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Am J Physiol Heart Circ Physiol 279: H692-H701, 2000;
0363-6135/00 $5.00
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Vol. 279, Issue 2, H692-H701, August 2000

Inhibition by nickel of the L-type Ca channel in guinea pig ventricular myocytes and effect of internal cAMP

Ion A. Hobai, Jules C. Hancox, and Allan J. Levi

Cardiovascular Research Laboratories, Bristol Heart Institute, and Department of Physiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, United Kingdom

The characteristics of nickel (Ni) block of L-type Ca current (ICa,L) were studied in whole cell patch-clamped guinea pig cardiac myocytes at 37°C in the absence and presence of 100 µM cAMP in the pipette solution. Ni block of peak ICa,L had a dissociation constant (Kd) of 0.33 ± 0.03 mM in the absence of cAMP, whereas in the presence of cAMP, the Kd was 0.53 ± 0.05 mM (P = 0.006). Ni blocked Ca entry via Ca channels (measured as ICa,L integral over 50 ms) with similar kinetics (Kd of 0.35 ± 0.03 mM in cAMP-free solution and 0.30 ± 0.02 mM in solution with cAMP, P = not significant). Under both conditions, 5 mM Ni produced a maximal block that was complete for the first pulse after application. Ni block of ICa,L was largely use independent. Ni (0.5 mM) induced a positive shift (4 to 6 mV) in the activation curve of ICa,L. The block of ICa,L by 0.5 mM Ni was independent of prepulse membrane potential (over the range of -120 to -40 mV). Ni (0.5 mM) also induced a significant shift in ICa,L inactivation: by 6 mV negative in cAMP-free solution and by 4 mV positive in cells dialyzed with 100 µM cAMP. These data suggest that, in addition to blocking channel conductance by binding to a site in the channel pore, Ni may bind to a second site that influences the voltage-dependent gating of the L-type Ca channel. They also suggest that Ca channel phosphorylation causes a conformational change that alters some effects of Ni. The results may be relevant to excitation-contraction coupling studies, which have employed internal cAMP dialysis, and where Ni has been used to block ICa,L and Ca entry into cardiac cells.

divalent; patch clamp


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