Ca and Na selectivity of the active membrane of rabbit AV nodal cells

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Abstract
The atrioventricular (AV) node is thought to have a slow ionic channel. These experiments were designed to measure the relative contributions of Na and Ca ions to inward currents in the AV nodal cells of rabbit heart superfused with Tyrode solution. The effects of tetrodoxin (TTX), Mn2+, and verapamil observed in this study were in agreement with reports by others. The overshoot of AV nodal (N) cells was related to external Ca, with a slope of 12 mV/decade, unchanged by addition of TTX. Similar dependence of overshoot on external Na was seen, with a slope of 20 mV/decade. The slope did not change on addition of TTX. Total removal of either Na or Ca from the solution abolished excitability. Using a constant field equation, we estimated relative permeability (P) of the membrane at the time of maximal overshoot to be PCa/PNa congruent to 60 similar to or approximately 100 and PK/PNa congruent to 1. Relative contributions of these ions to the currents were estimated as ICa congruent to 17%, INa congruent to 33% (inward currents), and IK congruent to 50% (outward current). In conclusion, AV nodal cells have "slow inward-current channels" that are selective for Ca over Na ions.