysical/pharmacological properties of I(Kss) indicate a TREK-like K(+) channel. The effect of noradrenaline on I(Kss) was abolished by combined β(1)-/β(2)-adrenoceptor antagonism (1 µM propranolol or 10 µM β(1)-selective atenolol and 100 nM β(2)-selective ICI-118,551 in combination), but not by β(1)- or β(2)-antagonist alone. The action of noradrenaline could be mimicked by β(2)-agonists (zinterol and fenoterol) in the presence of β(1)-antagonist. The action of noradrenaline on I(Kss), but not on I(CaL), was abolished by pertussis toxin (PTX) treatment. The action of noradrenaline on I(CaL) was mediated by β(1)-adrenoceptors via a PTX-insensitive pathway. Noradrenaline prolonged APD(30) by 52 ± 19% (n = 5; P < 0.05), and this effect was abolished by combined β(1)-/β(2)-antagonism, but not by atenolol alone. CONCLUSION: Noradrenaline inhibits a rat atrial TREK-like K(+) channel current via a PTX-sensitive mechanism involving co-operativity of β(1)-/β(2)-adrenoceptors that contributes to atrial APD prolongation

Evidence for a Novel K ϩ Channel Modulated by ␣ 1A - Adrenoceptors in Cardiac Myocytes

ABSTRACT Accumulating evidence suggests that steady-state K ϩ currents modulate excitability and action potential duration, particularly in cardiac cell types with relatively abbreviated action potential plateau phases. Despite representing potential drug targets, at present these currents and their modulation are comparatively poorly characterized. Therefore, we investigated the effects of phenylephrine [PE; an ␣ 1 -adrenoceptor (␣ 1 -AR) agonist] on a sustained outward K ϩ current in rat ventricular myocytes. Under K ϩ current-selective conditions at 35°C and whole-cell patch clamp, membrane depolarization elicited transient (I t ) and steady-state (I ss ) outward current components. PE (10 M) significantly decreased I ss amplitude, without significant effect on I t . Preferential modulation of I ss by PE was confirmed by intracellular application of the voltage-gated K ϩ channel blocker tetraethylammonium, which largely inhibited I t without affecting the PE-sensitive current (I ss,PE ). I ss,PE had the properties of an outwardly rectifying steady-state K ϩ -selective conductance. Acidification of the external solution or externally applied BaCl 2 or quinidine strongly inhibited I ss,PE . However, I ss,PE was not abolished by anandamide, ruthenium red, or zinc, inhibitors of TASK acid-sensitive background K ϩ channels. Furthermore, the PE-sensitive current was partially inhibited by external administration of high concentrations of tetraethylammonium and 4-aminopyridine, which are voltage-gated K ϩ channel-blockers. Power spectrum analysis of I ss,PE yielded a large unitary conductance of 78 pS. I ss,PE resulted from PE activation of the ␣ 1A -AR subtype, involved a pertussis toxininsensitive G-protein, and was independent of cytosolic Ca 2ϩ . These results collectively demonstrate that ␣ 1A -AR activation results in the inhibition of an outwardly rectifying steady-state K ϩ current with properties distinct from previously characterized cardiac K ϩ channels. The exceptional diversity of K ϩ channels has particular significance in the heart, where different currents contribute to distinct phases of the cardiac action potential The steady-state currents represent a diverse family that includes the so-called ultra-rapid delayed rectifiers (I Kur ) of mouse ventricular myocytes and human and canine atrial myocyte

Effects of ET-1 on rapid delayed rectifier K⁺ current tails.

<p>A. Upper traces show currents elicited on depolarisation to +30 mV and subsequent repolarization to −40 mV by protocol shown in lower trace. Deactivating tail currents on repolarization represent the I_Kr ‘tail’. Currents are shown in control solution and in presence of 10 nM ET-1. Insert shows an expanded portion of the traces to highlight the ‘tail’ currents (the horizontal arrow in the inset denotes the zero current level). B. Mean ‘tail’ current I–V relationships for 5 cells, in absence (control, filled circles) and presence (open circles) of 10 nM ET-1. I–V curves were fitted with equation 2 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033448#s2" target="_blank">Methods</a>) to derive V_0.5 values of −15.7±3.6 mV in control and −0.7±4.3 mV in ET-1 (p<0.05), with respective k values of 6.4±2.6 mV and 6.9±3.9 mV (p>0.9). The ‘tail’ current was significantly reduced in presence of ET-1 at all voltages ranging from −20 to +50 mV except +20 mV. C. Mean I–V plots for I_Kr tails in the presence of 1 µM BQ-123 without (filled squares; n = 5) and with 10 nM ET-1 (open squares, n = 5 for all, except at +40 and +50 mV where n = 4). Derived V_0.5 values were −12.0±5.1 mV and −2.9±6.4 mV for BQ-123 and BQ-123+ET-1, respectively (P>0.3), with associated k values of 4.9±4.4 and 8.4±5.8 (p>0.6). Asterisks in B denote statistical significance (p<0.05 *, p<0.01 **, p<0.001 ***).</p

Effects of tertiapin-Q (TQ) on the effect of ET-1 on spontaneous APs and ET-1 activated current.

<p>A. Continuous recording of spontaneous activity in control, in the presence of TQ (300 nM) before and with application of 10 nM ET-1 in the maintained presence of TQ. Note the absence of immediate hyperpolarisation and cessation of APs evident in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033448#pone-0033448-g001" target="_blank">Figure 1</a>. Bi, ii and iii show expanded records from recording in A, at time-points indicated: i taken during control, ii near the end of TQ alone and iii is taken at ∼13 seconds of ET-1 application (at which time-point cells exposed to ET-1 alone had hyperpolarised and become quiescent). Similar results were obtained from 7 cells. C. Mean I–V relationships for the 10 nM ET-1 activated instantaneous current in absence (filled triangles, n = 14) and in presence of 300 nM TQ (open triangles, n = 7. except at −80 mV where n = 6). TQ prevented this action of ET-1. Asterisks in C denote statistical significance (p<0.05 *, p<0.001 ***).</p

ET-1 effects on the hyperpolarisation-activated current I_f.

<p>A. Upper traces show currents elicited −120 mV in an I_f-expressing cell in control solution and 10 nM ET-1 by protocol shown in bottom trace. Note outward shift in holding current in presence of ET-1. Closed circles indicate control trace; open circles indicate trace in ET-1. B. Mean I–V relationships (n = 7) for I_f, plotted as time-dependent current during command pulses, in absence (control, filled circles) and presence of 10 nM ET-1 (open circles). The activating effect of ET-1 was significant only at −120, −110 and −100 mV. C. Mean I–V relationships for the instantaneous current recorded at the beginning of the test-pulse (Ci: in absence (control, filled circles) and presence (open circles) of 10 nM ET-1). Cii shows I–V relation for the ET-1 activated instantaneous current (Cii, filled circles), in cells also showing I_f (n = 7). ET-1 activates a large inwardly rectifying current. D. I–V relations for I_f in presence of 1 µM BQ-123 (n = 11) without (filled squares) and with 10 nM ET-1 (open squares, n = 11 at all potentials except at −50 mV, where n = 10). BQ-123 prevented stimulation of I_f by ET-1. E. Inhibitory effect of 1 µM BQ-123 on the ET-1 activated current in cells exhibiting showing I_f (open squares, n = 12 except at −50 mV where n = 11). Asterisks denote statistical significance (p<0.001 ***).</p