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 mu M 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 mu M. I-Ca(L) was increased by 100 mu M UD-CG 212 by 174.5+/-30.2% (n=10) with an EC50 value of 1.78 mu M in human atrial cells. These two agents inhibit PDE III selectively.
3 A selective PDE IV inhibitor, rolipram (1-100 mu M), did not itself affect I-Ca(L) in human atrial cells. However, 100 mu M rolipram significantly enhanced the effect of 100 mu M 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 mu M). These values were significantly lower than those obtained in human atrial cells (P < 0.0001). Rolipram (1-100 mu M) 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 mu M UD-CG 212 and 100 mu M rolipram. This value was almost 10 times larger than that obtained for the effect of 100 mu M 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 beta-adrenoceptor-mediated stimulation.