Contribution of phosphodiesterase isozymes to the regulation of the L-type calcium current in human cardiac myocytes

Abstract

1. To determine the contribution of the various phosphodiesterase (PDE) isozymes to the regulation of the L-type calcium current (I_Ca(L)) in the human myocardium, we investigated the effect of selective and non-selective PDE inhibitors on I_Ca(L) in single human atrial cells by use of the whole-cell patch-clamp method. We repeated some experiments in rabbit atrial myocytes, to make a species comparison.

2. In human atrial cells, 100 μM pimobendan increased I_Ca(L) (evoked by depolarization to +10 mV from a holding potential of −40 mV) by 250.4±45.0% (n=15), with the concentration for half-maximal stimulation (EC₅₀) being 1.13 μM. I_Ca(L) was increased by 100 μM UD-CG 212 by 174.5±30.2% (n=10) with an EC₅₀ value of 1.78 μM in human atrial cells. These two agents inhibit PDE III selectively.

3. A selective PDE IV inhibitor, rolipram (1–100 μM), did not itself affect I_Ca(L) in human atrial cells. However, 100 μM rolipram significantly enhanced the effect of 100 μM UD-CG 212 on I_Ca(L) (increase with UD-CG 212 alone, 167.9±33.9, n=5; increase with the two agents together, 270.0±52.2%; n=5, P<0.05). Rolipram also enhanced isoprenaline (5 nM)-stimulated I_Ca(L) by 52.9±9.3% (n=5) in human atrial cells.

4. In rabbit atrial cells, I_Ca(L) at +10 mV was increased by 22.1±9.0% by UD-CG 212 (n=10) and by 67.4±12.0% (n=10) by pimobendan (each at 100 μM). These values were significantly lower than those obtained in human atrial cells (P<0.0001). Rolipram (1–100 μM) did not itself affect I_Ca(L) in rabbit atrial cells. However, I_Ca(L) was increased by 215.7±65.2% (n=10) by the combination of 100 μM UD-CG 212 and 100 μM rolipram. This value was almost 10 times larger than that obtained for the effect of 100 μM UD-CG 212 alone.

5. These results imply a species difference: in the human atrium, the PDE III isoform seems dominant, whereas PDE IV may be more important in the rabbit atrium for regulating I_Ca(L). However, PDE IV might contribute significantly to the regulation of intracellular cyclic AMP in human myocardium when PDE III is already inhibited or when the myocardium is under β-adrenoceptor-mediated stimulation.

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