Dynamic Ca2+-induced inward rectification of K+ current during the ventricular action potential.
Inward rectification, an important determinant of cell excitability, can result from channel blockade by intracellular cations, including Ca2+. However, mostly on the basis of indirect arguments, Ca2+-mediated rectification of inward rectifier K+ current (IK1) is claimed to play no role in the mammalian heart. The present study investigates Ca2+-mediated IK1 rectification during the mammalian ventricular action potential. Guinea pig ventricular myocytes were patch-clamped in the whole-cell configuration. The action potential waveform was recorded and then applied to reproduce normal excitation under voltage-clamp conditions. Subtraction currents obtained during blockade of K+ currents by either 1 mmol/L Ba2+ (IBa) or K+-free solution (I0K) were used to estimate IK1. Similar time courses were observed for IBa and I0K; both currents were strongly reduced during depolarization (inward rectification). Blockade of L-type Ca2+ current by dihydropyridines (DHPs) increased systolic IBa and I0K by 50.7% and 254.5%, respectively. beta-Adrenergic stimulation, when tested on I0K, had an opposite effect; ie, it reduced this current by 66.5%. Ryanodine, an inhibitor of sarcoplasmic Ca2+ release, increased systolic IBa by 47.7%, with effects similar to those of DHPs. Intracellular Ca2+ buffering (BAPTA-AM) increased systolic IBa by 87.7% and blunted the effect of DHPs. Thus, IK1 may be significantly reduced by physiological Ca2+ transients determined by both Ca2+ influx and release. Although Ca2+-induced effects may represent only a small fraction of total IK1 rectification, they are large enough to affect excitability and repolarization. They may also contribute to facilitation of early afterdepolarizations by conditions increasing Ca2+ influx.
Zaza
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Dipartimento di Fisiologia e Biochimica Generali, Università degli Studi di Milano, Milan, Italy. zanto@imiucca.csi.unimi.it
Rocchetti
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Brioschi
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Cantadori
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Ferroni
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D000200:Action Potentials; D000818:Animals; D001464:Barium / Q000502:physiology; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D004095:Dihydropyridines / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004553:Electric Conductivity; D006168:Guinea Pigs; D008274:Magnesium / Q000502:physiology; D008564:Membrane Potentials; D011188:Potassium / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D016276:Ventricular Function
D002121:Calcium Channel Blockers; D004095:Dihydropyridines; D011943:Receptors, Adrenergic, beta; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D001464:Barium; D004533:Egtazic Acid; D008274:Magnesium; D011188:Potassium; D002118:Calcium