Left ventricular myocytes isolated from the heart of young (2-month-old) and old (18- to 20-month-old) spontaneously hypertensive rats (SHRs) 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 presence of a hyperpolarization-activated If-like current. 2. In control Tyrode solution, a time-dependent increasing inward current activated by hyperpolarization was recorded in myocytes from old SHRs showing a diastolic depolarization phase. A barium-insensitive, caesium-sensitive, time-dependent inward current was recorded in a minority (4 of 33) of 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 was blocked by extracellular CsCl (4 mM) in a voltage-dependent manner. Reducing [K+]o from 25 to 5.4 mM caused a shift of the reversal potential from -17.3 +/- 3.8 to -25.7 +/- 2.7 mV and a 60% decrease of current conductance. 4. These findings suggest that an If-like current is present in rat ventricular myocytes from old SHRs, where it might favour the occurrence of spontaneous action potentials.
