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Nakajima I, Watanabe H, Iino K, Saito T, Miura M: Ca2+ overload evokes a transient outward current in guinea-pig ventricular myocytes. Circ J. 2002 Jan;66(1):87-92.
There are 2 types of transient outward currents (Ito) in the hearts of various mammals: a 4-aminopyridine (4-AP) sensitive K+ current and a 4-AP resistant Ca2+ activated current, carried by Cl-, (referred to as I (to1) and I (to2), respectively). However, the I (to) has been considered to be absent in guinea-pig ventricular myocytes and so this study tested the hypothesis that I (to1) is generally absent in guinea-pig ventricular myocytes, but I (to2) appears under the condition of Ca2+ overload. Membrane currents were recorded by the whole-cell patch-clamp technique and Ca2+ overload was achieved by adding internal, and eliminating external, Na+ with subsequent enhancement of Ca2+ influx via the Na+-Ca2+ exchange. Under physiological conditions, I (to) could not be elicited by 300 ms-test pulse from -70 mV to 0 mV (n=32). However, under Ca2+ overload, a biphasic current resulting from the overlap of the L-type Ca2+ channel current and Ito was elicited (n=38). This I (to) was resistant to 4-AP (3 mmol/L, n=30) but sensitive to both anthrancene-9-carboxylic acid (9-AC, 3 mmol/L, n=8) and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (100 micromol/L, n=3). Replacing K+ with Cs+ on both sides of the membrane failed to abolish I (to) (n=38). I (to) disappeared by lowering the external Cl- (n=3). The amplitude of I (to) was dependent on that of the L-type Ca2+ channel current (n=4). Because Ca2+ release from the sarcoplasmic reticulum was prevented by caffeine (5 mmol/L), I (to) was negligible (n=6). These results suggest that I (to1) is absent, but Ca2+ overload evokes I (to2) in guinea-pig ventricular myocytes. |
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