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National Institute for Physiological Sciences, Okazaki, Japan.
1. The sensitivity to internal Ca2+ of the hyperpolarization-activated inward current (Ih or If) in rabbit single sino-atrial node cells was investigated by the whole-cell voltage-clamp method. 2. When the patch pipette contained an internal solution of pCa 10, the amplitude of If decreased by 74.8 +/- 3.3% in 10 min (n = 7) after rupture of the patch membrane. When the pipette contained an internal solution of pCa 7, If increased by 43.7 +/- 8.7% within 10 min (n = 5). 3. Increase of If by the higher Ca2+ internal solution was confirmed in the same cell using the cell dialysis method. Both If and its tail current were increased at every membrane potential. The amplitude of If increased most markedly between pCa 8 and 7. 4. The reversal potential and kinetics of If were unaffected by the internal Ca2+ concentration. Increase of If by the high internal Ca2+ concentration was sensitively blocked by Cs+. These findings confirm that the increased current is indeed If and not a newly activated If-like current due to elevation of internal Ca2+. 5. The activation curve of If shifted approximately 13 mV in a positive direction by elevating Ca2+ from pCa 10 to 7 (n = 21), indicating that the voltage dependence of If was modulated by internal Ca2+. 6. beta-Agonists also modulated If, but the underlying mechanisms of their effects on If differed from those of the internal Ca2+. The former affected the If kinetics rather than its amplitude, whereas the latter acted on the If conductance rather than on its kinetics. 7. The increase in If by the internal Ca2+ was unaffected by protein kinase inhibitor or calmodulin inhibitor, suggesting that the internal Ca2+ directly modulates If. 8. When the patch pipette contained pCa 7 internal solution, the maximum diastolic potential shifted towards a positive potential but the heart rate remained almost constant.
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