Apico-basal inhomogeneity in distribution of ion channels in canine and human ventricular myocardium.

OBJECTIVES: The aim of the present study was to compare the apico-basal distribution of ion currents and the underlying ion channel proteins in canine and human ventricular myocardium.
METHODS: Ion currents and action potentials were recorded in canine cardiomyocytes, isolated from both apical and basal regions of the heart, using whole-cell voltage clamp techniques. Density of channel proteins in canine and human ventricular myocardium was determined by Western blotting.
RESULTS: Action potential duration was shorter and the magnitude of phase-1 repolarization was significantly higher in apical than basal canine myocytes. No differences were observed in other parameters of the action potential or cell capacitance. Amplitude of the transient outward K(+) current (29.6+/-5.7 versus 16.5+/-4.4 pA/pF at +65 mV) and the slow component of the delayed rectifier K(+) current (5.61+/-0.43 versus 2.14+/-0.18 pA/pF at +50 mV) were significantly larger in apical than in basal myocytes. Densities of the inward rectifier K(+) current, rapid delayed rectifier K(+) current, and L-type Ca(2+) current were similar in myocytes of apical and basal origin. Apico-basal differences were found in the expression of only those channel proteins which are involved in mediation of the transient outward K(+) current and the slow delayed rectifier K(+) current: expression of Kv1.4, KChIP2, KvLQT1 and MinK was significantly higher in apical than in basal myocardium in both canine and human hearts.
CONCLUSIONS: The results suggest that marked apico-basal electrical inhomogeneity exists in the canine-and probably in the human-ventricular myocardium, which may result in increased dispersion, and therefore, cannot be ignored when interpreting ECG recordings, pathological alterations, or drug effects.