Functional remodeling in post-myocardial infarcted rats: focus on beta-adrenoceptor subtypes.

Cellular electrophysiological remodeling of the infarcted heart may lead to the deterioration of cardiac function and/or to arrhythmias. The present study was designed to characterize the functional expression of the hyperpolarization-activated current (I(f)) and its modulation by beta(1)-, beta(2)- and beta(3)-adrenoceptor (AR) subtypes, in patch-clamped ventricular myocytes isolated from the heart of post-myocardial infarcted (PMI) rats and sham-operated control (SHAM) rats. Maximum specific conductance of I(f) was significantly higher in left ventricular myocytes (LVM) from PMI rats compared to right ventricular myocytes from PMI rats as well as LVM and RVM from SHAM rats. All other basic properties of I(f) were similar. beta(1)AR stimulation with noradrenaline caused a rightward shift of V(H) in LVM from PMI rats which was significantly smaller (52.2%) than in LVM from SHAM rats. Incubation with pertussis toxin (PTX) largely restored the effect of beta(1)AR in PMI cells (86.6% vs. SHAM cells), but did not affect beta(1)AR response in SHAM cells. beta(2)AR response was significantly and equally increased by PTX-pretreatment (by 94% in SHAM and 87% in PMI cells). Conversely, beta(3)AR stimulation by the selective agonist SR 58611A caused a leftward shift of the activation curve which was significantly larger in PMI cells than in SHAM cells (P<0.01). beta(3)AR response was blunted by PTX-pretreatment, by incubation with N(G)-monomethyl-l-arginine acetate or by the selective beta(3)AR antagonist SR 59230A 1 microM. In conclusion, I(f) is significantly overexpressed in LVM from PMI rat hearts. In these cells, I(f) modulation by beta(1)AR is significantly depressed while beta(3)AR modulation is markedly enhanced, probably reflecting the increased activity of PTX-sensitive G(i) proteins in PMI cells.

Restoration of cardiomyocyte functional properties by angiotensin II receptor blockade in diabetic rats.

Recent evidence suggests that blockade of the renin-angiotensin system ameliorates diabetes-induced cardiac dysfunction, but the mechanisms involved in this process remain elusive. We investigated the effect of treatment with an angiotensin II receptor blocker, losartan, on the metabolic and electrophysiological properties of cardiomyocytes isolated from streptozotocin-induced diabetic (STZ) rats. Glucose uptake and electrophysiological properties were measured in ventricular cardiomyocytes from normoglycemic and STZ-induced diabetic rats given vehicle or 20 mg x kg(-1) x day(-1) losartan for 8 weeks. Insulin and beta-adrenergic stimulation failed to increase the glucose uptake rate in STZ cardiomyocytes, whereas the alpha-adrenergic effect persisted. Concurrently, a typical prolongation of action potential duration (APD) and a decrease of transient outward current (I(to)) were recorded in patch-clamped STZ myocytes. Treatment with losartan did not affect body weight or glycemia of diabetic or control animals. However, in losartan-treated STZ-induced diabetic rats, beta-adrenergic-mediated enhancement of glucose uptake was completely recovered. APD and I(to) were similar to those measured in losartan-treated control rats. A significant (P < 0.0001) correlation between metabolic and electrophysiological parameters was found in control, diabetic, and losartan-treated diabetic rats. Thus, angiotensin receptor blockade protects the heart from the development of cellular alterations typically associated with diabetes. These data suggest that angiotensin receptor blockers may represent a new therapeutic strategy for diabetic cardiomyopathy.

Treatment with irbesartan counteracts the functional remodeling of ventricular myocytes from hypertensive rats.

Changes in electrophysiological (action potential prolongation, decrease in transient outward current I(to), occurrence of the hyperpolarization-activated current I(f)) and contractile properties develop in hypertrophied ventricular myocytes, likely implicated in the increased propensity to arrhythmias. Angiotensin II is a key signal for myocyte hypertrophy; the effect of 8-week treatment with irbesartan, a type 1 angiotensin II receptor (AT(1)) antagonist, on cardiac remodeling was tested. Sixteen-month-old hypertensive rats (SHRs) were treated with irbesartan (20 mg/kg/d) or saline for 8 weeks. At the end of treatment, systolic blood pressure and heart weight to body weight ratio were reduced in irbesartan-treated compared with nontreated SHRs. Electrical and contractile properties were measured in isolated ventricular myocytes, by patch-clamp or video-dimension analysis, respectively. Action potential duration was significantly shorter in irbesartan-treated than in nontreated SHRs (at -60 mV: 119 +/- 24 ms vs 187 +/- 20 ms); correspondingly, maximal I(to) density was larger in irbesartan-treated than in nontreated SHRs (25.4 +/- 2.8 pA/pF vs 18.5 +/- 1.5 pA/pF). Maximal specific conductance of I(f) was lower in irbesartan-treated vs nontreated SHRs (24.8 +/- 3.0 pS/pF vs 35.2 +/- 4.0 pS/pF). Finally, the relaxation rate of shortening in field-stimulated intact myocytes was significantly faster in irbesartan-treated than in nontreated SHRs (7.3 +/- 0.5/s vs 5.7 +/- 0.3/s). Thus, AT(1) blockade with irbesartan, at an oral daily dosage that gave a slight but significant reduction of systolic blood pressure, largely counteracts the development of myocyte hypertrophy and associated functional alterations.

Selectivity of different calcium antagonists on T- and L-type calcium currents in guinea-pig ventricular myocytes.

Both L- and T-type calcium channels are present in the heart. In cardiac myocytes L-type calcium channels are blocked by the classical calcium channel blockers, while T-type calcium channels are thought to be insensitive to these drugs and to be selectively blocked by mibefradil. We aimed to compare the T/L calcium channel blocking selectivity of several calcium channel blockers by evaluating their effects on both components evoked in the same cell from a holding potential corresponding to the normal physiological value (-90mV). Currents were recorded in single patch-clamped guinea-pig ventricular myocytes, superfused with a Na(+)- and K(+)-free solution to abolish overlapping currents. Two dihydropyridines (amlodipine and lacidipine), verapamil diltiazem and mibefradil were tested; for each compound concentrations equieffective on L-type Ca(2+) current were used. All calcium channel blockers, at concentrations blocking less than 30% of L-type Ca(2+) current, inhibited a significant amount of T-type Ca(2+) current, varying from 0.8% (diltiazem) to 28% (mibefradil). We calculated for each compound the T/L ratio. As expected, mibefradil showed the highest T selectivity; lacidipine and diltiazem resulted to be L selective. Verapamil and amlodipine were not selective. Thus, the calcium channel blockers can be differentiated on the basis of their T/L selectivity.

Does recombinant human interleukin-11 exert direct electrophysiologic effects on single human atrial myocytes?

Recombinant human interleukin-11 (rhIL-11) treatment given to alleviate side effects of cancer therapy is associated with an increased susceptibility to atrial arrhythmias in elderly patients. To elucidate the mechanism underlying this action, we investigated the direct electrophysiologic effect of rhIL-11 on single human atrial myocytes (HuAM) using the patch-clamp technique. Action potentials (AP) at different driving rates were recorded in the perforated-patch configuration, and L-type calcium current (I(Ca,L)), outward potassium currents (I(to) and I(K)), and the hyperpolarization-activated pacemaker current If were measured in the disrupted whole-cell configuration. At therapeutic concentrations (i.e., 10-100 ng/ml), rhIL-11 did not modify AP parameters and cycle-length dependence of AP duration. I(Ca,L) (measured at 0 mV) was 370 +/- 45 pA in control and 379 +/- 48 pA and 368 +/- 42 pA in the presence of 10 and 50 ng/ml rhIL-11, respectively (p = NS). The amplitude and activation of I(to) were not modified by rhIL-11 (i.e., I(to) was at +60 mV: 2.1 +/- 0.2 nA in control vs. 1.9 +/- 0.2 nA and 2.1 +/- 0.2 nA in the presence of 10 and 50 ng/ml rhIL-11, respectively, p = NS). Similarly, late currents measured at the end of the pulse were unchanged in the presence of 10 or 50 ng/ml of rhIL-11. If activation was not modified by rhIL-11: maximal current was 173 +/- 34 pA in control and 159 +/- 35 pA and 117 +/- 14 pA in the presence of 10 and 50 ng/ml of rhIL-11, respectively; midpoint activation was -99 +/- 3 mV in control and -98 +/- 4 mV and -94 +/- 2 mV in the presence of 10 and 50 ng/ml of rhIL-11, respectively (p = NS). Thus, it is unlikely that direct alterations of membrane potential and currents of HuAM caused by rhIL-11 are the basis for the genesis of atrial arrhythmias observed in patients treated with this agent.