CHARACTERIZATION OF THE HYPERPOLARIZATION-ACTIVATED CURRENT, I-F, IN VENTRICULAR MYOCYTES ISOLATED FROM HYPERTENSIVE RATS

1. Left ventricular myocytes isolated from the heart of young (2-month -old) and old (18- to 20-month-old) spontaneously hypertensive rats (S HRs) were studied in the whole-cell configuration. Since multicellular preparations from old SHRs show a diastolic depolarization phase, we performed experiments to test whether it was associated with the prese nce of a hyperpolarization-activated I-f-like current. 2. In control T yrode solution, a time-dependent increasing inward current activated b y hyperpolarization was recorded in myocytes from old SHRs showing a d iastolic depolarization phase. A barium-insensitive, caesium-sensitive , time-dependent inward current was recorded in a minority (4 of 33) o f cells from young SHRs (membrane capacitance, 160 +/- 7 pF) but in 93 % (25 of 27, P < 0.01) of myocytes from old SHRs (membrane capacitance , 355 +/- 19 pF, P < 0.01). 3. The current was fully activated at -120 mV and voltage of half-maximal activation was -88.1 +/- 1.5 mV; it wa s blocked by extracellular CsCl (4 mM) in a voltage-dependent manner. Reducing [K+](0) from 25 to 5.4 mM caused a shift of the reversal pote ntial from -17.3 +/- 3.8 to -25.7 +/- 2.7 mV and a 60% decrease of cur rent conductance. 4. These findings suggest that an I-f-like current i s present in rat ventricular myocytes from old SHRs, where it might fa vour the occurrence of spontaneous action potentials.