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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Transient outward current carried by inwardly rectifying K⁺ channels in guinea pig ventricular myocytes dialyzed with low-K⁺ solution

Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7

Submitted 3 November 2003 ; accepted in final form 26 June 2004

There have been periodic reports of nonclassic (4-aminopyridine insensitive) transient outward K⁺ current in guinea pig ventricular myocytes, with the most recent one describing a novel voltage-gated inwardly rectifying type. In the present study, we have investigated a transient outward current that overlaps inward Ca²⁺ current (I_Ca,L) in myocytes dialyzed with 10 mM K⁺ solution and superfused with Tyrode’s solution. Although depolarizations from holding potential (V_hp) –40 to 0 mV elicited relatively small inward I_Ca,L in these myocytes, removal of external K⁺ or addition of 0.2 mM Ba²⁺ more than doubled the amplitude of the current. The basis of the enhancement of I_Ca,L was the suppression of a large transient outward K⁺ current. Similar enhancement was observed when V_hp was moved to –80 mV and test depolarizations were preceded by short prepulses to –40 mV. Investigation of the time and voltage properties of the outward K⁺ transient indicated that it was inwardly rectifying and unlikely to be carried by voltage-gated channels. The outward transient was attenuated in myocytes dialyzed with high-Mg²⁺ solution, accelerated in myocytes dialyzed with 100 µM spermine solution, and abolished with time in myocytes dialyzed with ATP-free solution. These and other findings suggest that the outward transient is a component of classic "time-independent" inwardly rectifying K⁺ current.

L-type calcium current; magnesium; spermine; ATP

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