Single-channel recording of inwardly rectifying potassium currents in developing myocardium

Properties of the inwardly rectifying K+ channel, which contributes to the maintenance of the resting membrane potential, were studied in neonatal rabbit ventricular myocytes using the patch-clamp technique. Inward rectification was evident in single-channel current-voltage (I-V) relations at potentials positive to the potassium equilibrium potential (Ek = 0 mV with [K+]o = [K+]i = 150 mM, [Mg2+]i = 2 mM). The single-channel conductance was 3.2 +/- 0.1 pS in physiological (5.4 mM) [K+]o. The zero-current potential shifted 48.4 +/- 2.4 mV for a ten-fold change in [K+]o in neonatal cells. External Ba2+ blocked the current in a dose-dependent manner. The voltage dependence, open-state probability and channel density of this channel were compared between neonatal and adult ventricular myocytes isolated by similar techniques. The open-state probability of the channel was approximately the same in neonatal (0.39 +/- 0.06, n = 13) as in adult cells (0.4 +/- 0.05, n = 11). However, in symmetrical transmembrane K+ concentration [( K+]o = [K+]i = 150 mM), the single channel conductance was significantly smaller in neonatal (25 +/- 0.3 pS, n = 25) as compared with adult cells (31 +/- 0.4 pS, n = 12). In addition, the relationship between resting membrane potential and [K+]o was measured in neonatal and adult myocytes. The resting membrane potential in the neonate was less dependent on [K+]o than in the adult. These results are consistent with an age-related change in resting membrane K+ permeability which may result from a developmental change in the single-channel conductance properties of the inwardly rectifying K+ channel.