Aim: To examine the effects of tachycardia remodeling and muscarinic regulation on IKH in PV and left atrial (LA) cells. Methods: Cardiomyocytes from canine LA and PV-sleeves were examined with patch-clamp technique. Western blotting was done with membranes of isolated cells. Rapid atrial pacing (RAP, 400 bpm) was performed for 1 week.
Results: IKH is composed of an instantaneous (INST) and a time-dependent (TD) component. Constitutively active TD was greater in PV than LA (at -100 mV: -1.3±0.2 vs-0.9±0.1 pA/pF, p<0.01) and increased with RAP to -2.1±0.3 in PV and -1.4±0.4 pA/pF in LA. The increase of TD in PV myocytes after carbachol was 2.7-fold in control and blunted after RAP (1.6 fold), so that maximally activated TD was similar in CTL and RAP (-3.1±0.4 vs-3.5±0.5 pA/pF, p=ns. In contrast, unstimulated constitutive INST was similar for LA and PV (at -100mV: -3.9±0.4 vs. -3.6±0.4 pA/pF, p=ns) and was not affected by RAP (-3.7±0.4 vs-3.8±0.7). Carbachol increased INST in PV 2.5-fold but only 1.7-fold after RAP, so that maximally receptor-activated INST was reduced (-9.1±1.3 vs-6.2±0.7 pA/pF, p<0.01, Fig.1A).
Western blotting showed no difference between LA and PV Kir3.1/3.4 protein levels. RAP lead to a slight reduction in Kir3.4 protein with no change in Kir3.1 but a significant reduction in M2 receptor (Fig.1B) and Giα. This may explain the reduction in carbachol activated INST with RAP.
(A) mean±SEM density of INST IKH in PV myocytes. (B) Western blots of M2 receptor protein
Conclusion: These results suggest that regional differences in constitutively active IKH are rather due to signaling than ion channel expression differences. G-protein and M2 receptor expression is altered in atrial tachycardia. The effects of remodeling on IKH may contribute to AF promotion, particularly in PV. The results of this study provide novel insights into regional and regulatory differences of ionic currents in AF.
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