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 ofI 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 Ba2+.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 inI h may contribute to changes in the modulation of neuronal excitability in adult versus neonatal rat intracardiac ganglia.