Developmental changes in hyperpolarization-activated currents I(h) and I(K(IR)) in isolated rat intracardiac neurons

The hyperpolarization-activated nonselective cation current, I(h), was investigated in neonatal and adult rat intracardiac neurons. I(h) was observed in all neurons studied and displayed slow time-dependent rectification. I(h) was isolated by blockade with external Cs(+) (2 mM) and was inhibited irreversibly by the bradycardic agent, ZD 7288. Current density of I(h) was approximately twofold greater in neurons from neonatal (-4.1 pA/pF at -130 mV) as compared with adult (-2.3 pA/pF) rats; however, the reversal potential and activation parameters were unchanged. The reversal potential and amplitude of I(h) was sensitive to changes in external Na(+) and K(+) concentrations. An inwardly rectifying K(+) current, I(K(IR)), was also present in intracardiac neurons from adult but not neonatal rats and was blocked by extracellular Ba(2+). I(K(IR)) was present in approximately one-third of the adult intracardiac neurons studied, with a current density of -0.6 pA/pF at -130 mV. I(K(IR)) displayed rapid activation kinetics and no time-dependent rectification consistent with the rapidly activating, inward K(+) rectifier described in other mammalian autonomic neurons. I(K(IR)) was sensitive to changes in external K(+), whereby raising the external K(+) concentration from 3 to 15 mM shifted the reversal potential by approximately +36 mV. Substitution of external Na(+) had no effect on the reversal potential or amplitude of I(K(IR)). I(K(IR)) density increases as a function of postnatal development in a population of rat intracardiac neurons, which together with a concomitant decrease in I(h) may contribute to changes in the modulation of neuronal excitability in adult versus neonatal rat intracardiac ganglia.