Transient outward current in catecholamine-induced cardiac hypertrophy in the rat

We have demonstrated that a daily injection of isoproterenol (5 mg/kg ip) for 7 days induces a 30% increment in heart weight-to-body weight ratio and prolongs the action potential duration (APD) in male Wistar rats. The underlying mechanism of the prolonged APD was investigated in this model of hypertrophy by measuring the transient outward potassium current (Ito) in left ventricular myocytes of the rat with whole cell voltage-clamp techniques. Cell membrane capacitance was increased by 39%: 122 +/- 3 (n = 23) and 171 +/- 5 (SE) pF (n = 20) in control and hypertrophy, respectively (P < 0.001). Ito was evoked in sodium-free solutions containing 0.5 mM Ca2+ and 2 mM Co2+ by step depolarizations from a holding potential of -80 mV. The amplitude of the 4-aminopyridine-sensitive Ito (at 70 mV) was reduced by 28% in hypertrophy: 3.2 +/- 0.3 (n = 23) and 2.3 +/- 0.4 (SE) nA (n = 20) in control and hypertrophy, respectively (P < 0.05). When normalized for cell capacitance, the reduction was much larger: 26.4 +/- 2.5 and 13.1 +/- 1.8 pA/pF in control and hypertrophy, respectively (P < 0.001). The voltage dependence of Ito was similar in both cell types. No change was observed in the steady-state activation and inactivation kinetics in the two groups, nor was there a change in the time dependence of inactivation. The recovery from inactivation of Ito when fitted with a monoexponential function was not changed significantly in hypertrophy: time constants = 8.2 +/- 0.4 (n = 13) and 8.3 +/- 0.3 ms (n = 12) in control and hypertrophy, respectively. These results show that Ito density is decreased in catecholamine-induced cardiac hypertrophy, but current kinetics are not affected. The reduced Ito density may underlie the prolongation of APD in this model of hypertrophy.