COMPARISON OF THE EFFECTS OF INTERNAL [MG2-K1 IN CAT AND GUINEA-PIG CARDIAC VENTRICULAR MYOCYTES(] ON I)

The effects of internal Mg2+ (Mg-i(2+)) on processes underlying the in ward rectification of the cardiac I-K1 in enzymatically isolated cardi ac ventricular myocytes (CVM) obtained from cat, guinea pig and rabbit were compared using the whole-cell and excised inside-out patch confi gurations of the voltage-clamp technique. In confirmation of the findi ngs of other investigators, Mg-i(2+)-sensitive outward I-K1 currents c ould be elicited from guinea pig CVM at 15 degrees C when Mg-i(2+) was reduced from 1 mM to less than 1 mu M, suggesting that Mg-i(2+) has a n important role in the inward rectification of I-K1 in guinea pig CVM at unphysiologically low temperatures. However, as temperature was ra ised to more physiological levels, (e.g., 30 degrees C), Mg-i(2+)-sens itive outward I-K1 currents could no longer be evoked. In contrast wit h the results obtained with guinea pig CVM, I-K1 remained inwardly rec tifying in cat and rabbit CVM independent of the Mg-i(2+) concentratio n ([Mg2+](i)) at 37 degrees, 15 degrees, 10 degrees and 5 degrees C, R educed [Mg2+](i) at low temperature (15 degrees C), shifted the voltag e dependence of guinea pig I-K1 activation in the depolarizing directi on; this resulted in an apparent linearization of I-K1 conductance. Wh en the range of the membrane potential examined was expanded to includ e voltages up to +80 mV, the guinea pig I-K1 was found to exhibit inwa rd rectification with the inflection in the I-V relationship that is f ound at similar to E(K) at normal temperature, also shifted 80 mV or m ore to much less negative voltages at low temperatures. In contrast, i rrespective of [Mg2+](i) or temperature, the voltage dependencies of t he I-K1 activation curves for both cat and rabbit myocytes were not ch anged from that defined when [Mg2+](i) was 1 mill and temperature was 37 degrees C. We propose that Mg-i(2+) affects inward rectification of I-K1 in cold guinea pig CVM by altering the voltage dependence of act ivation.