Epicardial cells are more susceptible to the electrophysiological effects of ischemia than are endocardial cells. To explore the ionic basis for the differential electrophysiological responses to ischemia at the two sites, we used patch-clamp techniques to study the effects of ATP depletion on action potential duration and the ability of ATP-regulated K+ channels in single cells isolated from feline left ventricular endocardial and epicardial surfaces. During ATP depletion by treatment with 1 mM cyanide (CN-), shortening of action potential durations was significantly greater in epicardial cells than in endocardial cells. Thirty minutes after initiating exposure to 1 mM CN-, action potential duration at 90% repolarization was reduced to 0.70 +/- 0.12 of the control value for endocardial cells versus 0.39 +/- 0.18 for epicardial cells (p less than 0.01), and action potential duration at 20% repolarization was reduced to 0.72 +/- 0.13 for endocardial cells versus 0.12 +/- 0.09 for epicardial cells (p less than 0.01). In both endocardial and epicardial cells, the shortening of action potential by CN- treatment was partially reversed by 0.3 microM glibenclamide; the magnitude of reversal, however, was much greater in epicardial cells. After exposure to 1 mM CN-, the activity of ATP-regulated K+ channels in cell-attached membrane patches was significantly greater in epicardial cells than in endocardial cells. To study the dose-response relation between ATP concentration and open-state probability of the channels, intracellular surfaces of inside-out membrane patches containing ATP-regulated K+ channels were exposed to various concentrations of ATP (10-1,000 microM). The concentration of ATP that produced half-maximal inhibition of the channel was 23.6 +/- 21.9 microM in endocardial cells and 97.6 +/- 48.1 microM in epicardial cells (p less than 0.01). These data indicate that ATP-regulated K+ channels are activated by a smaller reduction in intracellular ATP in epicardial cells than in endocardial cells. The differential ATP sensitivity of ATP-regulated K+ channels in endocardial and epicardial cells may be responsible for the differential shortening in action potentials during ischemia at the two sites.

Whole-cell voltage-clamp experiments were performed on enzymatically dissociated single ventricular myocytes harvested from feline endocardial and epicardial surfaces. The studies were designed to test the hypothesis that the differences in the amplitude of transient outward current (Ito) contribute to the difference in action potential configuration between endocardial and epicardial myocytes. In the control state, action potentials recorded from epicardial cells demonstrated a prominent notch between phases 1 and 2, and membrane current recordings displayed a prominent Ito, whereas in endocardial cells the notch in action potentials and Ito were small. External application of 4-aminopyridine (2 mM) reduced the amplitudes of notch and Ito in epicardial cells but not in endocardial cells. After application of 4-aminopyridine (2 mM) and caffeine (5 mM), the notch and Ito were abolished completely in both endocardial and epicardial cells. The first component of Ito (Ito1) was present in all epicardial cells studied (n = 20); it was absent in 12 of the 20 endocardial cells, and a small Ito1 was present in the remaining eight endocardial cells. The mean amplitude of Ito1 was significantly greater in epicardial than in endocardial cells. At a test voltage of +80 mV, the amplitude of Ito1 was 102.0 +/- 47.7 pA/pF in epicardial cells and 3.3 +/- 3.3 pA/pF in endocardial cells (p less than 0.01). The second component of Ito (Ito2) was present in all endocardial (n = 30) and epicardial (n = 30) cells studied. The amplitude of Ito2 was significantly greater in epicardial than in endocardial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

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1. A mechanism underlying reactivation of the adenosine 5'-triphosphate-sensitive K+ (KATP) channels by MgATP complexes after run-down was examined in guinea-pig ventricular myocytes using the patch-clamp technique with inside-out patch configuration. 2. After run-down was induced by exposure of the intracellular side of the membrane patch to Ca2+ (1 mM), channel activity was reactivated by exposure and subsequent wash-out of MgATP (2 mM). Addition of inhibitors of various serine/threonine protein kinases to the MgATP solution did not suppress reactivation of the run-down channels. 3. Non-or poorly hydrolysable ATP analogues were unable to reactivate run-down channels. 4. The degree of channel recovery was dependent upon the duration of MgATP exposure.The apparent half-activation value (K%) of MgATP for reactivation was decreased with increasing exposure time. 5. Various products of ATP hydrolysis were unable to reactivate run-down channels except a relatively low concentration (100 /M) of ADP exposure. 6. Other nucleotide triphosphates, in the presence of Mg2+, were unable to reactivate rundown channels.7. Fluorescein 5-isothiocyanate (50 /uM), which interacts with lysine residues of the nucleotide-binding site on various ATPases, inhibited KATP channel activity. After wash-out, channel activity recovered only slightly. 8. These data suggest that the hydrolysis of ATP is important for reactivation of rundown KATP channels but that protein phosphorylation by serine/threonine protein kinases may not be involved. Since no products of ATP hydrolysis could reproduce MgATP-induced channel reactivation and since the degree of channel recovery was dependent upon the duration of MgATP application, the hydrolysis energy appears to be utilized for channel reactivation.

Left ventricular hypertrophy (LVH) potentiates reperfusionassociated ventricular fibrillation. To study the mechanism responsible, patch-clamp techniques were used to evaluate transmembrane ionic currents during "reperfusion" after a CN --induced metabolic surrogate for ischemia in isolated myocytes from a feline model of experimental LVH. Reperfusion caused the generation of early afterdepolarizations (EADs) from an average take-off potential of -33 mV in LVH cells but not in cells from normal hearts. 10 min after initiating reperfusion of normal cells, action potential duration (APD) at 50% repolarization (APDTO) lengthened from 198±41 to 233±57 ms whereas in LVH cells APDjo lengthened from 262±84 to 349±131 ms (P < 0.05). Among the LVH cells, APDjo lengthening was significantly greater in the cells that had developed EADs. During reperfusion, steady state outward current in the voltage range of the action potential plateau (between -20 and +20 mV) was reduced from the control values in LVH cells but not in normal cells. Reperfusion-related reduction of steady state outward current in LVH cells was abolished under experimental conditions in which L-type Ca2+ current was isolated from other classes of currents whereas it was still observed under the condition in which pure K+ currents could be recorded. Thus, reduction of steady state outward current due to the reduction of outward K+ current over the action potential plateau voltage range appears to be responsible for an excessive prolongation of APD, leading to the development of EADs. (J. Clin. Invest. 1993. 91:1521-1531