Modulation of transient outward current by extracellular protons and Cd²⁺ in rat and human ventricular myocytes

Abstract

1. The effects of extracellular acidosis and Cd¥ on the transient outward current (Ito) have been investigated in rat and human ventricular myocytes, using the whole-cell patch-clamp technique. 2. In rat myocytes, exposure to acidic extracellular solution (pH 6·0) shifted both steady-state activation and inactivation curves to more positive potentials, by 20·5 ± 2·7 mV (mean ± s.e.m.; n = 4) and 19·8 ± 1·2 mV, respectively. Cd¥ also shifted the activation and inactivation curves in a positive direction in a concentration-dependent manner. 3. In human myocytes, the steady-state activation and inactivation curves were located at more positive potentials. The effect of Cd¥ was similar, but acidosis had less effect than in rat myocytes (e.g. pH 6·0 shifted activation by only 7·2 ± 2·2 mV and inactivation by 13·7 ± 0·5 mV; n = 4). 4. In both species, the effect of acidosis decreased with increasing concentrations of Cd¥ and vice versa, suggesting competition between H¤ and Cd¥ for a common binding site. 5. The data indicate that acidosis and divalent cations influence Ito via a similar mechanism and act competitively in both rat and human myocytes, but that human cells are less sensitive to the effects of acidosis.7786

Channels involved in transient currents unmasked by removal of extracellular calcium in cardiac cells

“…The "high" threshold for activation of the transient currents and their sensitivity to Iso also make it unlikely that they were due to monovalent cation flow through T-type Ca 2ϩ channels. Finally, although nifedipine has been shown to block a variety of native or expressed K ϩ channels with low affinity (10,32), extracellular divalent cations are not known to block I to [except from a shift of the voltage-dependent activation and inactivation (12,27)], and I to is not increased by ␤-adrenergic receptor stimulation. We are therefore left with the hypothesis that they are due to L-type Ca 2ϩ channels.…”

“…Membrane currents were measured as previously described (12,27) using the whole cell patch-clamp technique (11). Heatpolished borosilicate glass electrodes (horizontal puller, Zeitz Instrumente; Munich, Germany) with tip resistances of 1-1.5 M⍀ when filled with the internal solution were used.…”

Modulation of ventricular transient outward K+ current by acidosis and its effects on excitation-contraction coupling

“…We also studied the indirect action of these repolarization changes on I Ca,L and CaTs. In contrast to previous reports of pH effects on ventricular I to (24,50,58,59), our experiments were performed at physiological temperature with a myocyte preparation with a more humanlike AP than that of rat.…”

“…Previous work with rat ventricular myocytes, measured at room temperature, reported that both extracellular acidosis (50) and simultaneous reductions in pH o and pH i (24) (Fig. 3A), as also observed in rat ventricular myocytes (50).…”

“…In addition, previous work has shown that I to in rat ventricular myocytes, measured at room temperature, is altered by extracellular acidosis [low extracellular pH (pH o )] (50), intracellular acidosis [low intracellular pH (pH i )] (58,59), and simultaneous reductions in pH o and pH i (combined acidosis) (24). Low pH o elicits right shifts in inactivation (toward less negative potentials) causing increased I to at voltages of approximately ϩ50 mV to ϩ60 mV when initiated from depolarized potentials but has little or no effect when initiated from voltages near the normal resting potential (50). In contrast, intracellular acidosis (pH o 7.4), induced by pipette acid loading, is reported to decrease I to over the voltage range of Ϫ30 mV to ϩ60 mV (58,59).…”