Hangang Yu

Role of the Kv4.3 K+ Channel in Ventricular Muscle: A Molecular Correlate for the Transient Outward Current

The expression of 15 different K+ channels in canine heart was examined, and a new K+ channel gene (Kv4.3), which encodes a rapidly inactivating K+ current, is described. The Kv4.3 channel was found to have biophysical and pharmacological properties similar to the native canine transient outward current (I(to)). The Kv4.3 gene is also expressed in human and rat heart. It is concluded that the Kv4.3 channel underlies the bulk of the I(to) in canine ventricular myocytes, and probably in human myocytes. Both the Kv4.3 and Kv4.2 channels are likely to contribute to the I(to) in rat heart, and differential expression of these two channels can account for observed differences in the kinetic properties of the I(to) in different regions of rat ventricle. There are significant differences in the pattern of K+ channel expression in canine heart, compared with rat heart, and these differences may be an adaptation to the different requirements for cardiac function in mammals of markedly different sizes. It is possible that the much longer ventricular action potential duration observed in canine heart compared with rat heart is due, in part, to the lower levels of Kv1.2, Kv2.1, and Kv4.2 gene expression in canine heart.

Transient Outward Current, Ito1, Is Altered in Cardiac Memory

BACKGROUND Cardiac memory refers to an altered T-wave morphology induced by ventricular pacing or arrhythmias that persist for variable intervals after resumption of sinus rhythm. METHODS AND RESULTS We induced long-term cardiac memory (LTM) in conscious dogs by pacing the ventricles at 120 bpm for 3 weeks. ECGs were recorded daily for 1 hour, during which time pacing was discontinued. At terminal study, the heart was removed and the electrophysiology of left ventricular epicardial myocytes was investigated. Control (C) and LTM ECG did not differ, except for T-wave amplitude, which decreased from 0.12+/-0.18 to -0.34+/-0.21 mV (+/-SEM, P<0.05), and T-wave vector, which shifted from -37+/-12 degrees to -143+/-4 degrees (P<0.05). Epicardial action potentials revealed loss of the notch and lengthening of duration at 20 days (both P<0.05). Calcium-insensitive transient outward current (Ito) was investigated by whole-cell patch clamp. No difference in capacitance was seen in C and LTM myocytes. Ito activated on membrane depolarization to -25+/-1 mV in C and -7+/-1 mV (P<0.05) in LTM myocytes, indicating a positive voltage shift of activation. Ito density was reduced in LTM myocytes, and a decreased mRNA level for Kv4.3 was observed. Recovery of Ito from inactivation was significantly prolonged: it was 531+/-80 ms (n=10) in LTM and 27+/-6 ms (n=9) in C (P<0.05) at -65 mV. CONCLUSIONS Ito changes are associated with and can provide at least a partial explanation for action-potential and T-wave changes occurring with LTM.

Developmental change in the voltage-dependence of the pacemaker current, if, in rat ventricle cells.

Abstract Myocytes were isolated from newborn and adult rat ventricle. Using the whole-cell patch clamp, the two cell populations were compared for the presence of the hyperpolarization-activated pacemaker current if. As in other mammalian species, the threshold voltage in acutely dissociated adult rat myocytes was extremely negative (–113 ± 5 mV; n=12). In contrast, threshold in newborn cells was relatively positive, regardless of whether measured in acutely dissociated (–72 ± 2 mV; n=6) or cultured cells (–70 ± 2 mV; n=9). Current density was not reduced in the adult. These results suggest that with development the ventricle assumes its non-pacemaker function, at least in part, by a shift of the voltage dependence of if outside the physiological range.

Positive Chronotropic Actions of Parathyroid Hormone and Parathyroid Hormone–Related Peptide Are Associated With Increases in the Current, If, and the Slope of the Pacemaker Potential

BACKGROUND The classic calciotropic hormone parathyroid hormone (PTH) and its paracrine factor parathyroid hormone-related protein (PTHrP) both increase heart rate. METHODS AND RESULTS We used standard electrophysiological techniques to study the effects of PTH and PTHrP on isolated rabbit sinus node, isolated canine Purkinje fibers, and disaggregated rabbit sinus node myocytes. Sinus node maximum diastolic potential, activation voltage, and amplitude were unchanged by PTH or PTHrP (P>.05). However, the slope of phase 4 and the automatic rate were increased at PTH and PTHrP > or = 10 nmol/L (P<.05). Comparable results were seen in canine Purkinje fibers. We then used the perforated-patch technique to study the I(f) pacemaker current in sinus node. PTH 12.5 nmol/L and PTHrP 12.5 to 18 nmol/L increased I(f) at -65 mV by 68+/-41% (n=5) and 69+/-50% (n=5), respectively. Actions of both agents were reversible. The increase in I(f) appeared to result from a change in maximal conductance and not a shift in the voltage dependence of activation. CONCLUSIONS These observations provide, for the first time, direct electrophysiological support for the chronotropic actions of PTH and PTHrP. They suggest that classic hormones and paracrine factors can have multiple functions and that in the case of PTH and PTHrP, a newly recognized action is to alter automaticity directly.

Phosphatase inhibition by calyculin A increases if in canine Purkinje fibers and myocytes

SummaryThe actions of the phosphatase inhibitor calyculin A on the pacemaker current if were studied in canine Purkinje fibers and myocytes. Calyculin A increased if in response to hyperpolarizations toward the middle of the if activation curve. A three pulse protocol indicated this increase was due to a positive shift of if activation on the voltage axis. Taken together with our previous results (that kinase inhibition with H7 shits if activation in the negative direction on the voltage axis (2)), these results suggest that phosphorylation is an important regulator of the voltage dependence of if activation.

Epidermal growth factor increases i f in rabbit SA node cells by activating a tyrosine kinase

Our previous results have demonstrated that tyrosine kinase inhibition reduces i(f) in rabbit SA node myocytes, suggesting that tyrosine kinases regulate i(f). One receptor tyrosine kinase the EGF receptor kinase is known to increase heart rate. To determine if this action is mediated through changes in i(f), we examined the effect of epidermal growth factor (EGF) on i(f) with the permeabilized patch-clamp technique. 0.1 microM EGF increased i(f) amplitude in response to single-step hyperpolarizations in the diastolic range of potentials. This increase was 20+/-3%, n=11 at -75 mV. This effect is caused by activating a tyrosine kinase because 50 microM genistein, a tyrosine kinase inhibitor, eliminated this EGF action. A two-step pulse protocol showed that maximal i(f) conductance was increased by EGF. We further examined this conductance change by constructing the activation curve. The maximal i(f) conductance was increased by 23% with no change in midpoint, V(1/2), control=-74+/-2 mV, V(1/2) EGF=-74+/-1 mV. Thus EGF acts via a tyrosine kinase to increase maximal i(f) conductance with no change in the voltage dependence of activation. These results suggest that EGF effects on i(f) contribute to the positive chronotropic effect of EGF on SA node.